Aluminum Gear Shift Fork with Supporting Pad for light weighting in Commercial Vehicles Suresh Kumar K, Manger VE Commercial Vehicles Limited skkandreegula@vecv.in, Naveen Sukumar N, Dy.Manger VE Commercial Vehicles Limited nnsukumar@vecv.in Umashanker Gupta, Sr.Manger VE Commercial Vehicles Limited usgupta@vecv.in Abstract To compete with the current market trends there is always a need to arrive at a cost effective and light weight designs. For Commercial Vehicles, an attempt is made to replace existing Gear Shift Fork from FC Iron (Ferro Cast Iron) to ADC (Aluminum Die Casting) without compromising its strength & stiffness, considering/bearing all the worst road load cases and severe environmental conditions. ADC has good mechanical and thermal properties compared to FC Iron. Feasible design has been Optimized within the given design space with an extra supporting pad for load distribution. Optimization, Stiffness, Contact pattern has been done using OptiStruct, Nastran & Ansys for CAE evaluation. A 6-speed manual transmission is used as an example to illustrate the simulation and validation of the optimized design. Advanced linear topology optimization methods have been addressed as the most promising techniques for light weighting and performance design of Powertrain structures. The theoretical achievements are obtained both mechanically and mathematically. Nowadays, the great challenge lies in solving more complicated engineering design problems with multidisciplinary objectives or complex structural systems. The purpose of this paper is to provide an insight into the conversion of material and achieving an optimized design without compromising on functionality of the product using structural linear topology optimization. The advantage of the proposed method is that structural optimization on irregular design domains can be carried out effectively with ease. Furthermore, this method integrates the stress analysis and the boundary evolution within the framework of finite element methods. For this, I have used linear Topology Optimization technique with the help of Simulation tool Altair OptiStruct and lastly, tested the aluminum gear shift fork with supporting pad under VE Commercial Vehicles Ltd Standard Durability Cycles. Introduction Cast Iron existing/proven gear shift fork has no reported failures in LMD vehicles till date. For light weighting, we have changed material from Cast Iron to Aluminum without compromising in its strength & stiffness. The following are the challenges for converting material:- (1) Stiffness reduction directly by 1/3 rd, as modulus reduces to 1/3 rd. (2) It would be bulky to arrive at same stiffness as of CI with two pads. Hence, an extra supporting pad is introduced for load distribution. For this, we have adopted following approaches. Topology Optimization It is a mathematical technique that optimizes the material distribution for a structure within a given package space. This approach leads the design engineer to consider the design s most effective load paths, not just those from historical or competitive designs. This methodology has often shown radical departure from an incumbent design philosophy, producing a significantly more efficient design. Simulate to Innovate 1
The topology exercise creates successive concept design iterations until an optimal balance of mass, architectural efficiencies and structural performance is reached [1]. To obtain equal stiffness on both the legs, topology optimization has been carried out using Altair OptiStruct FE Solver [2]. From the results of optimization it has been observed that the design proposed by the topology optimizer is not feasible for manufacturing as shown in Fig. 1. Hence, we have gone for below approach. Figure 1: Topology Optimization Results Stiffness Evaluation Generally any transmission has 2 types of Forks i.e. Symmetrical and Un-Symmetrical Forks. For Symmetrical Forks, whose legs are same length - It s very simple to arrive at required Stiffness. For Un-Symmetrical Forks, we have followed the below mentioned approach. Page 2 of 9
Figure 2: Basic design calculation From the above equation I2, will be obtained by modifying the cross section using CAD tool (Pro-E Creo 5), taking care of packing and manufacturability constraints of minimum thickness for PDC Casting of Aluminum. Refer Fig. 2 for base design calculations. However there will be slight variation in results which could be updated in the next iteration. Within 2 to 3 iterations, it arrived at equal stiffness on both legs. This final design is used to check the strength for operating loads. Material details Aluminum die casting alloys have a specific gravity of approximately 2.7 g/cc, placing them among the lightweight structural metals. The majority of die castings produced worldwide are made from aluminum alloys. Six major elements constitute the die cast aluminum alloy system: silicon, copper, magnesium, iron, manganese, and zinc. Each element affects the alloy both independently and interactively. Alloy A380 (ADC10) is by far the most widely cast of the aluminum die casting alloys, offering the best combination of material properties and ease of production. It may be specified for most product applications. Some of the uses of this alloy include electronic and communications equipment, automotive components, engine brackets, transmission and gear cases, appliances, lawn mower housings, furniture components, hand and power tools. Alloy 383 (ADC12) is alternatives to A380 for intricate components requiring improved die filling characteristics. Alloy 383 offers improved resistance to hot cracking (strength at elevated temperatures). Page 3 of 9
FE Modeling Optimized Gear Shift Fork geometry is modeled in Pro-E, where the complete wireframe generated. The data is then translated to Initial Graphics Exchange Specification (IGES/STP) format and read into HyperMesh, where the Gear Shift Fork is FE modeled by 3D Tet mesh generation solid Element type. Fig.3 shows a typical CAE Tet meshed model of the Gear Shift Fork. Figure 3: Aluminum Gear Shift Fork - FE Model Structural Analysis Page 4 of 9
According to the applied loadings originating from different categories of mechanics, this linear elastic analytical procedure could further be divided into three load steps, (1) Stiffness Evaluation (Displacement & SPC forces) (2) Stress Analysis (3) Contact Pattern Simulation (1) Stiffness Evaluation (Displacement & SPC forces) Final design from Stiffness approach has been evaluated under operating loading condition to get the equal Displacement and Reaction force at both legs [3]. Section Modulus I2 is fine tuned to obtain equal deflection within acceptable variation of 0.01mm. It took 2 more iterations to arrive at the final stiffness. Fig. 4 shows the loads and boundary conditions - Fork rail constrained except translational Z direction because load applied in this point as a prescribed displacement of 10 mm in Z- direction this displacement is shared by both the legs. Bottom of the legs constrained in translational Z - direction with using spring element and inside face of the bottom legs constrained in translational X-direction. Figure 4: Gear Shift Fork Loads and Boundary Conditions Figure 5: Gear Shift Fork Displacement Results To maintain the same stiffness for both the legs, the displacements and Reactions should be equal for both the legs. Page 5 of 9
Fig. 5 shows, the Displacement results for Optimized design under operating loading condition Displacement s are same or acceptable for the both legs. Figure 6: Gear Shift Fork Reaction Force Results Fig. 6 shows, the Reaction forces results for Optimized design under operating loading condition Reaction s are same or acceptable for the both legs. (2) Stress Analysis Now, this final design is ready for Strength evaluation using Nastran FE Solver. Operating load is applied on each leg constraining all DOF at Fork rail. Stresses in each leg should be below the endurance strength of the material. As the constraint of bulky design already mentioned with two pads, a third pad has been introduced to share the load. But this is done in a phase wise manner in which the third pad will only come to contact after there is specified deflection in the main two legs. This third pad is located at the center of the fork. If the FOS is less than 1.0, then equal amount of thickness is increased on both the legs until we arrive at or above FOS 1.0 as shown in the Fig. 7. Figure 7: Gear Shift Fork Stress Results (3) Contact Pattern Simulation Contact analysis has been carried out to check the contact status using FE OptiStruct Solver. Surface to surface contact is provided between mating surfaces of forks legs & third pad with synchronizer ring. Contact starts at fork legs that will have a hinge effect before it touches third pad meeting the required deflection. Page 6 of 9
A minimum nominal gap is provided for 3 rd pad to share the load when max/abuse load appears on the load shifting Jaw. Gap could be adjusted if the FOS in the legs goes below 1.0 for the Max/Abuse Load as shown in the Fig. 9 and Fig. 10. Loads and Boundary conditions as shown in the Fig. 8. Figure 8: Gear Shift Fork Contact Pattern Loads and Boundary Conditions Figure 9: Gear Shift Fork Contact Patch Results Figure 10: Gear Shift Fork Contact Patch Displacement Results Page 7 of 9
Once the third pad come into contact stress pattern shifts from fork legs to the middle of the fork as shown in the Fig. 11. Stress induced should be below the Yield strength of the material (FOS > 1.0). If not, the web thickness will be increased to meet the requirement. Figure 11: Gear Shift Fork Contact Pattern Stress Results Experimental Verification As per VE Commercial Vehicles Ltd, standard durability duty cycle, rig has been setup as shown in the Fig. 12 and tested the transmission assembly in which Aluminum Gear Shift Fork s (1 st & Reverse Fork and 4 th & 5 th Fork) are the test components. Transmission assembly for operating load condition has been evaluated with defined duty cycles (For upshifting 2400 and down-shifting 1500 RPM) as shown in the below table 1. Duty cycle has been generated using RLDA (Road load data acquisition) collected on existing vehicle on a defined road map. This data from RLDA has been converted to rig duty cycle considering a total life span of vehicle as 5.5 lakh kms. Same duty cycle was run in 4 consecutive phases to complete the testing cycle. As per VE Commercial Vehicles Ltd, standard endurance test cycle has been carried out on 3 vehicles for 1.8 lakh kms each. Therefore a cumulative of 5.4 lakh kms (3*1.8lakh) has been covered on the vehicle without any failure. Hence vehicle validation has been concluded and certified as for design implementation. Table 1: Duty Cycle Shift Pattern Page 8 of 9
References 1. Altair OptiStruct help. 2. Rohit Kunal Gear Shift Fork Stiffness Optimization, SAE Paper No. 2011-01-2235 @ Commercial Vehicle Engineering Congress. 3. Cury, R. and Baruffaldi, L., Topological Optimization of Clutch Fork using Finite Element Analyses, SAE Paper No. 2012-36-0223. Acknowledgments The authors would like to express their gratitude to Mr. Rajinder S. Sachdeva, Executive Vice-President, Technology Development, VE Commercial Vehicles Ltd, for granting permission to publish this work. Special thanks to Mr. V. Chandrasekhar, GM-CAE for his guidance and encouragement to develop present work. Also, the contributions and support of all CAD, CAE engineers and respective testing engineers are highly appreciated. Definitions, Acronyms, Abbreviations ADC Aluminum Die Casting LMD Light/Medium Duty FOS Factor of Safety PDD Product Design and Development DOF Degrees of freedom RLDA Road Load Data Acquisition Keywords Gear Shift Fork, Supporting Pad, Stiffness Evaluation, Contact Pattern, Light Weighting and Commercial Vehicles. Publication Published @ The 11 th International Conference on Automotive Engineering, TSAE, Thailand (http://papers.sae.org/2015-01-0088/). Page 9 of 9