Design Validation of an Integrated Transmission System Vikas Manjrekar 1, S Shamasundar 2*, Samarth Shrivastava 3 1 Executive Vice President, R&D& Strategic Sourcing, AVTEC Ltd, Pithampur, MP. Email: vikas.manjrekar@avtec.in 2* Managing Director, ProSIM R&D Pvt Ltd, Bangalore. Email: shama@pro-sim.com 3 CAE Engineer, R&D, AVTEC Ltd, Pithampur, MP. Email: samarth.shrivastava_tl@avtec.in ABSTRACT Authors have worked on design and development of an integrated transmission system for a 4x4 SUV. Design, design verification and validation, design optimisation of torque transfer components - gear pairs, shafts, synchroniser, differential, bearings etc., and housings were carried out. Multi-body dynamic analysis has been carried out to determine time dependent bearing loads. Finite element analysis was conducted to determine the stresses, vibrations and validate the design. Durability assessment was carried out to determine the fatigue life of all the components in the transmission system, considering the duty cycle loads as per customer statement of requrements. Transient dynamic analysis of the transmission system was carried out. Resonance analysis is carried out to determine the critical speed and components causing resonance /noise. Acoustic analysis was carried out to predict the noise generated. Using virtual prototyping principles, the design validation of the transmission system was done and design parameters were optimised. Gear engagement analyses, synchroniser engagement behaviour, transmission efficiency, have been calculated. Physical prototyping and testing was carried out for design validation. Vibration, Noise, and durability were measured in the test rigs. Paper discusses the issues in modelling (multi-body dynamic simulation, finite element analysis, and durability) and also in the testing practices. Authors discuss the challenges faced in the development process and ways to overcome typical problems in the development. Commercially available software tools such as SIMPACK, ABAQUS, and FESAFE have been used in this study. Key Words: NVH, durability, fatigue life, gear box, transmission, differential, synchroniser, FEM analysis, resonance, noise isolation, SIMPACK, FESAFE, ABAQUS, Modal Analysis, Harmonic Analysis. INTRODUCTION: Gear box design and development is a complex exercise calling for interdisciplinary expertise. OEM demands on higher performance (such as NVH, durability and so on,) are continuously increasing. Using technology, methods, guidelines, and tools available at their fingertips, designers can develop superior designs. Only by effective utilization of research budget an organization can develop improved design, which can result in cost savings and systematic production [1]. The integrated transmission reported in this paper has been designed, verified and validated, and qualified deploying principles of virtual as well as physical prototyping and testing. These have been done as per the customer statement of requirement and customer prescribed testing standards. Effect of the accuracies of manufacturing gears, bearings, housing & all torque transfer on gear box performance are analyzed. The design objectives considered in design were the customer prescribed levels of load capacity, reliability, service life (which comprises of endurance life & fatigue life. Noise &
vibration test for this transmission was carried out for isolation / elimination of unwanted noise & vibration from the system. Design of Gearbox: The transmission (gearbox) discussed in this paper is an integrated transmission & transfer case & shown in Figure-1. This is different from conventional transmission in many aspects. Conventional transmission which has been used in AWD vehicles has transfer case separately attached where as integrated transmission has transfer case mounted directly on rear cover housing for compactness and NVH improvement. Design of a 5-speed transmission with an integrated transfer case has many challenges. First of all a new product development plan has to be made and the work flow has to be in line with the plan. Challenges like material selection for all the torque transfer parts, housing parts, synchronizer, bearings and other miscellaneous parts has to be considered appropriately. Environmental condition of usage from -30 /+50 degree Celsius for material was a major challenge for performance and durability. Engineering analysis of design such as DFMEA, PFMEA, and GD&T have been done since this was a new product. 5 speed manual transmission Motor shift transfer case Figure-1: Integrated Transmission & Transfer Case
The process for new product design and development has followed various stage gate process which capture voice of customer (VOC) and business viability, translating VOC in design followed by design for performance, fitment, manufacturing, (for quality and cost) and serviceability before signing off with customer. The main task of design process is to create a relationship between the product function, product structure & complimenting with requirement with other critical aggregates (engine, chassis, Axles etc) of vehicle. Product function for integrated transmission is to run vehicle in 5 forward speeds & 1 reverse speed within the specified torque & rpm range. It has also a transfer case with differential which makes the vehicle alltime 4WD and can accommodate differential speed in front and rear axle. To increase flexibility and feature in the system to fulfilling customer requirements the transfer case has been designed with 2 speeds reduction (low & high) & a differential. Differential facilitates in locking & unlocking the power flowing in forward & reverse axle. In lock state both the forward & reverse axle receive equal torque & in unlock state both the axle receive unequal torque & power transmission. This transmission has many design changes with respect to conventional 5 speed transmission. Therefore, tunneling of transmission in the vehicle body has been analyzed to improve the compatibility and performance of other critical aggregates. Corresponding system assemblies combining components for a satisfactory design structure has been developed to fulfill assembly requirements. One of the most weighted quality assessment parameter of gearbox nowadays is acoustic noise level. A gearbox is rated according to the noise level depending on the customer acceptability with bench mark vehicles / competitions [2]. A full consecrated test rig has been installed with 3 electric servo control motors for load & speed application. Durability cycle & fatigue cycle have been developed for accelerated testing by designers with field usage and B10 life specified by customer. Performance measurement has been done by developing durability cycles i.e. endurance cycle & fatigue cycle as per voice of customer. Table 1 illustrates load cases for durability testing transmission on rig. Table1. Load Cases of durability cycle for transmission S.no. % of total time Front Axle (% of total torque) Rear Axle (% of total torque) 1 10% 100% 100% 2 30% 120% 80% 3 30% 80% 120% 4 10% 140% 60% 5 10% 60% 140% 6 5% 200% 0% 7 5% 0% 200% Design Validation: Optimised design was finalized based on the simulation and modeling explained in the next sections. Prototypes of the optimized model were manufactured for design validation by testing. Durability cycle for integrated transmission has been developed & the transmission has been tested for 752 hours.
Durability & fatigue cycles have been done for 2 transmission prototypes. Figure-2 shows the test rig used for testing these transmissions. While testing the performance of a gearbox it is necessary to test the torque carrying capacity of the gearbox at different speeds and noise generation in normal running conditions. Theoretical calculations using ISO (International Standards Organization) & AGMA (American Gear Manufacturing Association) standards were used for specifying torque ratings. A validation test rig with AC regenerative motors simulating 4WD environment was used for validation. Since the gear performance is highly dependent on number of parameters such as material, design, manufacturing, operation & environment therefore a noise insulated acoustic chamber has been built for testing [3]. This rig also offers a versatile loading environment where actual working conditions like oil temperature sensing, shift feel, vehicle inclination, traction (loading) can be simulated in laboratory testing. Figure-2: Test Rig for Integrated Transmission Modeling and Simulation: A model of the complete integrated transmission system (consisting of gear pairs, bearings, shafts, housing, differential, synchronizer etc.,) has been developed to create virtual prototype and simulate all the tests virtually. Simulation involved in creating a digital mockup of the assembly, developing discretized models, conducting multi-body dynamic simulations, finite element analysis, acoustic analysis, and durability analysis and fatigue life assessment. Non-linear contact analysis has been performed to account for the contact friction conditions. Multi-body dynamic Simulation For dynamic analysis of drive train, multibody dynamics (MBD) simulation software SIMPACK is used to build the virtual model and to simulate. In a drive train, meshing of the gear teeth is a common source of internal excitation [4]. Gearbox housing, transfer case and shafts are considered as flexible when modeling in SIMPACK. Flexible models in FE can have a large number of degree of freedoms (DOF s), in the order of magnitude of ten thousand up to several hundred thousand. The reduction to a smaller set of DOF s, in the order of magnitude of one up to ten, which can be imported in to SIMPACK is done with the component mode synthesis (CMS) technique. The Craig-Bampton method is a well-established CMS technique [7]. This reduction involves the creation of a set of static constraint modes and fixed-interface normal modes with their corresponding Eigen frequencies [5]. The accuracy of the flexible body in the multibody model is determined by the accuracy of the FE model, and the number of modes that were taken into account during the CMS reduction [6]. Gear wheels are modeled in SIMPACK using inbuilt
modeling element. Backlash, multiple tooth contact and material properties are all taken into account in modeling. Shafts are mounted on the housing with nonlinear bearing stiffness and damping values. Synchronizer components (synchro cone, block rings, hub and sleeve) are modeled as sub-model and assembled with the main model. Synchronizer shifter force has considered to engaging different speed. Transmission and Transfer Case are modeled as sub assembly in Main model via sub structuring. Load cases is applied to the main model via sub structuring. Figure-3 shows the complete gearbox system topology in a 2D framework. Topology diagram helps to understand the load transfer path in an intuitive way. Figure-3. Topology diagram of complete gearbox system A multi-body dynamic (MBD) model thus created in SIMPACK, is used to extract time dependent bearing forces. Gearbox is mounted accounting for mounting stiffness and damping values. The sensor elements (used in vibration and noise testing) are modeled at the probe locations to extract acceleration. Input torque and speed are applied at the input shaft and for output shaft the reverse torque is applied. Finite Element Analysis, Durability assessment Authors have used a commercial software ABAQUS for finite element analysis and FESAFE for fatigue life (damage) and durability assessment. Standards assumptions such as isotropic material have been invoked. For FEM analysis, second order tetrahedral elements are used. The stress results from FE analysis is used in durability analysis of housings and TT components. The fatigue properties are assigned for all components and as per vehicle usage conditions the duty cycle load is considered during durability analysis. The fatigue life (number of cycles) is estimated for transmission assembly. The number of cycles observed in housing assembly is 10E 7 cycles. Issues associated with
fatigue damage (life) and durability assessment have been described by other researchers [8]. Care to be taken during fatigue life and fatigue hot spot assessment are described in [9]. Acoustic analysis is carried out to predict the noise generated by Finite element method. Major components contributing to the noise in the integrated Transmission housings, TT parts were considered in the analysis. Air volume is considered as a cylinder shape (acoustic structure) from transmission axis and air media properties are assigned to acoustic elements. The acoustic structure is discretized as per wave length criteria and analysis is done at the 1/3rd Octave band and the average sound pressure level (db) at the required probe location for different load cases were extracted at the corresponding frequencies. Results and discussion: Due to paucity of space limited results are presented here in this paper. Figure-4 shows the input torque, and output torques at rear and front drive shafts. Gear box transmission efficiency is calculated using the input and output torques. Efficiency of gearbox is associated primarily with tooth friction and lubrication churning losses. Churning losses are independent nature of gears and gears ratios. Input torque applied to input shaft and output torque are measured from the system. Efficiency of gearbox due to friction was found in the range of 98% to 99%. Time dependent bearing force history calculated by SIMPACK is shown in Figure-5. This is converted into frequency dependent forces by using FFT (Fast Fourier Transformation) in SIMPACK post processing. The Bearing forces are used to conduct structural analysis in FE program. Efficiency of the gearbox for different gear engagement was calculated considers the gear friction. The FFT converted bearing forces are used in acoustic analysis in FE program. Resonance analysis is carried out to find out critical components (which contribute more vibrational signals to the resultant vibration of whole system) in system. Acceleration at probe locations in gearbox housing are extracted from the SIMPACK sensor elements. Using order analysis in SIMPACK, the critical components in the system for all gear engagement was identified. For example, the Figure-6 shows, at a particular speed acceleration is higher for the 3 rd gear pair which is having meshing frequency 21Hz. From the same plot we can observe another gear pair from transfer case of meshing frequency 28Hz is accelerating. TC gear pair is accelerating at second harmonic speed can be observed at 56 th order. The speed synching time over the frictional torque for applied shifter force has analyzed for each engagement. Time dependent bearing load was applied to acoustic analysis to find out noise generated in each speed range.
Figure-4: Input and output torques for efficiency calculation. Figure-5: Bearing Load at TC Input Shaft Figure-6: Campbell Diagrams in 3D (order, RPM, acceleration) and 2D (Order, acceleration)
A modal dynamic analysis is conducted. Some of the dominant mode shapes of transmission system is shown in figure-7. Mode 1 (358.11 Hz) Mode 2 (362.73 Hz) Mode 6 (617.06 Hz) Mode 7 (642.89 Hz) Figure-7: Mode shapes of Transmission system. Von Misses stress observed on the transmission assembly for a particular static analysis case is shown in Figure-8. Stress / strain fields calculated by finite element analysis are used to carry out the fatigue life and durability analysis. Lifetime of transmission components was determined. Towards this the life available (described as number of cycles for fatigue crack initiation) at the fatigue hot spot had to be more than the prescribed limit, viz., 10E 7 cycles. This would mean available design life of the entire integrated system is higher than the life expected by customer, and would meet and pass the specification. Figure-8: von Misse s stress on transmission housing assembly
The noise level extracted for one of the load cases at a particular probe location is shown in Figure-9. Figure-9: Noise level for a particular gear engagement case at a probe location Conclusions: A methodology of design and development of a new integrated gear transmission system for 4x4 SUV (AWD) is explained. By using a combination of virtual and physical prototyping and testing, the design objectives set forth by the customer was achieved. This included o o o Achieving a durability for 10e 7 Cycles Achieving a noise level less than 73 dba Optimising the structure with sufficient factor of safety. Using simulation techniques cost and time saving has been achieved in the design and development cycle. References: 1. Hales Crispin, 2003. Respect Design or Expect Disaster! International Conference On Engineering Design, Iced 03 Stockholm, August 19-21, 2003. 2. Ognjanovic M. &Milutinovic M., 2013. Design for Reliability Based Methodology for Automotive Gearbox Load Capacity Identification, Journal of Mechanical Engineering, Vol 59(2013)5, 311-322. 3. Rosic et.al, 2005. Optimum Design of Multispeed Gearboxes and Modeling of Transmission Components, V International Scientific Conference, Heavy Machinery-HM'05 Kraljevo. 4. Alexander Neubauer,2014 Simulation Methods for NVH-Development of a Double Clutch Transmission Gearbox, SIMPACK User Meeting. 5. F. Vanhollebek, J. Helsen2, J. Peeters1, D. Vandepitte2, W. Desmet2, Combining multibody and acoustic simulation models for wind turbine gearbox NVH optimization
6. Dr. Berthold Schlecht, Tobias Schulze, Multibody system Simulation of Wind turbine for Determination of Additional Dynamic loads, SIMPACK User Meeting. 7. SIMPACK Documentation 8. Wufu Ma, "Strength and fatigue calculations of a planet gear carrier", (PERA Technology (Beijing) Co. Ltd., Chengdu 610016, Sichuan, P.R.China) 9. Fe-safe 6.5 User Manual. Dassault Systems