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Development of Wedge Roller Traction
Drive Units
Hiroyuki Ito, Satoshi Dairokuno, and Hideo Okano
Corporate Research and Development Center
ABSTRACT
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
Motor
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
Rotation
Output ring
2N w • sinα
Wedge angle Large guide roller
Output ring Nw
T i: input torque
Fw
Eccentric Nw
2F w • cosα
Fw
Ri T
Small guide roller
α Input shaft
α
0
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
• •
80
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
60
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
2
500 50
1.6 Input shaft
400 40 Output shaft
1.2
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)
References:
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)
826-831.
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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