Brake drums, brake canisters etc.

[Iver]

MC9. 
Ok, I have finished replacing the brake shoes on one tag axle wheel along with a new seal and new diaphragm for the brake canister.
Now I have moved on to one of the drive axle wheels.  I removed the outer and inner dual wheels thanks to a 30:1 geared wrench which I picked up recently.  Just simple arm power removed all outer and inner wheel nuts.

Luckily for me, the previous person who worked on the hub replaced all of the brake drum screws with new ones and just left them in loose.  So the drum came off easily.  It looks as though the shoes are fairly new so that turns out to be a good thing.   However, when I inspected the inner drum surface, it looks like it has hundreds of small checks(cracks) running from side to side.   The drum measures still within specs but the checks concern me.   Normal/not normal???

Also, how much play/slack is allowed on the s-cam shaft?  If the bushings have worn too far I assume the axle has to come out in order to remove the s-cam shaft so the bushings can be replaced?

And lastly,  I am going to inspect and replace the diaphragms on the drive wheel brake canister.  The book says release the park brake and then remove the air lines from the canister and then separate each section.  When I replaced the tag axle diaphragm, I left the air line attached when I separated the can.

So with the park brake released, there shouldn't be any air pressure at the canister?   Unless the pressure in the system drops enough to apply the park brakes??
          Anyway, thanks for any thoughts,   Iver.

[bobofthenorth]

I expect you know this already Iver but please don't take apart any canister unless you know for sure that it is NOT a spring canister.  As for the wear on the brake cams Bob (the mechanic Bob) grabbed hold of my slacks, wiggled them and said "that's an automatic fail".  I don't know what he based that on but the measurement that could have failed them was the total cam rotation measured with a dial gauge on the end of the cam shaft.  My understanding was that there is a total cam rotation (110 degrees) that would fail and that could be made up from shoe wear, drum wear, roller wear, S-cam wear and bushing wear.  But some of the real experts will likely get out of bed pretty soon and give you a real answer.

[gumpy]

I can't give you a measurement for allowable play in the s-cam shaft, but if you can feel it move, it's probably too much. I'd recommend you replace the bushings and seals regardless.

As for the spider cracks, they are normal.  You should have the drum turned when putting new shoes on, and then size the shoes to the drum diameter.

Air pressure is needed to release the parking brake. I believe you should be ok to leave the lines attached if you have enough room to remove the bands and separate the
chambers. Personally, I think it would be easier to replace the diaphragms with the can off the bus, though. It's a fight to get those bands back in place, and a stiff air line might
make it even more of a fight.

[DaveG]

sounds like the "cracks" in your drum are normal heat check cracks...no problem.

"S" cam up/down/fore/aft should be limited to about 0.030", although many rigs out there will have more....

[DaveG]

sounds like the "cracks" in your drum are normal heat check cracks...no problem.

"S" cam up/down/fore/aft should be limited to about 0.030", although many rigs out there will have more....

Craig, I am not sure how many folks turn drums and arc shoes anymore. Locally, the shops don't like to turn big drums because they generally run into hard spots that have to be ground out by hand during the turning process...labor, labor labor. 

In the big truck industry, new/relined shoes go on with new hardware (spring kit/rollers/anchor pins) and run on old drums...not perfect but kinda industry standards.
Also, as far as the "S" cam goes, I've never heard it refered to in degrees of rotation. The Highway Patrol/DOT or whoever is going to be concerned with stroke or travel on the air chamber...that is usually adjustment of the slack adjuster. If the initial set-up of the brakes are correct (push rod length) and the stroke or travel of the air chamber is within limits (less than 2" for a type 30 chamber) then all should be good.

As a general rule, for all to be aware of is brake lining thickness. On a brake shoe with two pieces of lining, the minimum thickness is 1/4". On a brake shoe with a single pieces of lining, 3/16"

[bevans6]

Rotation of the S-cam is important so that you don't get "cam-over".  If the cam can rotate too far before the brake is fully applied, the rollers can slip off the ends of the "S" and get hooked in behind, causing no end of grief.  This can happen even with the slack adjuster correctly adjusted if the S-cam is worn, the bushings are worn, or more probably the drum ID is out of spec, worn too much or cut too large, or if the shoes are worn.  Basically, the S-cam is being asked to expand the shoes beyond it's ability to do so.  the measurement would be total rotation of the S-cam, but I don't know the procedure to do that.

Brian

[bobofthenorth]

You need a little weighted dial gauge with a magnet on it.  You wind the brakes all the way off, put the dial gauge on the end of the S-cam and tighten the slack until the brakes are fully applied.  Its a required procedure for a vehicle inspection in SK & BC for sure & by extension I expect all of western Canada and likely Canada wide.  As I already posted, failure is 110 degrees and, PITA that it is to conduct the test, its actually a very good test because it measures such a variety of out of spec conditions plus it measures conditions that add together.  So you could be marginal on a bunch of individual conditions but fail the combined test.

[Fredward]

I replaced the diaphrams in my DD-3s off the bus. I aired up the bus with shop air and then released the brakes. THen I exhausted all the air from the system. When I got the DD-3 on the bench I applied shop air to the park  port which fully extends the push rod. The rollers lock it in place. Then I removed both rings and replaced both diaphrams.

Then I reinstalled the DD-3 in the bus with the actuator rod still extended. I just loosely connected the nuts and hooked up the air lines. Then I aired up the bus again using shop air and pushed the park button in; did a full brake application which retracted the actuator rod. Then I hooked the actuator back up to the slack adjuster and tightened up the nuts holding the DD-3 onto the bracket.

I did one side at a time.

Fred

[Iver]

I think it would be easier to replace the diaphragms with the can off the bus, though. It's a fight to get those bands back in place, and a stiff air line might
make it even more of a fight.

Thanks Craig and Fred as well.  I took the air lines off and removed the DD3.  As you said much easier to work on out of the coach.

Thanks to everyone else as well for the info on the s-cam etc. 

Another question about air lines......  Two of the air lines on the brake can were standard brass compression fittings.   The third and largest air line I had never quite taken apart before.  It was a two piece fitting and had a large nut collar with a spring wrapped around the air line down about 2 inches.  I wasn't sure which nut to remove so I removed the outer nut with the spring and under it was a large brass/copper ferule around the rubber air line which was pushed inside the upper part of the fitting.  I just pulled it apart to separate it.  Is that right?  And just push it on and tighten the nut to replace it??

You wind the brakes all the way off, put the dial gauge on the end of the S-cam and tighten the slack until the brakes are fully applied.

If I did as you say and maybe put a mark on the s-cam shaft inside the slack adjuster, with the brakes all the way off, then tighten the slack until the brakes are fully applied and note the position of the mark,  that would be a rough idea how much rotation the shaft makes?

Thanks,  Iver.

[gumpy]

Another question about air lines......  Two of the air lines on the brake can were standard brass compression fittings.   The third and largest air line I had never quite taken apart before.  It was a two piece fitting and had a large nut collar with a spring wrapped around the air line down about 2 inches.  I wasn't sure which nut to remove so I removed the outer nut with the spring and under it was a large brass/copper ferule around the rubber air line which was pushed inside the upper part of the fitting.  I just pulled it apart to separate it.  Is that right?  And just push it on and tighten the nut to replace it??

Yeah, that sounds right. The ferrule should be crimped around the hose. Inspect these hoses closely and replace if necessary. The fittings are typically reusable unless severly corroded. You just need to replace the hose and ferrules.

craig

[bobofthenorth]

If I did as you say and maybe put a mark on the s-cam shaft inside the slack adjuster, with the brakes all the way off, then tighten the slack until the brakes are fully applied and note the position of the mark,  that would be a rough idea how much rotation the shaft makes?

That's probably as accurate as the dial gauge anyway - it just makes the process look more official.