Mechanical Forensics Engineering Services, LLC

Cammed-Over Heavy Truck Brake

Being unable to find any good online photos of an S-Cam type brake which has "cammed over", I decided to post some of my own. Larger versions of the photos (approx. 140kB) can be viewed in a separate window by clicking on the thumbnail shots. The truck in question was a 2001 Freightliner "Century" 6x4 tractor which had been used in delivery service, and had missed its regular PM stops for months. Before pulling any wheels or drums, it was clear that there was a problem waiting inside 2R and 3R: the lower shoes were close to the drums but there was a great honkin' air-gap between the top shoes and the drums:

Shoe Gap on #2R

Shoe Gap on #3R

#2R Drum Off

These two photos (left and right) show the two wheel ends with the drums removed. There was significant rust inside the drum, indicating little or no shoe contact lately, as well as road-grime build up on the surface of the top shoes.

#3R Foundation Brake

Measuring #2R

#2R Drum Lip

The #2R drum was legal at the deepest part, but slightly beyond limits at the opening with the diameter measured to be 16.628 inches. The lip at the edge was pretty obvious, showing little scratches where the tips of the Frasier Gage moved around on the surface. The drum on #3R was much worse off, being nearly 0.230 inches over nominal (0.110" over the legal limit). Friction material on both wheel ends was well below the Fed-specified 0.25" minimum thickness. Interestingly, one of the left drums (not shown) was so far out of spec that it was off the gage, but that brake had not cammed over, and the shoes were still making contact with the drum face so the rivet heads were exposed and shiny blue (instead of being covered by roadgrime as these on the right were).

The photos on the right are the real money shots. They show the top shoes in both right wheels having "fallen over" the tips of their respective cams. Both of these shoes got stuck with one shoe fully cammed over and one shoe jammed at the tip of the cam. Braking on these wheels? No, thanks.

#2R Cammed Over

#3R Cammed over

Mechanical Forensics Engineering Services, LLC.

This page created 11-SEP-2009 and last modified 12-SEP-2009