Olin Electric Motor Sports
About
Olin Electric Motor Sports is a competitive undergraduate Electric Formula SAE Team. My first year on the team I was an engineer on the drivetrain responsible for a variety of subsystems on our fifth car. This year I am Mechanical Lead, responsible for leading the integration of the subsystems and organizing a rigorous testing plan to create a winning car.
Liquid Cooling Loop
To ensure the motor and motor controller run with optimal efficiency they must be continuously cooled.
Through testing, the team found the pervious system's flow rate was too slow. This caused the motor and motor controller to be less efficient.
I designed a liquid cooling loop to cool the system. To maximize airflow through the radiator, I mounted the radiator on the side of the car where it would be unobstructed. I used Solidworks to design 1/16” bent steel brackets for the radiator and pump. I also manufactured the brackets using a water jet, sheet metal brake, and laser cut welding jigs.
Since the radiator sticks out out from the car it has a high risk of being hit. By creating flanges on the bracket, I increased the polar moment of inertia as compared to a flat bracket, reducing risk of bending.
In order to keep the car as light as possible it is important the pump is only as powerful as needed to achieve the proper flow rate in the system. This flow rate is specified by the manufacturer. The pump used on prior cars did not achieve a high enough flow rate, but by decreasing pressure drop in the system, the same pump could safely be used.
I modeled the routing of the tubes using 3D splines in Solidworks to ensure the tubes would never be below its minimum bend radius. I also reduced the number of right angle connectors. Both of these reduce pressure drop and allow the cooling system to run at a safe flow rate.
Sprocket Design
The drivetrain uses a chain drive in order to transfer torque from the motor to the differential. It was my responsibility to design and manufacture both the driving and driven sprockets.
For the driving sprocket I chose a constant radius trilobe pattern to transfer torque from the motor shaft to the sprocket. Multiple options were considered, such as a keyed or hex shaft, but both options have sharp angles which cause stress concentrations. In addition, using the trilobe allows for easy machining on a 2.5 axis CNC mill, reducing the cost and time required to manufacture.
The driven sprocket weighs almost 3 lbs and has a large empty area, making it ideal for light-weighting. I used Solidworks FEA and the equation below to model the loads on the sprocket during the car’s operation.
Tk = T0 × {sin ø ÷ sin (ø + 2β)}^k-1
Where:
-
Tk = back tension at tooth k
-
T0 = chain tension
-
ø = sprocket minimum pressure angle 17 - 64/N(°)
-
N = number of teeth
-
2β = sprocket tooth angle (360/N)
-
k = the number of engaged teeth (angle of wrap × N/360); round down to the nearest whole number to be safe
The sprocket undergoes a cyclical load as compared to the static loads most of the car faces. To account for this in my analysis I assumed the endurance limit was the maximum stress the sprocket could endure. This ensured it would not fail even after millions of cycles. I then designed both a circular and triangular lightweighting pattern. While the triangle pattern had a higher max stress and deflection, both remained within the 1.5 factor of safety and allowed for over a 30% reduction in weight.
I machined both sprockets on a 2.5 axis CNC mill. The sprockets were held in place using toe clamps and a dial indicator was used to set the origin at the center of the sprocket.
Drivetrain Mounting Brackets
The brackets that mount the drivetrain assembly to the chassis react close to 10,000 newtons of force from the assembly. As the engineer in charge of this project, I did initial calculations to determine the maximum potential pre-load tension force of the bolts being used. With this, I calculated I would need 2 bolts to react the tension force, and one shear pin to react shearing loads.
Using Solidworks FEA, I simulated the stress and displacement of the bracket in a worst case acceleration and braking case of the car, ensuring I maintained a FOS of 1.5 (the minimum factor of safety for all parts in the drivetrain).
The initial design had high deflection in the center of the part. I overcame this issue by creating a slot where the bracket could be welded to the chassis.
The final bracket was made of water jet ¼” 1018 steel, which was chosen for its high strength, but also lower cost as compared to 4130.