study on mechanism of impact noise on steering gear...
TRANSCRIPT
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 [email protected] 2 [email protected] 3 [email protected] 4 [email protected] 5 [email protected]
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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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