Evaluation of Rotating Driveshaft Breathing Mode with Nearfield Acoustic Holography

Ming-Te Cheng, Liqun Na, Takeshi Abe, Chris Nouhan, Chuck Joseph Ford Motor Company, USA

Advanced Engineering Center Dearborn, MI 48121

ABSTRACT

One of the major noise sources/issues of automotive powertrain is gear whine, which is caused by the dynamic gear meshing forces due to transmission error of gears and gear train dynamics. The noise usually radiated through the housing of the transmission and structure-borne path. However it is also important to identify the significant path/amplifier to solve the problem efficiently. This paper focuses on the application of nearfield acoustic holography to identify the breathing mode and the root cause analysis of the high frequency gear whine issue that occurred on two RWD vehicle with one-piece driveshaft design.

INTRODUCTION

Gear noise induced from transmission gear whine or final drive axle gear whine is one of the major noise sources/issues for automotive powertrain system. The gear noise originated from the gear meshing excitation due to transmission error of gears and the gear train dynamics [1]. Although the noise usually radiated through the housing of the transmission/final drive axle, the driveshaft is also one of the major radiation path when the meshing frequency matches the resonance frequency of the shaft [2,3,4]. This paper describes two cases to demonstrate the use of the 144 channel nearfield acoustic holography (NAH) microphone array to measure the one-piece driveshaft breathing modes excited by shaker and by the real world gear mesh excitation. The 1st case is a subsystem test and the second case is a full vehicle test.

DRIVESHAFT BREATHING MODE IDENTIFICATION WITH SHAKER EXCITATION USING NAH

This case utilizes a one-piece driveshaft shaft with slipping tube in the center. So this case is different from the tradition one-piece single tube shaft. The purpose of this case study is to verify the shaft breathing modes with conventional shaker/impact excitation approach but use nearfield acoustic holography to get more detailed acoustic images. The boundary condition of the shaft is very important to reproduce the correspondent noise problem frequencies. Free-free case cannot represent the whole system very well. Figure 1 shows the setup with the microphone array. The driveshaft was connected to a driving dynamometer and the other end connected to the production axle. The shaker was attached to the center of the shaft. Due to the length of the shaft and the size of the microphone array, the test results were collected in two measurements and then combined to get the final acoustic images. Figure 2 shows the driveshaft used in this experiment. The longer tube on the right is the shaft attached to the shaker and the shorter shaft on the left has grooves outside the surface of the shaft. Figure 3 shows the acoustic frequency response function from impact test to identify the major modes that radiated noise. The excitation is at the driveshaft center Y and Z directions and the response is the driveshaft center nearfield sound pressure. The microphone is located two inches from the surface of the driveshaft.

Figure 1: One-piece driveshaft system with axle, the shaker located at the center of the shaft. 144-channel microphone array was on top of the setup

Figure 2: One-piece driveshaft with sliding tube

As shown in figure 3, three frequencies (856Hz, 1094Hz and 1450Hz) are identified to be the major acoustic radiating modes. After the acoustic impact test, a simple modal test was performed to identify the driveshaft mode shape at these three frequencies. Accelerometers were instrumented at opposite side of the shaft to differentiate the bending mode versus the breathing mode. Nearfield acoustic holography test was then done with shaker excitation at these three distinct frequencies.

Figure 3: Acoustic frequency response function from impact test

Figure 4 shows the 856 Hz results. The top picture shows the averaged sound intensity pattern with 30 Hz bandwidth. The 856 Hz mode is considered to be the second breathing mode of the system. The bottom figure shows the breathing mode corresponding to the radiation pattern. The modal results also verify the breathing mode because the accelerometers on both side of the shaft show same magnitude but with opposite phase. Figure 5 shows similar results but at 1094 Hz. The bottom figure also shows the modal result that is in line with the acoustic radiation pattern. Figure 6 shows the results at 1450 Hz. Due to the sliding tube design, this one-piece drveshaft is actually two tubes overlay to each other with grooves. For the 856 Hz mode, it is possible that the mode is the combination of two 1st breathing modes from both shafts. For the 1094Hz mode, the 2nd breathing mode from each shaft combined to form the 4th breathing mode of the system. For the 1450Hz, the 3rd breathing mode of the longer shaft at right is excited. Further tests for individual tubes are needed to verify the assumption.

Figure 4: Top: Nearfield sound intensity map of driveshaft at 856Hz, Bottom: the acoustic impact result of the driveshaft breathing mode at 856 Hz

Figure 5: Top: Nearfield sound intensity map of driveshaft at 1094Hz, Bottom: the acoustic impact result of the driveshaft breathing mode at 1094 Hz

Figure 6: Nearfield sound intensity map of driveshaft at 1450Hz

DRIVESHAFT BREATHING MODE IDENTIFICATION WITH GEAR MESH FORCE EXCITATION USING NAH

The second case uses the traditional one-piece driveshaft but investigating the shaft radiation pattern under real world operation conditions and with full vehicle setup. Figure 7 shows the bottom view of the drivetrain system. The focus of this section is on the root cause analysis of the high frequency gear whine issue that occurred on this RWD vehicle in overdrive gear position. The problem condition is the 23rd order of the driveshaft at 3210rpm (1230Hz). This order is the gear meshing order from the transmission. Impact test to measure the bending and breathing modes were conducted for baseline and baseline shaft with an inertia ring. The inertia ring was positioned at the rear end of the driveshaft. The impact excitation location is at the transmission extension Z direction and the response is at the driveshaft center nearfield sound pressure. As shown in figure 8, the inertia ring totally decoupled the fourth bending mode from the breathing mode. Figure 9 shows the effective response change at the driver's right ear sound pressure level (15 dB reduction) when adding the inertia ring. Due to the limitation of the microphone array, the measurement was done at three different positions to cover the full length of the driveshaft at the 3210rpm steady state condition. Figure 10 shows the nearfield sound intensity pattern of driveshaft at 1230Hz, measured when driveshaft rotating at 3210rpm speed. The peak sound intensity level is 95dB. The bottom part of figure 10 shows the modal result from impact of the driveshaft bending mode at 1212 Hz. Figure 11 shows the analytical simulation of the driveshaft bending mode at 1194 Hz which has very high correlation with the testing result. The analytical result also shows that the bending mode was shifted 30Hz when the inertia ring added. Figure 12 shows the nearfield sound intensity pattern of driveshaft with the inertia ring at 1230Hz, measured when driveshaft rotate at 3210rpm speed. Although the breathing mode still exist, the peak sound intensity level reduced 13 dB to 82dB when the bending mode was de-coupled.

Figure 7: One-piece driveshaft system with manual transmission and live axle

Figure 8: Impact test comparison: Excitation: Trans mission end, response: Driveshaft Nearfield

Microphone

Figure 9: Effect of the inertia ring for driver's right ear

Figure 10: Top: Nearfield sound pressure field of driveshaft at 1230Hz, measured when driveshaft rotate at 3210rpm speed, the peak sound intensity level is 95dB, Bottom: the impact result of the

driveshaft bending mode at 1212 Hz

Im p a c t T e s t : B a d T r a n s + W r o s t S h a f t @ 1 2 1 8 H z ; E x c ita t io n : D /S C e n te r

-4 0

-3 0

-2 0

-1 0

0

1 0

2 0

3 0

0 8 0

T r a n s E x t . D / S F r o n t D / S R e a r R e a r P i n i o n

Figure 11: Analytical simulation of the driveshaft bending mode at 1194 Hz

Figure 12: Nearfield sound pressure field of driveshaft with the inertia ring at 1230Hz, measured when

driveshaft rotate at 3210rpm speed, the peak sound intensity level is 82dB

CONCLUSION

This paper verifies that applying NAH method to visualize drivetrain air-borne noise is most effective to identify the driveshaft breathing modes because of sufficient acoustic information it provides. The NAH results can also facilitate the analytical model validation and improve the simulation accuracy as shown in the driveshaft air-borne noise analyses.

ACKNOWLEDGEMENTS

The authors would like to thank their colleague Bruce Bonhard at Ford Motor Company for providing the analytical result and Gene Derby for the testing support. Without their support, this paper cannot be completed.

REFERENCES

[1] Donald Houser, "Gear Noise" Dudley's Gear Handbook, chapter 14. [2] Takeshi Abe, Bruce Bonhard, Ming-Te Cheng, Mark Bosca, Chris Kwansniewicz and Liqun Na, "

High frequency gear whine control by driveshaft design optimization" SAE Paper 2003-01-1478 [3] Nancy Gagnon, Vic Gagnon, "Identification of Powertrain Noise Sources Using Sound Intensity and

Modal Analysis Techniques", Proceeding of the XV International Modal Analysis Conference. [4] Takeshi Abe, Ming-Te Cheng, Liqun Na, "Applications of nearfield acoustical holography (NAH) for

powertrain noise sources" Proceeding of the 32nd International Congress and Exposition on Noise Control Engineering.