Clutch Operation Schaeffler Symposium

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					     4       Release system
4        Release system                                                                     Release system   4


                                                                                  Clutch Operation
                                                                                            Roland Welter
                                                                                               Volker Lang
                                                                                            Bernhard Wolf

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4    Release system                                                                                                                                                                                           Release system   4

     Intro-                                                                                                   was presented as an option for targeted im-
                                                                                                              provement of the pedal characteristic. As an ad-
                                                                                                                                                                                                  Cam plate

     duction                                          150
                                                                                                              vantage of this solution, it was described that in
                                                                                                              tight space, a higher load compensation is
                                                                                                              achieved than with an over-center spring, and
                                                                                                              that the course of the curve can be changed al-
                                Pedal force Fp in N
     The articles on clutch         100
                                                                                                              most at will due to the free selection of the cam
     operation from the
                                                                                                              plate geometry. Thus, for example, a curve with
     2002 [1] and 2006 [2]                                                                                                                                                Pulley

                                                                                                              two zero cross-overs of the force is possible, by
     Symposia dealt with
                                                                                                              which means the minimum pedal force can be
     the trend towards
                                                                                                              raised when the pedal is disengaged (Figure 1).
     more torque, higher
     performance density               0                                                                      The principle has since been tested in different ve-                  Spring
     and thus also master-                                                                                    hicles with good results. The original design with
     ing higher release                                                                                       one leaf spring and one pulley has now become
     loads. As manufactur-                                                                                    two pulleys and two springs in the form of V-shaped
     er of complete clutch                                                                                    bent sheets (Figure 2). This avoids lateral forces on
     systems, LuK has fo-                                                                                     the pedal and pedal shaft.
     cused on carrying out               0         20         40           60          80       100    120    Compensation by means of a leaf spring was first
     the design of such
                                                                   Pedal stroke Sp in mm
                                                                          stroke                              developed for vehicles with high engine torque            Figure 2    Pedal force compensation by means of
     systems in a profes-
                                                                                                              and a strong need for force correction. Since                         springs, pulleys and cam plate
     sional manner, based                  Pedal force with compensation                                      such applications are less in demand from to-
     on simulations and                    Compensation spring                                                day’s perspective, the obvious thought was to
     vehicle tunings, and                  Pedal force without compensation                                                                                             duced energy and to achieve a targeted correc-
                                                                                                              change the design to meet the needs of middle
     on offering the right                                                                                                                                              tion of the pedal force curve by means of their
                               Figure 1 Pedal force curve with and without correction (example)               class vehicles and, to do this, to reduce the
     products to meet cus-                                                                                                                                              ramp contour. Three needle rollers are arranged
                                                                                                              achievable force level somewhat. At the same
     tomer requirements,                                                                                                                                                between the ramps and the cylindrical housing
                                                                                                              time, we had to reduce the space and design the
     such as self-adjusting                                                                                                                                             sleeve. When the pedal is operated, the three
                                                                                                              pedal and compensation mechanisms as a single
     clutches and hydraulic actuation systems. In ad-       Conventional clutch operation:                                                                              needle rollers roll down the ramps and, with
                                                                                                              unit. The refinement (Figure 3) uses a compen-
     dition, we have integrated functions which help                                                                                                                    the energy released from the springs, support
                                                            • Power density                                   sation mechanism which is arranged radially
     make the pedal force characteristic more ergo-                                                                                                                     the pedal release movement. When engaged,
                                                                                                              around the pedal shaft.
     nomic and reduce vibrations and noise. While           • Reliability and service life                                                                              energy is again stored in the springs. The prin-
     the development of these technologies contin-                                                            Multiple spring components of sheet metal are             ciple functions invariably and an assembly-
                                                            • Small vehicles
     ues, it has since fallen under the influence of                                                          stacked and the ramps designed as resilient               friendly solution integrated in the pedal is
     additional trends.                                     Consumption-optimized drives:                     bending bars. It is their job to store the intro-         about to be developed.
                                                            • Stop-start systems
     The changes in the world’s economy in 2008
                                                                                                                                                        Lower pedal position                   Upper pedal position
     and 2009 and the recognition that we must do           • Hybrid drives
     a better job at conserving our fossil fuels have
                                                            • Double-clutch transmissions
     led to a rising demand for small cars and lower-                                                                                                                          Spring
     consumption technologies for combustion en-
     gine drives in all classes of vehicles. Clutch sys-                                                                                                                       Housing
     tems for small vehicles are designed to be
     lighter and less costly today with no major com-
                                                            New Developments                                                                                                   Needle roller

     promise as far as comfort. On the other hand,
     consumption-optimized drives, such as stop-
                                                            for Conventional
     start systems, hybrids and double clutch trans-
     missions raise the level of technical challenge
     and complexity.
                                                            Pedal Ergonomics
     LuK can offer innovative solutions in both areas,
     classic manual operation and modern low-con-           In the Symposium 2006 [2], energy storage by
     sumption drives.                                       means of leaf spring and cam plate on the pedal   Figure 3   Unit for pedal force compensation arranged around the pedal shaft

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4    Release system                                                                                                                                                                                                                                                               Release system   4

     Unpleasant pedal vibrations and interior noises
     are excited by oscillations of the crank shaft and
     transferred via the release bearing through the
     hydraulic system all the way to the pedal. At low
     amplitudes, a simple change in the high pressure
     line by means of orifices or by adjusting the diam-
                                                                              Hydraulic pressure
                                                                                                                                                                   Engaging                                                                   Disengaging
                                                                                                                                                                                                                                              Disengaging                                               4
                                                                                                                                                               3                                                                          3
     eter and length is sufficient to correct the prob-

                                                                                                                                      Pressure loss p in bar
     lem satisfactorily. In more difficult cases, damping

                                                                                                                                                                                                                   Pressure loss in bar
     elements come into play. A distinction is made be-
                                                                                                                                                               2                                                                          2
     tween what are known as anti-vibration units
     (AVUs), which close the high pressure line in the
     manner of an automatic shut-off valve when the
                                                                                                                                                               1                                                                          1
     pedal is hold stable, and diaphragm units (Fig-
     ure 4). Both systems are in production, but have
     specific disadvantages. The anti-vibration unit                                           Elastomer valve
     raises the force hysteresis of the pedal character-                                                                                                       0         10          20          30           40                          0           10          20             30       40
     istic, while the broadband diaphragm unit raises                                                                                                                          Flow rate in l/min                                                           Flow rate in l/min
     the volume expansion and thus the release travel
     losses on the clutch.                                                                                                                                          Conventional AVU with elastomeric valve
     In order to avoid the disadvantages of the known                                                                                                               New AVU with piston valve
     solutions, a variety of simulations and tests were                                                                                 Figure 6                     Piston AVU with two pistons, one inside the other, and reduced pressure hysteresis
     run with a wide variety of action principles. A solu-
                                                                                                             Valve body
     tion related to the damper unit emerged as a pos-
                                                                                                                                        The previous AVU has likewise been refined (Fig-                                                  In addition to the actual release bearing, the cov-
     sible improvement. It is based on the effect of the
                                                                             Figure 4    Broadband diaphragm unit (top) and             ure 6) with the goal of reducing the pressure                                                     er-mounted release unit also requires a second
     Helmholtz resonators, known from acoustics, and                                     conventional anti-vibration unit (bottom)      losses while otherwise maintaining the same ef-                                                   bearing for support in the clutch cover. The drag
     is called a compact resonator. Here there is an ad-
                                                                                                                                        fectiveness against pedal vibrations. In the im-                                                  torque of the two bearings is generally supported
     ditional volume, which is connected with the high                       The compact resonator takes up only about 1/3 of
                                                                                                                                        proved version, the valves are made up of two                                                     by the high pressure line if the radial and axial vi-
     pressure line by means of a T-shaped connecting                         the volume of a conventional diaphragm unit. It is
                                                                                                                                        pistons, one inside the other, which are preten-                                                  bration amplitudes of the cover-mounted release
     piece of a defined length and diameter. In order to                      also significantly smaller and therefore easier to fit
                                                                                                                                        sioned in their seal seats by means of a spring.                                                  unit in the vehicle are not too great. In the pres-
     keep release travel losses as small as possible, the                    into the available space. An adaptation with a rigid
                                                                                                                                        As soon as the fluid exceeds the opening pres-                                                    sureless state, the integrated pre-load spring en-
     hydraulic capacity is minimized. The filter frequen-                     plastic high pressure line has succeeded many
                                                                                                                                        sure, the piston mounted in the direction of flow                                                 sures that the release bearings are pre-loaded
     cy and the damping is determined by the geometry                        times in cases that have previously required a
                                                                                                                                        moves, thus releasing a flow opening. Compared                                                    and that the contact between release bearing and
     of the hydraulic impedance.                                             steel-rubber line.
                                                                                                                                        to the known AVU with elastomeric valve, the                                                      diaphragm spring finger is also always dynamical-
                                                                                                                                        pressure losses in the piston anti-vibration unit                                                 ly present. Unpleasant noises and early bearing
                                                                                                                                        when open are considerably less and remain ap-                                                    damage can thus be effectively avoided. For as-
                                                                                                                                        proximately at the level of the opening pressure.                                                 sembling the engine and transmission, the high
                                                                                                                                        As a result, the piston AVU is practically imper-                                                 pressure line must be directed toward the
                                                        Pressure p in bar

                                                                            1,2                                                         ceptible, even when the pedal is pressed quickly,                                                 through-hole in the bell housing. Restrictions due
                                                                                                                                        since the pedal force is only slightly increased. A                                               to limited axial space when changing the clutch in

                                       Hydraulic                            0,8                                                         damping chamber between the pistons prevents                                                      case of repair did not occur in any of the cases
                                       impedance                                                                                        vibrations and noise.                                                                             tested. With the CMR, the evaluation ratings
                                                                            0,4                                                                                                                                                           against pedal shuddering improve so much that
                                                                                                                                        The best approach physically for avoiding pedal
                                                                                                                                                                                                                                          any other filter in the high pressure pipe can be
                                                                                                                                        vibrations is the complete decoupling of the axi-
                                                                                                                                                                                                                                          completely eliminated. In test vehicles, it has
                                                                              0         100    200    300       400     500     600     al crankshaft vibration by means of a cover-
                                                                                                                                                                                                                                          been found that a cover-mounted release unit
                                                                                                Frequency f in Hz
                                                                                                Frequency                               mounted release unit. Here the hydraulic reac-
                                                                                                                                                                                                                                          can also have a positive influence on the tendency
                                       Hydraulic                                                                                        tion forces are supported only on the clutch
                                                                                   Without resonator                                                                                                                                      to chatter and the excitation of gear rattle.
                                       capacity                                                                                         cover and not on the stationary transmission
                                                                                   With resonator                                       wall; an excitation of the hydraulic column does                                                  Based on dynamic simulation calculations of
     Figure 5   Compact resonator and frequency curve with and without compact resonator                                                not occur [1].                                                                                    clutch, release system and pedal, the necessity

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4    Release system                                                                                                                                                                                                       Release system    4

                                                                                          also has an axial stop         Male quick connector                                                                    plication     area    is
                                                                                          function, which is re-                                                                                                 restricted to constant
                                                                                          alized by means of a                                                                                                   temperatures of no
                                                                Feed line
                                                                                          notch in the pipe and                                                                                                  more than 120 °C. The
                                                                                          two half-shell pieces                                                                                                  volume expansion of
                                                                                          that lock together.                                                                                                    these lines changes

                                                                Piston with sealing       The connection be-
                                                                                                                                                                                                                 significantly above the
                                                                                                                                                                                                                 material’s glass-tran-
                                                                                          tween pipe and hose                                                                                                    sition temperature of
                                                                                          is traditionally made                                                                                                  approximately 40 °C,
                                                                                                                         Female quick connector
                                                                                          by means of a metal                                                                                                    therefore the line
                                                                                          intermediate      piece                                                                                                length is limited in
                                                                                          mechanically        em-                                                                                                compact engine spac-
                                                                                          bossed on the pipe.                                                                                                    es and use in right-
                                                                                          Precise metal-forming                                                                                                  hand drive vehicles is
                                                                                          tools now make it pos-                                                                                                 often not possible.
                                                                                          sible to form the end
                                                                                                                                                                                                               Newer lines use modi-
                                                                                          of the pipe with mul-
                                                                                                                                                                                                               fied PA 612 for the
                                                                Release bearing           tiple axial undercuts
                                                                                                                         Direct crimping                                                                       material and have a
                                                                                          so that a fabric-rein-
                                                                                                                                                                                                               geometrically      opti-
                                                                                          forced hose can be di-
                                                                Cover bearing                                                                                                                                  mized cross section
                                                                                          rectly pressed onto it
                                                                                                                                                                                                               (Figure 9). This mate-
                                                                                          using a sheet metal
                                                                                                                                                                                                               rial has a higher ther-
                                                                                          sleeve (Figure 8, bot-
                                                                                                                                                                                                               mal resistance than
                                                                                          tom). This direct con-
                                                                                                                                                                                                               PA 12H and is used at
                                                                                          nection has one fewer
                                                                                                                                                                                                               temperatures up to
                                                                                          sealing point and is
                                                                                                                                                                                                               140 °C. The bore di-
                                                                                          thus more operation-
                                                                                                                     Figure 8     Steel-rubber quick connector for clutch lines                                ameter of the lines is
                                                                                          ally reliable in princi-
     Figure 7   Cover-mounted release unit                                                                                                                                                                     made       just    large
                                                                                                                     section of 5 x 1 mm and 5.9 x 1.3 mm for longer                enough that the hydraulic losses are still accept-
                                                                                                                     lines.                                                         able at cold temperatures. Because of the small-
     of a filter, the design of the filter and the instal-   LuK has been making clutch lines of this design for
                                                                                                                                                                                    er diameter, the strain is less under internal
     lation site in the vehicle can be predicted very        about two years. Cutting the coil to size as needed
                                                                                                                     The lines made from PA 12H consist of an extrud-               pressure and the wall can be made thinner with-
     reliably. LuK recommends, therefore, that such          and highly flexible and automated bending tech-
                                                                                                                     ed and then thermally bent pipe with steel or                  out having to accept the disadvantages for
     calculation takes place at an early stage of a de-      nology minimize tool costs and response times in
                                                                                                                     plastic quick connectors on the ends. They are                 strength or volume expansion. The surface mo-
     velopment project to avoid frantic, last-minute         case of changes. Clutch lines in the steel-rubber
                                                                                                                     relatively inflexible and require routing with suf-            ment of inertia of the line is smaller so that the
     changes.                                                design will also remain attractive in the future for
                                                                                                                     ficiently large curves to compensate for move-                 lines are generally not thermally pre-bent, but
                                                             a number of applications.
                                                                                                                     ment between the engine and chassis. Their ap-                 instead are routed relatively freely in the vehicle
     Steel-Rubber Hydraulic
     Pressure Lines                                          Plastic Hydraulic Pressure                                                                                                     PA12           PA612           PA612

     Clutch lines made from steel pipe and rubber
                                                             Lines                                                                                              DxS                         8x2            5.9 x 1.3       5x1
     hose are used for long lines and in case of high                                                                                                           Volume expansion at        780             405             390
                                                             Plastic lines are less expensive than those made                                                   120 °C and 30 bar in mm³/m
     temperature load. On the ends, the lines have                                                                            D
                                                             from steel and rubber. However their application
     quick connectors to the clutch cylinders, which                                                                                                            Moment of inertia in mm4    189            54              27
                                                             area is restricted technically by the highest local
     are traditionally made of steel, but are now in-                                                                                                           Routing                     Preforming     Free routing    Free routing
                                                             temperature in the line, the volume expansion due
     creasingly designed as mounted plastic plugs.                                                                                                                                          necessary      possible        possible
                                                             to the temperature and the transfer behavior of
     The plugs shown at the top of Figure 8 ensure a
                                                             pressure pulses.                                                                                   Burst pressure at 120 °C in 80             90              85
     particularly high protection against leakage
     with an O-ring placed immediately between the           The materials used are temperature-stabilized           S                                          bar
     pipe and the counter-surface (e.g., in the hy-          polyamide 12 (PA 12H) with a cross section of 8 x
     draulic cylinder) as a seal. The quick connector        2 mm and polyamide 612 (PA 612) with a cross            Figure 9     Comparison of clutch lines of polymer materials

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4    Release system                                                                                                                                                                                                          Release system   4

                                                              are wear on the back of the seal and gap extru-         tion. For this, the concentric slave cylinder is                              Installation condition
                                                              sion with cracking.                                     provided with a panel to deflect the air flow
                                                                                                                      from the sealing point or a boot.
                                                              A new concept fundamentally improves the
                                                              sealing characteristics. The flanks of the seal         The durability life of the release bearing is large-
                                                              are protected from wear by reinforcement from           ly determined by the lubricity of the grease.
                                                              a thermoplastic seal carrier. Through the re-
                                                              duced friction on the back of the seal and the
                                                                                                                      There is no damage due to rolling fatigue. In or-
                                                                                                                      der to achieve the desired service life of grease,                                                                           4
                                                              smaller gap, the strain in the elastomer mate-          penetration to the bearing interior by dust, wa-
                                        Weld seam
                                                              rial and gap extrusion is also reliably avoided.        ter or brake fluid must be prevented. For appli-
                                                              As before, the carrier is connected axially to the      cations under central European conditions, this
                                                              piston with a defined lash, in order to keep mi-        requirement is generally met by means of a well-                           Operating pressure 45 bar
                                                              cro-movements during pressureless driving               matched labyrinth seal. In regions with lower
                                                              away from the seal. This reduces the cumula-            quality roads, it is recommended to seal the
                                                              tive wear travel of the seal and increases its          bearings on both sides with contact seals. This
                                                              service life. The connection prevents impermis-         results in a conflict of aims, with adequate seal-
                                                              sible enlargement of the lash between piston            tightness set against low friction heat and long
                                                              and seal during operation with a master cylin-          service life for grease.
                                        Weld seam
                                                              der with no resuctioning function. The cylinder
                                                                                                                      Specialized seals which meet these require-
                                                              faces and the seal itself are lubricated during
                                                                                                                      ments have been developed. The seal shown in
     Figure 10 Rotation-welded plastic quick connector        installation with a high-viscosity silicone oil and
                                                                                                                      Figure 13 on the left lubricates itself radially
                                                              PTFE, which does not dissolve in brake fluid and
                                                                                                                      with a flexible lip on the outer ring while the                  38 %        39 %              30 %         32 %
                                                              ensures a durable coating of the surfaces. The
     between the master and slave cylinder and af-                                                                    right seal next to the radially flexible lip is ad-
                                                              counter-faces of the cylinders are tribologically
     fixed with standard clips.                                                                                       ditionally protected against intensive splash
                                                              optimally designed. This means, especially with                                                                                         Max. Main strain
                                                                                                                      water by a mechanical protective collar. The
     The quick connectors are connected and pressure-         steel guide rods, that finishing is mandatory.
                                                                                                                      particular design of the radial seal lip makes its                                      0,40
     sealed with the pipe by rotation welding. This           The new sealing concept enables approximate-                                                                                                    0,35
                                                                                                                      moment of friction in the relevant range inde-
     method avoids the restriction of extrusion coating,      ly double the service life in a central release                                                                                                 0,30
                                                                                                                      pendent of the covering of the track. This limits
     in which only connectors with a straight hole can        system with an aluminum as well as a plastic                                                                                                    0,25
                                                                                                                      the maximum bearing heat and extends the
     be made because of the central core required by          housing.                                                                                                                                        0,20
                                                                                                                      lifetime of the grease. Release bearings of this                                        0,15
     the tool. In rotation welding, angular quick connec-
                                                              Depending on the application, the seal may              design withstand the harshest contamination                                             0,10
     tors can also be used (Figure 10) and the welding
                                                              also have to be protected against dust penetra-         tests and are suitable for speeds of up to                                              0,05
     machine can be easily integrated in a clean assem-
                                                                                                                      7000 1/min.                                                                             0
     bly line.
                                                                                                  Boot                                                                            Figure 12 Right: Concentric slave cylinder seal,
     Increase in Concentric Slave                                                                                                                                                           optimized for long life; Left: Previous design
                                                                                                                                     Splash water protection
     Cylinders Performance                                                                        Long life seal

     A long service life and high reliability of Concentric                                                                Sealing
     Slave Cylinders (Figure 11) is essential because of
     the cost and complexity of repairs in case of failure.                                                                                                                                                                       Standard
                                                                                                                                                                                    Installation tolerances
     The relevant factors are the wear of the central
     seal and the functionality of the release bearing.

                                                                                                                                                               Force Fp in N
     Considerable progress has been made in both ar-                                              Polymer housing

     The central seal is only one-sided and, due to                                                                                                                                                                     Friction
     the system design, wetted only with poorly lu-                                                                                                                                                                    optimized
     bricating brake fluid. With each clutch opera-
     tion, it moves approximately 8 mm along the                                                  Release bearing
     dry track and is exposed to environmental influ-
                                                              Figure 11 Concentric Slave Cylinder in full plastic                                                               Seal lip overlap s in mm
     ences such as high temperature, dust and mi-                       design with seal optimized for long service
     crovibrations. The resulting failure mechanisms                    life                                          Figure 13 Fully sealed release bearing with friction-optimized seals

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4    Release system                                                                                                                                                                                                    Release system   4

                   Finger run out                                                                                      Realized measuring principles utilize the Hall ef-
                   compensation                                                Magnet     Piston                       fect or are designed as linear variable displace-
                                                                                                                       ment (LVDT) sensors [4], [5]. In all cases, a mag-
                                                                                                                       net on the piston of the cylinder is required and
                                                                                                                       the sensor must be attached with a defined
                                                                                                                       spacing about in the middle of the measuring
                                                                                                                       path.                                                                                                                 4
                                                                                                                       There are two different principles of semicon-
                                                                               Travel sensor                           ductor elements used with the Hall sensors.
                                                                                                                       With the highly integrated sensors (2D Hall),
                                                                                                                       two Hall cells are orthogonal to one another
                                                                                                                       and combined into a single assembly with an
                                                             Figure 15 Clutch master cylinder with integrated travel   ASIC [6]. The previously trained position of the        Exciting coil
                                                                       sensor                                                                                                                  Receiving
                                                                                                                       magnets is calculated from the resulting field
                                                                                                                       vector. These sensors are compact, but the cali-                          coil 1
                                                             Innovation for                                            bration work in the assembly with the master
                                                                                                                       cylinder is comparatively high. In a different ar-
                                                                                                                                                                                                             coil 2
     Figure 14 Impact compensation bearing with contact
               washer for concentric slave cylinders
                                                             Consumption-Optimized                                     rangement, two identical Hall cells are posi-
                                                             Drives                                                    tioned at a defined distance from one another
                                                                                                                       in the measuring direction (Conti Helnet). Using                  Clutch master          Inductive
                                                                                                                                                                                            cylinder          travel sensor
     In a finger runout compensation bearing (Fig-                                                                     the arctan function of the signal of both ele-
     ure 14), a cup ring between the bearing and di-
                                                             Stop-start systems                                        ments, the position is calculated by means of a
     aphragm spring finger compensates for crook-            In order to maintain the usual type of operation          programmable ASIC. The advantage of this ar-
     edness. In this way an angled lift-off of the           with stop-start systems, early recognition of the         rangement is that metallic parts have a less dis-
     pressure plate is avoided and the geometrically         driver’s wishes is helpful. Travel sensors in clutch      ruptive effect in the surroundings and the cali-
     caused chatter is reduced. It is also helpful to        master cylinders fulfill this requirement and can         bration goes faster.
     use a contact washer of highly filled polymer           also take on additional functions which were
                                                                                                                       With the LVDT sensors, the magnetic field of a
     material in contact with the diaphragm spring           previously reserved for electrical position
                                                                                                                       primary coil is partially saturated by the mag-
     fingers to prevent metallic contact between the         switches in the pedals. A measuring accuracy of
                                                                                                                       nets and the change is measured by means of            Figure 17 Principle and application of the new
     diaphragm spring fingers and the bearing ring           about +/- 3 % and a high dynamic of the mea-
                                                                                                                       two secondary coils attached on the outer                        inductive sensors (Continental)
     and keep the friction low. Practical experience         sured signal are expected. An “intelligence” is
                                                                                                                       ends. The advantage of this principle is that the
     has shown that the contact washer also serves           demanded of the sensors, such as to diagnose                                                                     processed to the desired output signal in an ASIC
                                                                                                                       signal capture is relatively insensitive to varia-
     to minimize pressure pulsations and pedal vi-           broken cable or short circuit, which requires the                                                                specifically developed for this purpose. Because
                                                                                                                       tions in the distance of the magnets and to
     brations. Crankshaft vibrations are usually             use of a microcontroller or an ASIC. The output                                                                  of the low temperature sensitivity, these sensors
                                                                                                                       temperature drift. For this reason, LVDT sen-
     spring-cushioned relatively flexibly by the             signal is either analog or digital as needed. In-                                                                are as well suited for application in a Concentric
                                                                                                                       sors are used not only on master cylinders but
     bending of the diaphragm spring fingers. With           creased demands for redundancy can be taken                                                                      Slave Cylinder within the bell housing as for a
                                                                                                                       also particularly on slave cylinders in the warm
     high friction in contact between diaphragm              into account.                                                                                                    master cylinder.
                                                                                                                       environment of the bell housing for automatic
     spring fingers and re-
                                                                                                                       shifting systems.
     lease bearings, the                                                                                                                                                      Hybrid drives
     diaphragm spring is                                                                                               The magnet is a definite weak point in all de-
     prevented from a                                                                                                  scribed measuring principles, because it is rela-      In hybrid vehicles, a dry separation coupling on
     compensating       mo-                                                                                            tively expensive and the need for the mechanical       the combustion engine may be necessary de-
     tion, and a majority                                                                                              axis to be parallel to the magnetic axis is rela-      pending on the design of the drive train. The
     of the vibrations then                                                                                            tively high. In the future, it is conceivable to re-   hydraulic cylinders used are still custom solu-
     generate      pressure                                                                                            place the magnets with a simple metallic target        tions today. They are designed as release units
     pulsations. If the fric-        2D Hall                   HELNET                              LVDT                of aluminum if an inductive principle is used for      affixed to the clutch cover or the housing. Re-
     tion is low, on the                                                                                               the sensor. The function of this inductive sensor      lease units affixed to the clutch cover have the
     other hand, the dia-                                                                                              is based on an exciting coil, which generates a        advantage of a very compact construction and
     phragm spring fingers                Electrical coil                                                              magnetic field. Two receiving coils in the middle      relieve the stress of the operating forces from
     can compensate for                                                                                                record the change in the field, which results          the crankshaft. Figure 18 shows a version cur-
                                          Hall element
     the crankshaft vibra-                                                                                             from the movement of the metallic target along         rently in production with axially pre-loaded ball
     tions.                    Figure 16 Sensor principles                                                             the measuring axis. The electrical signals are         bearings. Here, the torque of the rolling bear-

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4    Release system                                                                                                                                                                                            Release system    4

                                   Cover-fixed release unit    is made especially large. So that fast operation     The engagement units used in connection with
                                                                                                                                                                                                           Radial offset
                                                               is nevertheless possible, particular attention       the lever actor are more compact to install [2]. In
                                   Release bearing             was paid to a low flow resistance in the feed        the version shown (Figure 21), the engagement           Engagement bearing
                                                               line. A travel sensor attached on the side per-      bearings are both taken up on the outside of the
                                                               mits a position control.                             guide rod such that an independent movement
                                   High pressure line                                                               in their operating range is possible. The sliding

                                   Cover bearing
                                                               Engager for double clutch                            sleeves are slit for this purpose and engage seg-
                                                                                                                    mentally with one another. A radial misalign-
                                   Drag torque support         While the operation units with conventional man-     ment compensation to the operating levers and
                                                               ual transmissions are often called “release sys-     the use of friction optimized plastics in the
                                                               tems,” in double clutches, the term “engagement      guideways reduce the hysteresis and enable a
                                                               system” has become established because of the        reproducible control of the clutches.
                                                               frequently used pressure-closed clutch. Engage-
                                                                                                                    The obvious design for a hydraulic engagement
                                                               ment systems require a higher number of roller
                                                                                                                    system consists of a central engager with two
                                                               bearings because of the high, longer-lasting bear-
                                                                                                                    pistons, one inserted radially inside the other.
                                                               ing force. In addition, higher stiffness of force
                                                                                                                    What appears conceivably simple at first glance
                                                               transfer, lower friction and precision in bearing
                                                                                                                    turns out to be tricky on closer examination.
                                                               guides is required to meet the demand for com-
     Figure 18 Cover-mounted release unit for a hybrid                                                              This solution often cannot be accommodated in
                                                               fortable shifting. Engagement systems can be di-
               vehicle                                                                                              the available radial and axial space and the
                                                               vided into mechanical and hydraulic actuation.                                                                Guide sleeve
                                                                                                                    travel sensors sometimes required for the con-
     ings is held by a separate torque support be-
                                                                                                                    trol can in any case be attached to the inner
     cause the line routed in a target space cannot                                                                                                                       Figure 21 Central engagement system for a double
     handle this.                                                    Pivot point                Sheet metal lever                                                                   clutch with lever actor
                                                                     in housing                    for clutch 1     The requirement can be met more elegantly if
     The release unit affixed to the housing supports
                                                                                                                    we let go the image of a ring piston and allow for
     itself on the transmission wall and thus is simi-                                                                                                                    self-ventilation. Travel sensors can be attached
                                                                                                                    a kidney shape instead (Figure 23). Every en-
     lar in principle to a conventional Concentric                                                                                                                        to the exterior of the housing using this design
                                                                                                                    gagement bearing is operated by a pair of kid-
     Slave Cylinder. The variants shown in Figure 19                                                                                                                      principle without problem for any clutch. The
                                                                                                                    ney-shaped pistons located opposite one anoth-
     consist of a housing and a piston made of alu-                                                                                                                       piston pairs need not necessarily be the same
                                                                                                                    er. The load transfer is similar to the known lever
     minum. The cylinder is designed for low pres-                                                                                                                        size. The housing can therefore be radially re-
                                                                                                                    mechanisms with two contact points with the
     sures up to max. 6 bar and operation with min-                                                                                                                       cessed and thus also fit into a radially narrow
                                                                                                                    bearings, but avoids radial friction forces. In the
     eral oil. It can therefore be controlled by the                                                                                                                      space with short intermediate shaft distance.
                                                                                                                    channels inserted in the regressive side of the
     hydraulic supply of a conventional automatic
                                                                                                                    housing provide the hydraulic fluid to the pres-      With transversely installed transmissions there is
     transmission. In order to achieve the required
                                                                                                                    sure chambers. The unit works with two pres-          naturally also the option of attaching a central en-
     operating loads of the clutch, the hydraulic face
                                                                     Engagement                 Sheet metal lever   sure connections, which are arranged to ensure        gager on the exterior of the transmission case (Fig-
                                                                       bearing                     for clutch 2

                                        Pressure chamber       Figure 20 Mechanical engagement system for a                                                                                                      Kidney piston
                                                                         double clutch by means of levers                                             Piston clutch 1                                               clutch 2

                                                               Mechanical action engagement systems are
                                                               available as assembly units with guide sleeve,                                         Piston clutch 2
                                                               two levers and two engagement bearings (Fig-
                                        Dynamic sealings       ure 20). The levers extend laterally out of the
                                                               bell housing and are operated electro-hydrauli-
                                                               cally. One bearing is taken up by a sliding sleeve
                                                               into the guide rod with side windows and the
                                                               other by a slider on the outside diameter. The
                                                                                                                                                                                                                 Kidney piston
                                                               advantage of the lever mechanism is in the use                                                                                                       clutch 1
                                        Travel sensor          of already known technology, the disadvantage
     Figure 19 Release unit attached to the bell housing for   in the space required by the swivel area of the      Figure 22 Hydraulic central engager with two ring     Figure 23 Hydraulic central engager with kidney-
               a hybrid clutch                                 lever.                                                         pistons for a double clutch                           shaped pistons for a double clutch

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4    Release system                                                                                                                                Release system   4

                                        Piston                 improved release systems provide greater comfort,
                                                               longer service life, achieve higher performance den-
                                                               sity at the same time as a lighter construction at rea-
                                        Pressure chamber       sonable manufacturing costs. Through the integra-
                                                               tion of electronic sensors and new drive types such
                                                               as hybrids and double clutches, completely new
                                                               technical solutions for operation systems are now
                                                               emerging, with a great deal higher complexity and
                                                               variety. At times exotic-seeming designs are now
                                                               leaving the development phase and will soon be
                                        Housing                mass-produced in meaningful numbers. This devel-
                                                               opment is driven by the changes that vehicle drives
                                                               are currently undergoing and will likely also contin-
                                                               ue dynamically in the near future.

     Figure 24 Central engager attached externally to the
               transmission                                    [1]     Zink, M.; Shead, R.; Welter, R.: Clutch Re-
                                                                       lease Systems, 7th LuK Symposium, 2002,
     ure 24). The clutch is operated by means of a draw
                                                                       pp. 35-47
     rod fed through the hollow shaft of the transmis-
     sion, which is connected with the piston by means         [2]     Zink, M.; Hausner, M.; Welter, R.; Shead, R.:
     of a thread. A spring provides a permanent preload                Clutch and Release System – Enjoyable
     of the engagement bearing, which is located inside                Clutch Actuation! 8th LuK Symposium,
     the bell housing. The exterior of the hydraulic unit is           2006, pp. 27-45
     protected against dirt and water by a plastic cover. In
                                                               [3]     Welter, R.; Hausner, M.; Grethel, M.: Pedal
     the version shown, a travel sensor records the pis-
                                                                       Force Support for Clutch Systems; ATZ
     ton position and enables automated clutch control.
                                                                       09/2007 Volume 109, pp. 768–777
                                                               [4]     Munzig, T.: Contactless Travel and Angle
     Summary                                                           Measurement in the Drive Train and Chas-
                                                                       sis, Elektronik Praxis, 8/14/2009
                                                               [5]     Determine the position without contact,
     While clutch operation would appear to be a well-
                                                                       Elektronik Net, 2009
     established matter, there is much going on techni-
     cally. In modern vehicles with manual transmissions,      [6]     Melexis; MLX 90333, Data Sheet, Jan 08

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