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4.0 Drivetrain
4.1 Overall Design
A Torsen T1 University Special differen6al was chosen to be the basis for the drive train. This differen6al was chosen due to the desirable characteris6cs of an automa6c torque-‐-‐-‐biasing differen6al when used in a racing environment. The Torsen T1 is used in many commercial applica6ons, including the center differen6al from an Audi QuaCro system, and is very affordable.
A differen6al case that uses a chain driven input was designed. The differen6al assembly was also designed to carry an inboard braking system on adapters connected to the differen6al housing. This design allows the cases to remain sta6onary while the differen6al housing, brake disk, and sprocket rotates together. A sta6onary case is beneficial to performance because it lowers the rota6ng mass within the drive train. A lower rota6ng mass can more efficiently deliver the maximum amount of horsepower to the 6res due to fewer losses to rota6ng iner6a.
The inboard braking system reduces unsprung weight by carrying the caliper on the frame and the disk brake on the differen6al. The connec6on between the brake rotor and the differen6al is effec6vely iden6cal to the connec6on between the drive sprocket and the differen6al. Both systems are solidly mounted to the differen6al housing. Inputs to the housing are then transferred through the gear set within the differen6al and into CV shaLs. The disadvantage of moun6ng the brake directly to the differen6al is the loss of brake torque distribu6on to each side of the chassis. In the event that a rear 6re leaves the ground, the differen6al will open up, and braking torque will only be applied to the 6re in the air.
Figure 20: Differen.al unit assembly
4.2 Sprocket Selection
An op6mum final drive gear ra6o will enable the formula car to be in the peak of the engines power band at every corner exit. The drive sprocket to driven sprocket tooth count ra6o on a chain driven system is the final drive ra6o. The final drive ra6o dictates the vehicle speed for a given engine RPM in each transmission gear. The controlling factors that contribute to the selec6on of a final drive ra6o are the engine’s usable power band, the vehicle’s 6re diameter, the engine’s maximum rpm, and the transmission ra6os. The Aprilia engine has a maximum engine speed of 13,500rpm and peak torque between 7,000 and 9,000 rpm. The formula car has been fiCed with 13-‐-‐-‐inch wheels and 20-‐-‐-‐inch diameter 6res. A 40/16-‐-‐-‐sprocket ra6o is used, providing the shiL points shown on the table below.
Table 8: Final drive ra.o selec.on table
4.3 Rear Brake Adaptor
The rear brake system is mounted inboard on the differen6al carrier. The differen6al case is sta6onary and will not be loaded by any braking torques. Torque will be transferred directly into the differen6al housing. The brake system has been aCached to the differen6al by a series of components, while the brake rotor is aCached to the differen6al supports, shown below in red, through a brake input adapter, shown below in gray.
Figure 21: Rear braking system, located inboard on the differen.al
The rear brake system consists of a single hydraulically actuated caliper mounted to the chassis. The maximum torque that can be delivered to the brake is dependent on the 6re radius and the weight of the car applying a force to the ground. Any greater torque will simply result in wheel slip. In the event that braking and accelera6on occur simultaneously, the maximum torque applied to the wheels will be a factor of output torque from the engine.
4.4 Estimation of Reaction Forces
Proper drive train design requires that all included components can withstand the peak loads generated during usage. The major forces on the drive train are the torques produced when the throCle or brake is applied. Due to weight transfer between the front and rear 6res, the largest torsional load on the rear axle and drive components will occur not during braking but while accelera6ng. This load depends on the torque output from the engine, as well as the gearing ra6o of the transmission and sprockets being used.
The maximum torque this engine can deliver, by calcula6on, to the drive train is 1068 N*m. This assumes the 6res do not slip. In order to check if the 6res can provide enough grip for these condi6ons, a force balance must be done on the car. Using an approxima6on of the car’s weight, loca6on of center of gravity, and wheelbase, it is possible to calculate the normal force pushing up on the rear 6res. These calcula6ons assume that the maximum amount of longitudinal accelera6on that could be produced is 1 g. This results in a combined load of about 2000N on the rear 6res. Using an es6mated coefficient of fric6on for the 6res and the radius of the wheel, the maximum torque that one or both 6res can provide before slipping is found.
It is found that the each 6re can resist up to 312Nm before slipping given these condi6ons. This means that 624Nm is the most torque that the drive train will experience in the absence of any shock loading. If both the brake and gas were to be applied at the same 6me, it would be possible for the components to experience the 1068 Nm of torque coming from the engine. These calcula6ons can then be used to find the maximum stress when tracking fa6gue failures.
4.5 Case Design
The main purpose of the differen6al case is to provide the Torsen differen6al with an enclosure that will hold the fluid needed to lubricate the differen6al gears and bearings. Keeping the case sta6onary and not allowing it to rotate with the brake rotor and sprocket reduce the rota6ng mass in the drive train. The case must contain bearings that allow the differen6al and the differen6al supports to rotate within it.
For assembly purposes, it is necessary that the case be made of two halves. By joining the two halves in the middle, it is also possible to shim behind the seats of the bearings to adjust the amount of preload on the tapered roller bearings. The bearings are press fit into the case using a very 6ght transi6onal fit. This will ensure that the races of the bearings do not rotate within the case while allowing for easy assembly and disassembly.
The two halves of the case slide over the differen6al supports and are joined in the middle by two flanges fastened together with six Allen head cap screws and threaded inserts. A free fit was chosen for the bolt holes. The ma6ng surfaces are sealed with the use of compression packing, similar to an O-‐-‐-‐ring. It is also necessary to seal between the CV shaLs and the adapters for the brake rotor and drive sprocket. This is accomplished by pressing a double-‐-‐-‐lip spring loaded shaL seal into the case from the outside. Each case half also incorporates series of ridges that contact with the assemblies moun6ng system, which prevents lateral movement.
6061 Aluminum is the material used for the case, chosen for its low weight, affordability, and ease of machining. The case is turned on a lathe from a solid slug of material. The bolt holes are drilled on a ver6cally milling machine and are then reamed to the final diameter. All cri6cal tolerances are verified using a coordinate measuring machine (CMM).
4.6 Input Adapters
The unique size and bolt paCerns of the brake rotor and drive sprocket necessitate moun6ng adapters. These adapters must connect the brake rotor and drive sprocket to the differen6al supports. The tolerances between the adapters and supports have an H11/h11 clearance fit. 4150 alloy steel is used, chosen for its high strength and low price. A sta6c analysis is considered to be sufficient on these parts, as they are not expected to experience a very large amount of life cycles or any major deflec6ons. Without performing fa6gue analysis on the input adapters, a large factor of safety is applied to all calcula6ons. Final selec6ons show the components are sized to a safety factor of 4.475.
4.7 Differential Supports
The differen6al supports are designed to transmit the torque of the drive sprocket and the brake rotor from the input adapters to the Torsen differen6al. Each support aCaches to the input adapters by the use of 8 pins around the circumference of their outer cylindrical shell. The supports must seal to the differen6al housing and to the CV shaLs that run concentrically through the center of them. The two supports are almost iden6cal to each other in design and will be placed on both sides of the Torsen differen6al.
Figure 22: Differen.al supports
The differen6al supports were also designed with a surface to hold both a tapered roller bearing inside the case. The inside face includes a polished surface for a bronze sleeve bushing to allow rota6on between the axle shaLs and supports.
Figure 23: Differen.al support assembly-‐-‐-‐ A: Tapered bearing, B: Inner and Outer Seals, C: Brass Bushing
Special considera6on was taken to meet the strict tolerances for the journal surface the bearing must rotate on. Since the mo6ons between the two rota6ng surfaces are usually less than 10 feet per minute, the plain bearings will be primarily in a boundary lubrica6on type of opera6on. This mode of opera6on usually results in a coefficient of fric6on between 0.08 and 0.14. Although a full fluid film is unlikely to exist, aCen6on must be put into making the journal eccentric. It is also determined that the journal must have a surface finish of 8 to 32 micro-‐-‐-‐inches for this type of opera6on.
The supports were machined on a lathe from a 4150 steel slug. ALer the machining process was complete, the supports were analyzed with the CMM to ensure that the accuracy and tolerances of the two adapters were met.
4.8 Bearing Selection
To determine the appropriate bearings, reac6on forces had to be determined. This is done by taking the moment about the mount on the rotor side of the assembly using the equa6on shown below. This assumes the moun6ng system and bearing are in line with any reac6on forces.
Using the equa6on shown above, the reac6on force (Fr) can be determined by solving for the reac6on force, since the max chain tension (Ft) is found to be 2300 lbs and the distances are known from Figure 4, shown below:
Photo 11: Cutaway view of a Torsen differen.al
The moment is taken about the brake rotor side because the greatest force applied to the differen6al will be coming from the sprocket end. Therefore the selected bearing can be used on both sides of the assembly. The equa6on shown below is used and the reac6on force is found to be 3220 lbs.
To calculate the combined radial and thrust load, the following equa6on is used: Where,
P is the equivalent load,Fr is the applied constant radial load (3220 lbs), Fa is the applied constant thrust load,V is a rota6on factor (1),X is a radial factor (1),and Y is the thrust factor
Since the ra6os between the axial and radial forces are less than .156, the thrust force can be ignored, which leaves the equivalent load (P) to be 3220 lbs.
Using the load-‐-‐-‐life rela6onship for a roller bearing, where L10 is the fa6gue life expressed in million of revolu6ons, the dynamic load, C, can be calculated:
The dynamic load is found to be 203,168 lbs. Using this value, the team determined the appropriate bearing for the sprocket side and rotor side using the McMaster Carr catalog. The appropriate bearing to be used for the inner race is part number 5709K33, with the matching bearing part number 5709K69.
Figure 24: Inner Race (5709K33) and Bearing (5709K69)
4.9 Mounting
The moun6ng system for the differen6al is designed to make the case easily accessible for installa6on and removal from the car. The moun6ng system also is designed to minimize case movement when torque is transferred through the bearings or seals to the differen6al housing. As torque is transmiCed through the CV shaLs, a thrust load is generated that tries to move the differen6al and it’s housing sideways. The mounts are located along the same plane as the differen6al bearings. The moun6ng system consists of four parts: two front mounts that connect to the cars chassis and two rear mounts that have tabs on the top and boCom to bolt into the other mount and secure the differen6al housing.
4.10 Hardware
Figure 25: Differen.al moun.ng brackets
Several bolts and pins are required to lock many of the rota6ng pieces together. Based upon the maximum torque analysis, the maximum torque possible on the sprocket is 1068 N-‐-‐-‐m. To solve for the force that will be applied to each bolt/pin, the equa6on shown below is used:
Tmax is the maximum amount of torque the engine can provide. r is the radius from the bolt to the differen6als’ axis of rota6on. Nb is the number of bolts or pins sharing the load.
The cross sec6onal area of the fastener and the shear stress can then be found using equa6ons.
The factor of safety is then calculated based on the yield strength and the shear stress of the fastener. Yield strength of 91000 psi was used for a grade 8 bolt.
This analysis is for sta6c loading. Ideally, the fasteners used in the drive train that are loaded torsionally should be analyzed for fa6gue. This is due to the fully reversed loading from braking and
accelera6ng the car. Since the number of cycles would be rela6vely low, giving the bolts and pins a high factor of safety would be sufficient.
4.11 Axles
The cars drive train required a set of axles to connect the Torsen differen6al to the Mazda Miata hubs that had been previously fiCed to the outer knuckles. ALer assessing the cost and 6me involved in producing axles, the decision was made to outsource the components to RCV Performance, a proven supplier and manufacturer of custom drive train components.
The axles are a varia6on of the Formula SAE Tripod Axle Kit currently in produc6on. They use an inboard CV housing and stub shaL that fits the Torsen differen6al. These splines came as a cut to fit applica6on from RCV, and were cut to fit the assembly. The outboard CV housing was modified to fit the spline paCern on the Miata hubs. The custom CV housings are shown below.
Figure 26: Custom CV housings adapted to fit the differen.al assembly
4.12 Drive train Conclusion
The final assembly of the differen6al case consists of the following components:
● Two halves of the case
● Sprocket Input Adapter
● Brake Input Adapter
● Two Differen6al Supports
The two halves of the case slide over the differen6al supports connect by a flange, enclosing the Torsen Differen6al along with its essen6al lubrica6ng fluid. The case is machined from aluminum and has cri6cal tolerances for the bearings and seals. Aluminum was chosen because it is a light material and easy to machine. The case is mounted to the rearmost horizontal members of the car. Both the input adapters and differen6al supports are constructed of 4150 steel. For the analysis of the differen6al supports, the outer diameter was sized to have a safety factor of 3.8. This minimum factor of safety was then used to develop the appropriate thickness of the brake and sprocket adapters and to select the appropriate hardware. A lower factor of safety used was for the pins connec6ng the differen6al supports to the brake and sprocket adapters, promo6ng failure at a loca6on that would result in minimal damage, expense, and injury if the drivetrain were to experience greater than expected loads.
The differen6al unit is designed to survive the abuse produced in a race environment and provides an effec6ve way of applying power to the ground, allowing the car to be as agile as possible.