TECHNICAL REPORT

Development of Spacer Balls for Rack Parallel TypeElectric Power Steering
M. ASAKURA

K. UCHIYAMAS. INOUE

We have developed rack parallel type electric power steering (RP-EPS) for large and premium class vehicles.
Premium class vehicles require high quality in terms of the steering feeling. In order to provide smooth steering,
spacer balls were adopted in the ball screw of RP-EPS.
Torque variation is reduced significantly as the spacer balls have the effect of reducing the slip between balls. Inaddition, in accordance with reduced slip, the durability of the ball screw was also improved.
Key Words: electric power steering, torque variation, ball screw, spacer balls

1. Introduction
Changes in the market such as a heighted awareness ofthe environment and sophistication of driver-assistancesystems in recent years has increased the demand forelectric power steering (EPS) in large and premium classcars. For large cars, the increase in vehicle weight isrelated to a demand for greater assist power in EPS.
JTEKT also developed the rack parallel type electric
power steering (RP-EPS, Fig. 1) as a product suitable forlarge cars and began its production in December 2016.
The RP-EPS is of a configuration whereby the motor
output that assists steering force is reduced in two stages
with a toothed belt and ball screw (Fig. 2). This product isalso more space-saving than conventional EPS and boasts
higher output.
Compared to small cars, etc. there is a demand forpremium class cars to offer high quality in terms of thesteering feeling. The RP-EPS developed by JTEKT has
adopted spacer balls on the ball screw in order to satisfy
this demand for high quality.

Fig. 1 RP-EPS

Toothed belt
Assist motor

Ball screw

Fig. 2 Reducer of the RP-EPSJTEKT ENGINEERING JOURNAL English Edition No. 1015E (2018)47

−Development of Spacer Balls for Rack Parallel Type Electric Power Steering−

Torque variation

2. Torque variation
If variation of steering torque occurs, the driver getsa feeling that something is not right through the steering
wheel. In the case of steering gears that use a ball screw
as the reducer, this feeling is felt after repeated small
angle steering movements (in the range of ±10 to 40 deg)such as that required for changing lanes.
Figure 3 shows the waveform of steering torque whentorque variation occurs. Torque variation occurs at aconstant cycle and that cycle is consistent with the ballmovement cycle, which indicates that the torque variation
is caused by the ball screw.
Figure 4 shows the results of X-ray observations of
the inside of the ball screw both with and without torquevariation. This reveals that the balls are evenly spaced
when there is no torque variation, but if there is torquevariation due to repeated small angle steering movements,
the balls will clump together in some spots but be wideapart in others. This is believed to be a result of the balls
gradually and continuously making contact with one
another due to repeated small angle steering movements.
If balls side-by-side make contact with each other due
to small steering, slip friction will occur between them.
This slip friction will have the effect of inhibiting ballrolling motion therefore repeated small steering will result
in continuous contact between balls. The slip friction
between balls is greater when multiple balls conjuncttherefore this will promote further continuous ball-to-ballcontact.
As described above, the slip friction that occurs
between balls greatly impacts torque variation.

Steering torque

Torque variation

Occurs at aconstant cycle

Steering angle

Fig. 3 Torque variation48

After repeated small steering

No torque variation

Ball-screw nut

GapBalls

Ball screw shaft

Fig. 4 X-ray observation of ball screw

3. The Principle of Spacer Ball Action
Using spacer balls as a method to reduce slip friction
between balls in a ball screw is already known andthere are some cases where such a method has beencommercialized in industrial ball screws, etc.
In a ball screw, load is transferred by the ball held
between the ball screw shaft and ball screw nut.
Spacer balls are the balls placed between two support
balls with a smaller diameter than that of a support ball.
The principle of spacer ball action is as per the following
explanation.
Figure 5 (a) shows a situation where spacer balls are
not used. At this time, each ball is rolling in the same
direction therefore slip friction occurs between the balls
which make contact with one another and this has asignificant impact on the torque variation and durabilityof the ball screw.
Figure 5 (b) shows a situation where spacer balls areused. The spacer balls have a smaller diameter than thesupport balls therefore are free of restraints related tothe ball screw groove and able to rotate in the opposite
direction to the support balls. As such, slip friction does
not occur between balls even in a state where balls comein contact with each other.
For the developed product, in order to maximize theinter-ball slip friction reduction effect of spacer balls, one
spacer ball is placed after every support ball. Figure 6
shows the waveform for steering torque on the steering
gear for which spacer balls are applied in the steering
gear which torque variation was observed in Fig. 3. It
is apparent that the spacer balls have had the effect ofsignificantly reducing torque variation.

JTEKT ENGINEERING JOURNAL English Edition No. 1015E (2018)

Ball rotational direction

Nut-side screw groove (rotating)

Shaft side screw groove (fixed)
Slip occurs between balls(a) No spacer ballsNut-side screw groove (rotating)Ball screw groove surface pressure−Development of Spacer Balls for Rack Parallel Type Electric Power Steering−Big

Increase greater than 10%of maximum surface pressure

Conventional productDeveloped productspecifications

Ball screw groove position

Fig. 7 Surface pressure distribution in ball screw groove
Shaft side screw groove (fixed)
Support ball Spacer ball(b) With spacer balls

Fig. 5 Principle of spacer ball action

Small torque variation

Steering torqueWith spacer balls

Steering angle

Fig. 6 Torque variation with spacer balls

4. Ball Screw Durability
Two factors that affect the durability of a ball screw are
the surface pressure of the screw groove surface and slip
friction.
When spacer balls are used, the number of support
balls decreases and the load distribution per support ballincreases. As with the developed product, if one specerball is placed after each support ball, the number of
support balls will halve. However, the contact between
the balls and the screw groove surface becomes contact
by curved surfaces therefore if the increase in contacting
surface area due to load increase is also taken into
account and the load is only borne by the support balls,
the maximum surface pressure of the ball screw groove
when a load is applied to the developed product will
increase by 10% or more (Fig. 7).
For this reason the below study was undertaken forthe developed product and the design implemented
accordingly.

JTEKT ENGINEERING JOURNAL English Edition No. 1015E (2018)

4. 1 Setting of ball diameter difference
The load that a steering gear is subjected to is notconstant, but rather decreases when vehicle speed ishigher and increases when steering angle is larger. In asimilar way, the load that the ball screw is subjected to isalso constantly varying.
As already mentioned in Section 2, a driver gets a
feeling that something is not right when torque variation
occurs in the ball screw primarily in situations of repeatedsmall angle steering movements, such as that required
when changing lanes, and when the vehicle is travelingat a high speed and the steering angle is small; in otherwords the load is relative low.
Considering this, on the developed ball screw, wedevised the approach of different action for the spacerballs in low-load and high-load areas.
Because drivers can easily sense torque variation
in low-load areas, spacer balls are able to fulfill theirfunction completely without being subjected to load,
minimize the slip between balls and facilitate smooth
operation of the ball screw.
In high-load areas, considering the durability of theball screw, the spacer balls are included as a portion towhich load is distributed in order to minimize a reductionin durability.
We examined the diameter difference between support
balls and spacer balls in order to balance these twomovements.
First, we determined the steering conditions, travellingconditions, etc. under which torque variation occurs
and calculated the load applied to the ball screw underthese conditions. From this load, we found the amount ofelastic deformation that occurs between the support balls,
ball screw shaft and ball screw nut and made this amountthe difference in diameters of the support balls and spacerballs.
By establishing this, we could assume that if the totalof the elastic deformation between the ball screw shaft
and support balls and surface balls and ball screw nut
respectively exceeded the diameter difference between

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−Development of Spacer Balls for Rack Parallel Type Electric Power Steering−

Ball screw groove surface pressure

the support balls and space balls, then the spacer balls
would begin to make contact with the ball screw groove
surface, become a portion subjected to load distribution
and minimize the reduction in durability.
Figure 8 shows the pressure distribution of the ballscrew groove surface during a durability test when thediameter difference is set. It was possible to minimize
the increase in maximum surface pressure to around 5%
compared to when no spacer balls are used.

BigMaximum surface pressure
increase is around 5%

With spacer balls
Without spacer balls
Support balls

Spacer balls(hollow)
Ball screw groove position

Fig. 8 Surface pressure distribution in ball screw groove withspacer balls⑴Slip inevitably occurs in ball screws due to the leadangle of the ball screw shaft and ball contact points
differing to that of the ball and nut contact points. Allball screws have this type of slip, therefore it is known
as “natural slip.”
This PV value is around a few MPa m/s with thespecifications of the developed product.
⑵Ball-to-ball slip occurs when balls come in contact
with one another.
In the developed product, the maximum PV value is thesame as ⑴ at around a few MPa m/s.
⑶When the balls come in contact with one another, amulti-point contact state is established whereby the ballhas more than three contact points (ball and ball screw
shaft, ball and ball screw nut, ball and ball).
At this time, the PV value of the ball and ball screw
shaft and the ball and ball screw nut, despite only beingaround a few MPa m/s in ⑴ above, could reach as highas several hundred MPa m/s.
⑷Because the PV value of the ball and ball screw shaft
and the ball and ball screw nut becomes extremely
large when ball-to-ball slip occurs, minimizing suchslip by using spacer balls is extremely effective in
improving ball screw durability.

4. 3 Results of a durability test on actual
equipment
4. 2 The effect of slip friction on ball screw
durability
It is already known that slip friction affects ball screw
durability.
For the developed product, with consideration
towards balancing torque variation and durability, theball diameter difference discussed in Section 4. 1 was
set therefore load is distributed to both spacer balls andsupport balls.
However, the distributed load of specer ball is smaller
than the support balls. Therefore, for the developedproduct, even if the balls come in contact with one
another during a durability test, the slip friction force that
occurs due to multi-point contact is relatively smaller thanthat of a ball screw which uses support balls only. In otherwords, when placed under high loads, the spacer balls of
the developed product can be thought of as functioning
as support balls at the same time as functioning as spacerballs; or in other words, possessing a function to alleviate
slip friction.
The slip friction alleviation effect of spacer balls underhigh load conditions can be contemplated using surfacepressure (P) and slip velocity (V) in the below way.

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Figure 9 shows the appearance of support balls after
a durability test on actual equipment. The state of thesurface is clearly improved when spacer balls are used.
The wear of the ball screw groove is considerably smaller
than when spacer balls are not used, suggesting thatspacer balls are functioning effectively.

With spacer balls

Without spacer balls

Fig. 9 Ball after durability test

JTEKT ENGINEERING JOURNAL English Edition No. 1015E (2018)

−Development of Spacer Balls for Rack Parallel Type Electric Power Steering−

5. Conclusion
On this occasion, JTEKT has developed spacer ballsfor RP-EPS application and helped to improve RP-EPS
performance and ball screw durability. Moving forward,
we plan to gradually roll these spacer balls out to RPEPS, for which the demand is predicted to increase.
*1 RP-EPS is a registered trademark of JTEKTCorporationReference1)

M. Izawa: Ball Screw Application Technology, Kogyo
Chosakai Publishing Co., Ltd. (1993) 75-90. (inJapanese)

M. ASAKURA* K. UCHIYAMA****JTEKT ENGINEERING JOURNAL English Edition No. 1015E (2018)

S. INOUE**Steering System Engineering Dept. 2, Steering Systems
Business Headquarters
Experiment & Analysis Dept. 2, Steering Systems
Business Headquarters

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