A Comprehensive CAE Analysis of Heavy Commercial Vehicle (HCV) Drive Axles Using HYPERWORKS Prabhakar M Div. Manager - ERC TML Drivelines Ltd C/o. Tata Motors Ltd Jamshedpur, -831010, India prabhakarm@tatamotors.com Vysakh Vasudevan Manager - ERC TML Drivelines Ltd C/o. Tata Motors Ltd Jamshedpur-831010, India vysakh.vasudevan@tatamotors.com Sandeep Kumar Veluguri Manager - ERC TML Drivelines Ltd C/o. Tata Motors Ltd Jamshedpur-831010, India sandeep.v@tatamotors.com Abstract The design validation of automotive aggregates in the digital space has now become critical to reduce the overall cycle time of development. Moreover, finite element method is considered as one of the well-established philosophies in digital validation around the globe. The advancements in the nonlinear finite element procedures have enabled the engineers around the globe to model much more accurately and reliably the actual nonlinear physical behavior of complex structures. TML Drivelines Ltd., a major player in the business of Heavy Commercial Vehicle (HCV) Axles and Transmissions, explores and employs the possibilities of finite element method in the validation of its product designs for structural strength and durability. The hot formed axle beam is an integral part of the axle which houses the final reduction gears, differential unit and the half shafts. Furthermore, the beam carries the static and accelerated vehicle load through the suspensions and transfers it to the ground. TML Drivelines Ltd., uses the nonlinear FEM capabilities of Altair Optistruct solver in modelling and analyzing the effects of assembly fit (Interference and Clearance) conditions in the steering knuckle assembly. Further, the nonlinear FE simulation has helped to achieve leakage prediction in Gearbox PTO cover. Briefly, Altair HyperWorks enables TML Drivelines Ltd. to validate engineering designs for improved product performance and functionality which ultimately define customer perception. Introduction Global competition and commercial pressure has led to the exploration of digital techniques in automotive product design evaluation which impart savings in the time and money invested in physical testing. The extensive progress that has happened in the area of finite element based simulations has equipped the automotive manufacturers to design and develop their products with superior quality and safety standards with less lead time. TML Drivelines Ltd., a major player in the business of Heavy Commercial Vehicle (HCV) Axles and Transmissions with global footprints employs advanced FE techniques and methodologies for strength and durability evaluation of its products. Many a times, the realistic simulation of the behavior of structural components under load, requires a deviation from the linear FE methodology towards the nonlinear philosophies. With the enhanced nonlinear capabilities of Altair Optistruct the Design Team have successfully predicted the contact behavior and effective stress states of its design components which enables them to deliver quality designs. In this paper, two case studies are presented where the nonlinear FE practices are made use in predicting the stress states and contact behavior in the components under loading. The HCV drive front axle, houses the transmission components and carries the knuckle assembly which enables proper steering of the front wheels. The steering knuckle assembly consists of the principal parts like joint flange, knuckle, tie-rod arm, plummer block, kingpins, bush and thrust bearing. The split kingpins connects the knuckle with the joint flange and allow the rotation of knuckle along the king pin axis. An interference fit of 50microns is defined between the kingpin and joint flange which keep the king pin in position. The kingpin maintains a clearance fit with the knuckle bush and thrust bearing which enables the rotation of knuckle. Design of the joint flange and knuckle requires deep examination of the stress states which is generated at the assembly fit locations under service loads. A linear static analysis is unable to solve the system with joints where an assembly fit is defined as it ignores the evolution of the boundary conditions in the fit definitions during the analysis. Also, the Nonlinear contact analysis with simple sliding contact defined between the mating faces of hole and shaft will make an inaccurate stress prediction in the joints. The resultant stress predicted will be even to all the revolving pairs as the load in the revolving pairs is not distributed as per the mating boundary conditions. A realistic simulation of the behavior of the joints with assembly fit definition is necessary for deciding the sections of the components which satisfies the strength and stiffness requirement. Altair Optistruct empowers the engineers to carry out a nonlinear contact 1
analysis with assembly fit definition which exactly simulates the behavior of the joints under load and presents an authentic stress situation on the components. The HCV Gearbox has a rear cover which jackets the main-shaft and is bolted to the gearbox housing. Improper design of the cover can lead to oil leaks, which is a major concern on the field. A non-linear contact simulation with various pretension values is conducted to predict the contact pressure generated between the housing face and the cover. Physical trials correlates the contact pattern predicted by Optistruct. Thus the oil leakage can be arrested with the proper contact pressure by application of the proper tightening torque or suitable gaskets. Process Methodology 1) Steering Knuckle assembly The fully built HCV live front axle is shown in fig. 1 (a). Plummer Block Knuckle Fig 1 (a) HCV live front axle King Pin Joint Flange Tierod 36µ clearance Fig 1 (b) Knuckle assembly 50µ interference 50µ interference 13µ clearance 13µ clearance Fig 1 (c) Sectional view of knuckle assembly with fit definition The axle housing encases the transmission components and carries the suspension mounting brackets. The steering knuckle assembly as shown in fig. 1(b) is bolted to the axle housing. The entire knuckle assembly is maintained at an inclination with respect to the vertical axis along the king pin centre line. The split king-pins acts as a rigid link between the joint flange and knuckle and at the same time enables the rotation of knuckle guided by the steering column. The fit definitions in the knuckle assembly are described in fig.1(c). A nonlinear analysis which replicates the true behavior of the joint under bending is carried out with Altair Optistruct solver. A contact interface is defined between the mating surfaces in the joints and appropriate fit conditions are applied through PCONT cards. 2
i) FE Modelling of Knuckle assembly The joint flange, knuckle, spindle, kingpins, plummer block tie-rod arm, bush and thrust bearing are meshed with 3D tetrahedral elements in Altair HyperMesh. The hole and shaft dimensions are meshed to the nominal dimensions. The joint flange hole surface which acts as the slave in contact interface, is meshed finer compare to the master kingpin surface. The hardware connecting the knuckle with plummer block, tierod arm and spindle are modelled with 1D beam elements of appropriate dimensions. Fig 2 shows the meshed model of the knuckle assembly. Fig 2 Mesh model of the steering knuckle assembly ii) Contact interface modelling and fit definition Fig 3 shows the contact interface groups generated in HyperMesh as per defined fits. Contact surfaces Fig 3 Contact interfaces generated in HyperMesh as per fit definitions A Master - Slave type Contact definition is implemented between the joint surfaces of interest. Contact surfaces are created on the faces which undergo contact. Five different Contact interfaces are defined between the King pin outer diameter and corresponding mating holes according to the specified fit situations. Contact interfaces are defined on the joint flange and tie-rod faces also where they rest on bush and bearing faces respectively. Preferentially meshed joint flange hole and faces are declared as Slave and Kingpin OD as Master in the contact definitions. 3
The Fit definition for each contact interface is introduced through PCONT property cards. The clearance and interference values for each fit is defined in PCONT property cards and mapped to the particular contact interface as contact property id definition. The contacts interaction between the resting faces of the components are defined as normal sliding type contact. The entire model is analyzed for 3g vertical loading condition and stress level on the components as well as the contact pressure pattern is studied. 2) Gear Box Rear Cover Leakage Analysis. Fig 4 FE model of the rear cover with contact surfaces with bolt pretension applied The Gearbox rear cover and the housing are meshed with 3D tetrahedral elements. The connecting hardware M8 and M10 bolts are modelled with 1D beam elements. A sliding contact is defined between the cover and housing faces making cover face as slave and housing face as master. Tightening torque of 25 Nm and 45 Nm are applied on the M8 and M10 bolts respectively by pretension process manager, and the contact pressure development is studied. Fig. 4 shows the FE model of the rear cover with contact surfaces. Analysis Results and Discussions 1) Steering Knuckle Assembly Under the application of 3g vertical load on the axle assembly the stress levels in the knuckle assembly is studied. The assembly fit definition in Optistruct has successfully simulated the realistic stress condition in the assembly under load. The contact pattern obtained by the fit simulation is shown in fig.4. Fig.4 clearly shows the realistic contact behavior under the interference and clearance conditions. The Kingpin OD and joint flange hole ID faces have experienced higher contact pressures of the order of which depicts the successful simulation of realistic fit conditions in the assembly. Similarly, the clearance fits between the Kingpin and knuckle bush as well as king pin and thrust bearing have resulted in a reduced contact pressures on their appropriate faces. 4
Clearance fit Interference fit Interference fit Clearance fit Fig. 4 The contact pressure contour showing the interference and clearance fit behaviour on the knuckle components and the kingpins under bending load. Fig. 5 compares the analysis results showing the stress states in the components without and with the fit definition. The simulation of the assembly fits brought in drastic changes in the stress states on the components under load, which are predicted inappropriately in case neglecting the fit definition. The first image shows the stress plot in the components without considering the assembly fits. This averages the stress among the components and shows a lower stress across the joint flange wall thickness. Also, the stress predicted in the knuckle joint is less realistic when considering the clearance fit condition. The second image depicts the stress condition in the components with proper assembly fit definition. The software clearly demarcates the zones with clearance and interference fit definition and calculates the stress generated under load more realistically. The more stress is predicted across the wall thickness in the joint flange which calls for the thickening of section. Thus, this advanced simulation of assembly fit condition has enabled the engineers in predicting the realistic stress condition under bending loads and optimize the sections for higher design strength. The knuckle hole with clearance fit is found to have less stressed compared to the interference zones which correlates with the realistic behavior under loading. Stress predicted evenly Localized stress prediction Fig. 5 (a) Stress contour of knuckle Assy without fit Fig. 5 (b) Stress contour of knuckle Assy with fit 5
2) Cover Leakage Analysis Optistruct nonlinear contact analysis successfully predicted the contact pattern for the cover face under the applied tightening torques. Bolt pretension simulation in Optistruct has effectively applied the tightening torque effect in the cover. Fig 6. Compares the FEA results with the actual test results, which draws an impressive correlation between the simulation and physical test results. The simulation image reveals the patches where there is a loss of contact under the field test data reveals that the oil leaks are happening in cases where the contact pattern on the mating faces between the cover and housing is not uniform. Fig. 6 Comparison of the contact pattern on the cover face predicted by Optistruct and field test An uniform contact pattern predicted by the Optistruct simulation again mapped to a never leakage case in the field trials. Fig. 7 shows the rear cover with an uniform contact pattern which reported no leakage on the field trials. Thus the Altair Optistruct nonlinear analysis with contact definition and bolt pretension simulation has helped to predict the oil leakages in Gearbox rear covers and directed the designers to take proactive steps in preventing the same on field. Fig. 7 Uniform contact pattern on the cover face predicted by Optistruct which has no leakage issues Benefits Summary In an automotive axle, the realistic geometric boundary condition of the knuckle assembly has to be simulated in the wheel hub ends, as the joint flange and knuckle are subjected to both the loads viz., the assembly stresses due to interference fits as well as the bending stresses due to vehicle payload. In linear static approach the stress pattern distribution of stresses at the kingpin Joint will be even to all the revolving pairs due to the absence of fit definition. This leads to the non-realistic distribution of stresses leading to strengthening of non-load bearing sections and weakening of load bearing areas. Non-linear analysis in Optistruct helps to define the actual assembly. A condition of a revolving knuckle assembly pair and simulates the realistic load distribution to design the joint flange and knuckle. 6
Also, the non-linear contact analysis applied to predict the contact pattern of the rear cover in the gear boxes helps the designer to decide the value of pretension to be provided in the bolts and requirement of additional gaskets to prevent leakage. The simulation is matching to the actual test contact pattern, thereby increasing the confidence level of designer to rely on the non-linear CAE simulation results. Future Plans The prototype sample of the live front axle assembly with the knuckle is in manufacturing stage. After successful completion of indoor rig testing and monitoring of vehicle trials the non-linear contact simulation of knuckle will be extended to front dummy axles also. Considerable reduction of field failure in knuckle assembly is expected. Conclusions: This paper successfully presents the use of non-linear finite element methods in the structural strength validation of transmission components. Contact non-linearity analysis is performed with the assembly fit definition to predict the realistic stress states developed on the components under load. Further the nonlinear capabilities of Optistruct helps in simulating gearbox cover leakage with the bolt pretension. Acknowledgements We Sincerely Thank M/s. Altair Technical support team, for guiding us to deploy the Non Linear Capabilities of Optistruct in our Design of Axle Assy. References: 1) Advances in nonlinear finite element analysis of automobiles K. J. Bathe, 0. Guillermin, J. Walczakt and Heng-Yee Chen 0, 1997 Elsevier Science Ltd. 2) Non-linear finite element analysis of solids and structures, vol. 2 M. A. Crisfield, Wiley publications. 7