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					    Development of Wedge Roller Traction
    Drive Units
    Hiroyuki Ito, Satoshi Dairokuno, and Hideo Okano
    Corporate Research and Development Center

  Our Wedge Roller traction drive unit utilizes a traction drive for attaining excellent features such as high efficiency,
and quiet, smooth power transmission by configuring the traction drive transmission to a motor. In this paper,
characteristics of our newly developed Wedge Roller traction drive unit are reported.

1. Introduction                                                  the input shaft. Two of the rollers, the large one and one
                                                                 small one, are fixed by bearings, and can rotate on their
  Power transmissions that incorporate a traction drive,         own axes.
which is capable of quiet and smooth operations with               When the input shaft rotates counterclockwise, the
minimum vibration, have been in the limelight in recent          wedge roller moves in the direction of the wedge in
years. In 1999, NSK succeeded in the commercialization of        proportion to the transmitted torque. Contact force is
a half-toroidal CVT for 3L turbo vehicles1). A traction drive    extended to the guide rollers through the input shaft and
transmits power by the shear force of the                        the inside surface of the output ring. Since the wedge
elastohydrodynamic lubrication (EHL) oil film that is            roller moves in the direction of the wedge, adequate
formed under a large contact force, which is generated in        contact force in proportion to the torque is constantly
the power transmission system. However, industrial               generated. Therefore, transmission efficiency is high, even
machinery, automotive actuators, and auxiliary equipment         when the torque is low.
have been gradually changing from hydraulic drives to              The wedge roller moves in the direction of the wedge
electric drives for energy saving benefits and because of        when input torque is generated, and the output ring,
the prevalence of sophisticated electronic controls. In 1998,    which contacts the rollers, is also moved slightly. This
NSK developed a Wedge Roller traction drive unit for             construction allows free movement of the output ring in
electrical power-assisted bicycles2), which only consisted of    the radial direction.
the reduction unit. In anticipation of increased                   However, when torque is transmitted from the output
applications that make use of high-speed motors combined         ring, the wedge roller moves away from the wedge. This
with a reduction unit, NSK developed a new Wedge Roller          mechanism prevents power from being transmitted,
traction drive unit that is compact and incorporates a           leaving the wedge roller idle, and thus never driving the
motor assembly with the reduction unit.                          motor in reverse.
                                                                   The placement of the housing between the motor and the
                                                                 reduction gear makes it possible to support the input shaft
2. Mechanical Description

2.1 General description
   Fig. 1 shows a cross section view of the Wedge Roller
traction drive unit, in which the reduction unit is on the          Wedge roller reduction gear     Input shaft
left, and the motor assembly is on the right.
   Power is generated in the brushless motor by a stator
and rotor, and transmitted through the motor shaft, which
is coupled to the input shaft of the reduction unit. Torque
is transmitted from the input shaft through three rollers
to the inside surface of the output ring. Slots are available
near the edge of the output ring. The reduced torque is
transmitted to the output shaft that fits in the slots.
   Fig. 2 illustrates the structure and function of the             Output shaft
reduction unit mechanism. The input shaft and output
ring are parallel to each other, but slightly offset
                                                                                   Output ring
(eccentric). One of the three rollers is slightly larger in
diameter than the other two, and one of the two small
rollers is movable with a wedge angle (wedge roller). All
three rollers fit in the space between the output ring and        Fig. 1 Wedge Roller traction drive unit

25       Motion & Control No. 14 (May 2003)
                                                                                                       Wedge roller
                                               Output ring
                                                                                                                2N w • sinα
    Wedge angle                                     Large guide roller

                                                                                  Output ring      Nw
                                                      T i: input torque
                                                         Eccentric                                                                Nw
                                                                                                           2F w • cosα
                                                                                                                                   Ri    T
                                                    Small guide roller

                                                                                                  α                                     Input shaft

           Wedge roller                                                                                                  R
                                 Input shaft

 Fig. 2 Structure and function                                             Fig. 3 Mechanism of contact force generation

retaining bearing and the guide roller holding pins. The                  2.4 Lubricant
housing also acts to separate and protect the motor from                    Any lubricant used in the traction drive unit must have
lubricant contamination.                                                  a high traction coefficient. The lubricant used in this
  Spring loaded pins move the wedge roller in the                         Wedge Roller traction drive unit is either traction oil used
direction of the wedge in order to provide contact force for              in half-toroidal CVTs, or traction grease that has similar
preloading.                                                               characteristics to traction oil3).

2.2 Principle of contact force
   Fig. 3 shows torque T from the input shaft being                       3. Performance Measurements
transmitted to the output ring by traction drive. The
wedge roller moves in the direction of the wedge (wedge                     Photo 1 shows two Wedge Roller traction drive units we
angle 2α) between the input shaft and the output ring. The                built to evaluate performance. Specifications for both units
input shaft and output ring then come in contact with the                 are listed in Table 1. Evaluation results of the motor for
guide rollers. Fw represents the tangential force                         the smaller traction drive unit (250W) are shown in Fig. 4.
transmitted from the input shaft and the output ring to                   Testing under practical operating conditions revealed an
the wedge roller. Nw represents the normal force that is                  efficiency of 80 to 85 percent. The efficiency of the
thus simultaneously generated. When we consider the                       reduction unit only was also evaluated4).
balance of the force being applied in the direction of the                  Efficiency of the larger traction drive unit (1kW) is
wedge, a contact force proportionate to the torque is                     shown in Fig. 5. Efficiency exceeded 90% in the low-torque
constantly generated because:
     2Fw·cos α = 2Nw·sin α
     ∴Fw = Nw·tan α
   The generated traction force Ft is given as
      Ft = µ·Nw
      µ : traction coefficient
   In order for the power to be transmitted without gross
slippage, the follow must be true:
      Ft ≥ Fw
     ∴α ≤ tan-1µ

2.3 Reduction ratio
  The reduction ratio, λ, is a ratio of the output ring and
the input shaft diameters:

    λ = Ro/Ri

                                                                          Photo 1 Wedge Roller traction drive units

                                                                           Motion & Control No. 14 (May 2003)                                         26
Table 1 Specifications of Wedge Roller traction drive units

                                                           Reduction gear                                                                                                    100
                                                                                Reduction gear
                                                           motor 1              motor 2                                                                                       95
 Rated motor output power                                      250 W                   1 kW                                                                                   90

                                                                                                                                            Efficiency, %
 Rated output speed                                           600 rpm                 600 rpm                                                                                 85
 Rated output torque                                           4Nm                    16 N m                                                                                                                                 1 000 rpm
                                                                    •                       •
                                                                                                                                                                                                                             2 000 rpm
 Input shaft diameter of reduction unit                       φ 8 mm                  φ 12 mm                                                                                 75                                             4 000 rpm

 Reduction ratio                                                6.0                     6.0                                                                                   70

 Lubricant                                                    Grease                    Oil                                                                                   65

                                                                                                                                                                                   0   2          4          6           8           10      12
                                                                                                                                                                                                      Input torque, Nm

                                               Power source: 42 V
                           1 000                                                  100                                               Fig. 5 Power transmission efficiency of drive unit
                             900                                                  90
                                                                                           Power source current: A Efficiency, %

                             800                                                  80
     Motor speed: 10 rpm

                                                                                                                                                                                                Output shaft speed, rpm
     Input: W Output: W

                                                                                                                                      Rotation speed fluctuations [DIN], %
                             700                                                  70

                             600                                                  60                                                                                               0       50               100              150             200
                             500                                                  50
                                                                                                                                                                             1.6                                              Input shaft
                             400                                                  40                                                                                                                                          Output shaft
                             300                                                  30

                             200                                                  20                                                                                         0.8

                             100                                                  10                                                                                         0.4

                               0                                                  0                                                                                           0
                                   0            0.5            1            1.5                                                                                                    0       400              800              1 200        1 600
                                                  Torque, Nm
                                                                                                                                                                                                  Motor speed, min -1
          Motor speed                  Input      Output       Source current          Efficiency

 Fig. 4 Drive unit with 250 W motor                                                                                                 Fig. 6 Fluctuation of drive unit

range and reached a maximum of approximately 97%. This                                                                             unit can be used for various applications. In addition, the
indicates that output torque of the motor can be                                                                                   traction drive unit can be further developed to make use of
transmitted with very little loss of efficiency. Motor speed                                                                       its low-noise and low-vibration features for use in high-
seems to have very little effect on efficiency, as well.                                                                           speed rotation applications.
Although not shown here, evaluations were also made for                                                                               The traction drive unit has thus far been developed as a
traction oil and traction grease. Both lubricants performed                                                                        unit for small motors. For applications in industrial
equally well with no marked difference in efficiency. Speed                                                                        machinery and automobiles, however, medium or high
fluctuations of the traction drive unit were measured and                                                                          power units need to be further developed.
reported in Fig. 6. Fluctuations decreased as speed was                                                                               In a unit having larger capacity, the motor will naturally
increased.                                                                                                                         be hotter, and the traction drive mechanism adjacent to it
                                                                                                                                   will require greater resistance to higher temperatures.
                                                                                                                                   Such a unit will have to be designed for better heat
4. Prospects for Future                                                                                                            radiation efficiency, and will require development of a
                                                                                                                                   lubricant applicable to higher temperatures.
  Although the above description is focused on the Wedge                                                                           Developments in geometric design, materials, and heat
Roller traction drive unit consisting of a motor-driven                                                                            treatment are very important to design a unit with higher
reduction unit, we can also consider usage of this unit as a                                                                       torque capacity over 10kN normal force at the power
generator when power is applied from the opposite side                                                                             transmission point.
(output shaft). It is also possible to transmit power in both
directions by converting the small guide roller into a
wedge roller, and by applying force to both of them. By
preloading each wedge roller with a spring, and by
controlling the positions of the supporting pins, the drive

27                         Motion & Control No. 14 (May 2003)
1) H. Machida and Y. Murakami, “Development of Traction Drive
   CVT POWERTOROS Unit—Report No. 1,” NSK Technical
   Journal No. 669 (2000) 9-20.
2) R. Otaki, K. Sakai, and H. Machida, “Wedge Roller Traction
   Drive Transmission,” Collection (VI) of Lecture Texts at 75th
   General Meetings of Japan Society of Mechanical Engineers,
   No. 98-1 (1998) 223-224.
3) M. Yamanaka, Y. Yamazaki, K. Inoue, and H. Hata,
   “Evaluation of Characteristics of Traction Grease at Low
   Temperature,” MPT2001-Fukuoka, The JSME International
   Conference on Motion and Power Transmissions, 1 (2001)
4) R. Otaki and H. Machida, “Study of Traction Drive
   Transmission Using Wedge Action,” MPT2001-Fukuoka The
   JSME International Conference on Motion and Power
   Transmissions, 1 (2001) 832-837.

                      Hiroyuki Ito

                      Satoshi Dairokuno

                      Hideo Okano

                                                                   Motion & Control No. 14 (May 2003)   28

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