Testing and validation of powder metal gears in a 6 speed manual transmission

Transcription

1 Testing and validation of powder metal gears in a 6 speed manual transmission Anders Flodin anders.flodin@hoganas.com Höganäs AB, Sweden Abstract A 6 speed manual transmission was redesigned and optimized for using powder metal steel as the gear material. The design work was very comprehensive and included deformations of housing, shaft, bearings, bolts and gears and varying temperature conditions. The analysis showed that powder metal (PM) gears would survive the durability cycle despite that no densification methods were used. In the second phase of the project, a SAAB 95 was equipped with a prototyped gear box according to the PM optimized design and 3 transmissions were built and tested in test rig for durability and vibrations. This paper will demonstrate the testing procedure and the results from testing. The transmission has also been benchmarked with the original transmission and comparisons have been made. Lessons learned will be discussed and how the design can be improved in future transmissions. Introduction The effort to design a transmission using PM has been presented previously [1,2], in brief the ambition was to understand: What durability levels are required? Can PM meet them and if so what would the manufacturing process be? Build the transmissions and put in demonstrator car Put transmissions in test rigs under OEM like conditions for validation Powder metal parts can have its strength tailored through processing to a greater extent than solid steel parts. The amount of processing required affects the part price, so every gear in a transmission can have different processing in order to be as cost efficient as possible while maintaining structural integrity. In order to be able to predict the strength and service life of gears there is one very important prerequisite; Fatigue data generated on gears, preferably on similar gears to those that are in the transmission. Fatigue data from standardized FZG C type gears and smaller spur gear had been generated to create 40 point S- n data curves as the lowest baseline level for pitting and bending fatigue. So when designing the gears it is also possible to calculate the safety margin against failure. However that safety margin calculation has little value if the test data is not of good quality. By performing the durability testing on the 6 speed manual transmission it is possible to get more understanding how the FZG S-n data relates to the durability of the 6 speed transmission, can it be used to predict safety margins in gear design? In this paper the focus will be on the testing and the results from testing. Transmission The transmission tested is a 6 speed manual designed for 320Nm. It has a 3 shaft layout and a transversal design for front wheel drive, see figure 1. Presented at WorldPM 2016 in Hamburg on October 12, 2016 Page 1

2 Figure 1. 6 speed manual that has been designed, prototyped and tested with PM gears. In the transmission all gears except the 1:st and 2:nd drive gear are in PM. The reason for not making the those gears in PM was that they were cut on the shaft and the difficulty would lie in attaching a PM gear on the shaft rather than in making the gear teeth themselves. The final drive and output shaft gears where not in PM either for similar reasons. Test rig Presented at WorldPM 2016 in Hamburg on October 12, 2016 Page 2

3 The test rig used for NVH, durability and overload/breaking load is depicted in figure 2. Figure 2. Test rig used for the testing in this paper. Here shown for testing of longitudinal transmissions and not transversal transmissions. A toothed belt is driving the input shaft and the differential is welded allowing output through each side shaft since connected to the brake dynamometers. Between the shafts and the dynamometers there were torque sensors fitted to measure the shafts torque and speed. The test object input shaft was connected through the spline interface to an adapter shaft with torque sensor sitting on the belt transmission tooth wheel. On the adapter shaft there was also a torque and speed sensor of the same type as for the output shafts. The oil temperature was measured at the drain plug with a PT 100 sensor. To track the wear of the transmissions during the test a Reilhofer delta- Analyzer vibration monitoring system was also used. Accelerometers were positioned on the gearbox housing and on the differential housing, see figure 3. Presented at WorldPM 2016 in Hamburg on October 12, 2016 Page 3

4 Figure 3. Accelerometer on differential housing. Tested parameters The transmissions was tested for durability, overload, vibration and breaking load. The durability cycle can be seen in table 1 Duty cycle 1st 2nd 3rd 4th 5th 6th Rev. Time[hours]* Rotations of 180 K 3.6 M 9.0 M 14.4 M 14.4 M 21.6 M 90 K input shaft Torque(Nm) typical European consumer car. Tabl e 1. Duty cycle used in test, The test torque for each gear is defined in table 1, overload torque was 450Nm. Only a few hundred revolutions was run at 450Nm for each gear to simulate abuse. Torques for 1,2+R are reduced, this is also done for the solid steel gears in the computer of the SAAB 95 by reducing Turbo pressure for these gears. This is a safety precaution to avoid to high abuse loads. For surface densified PM gears the torque can be increased to 320Nm, this has been tested in a separate project. As stated in the introduction, the strength of PM gears can be tailored to meet the requirements. Vibration was tested as well as overload, which is also presented below. Results The durability cycle in table 1 was followed. The torque levels for first, second and reverse have been reduced just as it is reduced in the car. The stresses on these gears are quite high so to avoid breakage the torque is reduced also for the solid steel original gears. The gear pairs 3-6 tested out well and survived the durability test on all 3 transmissions. 1:st gear+r (Astaloy 85Mo 0.25%C 7.25g/cc) First gear was a convoloid shape gear. In the design phase the simulations showed that lead crowning was not necessary which led into a design without lead crowning. In testing that proved wrong and edge contact due to misalignment gave pitting on one of the sides of all the flanks of driven gear. In retrospect it was the wrong decision to design the gear without lead crowning. 50% of the durability cycle was done for the 1:st gear, 1:st transmission. It was then decided to stop the test to avoid catastrophic failure which tend to damage other components in the transmission delaying the whole test program However in the second transmission the 1:st gear and reverse was run until runout, during inspection pits were found on the edge of the output gear again, but never on the reverse gear. Only 2 transmission were tested with convoloids. 2:nd gear (Astaloy 85Mo 0.25%C 7.25g/cc) Presented at WorldPM 2016 in Hamburg on October 12, 2016 Page 4

5 Second gear was asymmetric, it went all the way to runout but showed during inspection some pitting on the output gear, half of the teeth showed pitting on the edge. It looks like a concentricity or axial alignment problem. Same result for gearbox number 2. Only 2 transmissions were tested with asymmetric gears Gears 3-6 (Distaloy AQ+0.25%C and Hipaloy+0.25%C 7.25g/cc-7.5g/cc) The rest of the gears passed the testing with wear that can be considered as perfectly normal. Vibration The test cycle for vibration is shown in figure 4. Both drive and coast side was tested at different torque and speeds. Speed [rpm] Speed ramps Time[s] Torque Nm Speed High Torque "Low" Torque "High" Figure 4. Speed and torque ramps for vibration testing. High torque is 250Nm, low torque is 100Nm. Each ramp was repeated 6 times and the vector sum of the vibrations calculated and averaged. The collected data is then transformed from the time domain to the frequency domain using FFT and the acceleration data in the frequency domain is depicted in figure 5 for 4:th gear pair. In table 2 is some relative numbers calculated comparing original gears in solid steel to the PM gears and as can be seen PM gears outperform the steel gears. Presented at WorldPM 2016 in Hamburg on October 12, 2016 Page 5

6 0.30 F Order Gear acc Ref, gear 4, 100 Nm, rpm, up_av g max.: 0.20 µm F Order Gear acc Ref, gear 4, 100 Nm, rpm, up_av g max.: 0.04 µm F Order Gear acc Sintered, gear 4, 100 Nm, rpm, up_av g max.: 0.10 µm F Order Gear acc Sintered, gear 4, 100 Nm, rpm, up_av g max.: 0.01 µm 0.30 µm Amplitude (RMS) µm Amplitude (RMS) rpm DerivedTacho IN (DT1) F Order FD acc Ref, gear 4, 100 Nm, rpm, up_av g max.: 0.09 µm F Order FD acc Ref, gear 4, 100 Nm, rpm, up_av g max.: 0.02 µm F Order FD acc Sintered, gear 4, 100 Nm, rpm, up_av g max.: 0.05 µm F Order FD acc Sintered, gear 4, 100 Nm, rpm, up_av g max.: 8.05e-3 µm rpm DerivedTacho IN (DT1) Figure 5. Typical amplitude level curves showing first and second harmonics (1 st +2 nd multiple of mesh frequency) as a function of rpm. Upper graph is from accelerometer on transmission housing and lower graph is from Accelerometer on differential housing. Table 2. Difference calculations for 4:th gear. TEST Max. vibration displacement amplitude for TMR during sweep [µm] Difference calculations Gear Run Torque Reference PM gear PM gear relative Reference [absolute value] PM gear relative Reference [db] 20*log(µm sint /µm ref ) 4 up 100 Nm down 100 Nm up 250 Nm down 250 Nm From Figure 5 it can be seen that the vibrations are higher on the transmission itself than on the differential housing. The solid lines are measured on a brand new OEM transmission and the dashed lines are from a new PM transmission. Housing, shafts and differential was the same for both tests. Presented at WorldPM 2016 in Hamburg on October 12, 2016 Page 6

7 Breaking load Gears 4,5,6 were tested until breaking occurred, however some of the gears did not break and the test was stopped to protect final drive and rig. 4:th gear (from compaction tool),new gear stopped at 1154Nm (No failure), old gear durability tested 986Nm (Failure) 5:th gear (machined from blank), old gear durability tested 1160Nm (Failure) 6:th gear (machined from blank), old gear durability tested 1000Nm (No Failure, stopped to protect rig and final drive.) Abuse All gears were run at 450Nm for minimum 1000 revolutions to simulate abuse, all gears passed despite having just finished complete durability cycle before testing at 450Nm. Summary A 6 speed manual transmission have been designed for powder metal. 4 transmissions have been built, 3 for testing, 1 for a SAAB 95. The transmissions have been tested for durability, vibration, overload and breaking load with successful outcome at 230 Nm which is the maximum output torque from the engine in the SAAB L. Higher torques can be sustained but requires densification of surfaces. The breaking loads for gears 3-6 were 986 Nm or higher. The abuse load was set to 450Nm and all gears passed. In vibration testing the PM gears showed lower amplitude levels than the original steel gears. It appears as the 40 point FZG S-n data does not over predict the life and can be used for design. In this case 99% confidence level was used. Acknowledgement The design-work and testing work was performed by Vicura AB on the behalf of Höganäs AB. References: [1] Flodin. A, Andersson. M. Prototyping of Automotive 6 Speed Manual Transmission in Powder Metal -How does it compare to the original transmission. Proceedings World PM Orlando 2014, paper No 272. [2] Flodin. A. PM conversion of 6 speed manual in SAAB 9-5 -Lessons learned in prototyping, Proceedings EuroPM Salzburg Presented at WorldPM 2016 in Hamburg on October 12, 2016 Page 7