Pratt & Miller Engineering uses sophisticated fatigue analysis software to solve a brake pedal issue in a world-class race car. Various products from ANSYS, Safe Technologies, and Siemens PLM were used in tandem.

By Gary Latham
Pratt & Miller Engineering

In world-class auto racing, mechanics often reposition the brake pedal face to comply with driver preference. But moving the pedal off center introduces a significant twisting moment—a situation that increases stresses and can lead to metal fatigue.

Pratt & Miller Engineering is recognized around the world as a formidable force in both motor sports and high-level engineering. The Pratt & Miller team discovered that one of its race cars prematurely demonstrated small cracks on its brake pedal faces. Their initial investigation showed that the crack started near the pivot pin, which led the team to realize that mechanics were repositioning the pedal face. As the pedal face was moved further and further off center, the resulting twisting moment resulted in a crack at the high-stress location. Fortunately, engineers discovered the problem before any accidents occurred.

Since the original pedal design had not taken into account any after-market modifications, Pratt & Miller engineers set out to redesign the critical component. The goal was to greatly increase the fatigue life without compromising race performance. Simply over-engineering any part is not an option in motorsports, since weight is such a vital consideration. Race car engineers have to skirt the limit on parts: Weight costs speed, lap times and, ultimately, the race. With the next race scheduled just three days away, the part had to be designed, verified and machined quickly. To get an optimal redesign in the shortest amount of time, the Pratt & Miller engineering team employed structural mechanics simulation using the ANSYS Mechanical product together with fe-safe fatigue analysis software from Safe Technology Limited.

The original pedal part was used as a benchmark to compare various design alternatives. The team set up and evaluated the various options within the ANSYS Workbench environment. This platform made it especially easy to create the revolute joint at the pivot point on the pedal then apply a remote constraint to resist rotation where the balance bar connects to the master cylinders. Finally, a remote force was applied to represent the applied pedal force.

The team then parameterized the pedal in Siemens PLM’s NX CAD package with the rib thickness and fillet size as variables. ANSYS DesignXplorer software performed automated iterations with ANSYS Mechanical simulation and NX to determine the lightest possible design without exceeding the material limits. The integration went well, giving the design team confidence that the redesign would meet the new load requirements.

The fe-safe analysis clearly showed where the component needed to be strengthened in order to reduce the stresses. The simulation also determined how much the stresses must be changed by adding more material, and where the material must be added, to achieve the target design life of over 1 million cycles. The engineering team chose fe-safe because they thought it offered increased confidence in getting the design right the first time—in effect, out-pacing the competition by designing a lightweight part with a lower target fatigue life factor.

The pedal is constructed from normalized 4340 billet and machined with pockets and ribs to produce a light, stiff part. The loading was an assumed worst case, and the fatigue was a simple 0 to full load over 1 million cycles minimum required. The pedal design was improved by thickening the flange near the pivot to resist the twisting moment. The engineering team checked the redesign using fe-safe and found it to have a greatly improved fatigue life, well over the 1 million minimum load cycles. The original design predicted a life of 16,567 cycles, while the redesigned pedal has a predicted life of greater than 10 million cycles.

The new design is now is production and back on the race track. Pratt & Miller uses fe-safe in conjunction with ANSYS Mechanical software to optimize design, identify  stress factors, and determine where unnecessary material can be safely reduced to save weight, and where parts should be strengthened to prevent failure before the target fatigue life. §

Gary Latham is Design Office Manager for Pratt & Miller Engineering in Michigan.