Drum Brake System: Testing and Inspection
DRUMBRAKE DRUM OUT-OF-ROUND
A brake drum whose braking surface is out-of-round can cause vehicle vibration and brake pedal pulsation upon braking. To measure brake drum out-of-round, perform the following:
1. Remove the drum from the vehicle.
2. Using a brake drum micrometer, measure the inside diameter at several points around the inside of the drum on the machined surface. Record the measurements.
Drum braking surface out-of-round must not exceed 0.050 mm (0.002 inch). If the measurement is not within specification, reface or replace the drum. Always replace the drum if machining will cause the diameter to exceed drum maximum diameter. All brake drums are marked with the maximum allowable brake drum diameter (Fig. 40).
RUNOUT
A runout measurement can be done along with the out-of-round measurement.
1. Mount the drum on an off-vehicle brake lathe following the manufacturers instructions.
2. Mount a dial indicator on the lathe and position the indicator needle against center of the brake drum machined surface.
3. Slowly rotate the drum by hand recording high and low indicator readings through a 360° rotation.
Drum braking surface runout must not exceed 0.050 mm (0.002 inch). If the measurement is not within specification, reface or replace the drum. Always replace the drum if machining will cause the diameter to exceed drum maximum diameter. All brake drums are marked with the maximum allowable brake drum diameter (Fig. 40).