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CLUTCHES AND BRAKES

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					     CLUTCHES AND BRAKES
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         eys to effective con-      FRICTION TYPES ...............................................A129        clutching and braking func-
         trol and transmis-         ELECTROMAGNETIC TYPES ..............................A130                  tions in one unit, Figure l.
         sion     of    drive       MECHANICAL LOCKUP TYPES ..........................A131                    It has a friction plate and
torque, speed, and power in         OIL-SHEAR TYPES.............................................A132          disc combination for each
many rotating drive sys-            METHODS OF ACTUATION ................................A134                 function. As with many
tems are clutches and               CLUTCHES AND BRAKES ADVERTISING...........A136                            other types of clutch or
brakes. Their function is to                                                                                  brake, various mounting
either transfer torque from                                                                                   arrangements are possible
an input shaft to an output                                                                                   for clutch-brake units, and
shaft (clutching) or to stop and hold a   or stop a load. One surface is metallic, so are various means of actuation.
load (braking). Though offered as sep- generally cast iron and shaped into a                           In more complex designs, clutches
arate components, their functions are disc, band, or drum. The other surface and brakes use multiple discs and
often combined into a single unit re- has a friction facing made from molded friction plates to increase the working
ferred to as a clutch-brake.              organic material, and held together by (friction) surface, Figure 2. Often the
   Clutches and brakes can be catego- a heat-cured resin binder — usually a                         discs are submersed in oil to extend
rized by the technique used to engage plate, shoe, or pad. Friction facings can friction component life and increase
or stop the load (friction, electromag-   be made from any of several types of cooling efficiency.
netic, mechanical lockup), and by the     material, depending on application re-                       Some disc brakes use calipers in
method used to actuate them (me- quirements. Often, brass or aluminum place of a friction plate, Figure 3. An
chanical, electric, pneumatic, hy- chips are included to extend life and advantage to this design is that addi-
draulic, self-activating) .               improve heat dissipation.                                 tional calipers can be added to in-
                                                                                                    crease braking torque. Also, brake
                                                                                                    pads are easily serviced.
FRICTION                                       Disc
  The most popular type of clutch or             In simplest form, a disc-type clutch
brake uses the friction developed be-          or brake has a single friction plate
                                                                                              Drum
tween two mating surfaces to engage            and disc. A clutch-brake combines                 Drum clutches and brakes, Figure

                        Brake armature

 Brake magnetic coil




                                                   Clutch magnetic coil
                                                    Rotor

                                                    Clutch armature
                                                    Hub




 Output
 shaft




Figure 1 — NEMA C-face (left) and foot-mounted (right) electric clutch-brakes. To start, current applied to clutch coil generates
magnetic flux that clamps armature to rotor, causing it to rotate along with attached hub and output shaft. To stop, current is removed
from clutch coil and applied to brake coil. This clamps brake armature to magnet, which is fixed to housing, thereby stopping the load.

                                                                                                             1998 PT Design     A129
Figure 2 — Typical wet-disc clutch-brake
with multiple discs.                                                                  sively used in modern power trans-
                                                                                      mission systems.


                                                                                      ELECTROMAGNETIC
                                                                                         Three types of nonfriction electric
                                                                                      clutches and brakes are available:
                                                                                      magnetic particle, eddy current, and
                                                                                      hysteresis. Used primarily in applica-
                                                                                      tions that require variable slip, elec-
                                                                                      tromagnetic clutches and brakes use
                                                                                      electromagnetic attraction rather
                                                                                      than friction to perform their function.


                                                                                      Magnetic particle
                                                                                         The operating principles of mag-
                                                                                      netic particle clutches are illustrated
                                                                                      in Figure 6. The space between the in-
                                                                                      put and output members is filled with
                                           Figure 4 — Drum type clutch.               a mixture of dry iron particles. When
                                                                                      the coil is energized, the magnetic
Figure 3 — Caliper type disc brake.                                                   flux lines span this space and line up
                                                                                      the particles on the magnetic flux
4, have cylindrical friction surfaces      slightly tapered and coaxial with the      lines. This results in the magnetic
with a common axis (the shaft) on          shaft, and they are engaged in an ax-      particle chains locking the input and
which the unit is mounted. Drum            ial direction. In this respect, they are   output members together, causing
units are either constricting or ex-       a cross between drum and disc types.       them to rotate as a single unit. The
panding types; that is, the drum is           Cone clutches have light engage-        amount of particle bonding deter-
contacted on either its outside or in-     ment forces and high power transmit-       mines the amount of torque that can
side diameter to force engagement.         ting capabilities, but are difficult to    be transmitted and is directly propor-
Drum clutches and brakes wear              disengage. Cone units are not exten-       tional to the current flowing to the ro-
evenly and transmit high torque. The                                                                      tor. Torque can,
contracting type responds especially                                                                      therefore, be ad-
fast because centrifugal force helps                                                                      justed by varying
withdraw the shoes rapidly, thus                                                                          the amount of cur-
making them well suited for high                                                                          rent flowing to the
cyclic operations.                                                                                        coil. Magnetic par-


Cone
  Cone clutches and brakes, Figure 5,                                                                    Figure 5 —
have friction surfaces that are                                                                          Cone type clutch.


 A136 1997 Power Transmission Design
                                                                Hysteresis                 Figure 9 — Square jaw clutch.
                                                                 These are con-
                                                              stant torque devices         consists of square teeth that lock into
                                                              that can be used to          mating recesses in facing members. It
Figure 6 — Magnetic particle clutch.        provide any amount of slip, as long as         provides positive lockup, but because
                                            heat dissipation capacity of the unit is       it cannot slip, running engagement is
ticle clutches and brakes are useful in     not exceeded. Hysteresis losses trans-         limited to speeds under 10 rpm. Sizes
tensioning and positioning applica-         mit torque in this type of clutch. A coil      accommodating from 1 to 260 hp per
tions where continuous changes of           on the input rotor generates a mag-            100 rpm are available.
speed are required.                         netic field in the rotor and drag cup,
                                            Figure 8. The hysteresis losses in the
                                            drag cup cause the flux to change
                                                                                           Spiral jaw
Eddy current                                more slowly through the cup than the              Spiral jaw clutches use sloping en-
   Primarily used in variable speed         rotor; thus torque is transmitted              gagement surfaces to overcome the
devices, these clutches and brakes          through the drag cup. Hysteresis               limited engagement speeds of square
cannot be operated at zero slip. They       brakes provide constant torque for a           jaw types. Though the smooth, sloped
consist primarily of an input drum,         given control current. Torque is inde-         engagement surfaces allow for en-
stationary field coil, and a coupling       pendent of speed up to high speeds.            gagement speeds of up to 150 rpm,
pole assembly that acts as an output        Used primarily in fractional horse-            they can operate in only one direction,
rotor, Figure 7. When the field coil is     power applications, these brakes ex-           and have a tendency to freewheel.
energized, magnetic flux links the in-      hibit virtually no wear and almost un-
put drum with the coupling pole as-         limited life.
sembly. Eddy currents, developed
                                                                                           Multi-tooth
when the input drum rotates, create a                                                         These clutches offer the advan-
new magnetic field that interacts
                                            MECHANICAL LOCKUP                              tages of mechanical lockup clutches
with the field in the pole assembly,           A direct mechanical connection be-          combined with the advantages of elec-
creating coupling torque proportional       tween input and output components is           tric, pneumatic, or hydraulic actua-
to coil current.                            used by mechanical lockup clutches to          tion. Running engagement speeds of
   At zero slip, the eddy current brake     transmit torque. Many use centrifugal          up to 300 rpm are possible. They are
has no torque, thus it cannot be used       force, a wedging action, or wrapping ac-       available in capacities to 300 hp per
where holding a load is required. They      tion to lock the input component to the        100 rpm, Figure 10.
are useful for providing drag loads         output component, and are often re-
needed in applications such as ten-         ferred to as self-activating types. Speed,
sioning.                                    difference in speed between input and
                                                                                           Sprag
                                                                 output members, or           The typical sprag type clutch has
                                                                 direction of rotation     cylindrical inner and outer races,
                                                                 are used to engage        with sprags filling the space in be-
                                                                 the torque transmit-      tween, Figure 11. The sprags are
                                                                 ting components. An-      sized, shaped, and mounted in a man-
                                                                 other type of mechan-     ner that assures they will wedge be-
                                                                 ical lockup clutch, the   tween the two races when rotation oc-
                                                                 multi-tooth design,       curs in the correct direction.
                                                                 uses electric, pneu-
                                                                 matic, or hydraulic
                                                                 actuation.
                                                                                           Wrap spring
                                                                                              In wrap spring clutches, the input
                                                                                           shaft and output shaft are connected
                                                                Square jaw                 by a coiled spring whose inside diame-
Figure 7 — Eddy current         Figure 8 — Hysteresis clutch.     The square jaw           ter is smaller than the outside diame-
clutch.                                                         clutch, Figure 9,          ter of the two shaft hubs, Figure 12.


                                                                                       1997 Power Transmission Design      A137
                                                    sleeve to the intermit-       ment. Wear is greatly reduced by the
                                                    tently driven hub. When       oil film, which lubricates while trans-
                                                    the clutch is disengaged,     mitting most of the dynamic torque of
                                                    the roll cage forces the      engagement.
                                                    rolls down the ramps             The heat of engagement is gener-
                                                    away from the sleeve.         ated in the oil film rather than on
                                                    Roll-cage position is con-    working surfaces of the rotating ele-
                                                    trolled by an external        ments, so the heat can be readily re-
                                                    trip cam and trip lever,      moved and the drive’s thermal capac-
                                                    which may be actuated         ity is high. Also, the oil absorbs the
                                                    manually or by electric,      shock of engagement as the film is
                                                    hydraulic, or pneumatic       squeezed between discs and plates,
                                                    solenoid.                     thus reducing drive-train stress.
                                                                                  Small-diameter, multiple-disc designs
                                                                                  produce high torque-to-inertia ratios.
                                                    OIL-SHEAR                        These characteristics suit oil-shear
                                                    CLUTCHES AND                  drives to applications ranging from
                                                    BRAKES                        high inertia to high cycle. With ordinary
                                                                                  control logic, drives handle 50 to 100 cy-
Figure 10 — Multi-tooth clutch.           In a basic oil-shear drive, torque is   cles/min constant duty easily. You can
                                                     transmitted through          raise that significantly with various
                                                     shearing of an oil film      valve and manifold arrangements.
                                                     between two discs. As           Common applications include in-
                                                     the rotating input disc      dexing and reciprocating drives, con-
                                                     moves toward the sta-        veyors, hoists, shuttles, packaging
                                                     tionary output disc, the     machines, palletizers, welders, and
                                                     shearing allows the out-     machine tools. Oil-shear units serve in
                                                     put disc to begin rotat-     glass, wood products, steel, automo-
                                                     ing. There is no friction-   tive, textile, building material, food-
                                                     material-to-metal            packaging, and other industries. They
                                                     contact until input and      are well-suited for constant-slip ten-
                                                     output disc speeds are       sion uses and adjustable-speed drives.
                                                     nearly equal. Then the          It can be shown that output torque
                                                     oil film breaks down, al-    is directly proportional to viscosity,
                                                     lowing full static engage-   relative surface velocity, and area in
                                                                                  contact; inversely proportional to oil-
                                                    Figure 13 —                   film thickness.
                                                    Roller-ramp type clutch.         Clutch or brake output torque is
Figure 11 — Sprag clutch.                                                         proportional to the disc diameter as
                                                                                          well as contact area. For a
                                                                                          given disc diameter, you can
Rotation in one direction tightens the                                                    increase torque capacity by in-
spring to transmit torque. Rotation in                                                    creasing the number of discs.
the opposite direction loosens the                                                        The modern oil-shear clutch-
spring and disengages the unit.                                                           brake has 6 to 36 working sur-
                                                                                          faces. Actuation may be by
                                                                                          pneumatic, hydraulic, or
Roller ramp                                                                               spring pressure.
   Roller-ramp type clutches transmit                                                        Figure 14 shows such a
torque through rollers that ride on the                                                   clutch-brake. Compressed air
ramped surface of a hub, Figure 13.                                                       or hydraulic fluid pressure in-
When the clutch is engaged, a roll                                                        troduced in the clutch makes
cage positions the                                                                the nonrotating, centrally located pis-
rolls at the top of                                                               ton exert clamping pressure on the
the ramps and                                                                     clutch disc pack. The pack consists of
torque is transmit-                                                               steel discs (drive plates) keyed to the
ted from the con-                                                                 input shaft, and alternate friction-
tinuously rotating                                                                material-faced discs splined to the
                                                                                  output shaft. During acceleration,
                                                                                  torque transfers from the input to the
        Figure 12 —
                                                                                  output shaft through viscous shear in
  Wrap spring clutch.
                                                                                  the oil between friction surfaces.
                                                                                     For braking, compressed air, hy-


 A138 1997 Power Transmission Design
                                                  Brake pack
        Clutch pack

                                                                                       Figure 15 — In an oil-shear unit, oil flows
Figure 14 — Typical oil-shear clutch-      short as 50 milliseconds or as long as      through the hollow barrel of clutch or brake
brake unit has multiple-disc clutch and    several minutes.                            hub, then to the friction discs. Here the
brake packs.                                  Also, the dynamic torque rating of       clutch is engaged, and fluid flows in the
                                                                                       brake stack in preparation for brake
                                           an oil-shear unit can be modified with
                                                                                       engagement.
draulic fluid, or springs force the pis-   different combinations of oil and fric-
ton toward the brake end of the unit,      tion materials. And, because output
as air or hydraulic fluid exhausts         torque varies inversely with film           amount of drag is a function of viscos-
from the clutch piston chamber. The        thickness, control of clamping pres-        ity, disc size, relative slip speed, and
piston clamps the brake disc pack,         sure allows easy control of torque.         running clearance. Where residual
which is similar to the clutch pack ex-       In a drive with a speed reducer,         drag may be a problem, steps can be
cept that the steel plates are keyed to    load inertia referred to the drive is re-   taken to minimize it.
the housing.                               duced by the square of the reduction           Most often, oil-shear drives are ap-
   For units that use external actua-      ratio. There, 50% or more of the motor      plied in cycling applications where
tion of the brake, light-duty return       output may go to accelerating the           frequent stopping and starting can
springs are often used to move the         drive; 50% or more of the heat gener-       quickly burn up a dry-friction unit or
piston. In a spring-applied safety-        ated comes from starting and stop-          make it lose positioning accuracy.
brake setup, heavy-duty springs re-        ping the cyclic parts of the clutch-        Conversion to an oil-shear drive is
place the light-duty springs. Loss of      brake — important in high-cycling           simplified, because the self-contained
electric power or actuating pressure       work. Because inertia increases by          unit has its own bearings, housing,
automatically releases the clutch and      the fourth power of diameter, and           and input and output shafts.
sets the brake. Clutch-and-brake           torque increases proportionally to di-         Many oil-shear drives serve con-
overlap is mechanically impossible.        ameter or number of discs, the small-       stant-slip applications. In start-stop
   Oil-shear performance requires          diameter, multiple-disc design can de-      uses, the objective is to accelerate or
positive oil circulation through the       liver high torque with minimum              decelerate the load with full engage-
disc packs, Figure 15, to cool and lu-     inertia. Drive rating increases almost      ment of the clutch or brake. But in
bricate friction surfaces and bearings.    in proportion to the number of work-        constant-slip applications, the objec-
Typically, transmission fluid is dis-      ing surfaces, with no derating for          tive is to slip continuously without
persed radially across the faces of the    multiple-disc construction. You can         full engagement. Typical uses include
friction surfaces. The fluid then flows    get many torque ratings from one            unwind brakes, rewind clutches, dy-
to a sump where heat dissipates            brake size by adjusting the number of       namometer absorbers, tension-con-
through the housing. An internal wa-       discs and springs, or adjusting clamp-      trol brakes, constant-tension drives,
ter-to-oil heat exchanger can increase     ing pressure.                               and adjustable-speed drives.
thermal capacity.                             The totally enclosed, sealed hous-          Unwind brakes, for example, main-
   Not all wet clutches are oil-shear      ing prevents contamination from the         tain fairly constant tension in a mate-
units. In some friction-type drives, oil   outside. Also, normal maintenance re-       rial web even though the moment
may be used to cool the friction sur-      quires only maintaining oil level.          arm, and thus the torque required,
faces. Unless the clutch-brake main-          In general, dynamic coefficient of       changes as roll size diminishes.
tains an oil film between surfaces, it     friction can vary even at seemingly         Torque in the oil-shear drive can be
behaves like a friction-type clutch-       constant speed conditions, and even         controlled by manual adjustment of
brake. Torque transfers mechanically       more so with varying speed and tem-         clamping air pressure. You can get ac-
from one friction surface to another,      perature. The coefficient for a wetted      curacy of 10%. Better accuracy with
and heat will build up in the working      surface tends to be more constant           open-loop control is impossible;
surfaces, which must then be cooled.       than that for a dry surface. That con-      torque varies also with oil viscosity
   Besides the low wear and cush-          sistency extends to oil-shear drives.       and slip speed. Closed-loop control
ioned engagement inherent in oil-             With a clutch or brake disengaged,       with load-cell feedback to a suitable
shear drives, engagement time can be       the oil film, which is always present,      PID set-point controller can produce
set as needed. Depending on control        continues to transmit some torque.            0.25% accuracy.
logic and methods of actuation and         This can be a limitation in high-              Oil-shear drives make excellent dy-
cooling, engagement times may be as        speed, constant-slip applications. The      namometer absorbers because they


                                                                                   1997 Power Transmission Design         A139
can develop high and low torque over        Electric                                    outputs for smooth transitions be-
a wide speed range, with low inertia.                                                   tween clutch and brake engagement
In particular, they can develop high           Electrically actuated clutches and       in combination units. A control with a
torque, without chatter, at speeds be-      brakes permit extremely fast cycling        potentiometer permits torque adjust-
low 1 rpm, where other absorbers            rates (to 1,600 cpm in some uses).          ment for the clutch-brake and is often
can’t function.                             However, they do not provide the            used to soften starts and stops of sen-
                                            torque range of pneumatic or hy-            sitive loads.
                                            draulic units. Electric clutches and           Though 60 cpm is a routine cycle
METHODS OF ACTUATION                        brakes are more convenient for much         rate for electrically actuated clutches
   You must consider several factors        automatic machinery, where control          and brakes, rates 10 times as high are
when selecting a clutch or brake for a      commands are electric. Electric actu-       often encountered. One special solid-
given application:                          ation also works better in remote ap-       state control, the overexcitation
   • How much torque must the unit          plications where piping would be            (OEX) control, increases cycle rate
accommodate?                                cumbersome or costly.                       performance by reducing the mag-
   • What engagement methods are               Some types of electric clutches and      netic field buildup time of the electro-
available?                                  brakes provide closely controlled ac-       magnet coil. The OEX control oper-
   • Does the job need remote or elec-      tuation rates that would quickly wear       ates initially by applying over-voltage
tronic control?                             out other types. Conversely, they do        to the coil, then reducing the voltage
   • What is the response-time re-          not provide the “feel” of engagement        until rated voltage is reached. Suffi-
quirement?                                  possible with other types.                  cient voltage is applied to the coil for a
   • What are the environmental con-           An electromagnetic clutch or brake       few millisec, after which the coil has
ditions?                                    has two basic parts: an annular electro-    reached flux saturation point. The
   • What thermal capacity is               magnet and an armature. The electro-        overexcitation time is too brief to
needed?                                     magnet is a wound copper coil embed-        harm clutch-brake life.
   • Are there space or weight restric-     ded in a donut-shaped iron shell, and
tions?                                      fitted with lining material to retard
                                                                                        Pneumatic
   • What is the maximum operating          wear. When voltage is applied to the
speed?                                      coil, the magnetic field formed by the         Air actuation is the most common
   • What is acceptable service life for    electromagnet shell and armature en-        method for industrial equipment. Most
the unit or its friction elements?          gages the two friction surfaces and         factories have compressed air that can
   • How much routine maintenance           locks them together as a rotating unit,     serve clutches and brakes. Pressures
will be needed?                             Figure 16. If the armature is allowed to    to 200 psi are used to inflate tubes or
   Most clutch and brake types are          rotate, and the electromagnet is held       act on pistons to engage or disengage
available for applications from frac-       stationary, the unit works as a brake.      friction surfaces. Some are air-engaged
tional to 50-hp capacity. Thus, you            Electromagnetic clutches and             and spring-disengaged; others, spring-
must consider the method of actua-          brakes can have rotating or station-        engaged and air-disengaged. Some
tion and operational characteristics        ary coils. Rotating coil types are com-     combination clutch-brakes use air
typical of that method in order to opti-    mutated with slip rings and brushes,        pressure to engage the clutch, and
mize selection.                             and cannot be used in explosive atmo-       spring pressure to disengage the
                                            spheres. In the stationary field clutch,    clutch while simultaneously engaging
                                            the magnetic coil is fixed and, thus,       the brake.
Mechanical                                  does not need rings.                           Though some pneumatically actu-
   This is the simplest, least costly way      The simplest type of clutch or brake     ated clutches and brakes operate at
to engage a clutch or brake. Actuation      control consists of a plug-in module        rates to 80 cpm, they usually serve in
is by rods, cables, levers, or cams.        that converts ac line voltage to dc volt-   applications needing 20 cpm or less.
   Besides cost, a major advantage of       age and provides for on-off switching.         Perhaps the greatest asset of an
this type of actuation is the “feel” of        More complex controls include            air-actuated clutch or brake is low
engagement that the operator gets           solid-state input modules with inte-        heat generation in the actuator. Un-
through a pedal or lever, and the abil-     gral 0, 50, and 100-msec time delay         like an electric unit, there is no heat
ity to judge the amount of braking                                                                           generation during
force or slip needed.                                                                                        a long period of en-
   Because mechanical actuation de-                                                                          gagement. Static
pends on human strength, actuation                                                                           pressure stays con-
force is limited to about 75 lb. This low                                                                    stant after the pis-
clamping force limits response times                                                                         ton chamber is
and cycling rates. Normally, mechani-                                                                        filled, so the power
cally actuated clutches cannot be cy-                                                                        needed to sustain
cled more than a few times per minute
without wearing out clutch elements
or tiring the operator. As a result, me-
chanically actuated clutches are re-                                                                        Figure 16 — Basic
stricted to vehicles and small indus-                                                                       elements in a disc-
trial equipment such as hoists.                                                                             type electric clutch.


 A140 1997 Power Transmission Design
Figure 17 — Typical pneumatic clutch
installation.                                                     Figure 18 —                              Actuating fluid
                                             • Exhaust muf-
                                           flers — To quiet       Typical multiple-disc
torque is almost nothing.                  noisy exhaust          hydraulic clutch.
   As with mechanical actuation, a de-     valves, they typi-
gree of touch control or “feel” can be     cally reduce sound
gained with air clutches and brakes        pressure levels by
by channeling pressure through hand        up to 16 dB.
or foot-operated throttles. Where re-         A disadvantage                       Cylinder-piston
mote control is desired, electrically      of air-actuated                         assembly
actuated solenoid control valves may       clutches and brakes
be used to pressurize a clutch or          is the support
brake, Figure 17.                          equipment needed
   To operate efficiently, air-actuated    and the mainte-
clutches and brakes need several sup-      nance costs that go
porting components:                        with it. The
   • Pressure regulators — Selected        clutches and brakes
in accordance with their flow capabili-    themselves, how-                                                                  Separator
ties, these valves set pressure ranges     ever, use actuators                                                               plates
so that clutches and brakes deliver        that are typically                                                                splined to
                                                                                                                             hub
correct torque.                            no more complex
   • Filters — Installed ahead of the      than a cylinder and
pressure regulator, a filter needs at      piston sealed with
least 5-micron filtration capability.      O-rings or U-cups.
   • Lubricators — Located between         Thus, no special
the pressure regulator and control         skills are needed to
valve, a lubricator injects oil into the   maintain them.
air when pressure drops.
                                                                  Friction discs
   • Control valves — Depending on         Hydraulic
the clutch or brake configuration, the
control valve may be 3, 4, or 5-way           Principles of operation for hy-
solenoid type. A 3-way valve controls      draulically actuated clutches and
either a clutch or brake. A 4-way valve    brakes are similar to those of pneu-            Self-actuating
controls a combination clutch-brake,       matically operated units. Oil pressure
providing air to either component. A 5-    as high as 500 psi is used in conjunc-             These clutches rely on centrifugal,
way valve is used where clutch and         tion with a mechanical friction brake           wedging, or wrapping action to lock in-
brake require different pressures.         or clutch. In these applications, oil           put and output members together to
Generally, a control valve is located as   pressure is delivered to a piston that          transmit torque. (See “Mechanical
close to its clutch or brake as possible   acts against a rod, lever, or cam to en-        Lockup.”) They are automatic, needing
to assure least lag time.                  gage or disengage the clutch or brake.          no external control. Actuation or re-
   • Quick-exhaust valves — Installed         Hydraulic actuation usually pro-             lease occurs due to speed, difference in
at the clutch or brake inlet port and      vides fast response, and you can get            speed between input and output mem-
used where long lines are necessary,       smooth engagement by controlling                bers, or change in direction of rotation.
these valves improve response time be-     the rate of pressure buildup with a                Self-actuating clutches are best ap-
cause air need not travel back to the      pressure control valve.                         plied to applications where motor
control to be exhausted. Instead, it ex-      As with pneumatic actuation, the             speed is an adequate clutch control
hausts at the quick-exhaust valve          main disadvantage of hydraulic actu-            parameter, gentle starting with slip is
whenever supply pressure drops by,         ation is maintenance of support                 desirable or tolerable, and energy sav-
say, 3 to 5 psi.                           equipment.                                      ings are important.                    s


                                                                                      1997 Power Transmission Design           A141

				
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