TECHNICAL REPORT

Higher Performance of Traction Drive Units for Printing Machines
H. WATANABE

The traction drive unit is a power transmission device utilizing the rolling contact of rollers which has been
adopted in printing machines and the like for many years now due to serving as a smooth and backlash-free
reducer.
In commercial printing, high-performance color laser printers for on-demand printing have been used due tothe market needs of high-mix, small-volume production lots and short delivery time. In recent years, other demands
have emerged for high performance including adoption of high-quality images, special colors, paper versatility andhigh speed, and the traction drive unit used to meet such demands is also required to improve rotational accuracy
and to extend life.
In this report, we will introduce the planetary roller style traction drive unit technology developed by JTEKT tosatisfy these needs.
Key Words: traction drive unit, planetary roller, reducer, rotational accuracy, printing machines

1. Introduction
The traction drive unit is a power transmission device
utilizing the rolling contact of rollers which has beenadopted by industry for many years due to serving as asmooth and backlash-free transmission. JTEKT began
development of traction drive units in the 1980s. In 1990,
it commercialized a speed-increase spindle for machining1)
centers as a high-speed increaser , and this was also
adopted in the printing field as a high-precision film2)
feeder for plate-making machines . In 1993, the productbegan being mass produced as a quiet reducer for motor3)
assisted bicycles . JTEKT’s traction drive unit has also
been adopted in electric power steering systems, etc.
However, due to market needs for transmissions to
be lightweight, compact and low cost, there have been
efforts to increase the performance of resin gears, etc.
even further, and in recent years, traction drive units
are practically no longer used as general-purposetransmissions.
Meanwhile, there are no reducers capable of surpassing
traction drive units in terms of transmitting powersmoothly, therefore there are still areas in which demand
for this product is ongoing.
In commercial printing, there is an increasing need foron-demand printing due to the needs of high-mix, smallvolume production lots and a short delivery time. Here,high-performance color laser printers are used and recent
demands for higher performance, such as high-definition,
special colors, paper versatility and high speed have led todemands for the traction drive units used in such printers

JTEKT ENGINEERING JOURNAL English Edition No. 1016E (2019)

to have better rotational accuracy and longer life.
This report will introduce the planetary roller styletraction drive unit technology developed by JTEKT tosatisfy these needs as a high-accuracy feeder.

2. What is a Traction Drive Unit?
2. 1 Principle and Structure
A traction drive unit is a power transmission devise thatutilizes the rolling contact between rollers, whereby highprecision and high-strength rollers are assembled throughelastic deformation to create a high contact force between
rollers. Furthermore, by adopting traction grease which
achieves high viscosity in high pressure conditions, powertransmission is achieved whilst simultaneously securing
lubricating performance (Fig. 1).
JTEKT has used this principle to construct a planetary
structure, thus achieving the commercialization of aplanetary roller transmission with extremely small
rotational speed variation to replace the planetary gear
(Fig. 2).
Gear ratio is determined from the sun shaft diameter(di) and stationary ring internal diameter (do), i.e.
(di+do)/di, and normally ranges from 3 to 12 per stage.
Moreover, by making transmissions multi-stage, a large
reduction ratio is possible. With a planetary roller styletraction drive unit, the input shaft and output shaft are
the same axis, therefore a multi-stage structure is easily
achieved, and we have successfully developed a productwith a gear ratio of 1 000.

55

−Higher Performance of Traction Drive Units for Printing Machines−

Drive P
High-viscosity
lubricant
Driven P

Traction force T(rolling transmission force)

Traction force T ＝l P
l：Traction coefficient
P ：Contact force

Fig. 1 Principle of traction force
Bearing or bushing
Sun shaft

Input side(sun shaft)

１Material, heat treatment technologies

3ｖHigh-precision
machining technology

2 Tribology technology

4 Analysis technology

Output side
(revolution of planetary roller)

１ High-strength material
1 Optimal heat treatment
2 Optimal contact surface characteristics
3 High-precision machining

D2 Lubrication theoreticalanalysis (high traction grease)ｄPlanetaryroller
Stationary ring
Lubrication:
Traction grease

Fig. 2 Structure of a planetary-type traction drive unit

2. 2 Technologies Demanded for Traction Drive
Units
The planetary roller style traction drive unit adopts asimple structure yet requires a variety of technologies
in order to maintain high performance. Figure 3 shows
the core technologies that are required. Below are theequations used for transmission torque.
T i = l・P ・Z ・d /2orT o = l・P ・Z （d +D ）/2Here,T i: Transmission torque on the high-speed shaft side
T o: Transmission torque on the low-speed shaft side
l: Traction coefficient
P : Contact force for planetary rollers
Z : No. of planetary rollers
d : Sun shaft outside diameter
D : Stationary ring inside diameter
In order to maintain high transmission torque, there
is a need to increase contact force between rollers anduse lubricant with a high traction coefficient, however
increasing contact pressure tends to reduce rolling fatigue
life. Moreover, the traction coefficient of lubricant
differs depending on its ingredients, as does temperature
properties, durability, etc., therefore there is a need to
select the optimal oil by considering operating conditions.
Furthermore, in order to maintain rotational accuracy,
there is a need to control various geometrical tolerancessuch as dimensional accuracy, roundness, cylindricity andperpendicularity for each roller, as well as roughness atand below the sub-micron level.
JTEKT has developed a traction drive unit capable of56

higher performance by applying core bearing technologies
accumulated over the years such as high-strength
material, heat treatment, high-precision machining andoptimal surface pressure calculation.

4 Optimal surface pressure calculation

4 EHL theoretical analysis

Minimal rotation irregularity low noise/low vibration

No backlash

Fig. 3 Core technologies required of traction drive units

2. 3 Features
In general, traction drive units have the followingfeatures.
①Minimal rotation irregularity②Low noise/low vibration③Minimal backlash④High-speed rotation
Table 1 shows a comparison with the planetary gearand it can be seen that the torque capacity is around onefifth of a planetary gear of the same build.
Typical characteristics, including actual measurement
results, are given below.

Table 1 Comparison with planetary gear
Mechanism

Rotation

Noise/irregularity vibration

BacklashHigh-speed

Torqueperformance capacity

Tractiondriveunit
Planetary◎

◎◎

○△

△

△

△

△

◎gear
◎ : Extremely advantageous○ : Advantageous△ : Disadvantageous

JTEKT ENGINEERING JOURNAL English Edition No. 1016E (2019)

−Higher Performance of Traction Drive Units for Printing Machines−

2. 3. 1 Rotation Irregularity (Rotational Speed
Fluctuation Coefficient)

2. 3. 2 Noise
Unlike gears, traction drive units have no vibration
or noise caused by meshing, therefore offer excellent
quietness. Figure 5 shows the results of a noisecomparison with a planetary gear.
As the measurement method, we used the same
stepping motor to measure sound pressure while
continuously increasing the rotational speed, thenperformed a frequency analysis. The measurement results
showed that, whilst the traction drive unit retained thenoise of the drive motor rotating, there was no noise
created on several degrees, such as that observed in aplanetary gear.

Rotational speed fluctuation
coefficient, %The greatest advantage of a traction drive unit is its
extremely small rotational speed fluctuation. Figure 4shows comparison results with a planetary gear of thesame reduction ratio.
We used a stepping motor as the drive motor andmeasured the rotational speed of the output shaft witha rotary encoder then displayed a waveform of therotational speed via an FV convertor. As a result, weobserved that the rotational speed fluctuation coefficient
was reduced by around 80%.

210−1−200.1

0.2

0.3

0.4

0.5

Reduced by 80%

0.6Time, sRotational speed fluctuation
coefficient, %Precision planetary gear2

Rotational speed fluctuation coefficient (%) =(rotation irregularity)

1

Speed fluctuation
×100
Average rotation speed of the output shaft

0−1
−2

0

0.1

0.2

0.3

0.4

0.5

0.6Time, sTraction drive unit

Fig. 4 Rotation irregularity
Gear noise
disappearance

40

1k30
Motor noise

500

20

Rotational speed, min−1

1.5 k5250

2k
Gear noise
disappearance
1.5 k40

1k30
Motor noise

5002014400

1k

1.5 k
2k
Frequency, Hz

2.5 k〈Planetary gear〉

Sound pressure, dB

5250

Gear noiseSound pressure, dB

Rotational speed, min−12k14400

1k

1.5 k
2kFrequency, Hz

2.5 k〈Traction drive unit〉

Fig. 5 Noise comparison results of a planetary gear versus a traction drive unit

JTEKT ENGINEERING JOURNAL English Edition No. 1016E (2019)57

−Higher Performance of Traction Drive Units for Printing Machines−

Toner
Transfer belt

Photoreceptor drum

Photoreceptor drum
Motor
Traction drive unit

Printed materialFig. 6 Structure for attachment to printing machine

23.5

3. Higher-Performance Traction Drive
Units for Printing Machines
A laser printer works by applying toner to a virtual
image drawn onto a photoreceptor drum which uses alaser beam to form an image. The color printer adopting
a traction drive unit that will be introduced here works
by transferring the images from photoreceptor drums foreach color onto a transfer belt and forming a color image.
Normally, four colors are used, and in order to print
a high-resolution image, the respective photoreceptor
drums must rotate synchronously, therefore a tractiondrive unit, which offers minimal rotation irregularity, isadopted (Fig. 6).
Recently, in line with advancements in digital printing
technology, demands have emerged for even higher
accuracy and longer life to achieve maintenance-freeprinters.

3. 2 Compact Design
Figure 7 shows the specifications of the developed
product, while Fig. 8 shows its structure. The traction
drive unit for high-precision feeding developed by JTEKT
adopts a structure whereby multiple rows of ball bearings
are inserted between the planetary roller and drive pin2)
to fill the gap in order to reduce backlash . However, forthis particular application, rotation during operation is
unidirectional so backlash has no particular effect. Assuch, we adopted a bushing instead of ball bearings. This
allowed us to develop a compact product with a reduced
number of components, thus making assembly easier.

58

u58

3. 1 Reason for Applying to Printing Machines[Specifications]・Reduction ratio:
1/10
・Output shaft rotational speed:
100 min−1
・Output shaft load torque:
2.0 Nm maximum

Fig. 7 Specifications of developed productOil-filled rollerBushing
Drive pinStationary ring
Sun shaft

Planetary roller

Carrier assembly
Hexagon socket head bolt

Planetary roller Motor

Fig. 8 Structure of developed product3. 3 Improved Rotational Accuracy (Feed Error)
Rotational accuracy is the quality characteristic withthe greatest impact on the quality of images printed by aprinting machine, therefore as minimal error as possible
is preferable.
Compared to gears, traction drive units, which arepower transmission devices utilizing rolling contact, haveextremely minimal speed fluctuation, however in printing
machines which use multiple, synchronized traction driveunits, the rotational angle of the output shaft must also
have small relative error during operation. In other words,
there is a need to reduce the lead-lag (feed error) of theoutput shaft in relation to the motor’s commands.

JTEKT ENGINEERING JOURNAL English Edition No. 1016E (2019)If center misalignment occurs,
there is repeated lead-lagof the drive pin inside theplanetary roller

ACentermisalignment

0.05
0.04
0.03
0.02
0.01
0.00
−0.01
−0.02
−0.03
−0.04
−0.05

Feed error at center misalignment00.2 0.4 0.60.8

1

1.2 1.4 1.6 1.8Time, sec

Feed error, deg

I n t r a c t i o n d r ive u n i t s w i t h a bu s h i n g - b a s e dstructure, there is clearance with the planetary rollerinternal diameter and feed error can deteriorate due tomisalignment of the relative positions of the revolving
drive pin and planetary roller. As the mechanism forthis depicted in Fig. 9 shows, this is affected by themisalignment of the respective revolving centers of thedrive pin and planetary roller.
Figure 10 is a schematic of a feed error measurement
device. Figure 11 shows measurement results for feed
error due to center misalignment.
It can be observed that feed error increases with center
misalignment of several tens of microns. JTEKT has
developed its own original center alignment unit to securehigh rotational accuracy.

Feed error, deg−Higher Performance of Traction Drive Units for Printing Machines−

0.05
0.04
0.03
0.02
0.01
0.00
−0.01
−0.02
−0.03
−0.04
−0.05

Feed error when center misalignment is 20 lm

00.2 0.4 0.6

0.811.2 1.4 1.6 1.8Time, sec
A

Cross-section A-A

Feed error, deg

Centermisalignment

The revolving angle ofthe drive pinchanges due
to different
contact positions

0.05
0.04
0.03
0.02
0.01
0.00
−0.01
−0.02
−0.03
−0.04
−0.05

Feed error when center misalignment is 100 lm

00.2 0.4 0.6

0.8

11.2 1.4 1.6 1.8Time, sec

Fig. 9 Mechanism of deteriorating feed error due to center
misalignment

Fig. 11 Comparison of feed error due to center misalignment

3. 4 Longer Life
Brake

RotaryencoderDummy drum

Traction drive unit
Motor

Torque meter
Alignment jig
FV converter
Torque display unit

Computing device(integrates speed data)
Waveform display unit
(displays feed error waveform)

Fig. 10 Feed error measurement device

JTEKT ENGINEERING JOURNAL English Edition No. 1016E (2019)

In planetary roller style traction drive units, which havea similar structure to cylindrical roller bearings, factorsthat affect life are considered to be the flaking and wearof the raceway.
Another factor is believed to be seizure on the sliding
portion between the planetary roller internal diameter anddrive pin.
JTEKT applied the below technologies to achieve
longer product life.
・Prevention of seizure due to the bushing・Prevention of flaking due to the appropriate surfacepressure of each roller raceway
・Prevention of wear due to a raceway oil-replenishment
mechanism
In theory, wear should not occur on traction drive units
due to the existence of an oil film between rollers. In59

−Higher Performance of Traction Drive Units for Printing Machines−

reality, grease is used as lubricant due to its maintenancerelated advantages, however if the raceway becomes voidof grease, the oil film will break and wear will occur.
If wear occurs even once, the raceway’s contact force
decreases, and there is increased slip between rollers,causing the wear to progress and leading to insufficienttransmission torque. Currently, most cases of malfunctionin planetary roller style traction drive units are due to
raceway wear caused by lubricant shortage.
As a method of preventing lubricant depletion, JTEKT
4)
adopted oil-filled rollers made from resin and filledwith a large amount of oil to contact the planetary roller
raceway, thus successfully extending product life. We had
used oil-filled rollers on motor-assisted bicycles in thepast, however this time, in parallel to development of anew grease, we also developed oil-filled rollers using thesame type of base oil. Moreover, the grease replenishment
structure was improved in order to extend life.
Figure 12 shows the results of a durability test, whileFig. 13 shows the durability test equipment used. Results
show that the traction drive unit adopting the new grease
and oil-filled rollers in combination have three times
longer life than traction drive units using the conventional
grease.

Conventional
grease4. Conclusion
In the field of high-precision feed, there are applicationsin addition to printing machines which require preventionof rotation irregularity and reduced vibration, therefore
traction drive units are an effective measure. This paper
has introduced JTEKT’s initiatives to achieve highaccuracy and longer life, but it is predicted that demands
for higher performance in other areas will also intensify.
We shall continue development of this product as asolution to issues faced by customers in fields requiring
minimal rotation irregularity, low noise and low vibration.

References
1)
2)
3)

4)

K. UEDA, S. YASUHARA: Koyo Engineering Journal,
No. 137 (1990) 63 (in Japanese).
A. KAWANO: Koyo Engineering Journal, No. 165E
(2004) 59.
Koyo Seiko Co., Ltd.: Traction Drive Reduction Unit,
Koyo Engineering Journal, No. 147 (1995) 86 (inJapanese).
R. NAKATA: Koyo Engineering Journal, No. 149 (1996)
31 (in Japanese).

× Stops due to raceway wear/major slip

New grease+oil-filled rollers
0Can be used
continuously12345Life ratio, times

Fig. 12 Durability test result

BrakeTraction drive unit
Motor
Torque meter

Speed sensor

Data recorderFig. 13 Durability test equipment

H. WATANABE**60

Industrial Machinery Application Engineering Dept.,
Bearing Operations Headquarters

JTEKT ENGINEERING JOURNAL English Edition No. 1016E (2019)