Ultrasonic Leak Test for Automotive Brake Caliper

Abstract

Leak path defects in brake caliper castings were observed and an efficient method of detecting them was needed. The standard detection technique used a differential decay method at pressures up to 1000 psi but this technique could not detect all of the leak path defects. The only known method that could detect all of the leak path defects was a technique that used brake fluid, and required pressures up to 3,000 psi. Because a fluid medium used to conduct this test, regulations precluded the calipers from being used on a vehicle after the inspection process. This was the motivation for a Capstone Project in the Mechanical Engineering Technology Program at Central Washington University. For safety and economic reasons, a nondestructive test which utilized acoustics was chosen. It was decided that the main focus for this project would be the design of the fixture. The prototype fixture was designed to utilize a toggle clamp and a backing plate (manifold) with O-rings. The clamping system was produced at INTERMET Corporation. After the fixture was completed it was sent to Central Washington University. To verify predictions, strain gauges were used at different parts of the system during normal use. Then the placement of the acoustic listener was determined, completing the test fixture. In order to obtain a good seal (O-ring to cavity lip) the fixture required deflections less then .0045” (as determined by tolerance stacking and O-ring deformation requirements). A magnetic pin gauge was used to measure this displacement as it the fixture was pressurized to 82psi (available shop pressure). To demonstrate the safety of the device, a strain gauge was mounted on the backing plate to measure the force being counteracted by the toggle clamp. The maximum stress on the backing plate was approximately 10% of the material's yield strength (~ 36 ksi for A36 steel); producing a safety factor of about 10. No more than 3750 psi was measured. The maximum strain of the backing plate was 0.003 in/in. Upon completion, the fixture was sent back to INTERMET Corporation where a production model was designed and implemented.