Project Description

This project was completed for my mechanical design class, focusing on conceptual design & analysis of a hollow steel torsion bar suspension system. The project covers material selection, fatigue life, spline design, bearing support, bolted joints, axel sizing, and engineering safety margins. 

The Problem

The requirements for this project where to design a torsion bar suspension system to support a wheel and tire assembly on an axle. The bar is to twist with a range of +/- 2 degrees, at a 47 degree resting position. It must also survive 10^6 unidirectional torque cycles at 99.9% reliability, as well as meet minimum factors of safety for all components. 

My Approach

For this project I selected a steel from the ANSI material tables and sized a hollow shaft, with the max OD of 3in. This was to maintain a factor of safety of 1.15 in torsion. The hollow design allows for a reduction in weight, while maintaining torsional stiffness. Fatigue analysis was performed using the DE-Goodman criterion to maintain a minimum fatigue factor of safety of 1.05 across 10^6 load cycles. A full S-N curve was also created for the shaft's cross section when loaded. 

The design calls for spline connections at both the lever arm and fixed coupling, these were analyzed for shear and compression of spline teeth and shaft stress at spline root. The three failure modes were all evaluated with a minimum factor of safety of 1.05. 

A Symmco sleeve bearing and an aluminum casting support lug where designed with a minimum FOS of 3.75, a casting factor of 0.45 for bearing strength and to mitigate overloading. In addition, join stiffness at the lug-to-baseplate connection was calculated with all factor of safety where verified to be above 1.15.

The bar was mounted to the wall using a flange, proper bolt size and pattern where calculated using reactions at the wall connections. Proper steel bolts where selected to minimize cost but satisfy at factor of safety of 1.25. Bolt preload was set at 75% of proof load per standard practice. 

Finally the axle shaft was sized under the distributed wheel load with proper shear and bending moment diagrams constructed to identify critical cross-section. A minimum shaft diameter was determined in order to maintain a similar factor of safety of 1.15. 

Engineering Design Decisions

In to the design components, this project focused on aditional engineering design decisions that reflect real-world trade offs. 

• Economy: Bolts that met the minimum safety factor were needed to reduce overall cost without compromising integrity. 

• Weight: A hollow shaft was chosen to reduce material use and weight while still maintaining torsional requirements, applicable in real world vehicle weight budgets.

• Safety: Safety margins where hit and exceeded, including 99.9% reliability targets to reflect real industry practice of critical components.

• Complexity: Standard materials and components where chosen specifically to be off-the-shelf components to reduce manufacturing waste and sourcing complexity.