CETYS Universidad Baja SAE Design Report

Transcription

1 Copyright 2007 SAE International Vehicle #28 CETYS Universidad Baja SAE Design Report Gustavo Ovies, Andres Magaña, Alejandro Burgas Iván Pulido, Iván Williams, Bernardo Valadez Mechanical engineering division of CETYS Universidad, Campus Mexicali ABSTRACT The Baja SAE Series is a competition sponsored by the Society of Automotive Engineers (SAE) which gathers students from universities all around the world. The teams are challenged to design, build and test a recreational off-road vehicle that conforms to the limitations established by the Baja SAE Rules 2012 (SAE International, 2011) There are several regional competitions of the Baja SAE Series; CETYS Universidad has chosen to participate this year in Baja SAE International at Portland, Oregon, USA. In the present document it is outlined the design and development process for creating the Z7 prototype. For general specifications please refer to Table 5 at the appendix, page 12. INTRODUCTION CETYS Universidad has built seven prototypes for the series, being the Z7 the seventh member of the family. Figure 1 shows the Z7 Solidworks model. A lifecycle for Z7 as a product has been methodologically planned. It Figure 1. Z7 SolidWorks Model gives a clearer vision of how the project is organized. As shown in Figure 12, at the appendix, the lifecycle includes six stages: project plan, product redesign, manufacturing process design, competition, maintenance, recycle and disposal. The present design report explain only two stages, the first one which is the product redesign and the third one the manufacturing process design. The design stage is based in the dynamic analysis of the components, strain-stress results, deformation results, and fatigue, but also taking in consideration an easy manufacturability and low cost. 1

2 This report describes the process undertaken by the CETYS Universidad team in the design and manufacture of the Z7. The purpose of this competition is to simulate a real world engineering design in which collegiate teams design and manufacture a prototype of a rugged, single seat offroad recreational vehicle intended for sale to the non-professional week-end off-road enthusiast as stated in the Baja SAE official rules. The objective of the design is to create a durable, safe and easy to maintain vehicle that is able to deal with rough terrain in any weather condition. Because there is a separate report which deals with costs of parts and manufacturing details, this report will only consider costs when it becomes a crucial factor in a design decision. FRAME DESIGN OBJECTIVE The chassis is the component in charge of supporting all other vehicle s subsystems with the plus of taking care of the driver safety at all time. The chassis design need to be prepared for impacts created in any certain crash or rollover. It must be strong and durable taking always in account the weight distribution for a better performance. DESIGN This year, the chassis design presents a revised model from the one used last year in Peoria, Illinois The chassis contains improvements from the last iteration. In order to allow the new rear 3 link suspension, the new powertrain and drivetrain relocation, plus the new rack & pinion positioning in order to reduce the turning radius. Looking to improve the frame resistance while maintaining the weight minimum as possible with our design, we took this fellow materials and sizes as shown in Table 1. Material 1018 Steel Outside Diameter Wall Thickness Bending Stiffness Bending Strength Weight per meter cm cm Nm Nm kg 4130 Steel cm cm Nm Nm kg Table vs steel (CES Edupack ) 4130 Steel cm cm Nm Nm kg The previous table shows different Bending Strengths comparing the 1018 vs. the 4130 steel. By selecting the 4130 steel with a larger diameter and a smaller thickness, the inertia moment of the tubes are improved with the plus of a 27% weight reduction per foot. In order to further reduce the weight of the frame, we decided to use a smaller diameter tubes with 1.651mm (0.065in) wall thickness in non-critical parts of the frame, and using the 2

3 3.175cm (1.25in) OD tube only in the main cage members. Taking in account the new selection of materials we achieved a weight reduction in the frame from kg (135 lbs.) to 40.37kg (89 lbs.) making a 34% reduction. FINITE ELEMENT ANALYSIS (FEA) In order to prove the safety of our chassis design we decided to use GeoSTAR, due to its low memory requirements. After the static analysis, as shown in Figure 2, we found out which members of the frame were the ones that suffered the most stress and decided to make some modifications as shown in Figure 3. (1in) diameter tube to 3.175cm (1.25in) diameter tube. Also, reinforcements were added under the driver s seat. Figure 3. Second static analysis, with revised frame design for optimum stress distribution. Further analysis proved an impact factor of 9.5, which means that the stress during impact would have to reach more than nine times the normal working condition to cause plastic deformation or break. This could be translated to an impact to the ground from a height of 355.6cm (140in). Figure 4 shows the FEA reaching the material s yield point. Figure 2. Static analysis of the previous design iteration. The second analysis was made after applying the modifications to the lower frame. The stress graph shows a decrement in the overall stress suffered to an acceptable level. To achieve this proper distribution the lower rear members of the frame were changed from 2.54cm Figure 4. FEA provides an impact factor of 9.5, an improve of 35% from the previous design iteration. 3

4 MANUFACTURING PROCEDURES Every element of the frame was machined at the ends to fit perfectly in its position using a mill at the university workshop. The whole frame was welded with MIG using mild steel filling material. In this process, the use of Chromalloy becomes even a better choice because of its weldability which allows the material to maintain its mechanical properties after being welded. For chassis model please refer to Figure 13 at the appendix, page 13. SUSPENSION OBJECTIVE The suspension is responsible for dissipating the energy obtained from the impacts absorbed by the shocks. These impacts are caused by the uneven terrain. It is also responsible for maintaining the vehicles stability and ride height when managing obstacles. Another point is to reduce vibration for the vehicles durability and drivers comfort. DESIGN The rear suspension was a major improvement in design over the previous car. A three link suspension was opted in order to work in conjunction with the new drive train, as shown in Figure 5. This configuration gives us better bump absorption due to its long trailing arm, 63.5cm (25in). Both, front and rear, arms are made out of 2.54cm (1in) OD tube 4130 Chromalloy steel. Front arms have a wall thickness of 1.651mm (0.065in) and rear trailing arms have a thickness of 2.108mm (0.083in), as shown in Figure 5. Figure 5. Rear suspension. The front suspension works with a double A-arm system. Both upper and lower arms have identical length so the wheels vertical plane is maintained at all times during shock travel. Front suspension is equipped with two FOX 2.0 Air shocks with 11.43cm (4.5in) of travel. This setup gives us 26.67cm (10.5in) of total wheel travel, giving the car great ability to manage rocks, bumps and other obstacles while maintaining good traction. ANALYSIS The vehicles weight distribution is 33% in the front and 66% in the rear; therefore the rear shocks must be stronger than the front shocks. The use of the FOX 2.0 Air shocks allow the team to easily adjust the spring rate of the shocks at any time by adding or extracting nitrogen. The spring rate of the shocks is equivalent to kg/m (1.105 lb/in) per 6.895kPa (1psi) of 4

5 nitrogen. The working pressures of the shocks in normal condition are 1.296MPa (188psi) in each shock in the front and 1.551Mpa (225psi) in each shock in the rear. Figure 6 shows the analysis made with Solidworks Simulation to prove the resistance the A-arms considering a 4.448kN (1000 lbf). The rack is connected to 2 tie rods working in front of the shocks for reduced weight. The rack travels 8.89cm (3.5in) from lock to look to make the wheel turn. The front wheels configuration has a 3.5 camber angle and an 11.5 caster angle. The caster tends to drive the wheels forward, which makes it easier to maintain the car in a straight direction, also the inclination of the knuckle helps to reduce the turning radius to cm (78in), as shown in Figure cm Figure 6. Front suspension analysis in Solidworks Simulation. STEERING OBJETIVE The steering subsystem is responsible for the control of the vehicle. In the design process of this process of this subsystem the goal is to achieve a small turning radius and steering stability. The speed of response and the driver s input are also prime factors for the design of the steering system. DESIGN The steering system works with a VW off-road rack and pinion. The rack travels one and a half turns from lock to lock which allows good control of the vehicle and good responding speed. Figure 7. Turning radius calculation. DRIVETRAIN OBJECTIVE The objective of the drivetrain is providing to the driver more than the enough torque to the wheels from the engine to the wheels. The calculations were made in order to select the proper components that satisfy a top speed of m/s (30mph) to m/s (35mph) and to provide the car the enough strength to climb a 60 incline. DESIGN The main component of the drivetrain is the Briggs & Stratton engine which gives 5

6 19.66Nm (14.5 lb-ft) of torque at 3800 rpm and 10 hp at 3800 rpm, as shown in Figure 8. Component Hi Ratio Low Ratio CV-Tech Pulley DANA Transaxle Total Reduction 7.5:1 41.3:1 Table 2. Drivetrain system. (CV-Tech &DANA ) ANALYSIS For the evaluation of the torque required to obtain the enough strength to climb the 60 incline we made a simple study case, as shown in Figure 9. Figure 8. Power and Torque Curve. (Briggs & Stratton ) The system is composed by a CV-Tech CVT Pulley System with a PWB50 drive pulley and a TAS-99 driven pulley, which gives us a ratio of 0.65:1 at the hi ratio position and 3.6:1 at the low ratio position. After the driven pulley we use an H-12 FNR Independent Suspension Transaxle from DANA. This component includes the transmission, which allows the vehicle to reverse. This component also includes the differential, with a total reduction of 11.47:1. The use of the transaxle gives to the system a lot of reliability, strength and a high factor of safety. Table 2 shows the total reduction at the hi and low ratio. Figure Climbing case. (Reference #9) Also we evaluate the Gravity Center of the car, as shown in Figure 10, in order to reach the closes value to 60 between the GC of the car and the rear axle to obtain stability. Figure 10, Gravity Center. (GC) 6

7 RIMS AND TIRES OBJECTIVE The function of the rims and tires of the vehicle is first, to convert the torque given by the drivetrain into a push force to accelerate the car. Also the wheels work as a plus to help the suspension in reducing vibration. Last but not least, the correct selection of tires helps to keep traction in different types of terrain to keep the vehicle moving at any time. DESIGN The Z7 prototype is designed to work with two 58.42cm (23in) diameter tires and two 63.5cm (25in) diameter tires. This allows the vehicle to reach a higher top speed by sacrificing some push force. The tires selected are the ITP Holeshot ATR AT and ITP Holeshot XCT AT, as shown in Table 3, because they have the lowest weight in their class and they are reliable. Also the thread pattern of the wheel gives maximum traction on mud and loose terrain, the types of terrain you would encounter in an off-road race. The wheels selected are made of aircraft grade aluminum alloy, which gives enough strength to endure rough terrain while reducing the weight considerably. Component Size Weight Douglas Aluminum Wheels Douglas Aluminum Wheels ITP Holeshot ATR AT Tires ITP Holeshot XCT AT Tires 12x8 10x5 25x10R12 23x8R kg 1.588kg 9.979kg 7.167kg Table 3. Tires and Rims Selection. BREAK SYSTEM OBJECTIVE The breaking system of the Z7 is designed to lock all four wheels quickly to provide safe breaking. DESIGN For the breaking system we used two independent hydraulic system, both working with a VW 19mm master cylinder and a single pedal. The master cylinders make a cross connection, each controlling a front wheel and the opposite rear wheel as shown in Figure 11. By working in this way can ensure that in event of failure of one cylinder the car will not tend to turn out of the road. All four wheels have a Honda caliper with rotors to provide breaking force. The pedal has length of 38.1cm (15in) from foot position to bias bar contact, which makes it easier to provide enough breaking force. Figure 11. Braking system. ELECTRIC SYSTEM The electric system contains the breaking lights; reverse light, reverse alarm, and emergency stop kill switches. 7

8 There are two kill switches in the vehicle one over the wheel reach of the driver, and the second one outside of the car at the top right side of the rear body panels. This second location is easily accessible to team members and competition judges in case of emergency. The kill switches work by closing the circuit and killing energy to the engine causing immediate stop. However, pressing the kill switches does not kill the lights. For the electric system diagram please refer to Figure 14 at the appendix, page 14. SEAT AND RESTRAINTS OBJECTIVE The objective of the seat is to provide comfort and safety to the driver, while the restraints have to keep the driver inside the cockpit and on his seat at all times. SEAT AND RESTRAINTS SELECTION The seat is selected in order to provide the pilot the necessary comfort during the whole race and the minimum weight possible. The seat is slotted for a five-point harness restraint. We chose the CROW latch-and-link-point harness because of the previous experience we have with it. CROW provides good quality and reliability at a fair cost and that is why it has been considered as our first choice in safety for now. GUARDS OBJECTIVE The main objective of the guards and body panels is to keep the drivers safe debris and mechanical system, among other things. MATERIAL SELECTION All of the body panels, as well as the firewall and roll cage guards are made of aluminum sheet. We use a 0.508mm (0.020in) thick sheet to reduce weight while maintaining resilience in the panels. The skid plate is made out of textured aluminum sheet with a thickness of 1.651mm (0.065in) to provide good support for the driver and to ensure good grip at all times. All of the moving parts in the drivetrain are covered with regular 1010 steel expanded metal 1.27cm (1/2in) # mm(0.157in) thickness case, which provides excellent protection in case of drivetrain failure. Table 4 is a comparative of different materials selected as options for the drivetrain guards, based on weight and energy absorption at rupture as shown in Figure 15. Table 4. Material Comparison (CES Edupack ) 8

9 CONCLUSION The process of designing a vehicle is not a simple task; as a matter of fact it takes a lot of effort from all members of the team to achieve a successful design. The final prototype was the product of a collaborative multidisciplinary team design. The goal of the project was to create an off-road recreational vehicle that met o exceed the SAE regulations for safety, durability and maintenance, as well as to achieve a vehicle performance, aesthetics and comfort that would have mass market appeal for the off-road enthusiast. All of the design decisions were made keeping these goals in mind. The selection of components were made using engineering knowledge achieved through with offroad enthusiast and engineering advisors, taking as parameters first of all, safety, performance, weight, reliability and last of all cost. To see an overall technical description of the Z7 see Table 5. Computational design became the most important part of the process; by using CAD software we were able to print our ideas before constructing any prototype, plus the CAE packages and FEA allowed the team to recreate actual working conditions of some of the subsystems to ensure their durability, finally the CAM allowed is to fabricate some components at the CNC mill in the CETYS machine shop. Being part of a project of this nature is an experience that can be hardly matched to any other extracurricular project, as it allows the engineering student to exploit all of his/her knowledge while gaining even more, not only in the engineering discipline, but also in project management, team work, accounting and even marketing sales. The multidisciplinary gain of this project is what makes it successful and surely an experience to remember for the rest of your life. ACKNOWLEDGMENTS Proyecto Zorro would like thank the engineering department of CETYS Universidad first of all for their support in development of the Z7 project. We extend our thanks and appreciation to our sponsors, especially to Honeywell MRTC, Ramsey Products Corporation, and Mexicana Logistics, for their special contribution as the project could not have been completed without their contribution. We also thank the rest of our sponsors, ASCOTech Mexicali, Kenworth Mexicana, Energy & Lighting, Persal Manufacturing Solutions, FMM and Urbi, for taking part in this challenge. Last but not least we would like to thank SAE, Briggs & Stratton and all of the people that make these competitions possible for the opportunity they give to engineering students around the globe. 9

10 REFERENCES 1. SAE International (2011) Baja SAE Rules Collegiate Design Series. 2. SAE International (2009) Baja SAE Rules Collegiate Design Series. 3. Briggs&Stratton Series TM Engine. From: 4. Shigley, J.; Mischke C. ; Budynas, R. (2003) Mechanical Engineering Design. Seventh edition. McGraw Hill. 5. Spotts, M.F.; Shoup, T.E. (2004) Design of Machine elements. Seventh edition. Prentice Hall. 6. Dixon, J. (1999) The shock absorber handbook. Second edition. SAE publications Product lifecycle management. McGraw Hill. 7. Birch, T. (1999) Automotive Suspension & steering systems. Third edition. Delmar Editorial. 8. Gillespie, T. (1992) Fundamentals of vehicle dynamics. SAE International CONTACT Alejandro Burgas Mechanical Engineering Student (a_burgas@hotmail.com) Andres Magaña Mechanical Engineering Student (andre.magana@gmail.com) Gustavo Ovies Mechanical Engineering Student (gustavo_oz@hotmail.com) ADITIONAL SOURCES Matlab Simulink Solid Works SolidworksSimulation Geo Star CES Edupack DEFINITIONS, ACRONYMS AND ABBREVIATIONS Camber: is the angle of the wheel relative to vertical, as viewed from the front or the rear of the car. Caster: is the angle to which the steering pivot axis is tilted forward or rearward from vertical, as viewed from the side. Energy absorption at rupture: is defined as the amount of energy that material can absorb before cracking or breaking. It is also the area below the stressstrain curve. (See Figure 15) 10

11 SAE- Society of Automotive Engineers CETYS- Centro de Enseñanza Técnica Y Superior MIG- Metal Inert Gas CVT- Continuously Variable Transmission CAD- Computer Aided Design CAE- Computer Aided Engineering CAM- Computer Aided Manufacture CNC- Computer Numerical Control FEA- Finite Element Analysis RHO- Roll Hoop Overhead Member RRH- Rear Roll Hoop LC- Lateral Cross Member FBM- Front Bracing Member LDB- Lateral Diagonal Bracing LFS- Lower Frame Support FLC- Front Lateral Cross Member SIM- Side Impact Member 11

12 APPENDIX ENGINE Model Briggs & Stratton OHV Intek Displacement 305 cc Compression Ratio 8:01 Power 10HP Torque 19.66Nm (14.5 ft-lbs) DRIVETRAIN DANA Transaxle 11.47:1 Ratio CV Tech Pulleys 3.6:1 to 0.65:1 Ratio Total reduction 41.3:1 to 7.5:1 DIMENSIONS Overall Length 254cm (100in) Wheel Base cm (69.5in) Overall Width cm (63.5in) Ground Clearance 35.56cm (14in) Weight kg (475 lb) SUSPENSION Front Suspension Double A-arm, 26.67cm (10.5in) travel Rear Suspension Three link, 17.78cm (7in) travel Front Shocks FOX 2.0 Air Shocks, 11.43cm (4.5in) travel Rear Shocks FOX 2.0 Air Shocks,11.43cm ( 4.5in) travel STEERING VW off-road Rack & Pinion Rack 8.89cm (3.5in) travel Camber Angle 3.5 Caster Angle 11.5 WHEELS AND TIRES Front Wheels Rear Wheels Front Tires Rear Tires BREAKS Master Cylinder Calipers ELECTRIC Kill Switches Lights Reverse Alarm PERFORMANCE Max speed Turning radius 10 x 5 Douglas Aluminum Wheels 12 x 8 Douglas Aluminum Wheels 23 x 8 R10 ITP HOLESHOT XCT AT Tires 25 x 10 R12 ITP HOLESHOT ATR AT Tires VW 19mm Honda 2009 TRX450R w/rotors Ski Doo kill switches Breaking and reverse Back up alarm 97db m/s ( rpm cm (78in) Table 5. Z7 General Specs. 12

13 Figure 12. Project lifecycle (Grieves, 2006) Primary members Added after FEA Rear bracing 3.175cm (1.25in) OD Secondary members Removed and replaced for the blue members 2.54cm OD (1in) OD Figure 13. Z7 Chassis 13

14 Figure 14. Electric System Diagram Figure15. Energy absorption at rupture, 1010 Steel. (SAE International, 2009) 14