Clutch Damper High Cycle Fatigue

Transcription

1 Abstract Clutch Damper High Cycle Fatigue KD Wang LUK Incorporated Kalyan Bairavarasu LUK Incorporated The clutch damper of the manual transmission system is prone to vibration related fatigue damage when subjected to an overload torsional fatigue test. The amount of damage caused may be very significant. Because it would strike stop pins at the edges, which can likely cause detrimental vibratory stress in the damper. This vibratory stress may produce fatigue damage at a higher frequency than drive/coast torque may do. We term this vibration related fatigue damage as High Cycle fatigue (HCF) damage and the coast/drive related fatigue damage is called Low Cycle Fatigue (LCF). The HCF can significantly reduce the life of the damper in addition to LCF. In this paper an attempt is made to develop a new procedure using ANSYS finite elements to determine the damper life subjected to a HCF. In order to improve the turnover time, cam and linkage are not included in the model. Instead, mass of the center shaft has been increased by 80 times of its original mass to account for moment of inertia of the cam as shown in Figure 1.

2 Figure 1 - Finite Element Model Firstly modal analysis is done on the damper to determine its mode shapes and frequencies. The results obtained from the analysis are compared with the hammer test to confirm the dominating frequency. Transient dynamic analysis is done to get the vibratory stress and its stress distribution in order for the subsequent fatigue life estimate for damper at various loading. Depending on the damper speed and damping, the number of cycles can vary from none to a large number as shown in Figure 2. This procedure is capable of finding the Clutch damper life in a combination of LCF & HCF.

3 Figure 2 - Mode Shapes

4 Introduction High Cycle Fatigue (HCF) results from vibratory stress cycles at frequencies, which can reach thousands of cycles per second and can be induced from various aeromechanical sources. It is a widespread phenomenon in aircraft gas turbine engines that historically has led to the premature failure of major engine components (fans, compressors, turbines) and in some instances has resulted in loss of the total engine and aircraft. This paper presents a Novel approach to deal with High cycle fatigue on a Clutch damper High Cycle Fatigue/ Drive-Coast Fatigue When a damper is subjected to an overload drive torque during torsion durability test, stop pins would impact the damper edges, which likely cause detrimental vibratory stress. The vibratory stress may produce fatigue damage at higher frequency than drive/coast torque may do. This vibration related fatigue damage is termed as High Cycle Fatigue (HCF) damage and the drive/coast related damage is called Low Cycle fatigue (LCF). This HCF damage can significantly reduce the useful fatigue life of flange in addition to the LCF damage. It is essential to develop a Finite element procedure to estimate the life of the flange subjected to an over-load torque. Procedure A typical flange is selected for this purpose. The 5K torsional durability test machine is selected for the analysis. Firstly a Modal analysis is performed on the flange along with the Cam Mass. In order to reduce the computational time in Ansys, the Cam mass is transferred to the splined shaft. To obtain the transfer of Mass, the moment of Inertia of the mass is calculated. Then the mass of the center of the shaft is increased by 80 times in order to accommodate for the moment of inertia of the Cam mass as shown in Fig 1. Modal analysis (FEA and Hammer test) was done on a flange to acquire the dominating vibration modes and their response that can cause HCF damage. The results obtained from the modal analysis are compared with the hammer test to obtain the dominating frequencies. The stress distribution obtained from the transient dynamic analysis was input to FESafe to get the durability of the flange under HCF & LCF Analysis Results & Discussion Modal Analysis Modal analysis is done on the flange to find out the frequencies that are excited due to the impact load. The mode shapes are shown in fig 2. From the modal analysis a set of mode shapes are obtained. In order to find the dominating frequency from this set, a hammer test is done on the flange. Transient analysis Transient analysis is done on the flange to get the vibratory stress as shown in fig 7, and also static analysis is made on the flange to get the stress due to drive side and coast side torque as shown in fig 8. The response Vs Time plot is obtained from the analysis is shown in fig 9. Hammer test Hammer test is done on the flange to excite the various modes in the flange as shown in fig 3. From the hammer test, the frequencies obtained were found to be in the range Hz. the results obtained from

5 the hammer test are shown in fig 4 & fig 5. The damping ratio from the hammer test was found to be in the range % as shown in fig 6. Figure 3 - Hammer Test on the Flange

6 Figures 4 & 5 - Frequencies from the Hammer Test.

7 Figure 6 - Damping Ratio from Hammer Test

8 Figure 7 - Vibratory Stress Figure 8 - Drive-Coast Stress

9 Figure 9 - Response Vs Time Plot obtained from Transient Analysis Durability of the Flange The stress results obtained from the transient analysis are then input to the FESafe to calculate the durability of the flange. Depending upon the flange speed and damping, number of vibration cycles can vary from none to a large number. It is important to find out the fatigue life of the flange under various loadings. Loadings are: Only one drive/coast cycle: If the cycle speed were too slow to dynamically excite the flange, there would be no vibration stress and consequently no HCF. This loading is considered to cause only LCF damage. It was found to be infinite cycles (Fig 10) 1 Vibration cycle in one drive/coast cycle: if the damping is so large that only allow one vibration cycle to complete. It is found to be 3,184,197 cycles (fig 11). 2 vibration cycles in one drive/coast cycle: if two vibration cycles are allowed to complete, it is found to be 16,000 cycles (fig 12).

10 Figure 10 - Static Loading Cycle Figure 11 - One Vibratory load and Drive-Coast Load Figure 12 - Two Vibratory load and Drive-Coast Load

11 Conclusion The new procedure can determine combined damages that are caused by LCF & HCF. HCF can produce significant damage to the flange compared to LCF. Damping ratio has a profound effect on HCF, if damping is fast enough to catch the vibration Impact duration is very significant.