Air Brakes Experts

[Booyah45828]

Originally Posted by wireguy

I have come to the conclusion there is a geometry between the S cam and rollers that is different than the geometry on a engine camshaft and rollers or lifters, which is what I am familiar with. With those once your roller goes over the top the lifter is headed downhill again. I think being familiar with that geometry has kept me from seeing that the S cam uses a different geometry with respect to the rollers and that even if the roller is over the top of the curve it is still pushing the rollers out, maybe because of the length of the cam face.
Booyah45828 started me thinking that direction, that I wasn't seeing the geometry correctly.

Yes!

If you're looking at it like it is similar to a cam lobe for an engine, then it would be confusing. Unlike an engine camshaft, there is no down hill side, it's pushing out nearly the entire length of the roller surface, until it falls off the edge and back to 0. It might not look that way because of how the roller interacts with it, but it does.

Anyone who is still confused, plot it with an indicator in the way that I described and you can see for yourself.

[wireguy]

That singular fact, that the cam doesn't stop pushing after the rollers go over the top, has been the biggest source of my misunderstanding. In all the thousands of words I have read and many videos I have watched, no one has explained this until you did. I'm breathing a lot easier now that I have some confidence my brakes are fine. Thanks again.

[okinawascuba]

deleted... meant to quote, not quick reply.

[okinawascuba]

Originally Posted by Booyah45828

Yes!

If you're looking at it like it is similar to a cam lobe for an engine, then it would be confusing. Unlike an engine camshaft, there is no down hill side, it's pushing out nearly the entire length of the roller surface, until it falls off the edge and back to 0. It might not look that way because of how the roller interacts with it, but it does.

Anyone who is still confused, plot it with an indicator in the way that I described and you can see for yourself.

You are correct, and it should NEVER fall off the edge and go back to 0 even with no meat on the shoe left. It'd hit metal for a LOOOOONNNGG time before ever "falling" off the back of the cam.

[wireguy]

For the limited use we will put it to they will probably never wear out. Still I'm keeping an eye out for a set (4) of those 10" shoes. Still out there as are the drums but talk about expensive. I have time on my side now to find a set that isn't priced in the stratosphere. The six inchers on the front are common.

[Booyah45828]

Shoes typically come in kit format where they'll enough shoes and hardware to complete half an axle. So you would have to look for 2 kits. Good kits will have the springs, rollers, pins and bushings included, while you're cheaper ones might not.

okinawascuba, I've seen s cams flip. It was on a pusher axle for a dump truck. Definitely wasn't metal on metal, had over 1/4 shoe thickness left, which is why I advocate for replacement at anything over 120 degrees s cam rotation. At the bare minimum they should be inspected when they get to that point.

[okinawascuba]

Originally Posted by Booyah45828

I've seen s cams flip. It was on a pusher axle for a dump truck. Definitely wasn't metal on metal, had over 1/4 shoe thickness left, which is why I advocate for replacement at anything over 120 degrees s cam rotation. At the bare minimum they should be inspected when they get to that point.

I believe you, but I have never seen it nor could I even see it happening but it may depend on the manufacturer or design. A cam over should never happen. I completely agree with you about replacement or at least inspection if it gets close to that point.

You know what... My shops have never used turned drums, but I guess in hind sight that could potentially give you more distance to over rotate.