Study on Mechanism of Impact Noise on Steering Gear While

Turning Steering Wheel in Opposite Directions

Jeong-Tae Kim1; Jong Wha Lee2; Sun Mok Lee3; Taewhwi Lee4; Woong-Gi Kim5

1 Hyundai Mobis, South Korea

2 Hyundai Mobis, South Korea

3 Hyundai Motor Company, South Korea

4 Psylogic, South Korea

5 Virtual Motion, South Korea

ABSTRACT

This study is focused on the cause of the clanking noise which called "Tuk" in a vehicle. The noise was

generated from a steering gear system under high load conditions while a steering wheel was turning in the

opposite direction. In order to identify the mechanism of the noise, both experimental and simulational

studies were performed on a steering gear system in lab-testing conditions. A simulation model was

constructed based on modal testing and deflection data measured at several points in operating conditions.

The detailed behavior of each component such as a rack bar, a yoke and a housing was able to be

investigated with the help of the transient analysis. As a result of the testing and the simulation, it was

concluded that a vibration was caused by the collision of a rack bar and a pinion gear. The vibration which

started from the gear interface was transferred to the neighboring parts and the noise was radiated mainly at

the housing. The impact phenomenon was additionally confirmed with the measurement of transmission

errors between the gears. Further studies to suppress the noise have been successfully performed with the

help of the analysis model obtained from this study. The effectiveness of final countermeasures was also

verified with testing.

Keywords: Steering Gear, Impact Noise, CAE I-INCE Classification of Subjects Number(s): 76.9

1. INTRODUCTION

The most significant noises that cause customer’s complaints in the steering gear of an

electronically assisted power steering (EPS) system are rattle and clanking noise. The rattle is

periodic noise due to the impact between gear teeth when a vehicle is driven on an unpaved road, and

the researches on the gear rattle including other applications have been carried out for many years

(1-4).

The clanking noise, called “Tuk”, can be heard inside a cabin when turning the steering wheel in

the opposite direction in an engine idle condition. The noise is assumed to be caused by the impact

between internal moving parts in the steering gear system. The clanking noise has been major issue

for many years, however it is challenging to clearly identify the source because of numerous contacts

between the parts such as a rack bar, a pinion shaft, a yoke and etc. For this reason, the research on

the mechanism of the clank noise was rarely done, and the reduction of noise was made mainly rely

on testing based on trial and error.

1 jtkim@mobis.co.kr 2 jongwha@mobis.co.kr 3 sunmk@hyundai.com 4 thlee@psylogic.co.kr 5 woonggikim@virtualmotion.co.kr

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The purpose of this study is to identify the mechanism of a clanking noise in the steering gear of

an EPS system in order to find effective method to reduce the noise. Both testing and simulation

were performed on a steering gear system in lab-testing conditions. First, the motion of rack bar and

yoke body was measured while the steering wheel was turning left to right and right to left,

repetitively. Noise and vibrations at some points on the housing were also monitored to see the

relationship with the motion measured in operating condition. Based on the testing results, a

simulation model was built to observe detailed behavior of each part and the mechanism of the

collision between each part were found. Finally, the impact mechanism was additionally confirmed

with the help of transmission errors between the rack and pinion gears.

2. MEASUREMENT OF BEHAVIOR

2.1 Measurement of Displacement

Lab testing was carried out to observe detailed motion of moving parts such as a pinion shaft, a

rack bar, and a yoke body as shown in Figure 1. Figure 2 shows the equipment of the lab testing

where external loads were exerted to the end of the tie rod to simulate tire reaction force of a real

vehicle during a steering operation. The angle of the pinion shaft and the displacement of the rack

bar were recorded using encoder, and displacements at several major points were measured with

LVDT’s. The sensors were installed with additional fixtures and their detailed locations including the

directions were shown in Figure 3. The load cell was also inserted in the left and right tie rods of the

steering gear system to obtain the external loads transmitted from the motor for tire reaction forces.

Figure 1 – Structure of steering gear system.

Figure 2 – Lab testing equipment.

Figure 3 – Sensor location.

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The testing was performed for the steering gear system while the steering wheel was repetitively

turning in the opposite directions from left to right and right to left. As a result of the testing, the

motion with time is plotted in Figure 4. The greatest clanking noise was generated between 11.25 ~

11.3 sec and a high level of the acceleration on the housing appeared at the same time. The yoke and

the rack bar also showed abrupt change in displacement at that time interval. Before the

measurements were performed, the noise was supposed to occur at the instance of changing

directions by a steering wheel. The noise, however, was generated when the steering wheel is

passing by right before the center location where the external load is close to zero.

Figure 4 – Measured angle and deflection of steering gear.

The yoke gap in Figure 1 is defined as the absolute difference between the maximum and

minimum displacement in the X direction. The yoke gap is designed to ensure low steering efforts

under a high load condition but it causes unwanted motion of the yoke and its surrounding parts. The

yoke was stayed at its -X extreme position by the amount of the gap most of the steering operations.

It can be seen that the yoke moved toward the center of the pinion shaft in +X direction and the

displacement of the yoke was decreased abruptly right before the peak of the vibration. The

displacement of the rack bar at the L1 position was closely related with the X displacement of the

yoke, and hence it is assumed that the yoke was in contact with the rack bar in the X direction. The

rack bar was bended during the steering operation and its deformed shape was presented in Figure 5

when the maximum deflection reached. The figure shows that the amount of deflection was

increased with the increment of the yoke gap.

Figure 5 – Deformation of rack bar

2.2 Investigation on Noise

In order to investigate noise characteristics during the clanking noise event, sound pressure levels

and accelerations were measured at the locations shown in Figure 6.

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Figure 6 – Noise and vibration measurement

As shown in Figure 7, the acceleration of the rack bar in the X direction was the greatest

compared to those of the Y and Z directions. Consequently, it was considered that the clank noise

was mainly originating from an impact in the X direction. The peak of the acceleration on the center

of housing followed the peak of the acceleration on the rack bar or on the housing near the yoke after

about 0.003 sec. It can be seen that the impact took place around the yoke or the rack bar and the

vibration was transferred to the center of the housing.

Figure 7 – Acceleration and Sound Pressure

The measured noise was analyzed in both time and frequency domain using wavelet

transformation as shown in Figure 8.

Figure 8 – Noise spectrum

The frequency at the peak of the noise was closely related to the resonant frequency of the

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housing. The relationship can be seen more clearly if the noise and vibration spectrums which

measured at each component were compared. The vibration spectrum is shown in Figure 10. The

instant timing at the vibration peak of the housing differed from those of the rack bar to the pinion

shaft.

Figure 9 – Vibration spectrum

Figure 10 – Sound visualization

The noise was also visualized using a sound intensity meter in order to find where the noise was

mainly radiated on the housing surface. The noise was radiated from the center of the housing as

shown in Figure 10. As a result of the testing, it can be concluded that a vibration occurred near the

rack and pinion gears transferred to the housing and it leads to the noise radiated from the center of

the housing.

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3. ANALYSIS

3.1 Modeling

In order to investigate detailed motion of the rack bar, pinion shaft, the yoke and etc with time ,

transient analysis on the steering gear system were performed using commercial S/W, DAFUL. The

analysis model was constructed as shown in Figure 11. The rack bar was modeled with beam

elements to realize the bending as observed in the testing. Mount bushes and O-rings were modeled

as springs with 6 degree of freedom. The rack bush and the P-bush were also modeled with spring

elements and contact conditions were applied at the surface adjacent to the rack bar. Furthermore,

contact conditions were applied at the interfaces of other parts including the yoke as shown in Figure

12. Through the whole analysis process, the impact phenomenon of each part can be described

effectively.

Figure 11 – Analysis model.

Figure 12 – Modeling of rack and pinion gear.

3.2 Load and Boundary Conditions

Figure 13 – Load and boundary conditions

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For load conditions of the analysis model, measured data from lab testing were used . The loading

data measured at the tie rods showed that the forces increased up to 2500N with enlarging angle of

the steering wheel, and decreased as the steering wheel returned to its neutral position. In the

analysis, external forces were applied at the tie rods as shown in Figure 13. Fixed boundary

conditions were applied to the mounding of the housing and the hinge connectors were added to

simulate lab testing conditions. Also, the measured angle was input to the pinion shaft up to 0.6 sec

which corresponds to almost one cycle of the steering position from neutral to right-hand, right-hand

to left-hand, and left-hand to neutral position. Basically, the time step was set to 0.005 sec during the

analysis but it was refined to 6.6x10-6

sec for the time range from 0.27 to 0.3 sec where the impact

was expected from the testing results.

3.3 Results

The transient analysis was performed with the input data obtained from the testing referred to the

previous section 2.1. The Figure 14 shows a comparison between the displacements of the testing

and those of the analysis at the yoke and the rack bar. Generally, there were similar behaviors

between the testing and analysis. As shown in the case of the testing, the analysis results also showed

that the yoke stayed in contact with the plug for most of the steering, especially under the high load

condition. The yoke moved towards the rack bar and then there were sudden change in displacement

at 0.2812 sec when the steering wheel is turned from right-hand to neutral position or left-hand to

neutral position.

Figure 14 – Displacements of testing and analysis; a) yoke, b) rack bar

Figure 15 – Deformed shape of rack bar(50 times magnified); a) 0.2762 sec, b) 0.2796 sec, c)

0.2812 sec.

a)

b)

a)

b)

c)

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The scaled deflection shape of the rack bar is presented in Figure 15 to illustrate how the rack bar

deformed and where the impact occurred with time. Figure 14a shows the case when the steering

wheel angle was decreasing from 41 deg to 10 deg. The yoke maintains the position in contact with

the plug because the external load transferred from the rack bar is greater than the spring force which

pushed the yoke in the +X direction. The rack bar undergoes bending deformation similar to 2nd

bending mode shape because the rack bar was supported by the yoke in the left and the rack bush in

the right. As the steering wheel angle decreased less than 10 deg, the external load decreased

gradually. This lead to the yoke motion toward the pinion gear because the spring force became

greater than the external load transferred from the rack bar as shown in Figure 15b. The left-hand

side of the rack bar also moved toward the pinion gear because the rack bar was always in contact

with the yoke. The motion of the rack bar was more accelerated because external load was

continuously decreasing and the restoring force of the rack bar helped the rack bar motion. Finally,

due to the accelerated rack bar, the teeth of the rack bar collided with the teeth of the pinion gear as

shown in Figure 15c.

The impact between the rack and pinion gears is described in more detail in Figure 16 where the

red arrows indicated contact forces.

Figure 16 – Contact force of the rack and pinion gear; a) before collision, b) after collision.

Before the collision, as shown in Figure 16a the right teeth of the pinion gear were contact with

the left teeth of the rack gear. After the collision, the additional contact between the left teeth of the

pinion gear and the right teeth of the rack gear took place as shown in Figure 16b,.

Figure 17 – Velocity results; a) velocity of yoke bar with time, b) velocity distribution of rack bar

before collision.

The vibration induced by the impact is known to be proportional to the collision velocity. The

yoke was in contact with the rack bar during the collision and it could be clearly defined compared

to the velocity of the rack bar. That’s why the yoke velocity could be calculated in Figure 17 in order

to evaluate the strength related to the source of vibration. The velocity of the yoke increased

consistently until it reached its maximum speed of 40 mm/sec. after the collision, the velocity of the

a)

b)

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yoke dropped rapidly and then kept around 0 mm/sec.

The velocity distribution of the rack bar also plotted in Figure 17. This graph indicates that the

velocity profile was nearly linear and the velocity at the rack bush was almost zero . Consequently, it

can be seen that the rack bar was moving like a rigid body in rotational motion hinged at the rack

bush right before the collision occurred.

The vibrational characteristics of the analysis were plotted in Figure 18. The acceleration of the

rack bar and the housing in Figure 18 showed similar tendency with time compared with the testing

results in Figure 7. A large vibration was also observed after the rack bar was collided with the

pinion shaft.

Figure 19 shows how the vibration generated at the interface of rack and pinion gear was

propagated with time. Before 0.2811 sec, there was no vibration on the housing. The vibration

started at 0.2812 sec from rack and pinion gear and the vibrational wave traveled to the opposite side

of the housing. After the reflection wave started from 0.2813 sec, a standing wave started to be built

up in the housing.

Figure 18 – Analysis results; a) acceleration at rack bar, b) acceleration at housing.

Figure 19 – Acceleration contour on housing with time.

a)

b)

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4. TRANSMISSON ERROR

In order to confirm the impact between the rack and pinion gear when the clanking noise occurred,

transmission error of the gear was examined using the data obtained in the section 2.1. Transmission

error, TE of the rack and pinion gear can be defined as follows.

rLTE (1)

Where θ is pinion angle, r is radius of pitch circle, and L is translational displacement of rack

bar. Transmission error and its derivative with respect to time (DTE) are presented in Figure 20.

From this figure, it is seen that the transmission error increased with increasing external load. This is

because the distance between the gear axes increased depending on the external load. The instant

timing of clank noise was exactly coincident with the timing of peak of DTE.

Figure 20 – Transmission error of rack and pinion gears; a) transmission error and displacement,

b) time derivative of pinion and rack bar during collision.

The speed of the rack bar decreased while the rotational speed of the pinion shaft increased

before the 1st

peak of the acceleration between 1.299 sec and 1.3 sec. After the 1st peak of the

vibration, the speed of rack bar turned to increase with time and the rotational speed of the pinion

shaft started to decrease. This implies that the distance between the gear axes altered with time after

the 1st

peak of the vibration. From the analysis on the transmission error, it is reconfirmed that the

clank noise was induced by the collision of the rack bar and the pinion shaft.

5. CONCLUSIONS

In order to identify the clank noise in the steering gear system, both lab testing and transient

analysis were performed. As a result, the detailed motion of the steering gear system was

investigated and the following facts were found.

1) The cause of the clank noise was identified as a low speed collision between the rack bar and

pinion shaft.

2) The noise was generated when the steering wheel was passing the center position, not when

a)

b)

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the steering wheel was changing a rotational direction.

3) The mechanism of noise generation was as follows.

- The rack bar underwent bending deformation and the yoke was in contact with the plug

when the steering wheel was turned left or right.

- As the steering rotated to the center, yoke and the left of the rack bar start to move toward

the pinion and accelerated due to the restoring force and the decreasing external force.

-The moving rack bar collided with the pinion gear teeth at a relatively low speed.

-Vibration was generated from the gear interface as a result of the impact

-The vibration was transferred to the housing and a clanking noise was radiated from the

housing.

4) The analysis model to predict the clank noise was built and it showed close correlation with

the test results.

By applying the analysis model obtained from this study, the following further s tudies to reduce

the clanking noise have been successfully performed. As a result of a parameter study several

improved designs were proposed. Finally their effectiveness was verified with additional testing.

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Technical Paper 2011;2011-01-1584.

3. da Silva J, Zanini J. Design of Experiments Application (DOE) to Prevent Mechanical Noise in Power

Rack & Pinion Steering Systems, SAE Technical Paper 2004;2004-01-3377.

4. Ognjanović1 M, Kostić SĆ. Gear Unit Housing Effect on the Noise Generation Caused by Gear Teeth

Impacts, Journal of Mechanical Engineering 2012;58 (5):327-337.

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