Needle roller bearing design optimization through identifying the contact stresses along the roller length

Transcription

1 Needle roller bearing design optimization through identifying the contact stresses along the roller length Sarbjot Singh Bajwa, Escorts Agri Machinery, Faridabad, India 1

2 Needle roller bearing design optimization through identifying the contact stresses along the roller length 1. Summary: For long life and smooth running of bearing, there must be uniform contact stresses along the length of needle rollers. In case of any uneven contact stresses, roller pitting will be initiated at the point of high contact stresses which consequently will lead to bearing failure. Uneven contact stresses on roller occurs because of misalignment of shafts. Shaft misalignment may occur because of system deflections under load and due to needle bearing clearances which is the functional requirement. This paper elaborates the method to check the contact stress along roller length using KISSsoft/KISSsys software. One practical concern of bearing failure due to uneven contact stresses along the roller length has been considered. In this case, two needle roller bearings are supporting afreerotatingdoublegearonshaft.acoupleisgeneratedduetogearforces,whichinducesthetiltingofdoublegear.duetogeartiltingthereis uneven contact stress along the roller length which leads to bearing failure. Complete system was modeled in KISSsys and bearing calculation was done as per ISO/TS Bearing contact stresses were calculated considering system deflections. It was observed that KISSsoft calculated stress pattern on rollers was same as observed in actual failed parts. In current arrangement, there are two double row needle cage bearings. From failed bearing samples, it was observed that, outer bearing rows of both bearings were highly damaged as compare to inner rows. From analysis results, same observation was made that only outer bearing were loaded and there was negligible load on inner bearings. So based on analysis results and by doing different iterations, bearing arrangement was optimized. Since inner bearings was not taking any loads, so existing two double row bearing arrangement was optimized to two single row bearing arrangement. Optimal bearing position, roller length and roller diameter was selected to achieve best possible contact stress pattern and desired bearing life. Also crowning was introduced along roller length to avoid any edge loading. 2

3 2. Introduction Bearing life calculation method 2.1 Basic Life Rating: For calculation of bearing life, generally method of basic life rating calculation(c/p method) is used which was developed in 1940 s. L 10 =(C/P) p Where L 10 :isratedfatiguelifeat90%reliabilityinmillionrevolution. C:isbasicdynamicloadratingofthebearing. P: is dynamic equivalent load on the bearing. p: Life equation exponent.(p=3 for ball and 10/3 for roller) Assumptions: Zero Radial Internal Clearance No Significant Misalignment RadialLoadOnly Uniform Stress Distribution Along Rollers Good lubrication P<Coor0.5C,whicheverislower Since assumptions made in basic life rating calculation method are not applicable in actual case, so life calculated through this method will not give reliable results and actual life of bearing may be far different from calculated life. 3

4 2. Introduction Bearing life calculation method 2.2 Modified life rating: ISO 281/TS16281 This is the latest bearing life calculation method considering all known parameters affecting bearing life. Actual clearances, misalignments, contact stresses and lubricant characteristics are considered instead of assumption made in basic life rating. -Following Parameters are considering for modified bearing life calculation method: System loads on bearing Bearing dynamic load capacity Bearing roller geometry Bearing Clearances System misalignments Load distribution among rolling elements** Load distribution and contact stresses along the roller length(for roller bearing)** Lubricant Lubricant Contamination **Load distribution among different rolling elements and along the rollers length is affected by system deflection. Following parameters contribute to system deflections: System loads Shaft Stiffness Housing stiffness Bearing internal stiffness Bearing arrangement Bearing internal clearances Bearing fit clearances 4

5 3. Problem Definition Repeated failure of NRB was reported in one of our transmission running under testing for enhanced HP range and severe field application where usage of this particular bearing is also higher. In system arrangement, there is free rotating cluster gear on counter shaft and cluster gear is mounted on counter shaft through two double row needle roller bearings. These double row needle roller bearing were failing in the field. To identify the failure mode, in-housing testing was done and regular observations were made after short interval (25Hrs) of testing. It was observed that surface failure was initiating from outer row needle rollers and there were no failure marks on the inners row needle rollers. To study and simulate the failure in theoretical analysis, arrangement was modeled in KISSsys. The system deflections and bearing contact stresses were observed considering bearing clearances and stiffness. It was observed that loads were being taken by outer rows of double row needle roller bearing and there was negligible load on inner rows of needle bearings. Pitting initiated at outer rows No failure initiated at inner rows Tested NRB S 5

6 4. Impact of bearing clearances on system deflection.. Cluster Gear: Part under consideration for deflection H_L Input Gear Output Gear Needle bearings Sense of Rotation: CCW looking from this side Bearing 1 Bearing 2 Counter Shaft Counter Shaft arrangement 6

7 4. Impact of load and bearing clearances on system deflection Case1 : No bearing clearances considered. Deflection is due to shaft and bearing stiffness only Case3 : In addition to case 2 bearing clearances of NRB considered Deflection is due to combined impact of stiffness bearing clearances of bearing 1, 2 and NRB. Case 2: Bearing 1 & 2 Clearances considered. Deflection is due to combined impact of stiffness and clearances of Bearing 1 & 2 Observation: 1. There is major impact of bearing clearances on the behavior of system deflection. 2. Misalignment of Double gear w.r.tcounter Shaft: -In Z plane, force on input and output gear are in same direction (Gear separating force) so there is negligible misalignment of double gear w.r.t main shaft in Z plane. 3. -In X plan, force on input and output gear are in opposite direction and so a couple is generated which induces double gear misalignment with counter shaft 7

8 5. Impact of system defections on load distribution along the bearing roller length NRB 1 outer row NRB 1 inner row NRB 2 inner row NRB2outerrow NRB 1 NRB 2 Double Gear Counter Shaft Observation: 1. Due to misalignment of double gear w.r.t main shaft, load distribution on all needle bearing is not uniform. 2. Load is being taken by outside bearing only and there is negligible load sharing by two inner bearings. 3. Contact stresses are higher at extreme ends of outer rows of Needle rollers. 4. Calculated life in KISSsoft is having very good correlation with actual testing life of bearing. 8

9 6. Solution: Sinceincurrentdesign,doublerowneedlerollersbearingwereusedconsideringthatloadwillbesharedbybothrowsbutbasedontheKISSsoft analysis and actual testing observation, it has been observed that negligible load is being shared by inner rows of double row bearing. Now wholeloadisbeingsharedbyouterrowsonlysoduetolowerdynamicloadcapacityofsinglerowofbearing,pittingfailurewasinitiating.alsoit has been observed that due to coupled tangential forces on cluster gear, contact stresses are higher at extreme outer ends of needle rollers. So to enhance the bearing life following design correction required. -Optimize the position of needle rollers to reduce the impact of coupled tangential forces at cluster gear on contact stresses along roller length -Optimize the length of needle roller, so that full roller length bear loads. -Select appropriate size of needle roller(roller diameter and no of rollers) having required dynamic capacity to bear system loads. -Select optimum clearances -Optimization of roller profile by providing crowning to avoid extreme edge peak loads. -Ensure sufficient and clean lubrication and also ensure to consider the correct lubricant properties(grade, Temp, Impurities) for bearing life calculation. NRB 1 NRB 2 Double Gear NRB 1 NRB 2 Double Gear Counter Shaft Counter Shaft Old Design New Design 9

10 6. Solution: In new design, Shape of cluster gear has been changed to bring the NRB under the gear position so that to avoid overhang and thus to reduce impact of coupled tangential force. Bearing span increased helping greater stability. Double row needle bearings replaced with single row needle bearings. Length of needle roller finalized through different iterations. Rollers with different length were analyzed for contact stress to check whether full roller length is taking loads and accordingly optimum length was finalized in which full roller length was taking loads. Roller diameter and number of rollers optimized to achieve required dynamic capacity of bearing. Profile crowning along the roller length added to avoid edge stresses. Design of lubrication holes optimized to ensure optimum lubrication and to avoid any damage of roller due to edges of lubrication holes in high contact stress area. NRB 1 NRB 2 Max Contact stresses on rollers in proposed design has been decreased by 40%. Contact stresses are comparatively higher on NRB 2 as compare to NRB 1. Also contact stresses are higher at outer side of rollers and deceases gradually inwards. This behavior is due to combined effect of bearing clearances and coupled tangential forces. Since minimum bearing clearances are required for bearing operation, so this behavior 10 can t beeliminatedbutmaxstressescanbereducedbydesignoptimizationasdidinnewdesign.

11 7. Testing Results and its correlation with KISSsoft results Parts developed as per new design and transmission tested at test rig. At particular load condition, failure was initiating within 50 Hrs in old design of double row needle rollers. With new design of needle rollers, test rig testing of target 500 Hrs completed successfully. SmallpittingonNRB2wasinitiatedafter570hoursatacceleratedload.AsperKISSsoftanalysisitwasobservedthatmaxcontactstressatNRB2 is 20% higher than NRB 1 and also contact stresses are maximum at outer side of rollers and gradually decreasing towards inwards. Similar trend has been observed in tested bearings. Small pitting has been initiated at outer side of NRB 2 and NRB 1 still found OK thus showing a good correlation of KISSsoft analysis results with actual testing results. Pitting initiated at outer rows No failure initiated at NRB 1 Small pitting initiated at NRB 2 Pitting on Double row NRB within 50 Hrs of testing Small pitting initiated after completing 510 hours of testing Old Design New Design 11

12 8. Conclusion Bearing analysis as per IS0 281/TS method which took into consideration the bearing clearances, misalignments and lubricant properties gives much reliable bearing life results as compare to basic life rating calculation method in which many of the important parameters having great impact on bearing life are just assumed. Following guidelines has been explained in this paper to optimize the needle roller bearing arrangement. -Analyze bearing life using IS0 281/TS Optimize the position of needle rollers to reduce the impact of coupled tangential forces at cluster gear on contact stresses along roller length -Optimize the length of needle roller ensuring that full roller length bears load. -Select appropriate size of needle roller(roller diameter and no of rollers) having required dynamic capacity to bear system loads. -Select optimum clearances -Optimize the roller profile by providing crowning to avoid extreme edge peak loads. -Ensure sufficient and clean lubrication and consider the actual lubricant properties in analysis. This paper illustrates a very critical practical case of bearing failure which has been resolved through analysis as per IS0 281/TS using KISSsoft Software. Bearing design optimization and finalization done through KISSsoft and very good correlation between KISSsoft analysis and actual testing results has been observed. Acknowledgement: Author wish to thank Mr. Neeraj Vij(Head transmission and hydraulic design) for his permission to present this article and support of Mr. Nalin Tiwari, Mr. Sayyed Suheal, Sukhjit Singh, Digendra and Ankur Tyagi to make this exercise successful. References: 1. ISO/TS 16281, Roller bearings- Methods for calculating the modified reference rating life for universally loaded bearings, ISO 281, Roller bearings- Dynamic load ratings and rating life, KISSsoft, Calculation programs for machine design, 4. Antonio Gabelli, Armel Doyer, Guillermo E Morales: The modified life rating of rolling bearings: A criterion for gear box design and reliability optimization Power transmission engineering March Pankit B. Kondhiya, Prof.P. H. Darji, Contact stress analysis of needle roller bearing used in synchromesh gear box International Journal For Technological Research In Engineering Volume 1, Issue 10, June Riza Jamaluddin, Bearing Analysis Solutions- Introduction to theory and advanced methods in RomaxDESIGNER Webinar 27th May