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 Machining of Brake Drums
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raytobe
Advanced Member

USA
280 Posts

Posted - 06/10/2006 :  8:12:42 PM  Show Profile  Visit raytobe's Homepage  Reply with Quote
My own little survey for my own curiosity. Do you or your shop machine brake drums on a reline? If not can you briefly let us know why, (no lathe, time constraints, cheaper to replace, safety, feel it's not necessary, etc.)

Our shop does not machine bus drums, our fleet is all air(cam)/drum and we have the capacity but drums are replaced regardless of if they can be machined to a safe tolerance. Time constraints dictate this (I feel).

Thanks in advance for your input.

IBTMech
Top Member

USA
973 Posts

Posted - 06/11/2006 :  04:14:32 AM  Show Profile  Visit IBTMech's Homepage  Reply with Quote
We have Rockwell Q and Q-Plus with FF blocks on all of our buses and we find that by the time the shoes are shot, the drums are beyond machining. Depending on the driver and the route, we get anywhere from 30K to 120K on a set of brakes.

If it doesn't fit, FORCE it.
If it breaks, well, it needed replacing anyway.
Pullin' wrenches for 45 years.
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ModMech
Top Member

USA
948 Posts

Posted - 06/11/2006 :  09:59:15 AM  Show Profile  Visit ModMech's Homepage  Reply with Quote
We have never machined AIR brake drums, but we always did do the hydraulic brake ones. We were 90% juice, and did not machine rotors either.

It is my opinion that it is *technically* more correct to machine friction surfaces at every re-line, but let's geat real, we just cannot afford to place a vehicle our for repairs for an entire day just because the brake pads/shoes are worn out.

My policy is reline only once, replace rotors/drums the second time. The drums are generally worn out by 120,000 miles anyway (2nd brake job), and the rotor "fins" are so thin that the rotor's mass has been substantially reduced by 40,000 miles (2nd brake job, if not 3rd), so they NEED to be replaced at that time to prevent major failure (cracks).

I feel very strongly that it is ideal to resurface the rotor/drum, but replacement every other brake job is our best effort to stay very much on the safe side.

Also, in truth, I cannot tell any difference between a turned rotor/drum and unturned rotor/new drum after a brake job. That does not mean there is no difference in brake performance, just that it is too small for me to notice.

If you want customer service, you NEED an International!
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raytobe
Advanced Member

USA
280 Posts

Posted - 06/13/2006 :  5:42:59 PM  Show Profile  Visit raytobe's Homepage  Reply with Quote
Thanks for your input guys. This question was bought on by a brake job that I did (Van) that came back with the steering wheel slightly pulling to one side. I had not machined the rotors when I installed loaded calipers (they *looked* OK). Tore it down and machined the rotors-- problem solved.
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tigger2
Advanced Member

USA
286 Posts

Posted - 06/17/2006 :  4:16:26 PM  Show Profile  Reply with Quote
Our fleet of busses is all air brake at rebrake time we change shoes, drums, and all hardware. safty first and always.
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Brad Barker
Administrator

USA
838 Posts

Posted - 06/22/2006 :  11:46:20 AM  Show Profile  Visit Brad Barker's Homepage  Click to see Brad Barker's MSN Messenger address  Reply with Quote
Technically speaking but not necessarily practically speaking you should turn the lining if you turn the drums. Don't ask me how? My point is that a worn drum or turned drum will not match the curvature of a new set of shoes, so in all actuality you are loosing brake performance if you do not replace the drums. A worn drum, say .060 to .080 will only provide about 75% friction surface on a reline if drums are reused. If drums are bell shaped, grooved from rivel contact, etc. Brake performance is even worse. Reusing a worn or turned drum also causes stress on the reinforcing webbs of the shoes which can cause bending of the shoe which may result in cracked lining or cause other problems such as uneven braking.

Why take the chance of having a brake failure or lessening the life of a brake reline. Replace shoes, drums and all hardware at each reline.
The only time I would reuse a drum is if I had a premature failure of a shoe assembly within just a short time of a reline that did not damage the drum. If shoes are more than 50% but are removed due to a problem such as cracked lining I may reuse the drum. Always replace all brakes on the same axle each time a job is done. If hubs are pulled always replace the axle seal.

Brad A. Barker
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IBTMech
Top Member

USA
973 Posts

Posted - 06/22/2006 :  7:27:59 PM  Show Profile  Visit IBTMech's Homepage  Reply with Quote
An old friend of mine used to work for the local transit bus district long before there were rules governing the limits of brake drum machining. They would machine their drums 3 times before junking them and cut their brake blocks (all bolt on back then) to fit the drums. That was back in the bad old all asbestos and brass days where you would wear out two sets of shoes before the drums needed turning.

With today's semi-metallic linings, the drums are junk by the time the linings are worn out.

If it doesn't fit, FORCE it.
If it breaks, well, it needed replacing anyway.
Pullin' wrenches for 45 years.

Edited by - IBTMech on 06/22/2006 7:29:11 PM
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flyboy
Senior Member

USA
147 Posts

Posted - 07/14/2006 :  04:46:15 AM  Show Profile  Visit flyboy's Homepage  Reply with Quote
We have found that by the time the brake block is worn to replacement thickness that the drums are worn enough to warrant replacement. Additionally we use Marathon Heat Star which is recommended by the manufacturer to be installed with new drums to achieve maximum contact and optimum surface friction. Because the air brake shoe anchors are fixed in place drum wear results in progressively less contact for the shoes unless they are cam ground to match drum diameter. Cam grinding rstores contact area but removes lining material subsequently reducing service life. Besides a machine big enough to cut bus drums costs a lot of money and the labor involved makes the cost of new drums very cost effective. Buy your drums by the skid and save money.
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raytobe
Advanced Member

USA
280 Posts

Posted - 07/14/2006 :  2:34:29 PM  Show Profile  Visit raytobe's Homepage  Reply with Quote
quote:
Originally posted by flyboy

Besides a machine big enough to cut bus drums costs a lot of money and the labor involved makes the cost of new drums very cost effective. Buy your drums by the skid and save money.



I think you would be surprised at the size of the lathe that would turn a bus drum. I used to work at a different district and we had the huge lathe for them but where I am at now this is accomplished by using an extension arm that attaches to the lathe. The machine itself is your standard brake lathe. Your point about contact wear is well taken.
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quinn
Active Member

USA
17 Posts

Posted - 07/19/2006 :  07:39:33 AM  Show Profile  Reply with Quote
most brake mfg dont want reground drums used with automatic slack adjusters.too fine of tolerances on adjusters.wont adjust properly leaving you with little or no brakes
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raytobe
Advanced Member

USA
280 Posts

Posted - 07/19/2006 :  6:06:55 PM  Show Profile  Visit raytobe's Homepage  Reply with Quote
quote:
Originally posted by quinn

most brake mfg dont want reground drums used with automatic slack adjusters.too fine of tolerances on adjusters.wont adjust properly leaving you with little or no brakes



Heck, a properly working auto adjust slack adjuster is a rare thing for me now as it is. I have to adjust most of them up manually every PM anyway.
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Joe Hartnett
Advanced Member

USA
359 Posts

Posted - 07/20/2006 :  5:19:34 PM  Show Profile  Reply with Quote
I dont understand how a drum that is machined and trued to the hub would cause an automatic slack adjuster not to work.

Edited by - Joe Hartnett on 07/21/2006 5:39:04 PM
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Mechan1c
Top Member

USA
701 Posts

Posted - 07/22/2006 :  1:28:15 PM  Show Profile  Reply with Quote
In our all-air brake fleet, we never re-use drums after their first run. They're pretty much gone, not to mentioned heat checked. As far as auto-slacks go, we never adjust them. When one comes in past the legal stroke, it comes off. When we transitioned to this "no-adjust" policy shop-wide, we went through some slacks...now its rare to replace one. A properly installed Haldex slack is a good piece in my fleet. We have a big Coats machine that will do the drums up to 16.5 X 10 but we haven't used it for big drums in years.
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raytobe
Advanced Member

USA
280 Posts

Posted - 07/22/2006 :  2:08:51 PM  Show Profile  Visit raytobe's Homepage  Reply with Quote
quote:
Originally posted by Mechan1c

As far as auto-slacks go, we never adjust them. When one comes in past the legal stroke, it comes off. When we transitioned to this "no-adjust" policy shop-wide, we went through some slacks...now its rare to replace one. A properly installed Haldex slack is a good piece in my fleet.



So is it your belief that slacks were installed incorrectly to begin with (from the Dealer)or is it a longevity issue? I have adjusted up auto slacks on fairly new busses with low miles. The short body Genesis are terrible for this, especially on the rears.
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IBTMech
Top Member

USA
973 Posts

Posted - 07/22/2006 :  4:56:56 PM  Show Profile  Visit IBTMech's Homepage  Reply with Quote
Back in '92 when the first automatic slacks were coming out I asked the State Police their opinion of them and their reply was "Yank them off and put manual slacks on".

Which I did until '97. At that point the Staties changed their minds. Rockwell and Haldex have gotten their act together a little better since '92 but I still adjust them manually at every service interval.

The Rockwells seem to do better than the Haldex in keeping a consistant adjustment. I haven't had any experience with the Bendix auto slacks, though.

If I were to have a choice, I would go with all manual slacks. Federal law dictates differently.........

If it doesn't fit, FORCE it.
If it breaks, well, it needed replacing anyway.
Pullin' wrenches for 45 years.

Edited by - IBTMech on 07/22/2006 5:00:21 PM
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Mechan1c
Top Member

USA
701 Posts

Posted - 07/22/2006 :  5:37:49 PM  Show Profile  Reply with Quote
I know these things cause a lot of dissagreement, so I'll just say what works for us...20 years ago I thought the Bendix held adjustment better than the Rockwell. On our big buses, the 6" Bendix came OE on the front Eaton axle assy, and the 6 1/2" Rockwells on the rear because we ran the Neway air ride and the 16.5 X 10" Q equiped axle was the only option. When the buses came in for service the rears were always right under 2" and the fronts would be around 1" or 1 1/4". I used to just tighten 'em up, and back 'em off like in the manual days. The State guys would come in and mash down on the brakes and we always seemed to loose a rear or 2, never a front. It was my contention that they were not testing properly...but you know how that went over. I used to just make sure there were none even close.

Because Bendix didn't make a 6 1/2" ASA, we tried some Haldex on the rears and they seemed to come back in the shop at 1 5/8"- 1 3/4" and rarely anywhere near 2" When Bendix started making the ASA-5's in China, I started replacing those with Haldex as well. Most of the OE stuff over the last 5 years or so is Bendix all the way around because we quit spec'ing air ride. They do ok, but we still replace with only Haldex. Setting them up is fairly easy after you do a few, and they seem to retain good clearance and stroke. We also use a grease from Rockwell...of all people, that was originally designed for the Stopmaster wedge brakes you see on transit stuff. I think this helps the ASA's internals compared to general purpose chassis lube.

Raytobe: " So is it your belief that slacks were installed incorrectly to begin with (from the Dealer)or is it a longevity issue? I have adjusted up auto slacks on fairly new busses with low miles. The short body Genesis are terrible for this, especially on the rears."

I would be hesitant to say that that it's an OE problem, but all my experience is with Thomas, and out of the gate they seem to be ok the first couple of years. My theory (I can't prove any of it) is the combination of grease type, too much manual adjustment, excessive heat (esp. in the rear), and wearing foundation brakes are what cause shortened slack life. As far as improper set-up goes, I've seen most problems at the replacement stage, not on new buses. Do you have quite a bit of heat in the back of the Genesis?
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raytobe
Advanced Member

USA
280 Posts

Posted - 07/23/2006 :  5:08:49 PM  Show Profile  Visit raytobe's Homepage  Reply with Quote
quote:
Originally posted by Mechan1c

I know



I would be hesitant to say that that it's an OE problem, but all my experience is with As far as improper set-up goes, I've seen most problems at the replacement stage, not on new buses. Do you have quite a bit of heat in the back of the Genesis?



First off-- I *never* measure slack stroke. On the front I always tighten them up till the shoes stop the drums then I back off about a quarter turn until wheel spins free. On the rears I don't even jack the bus up anymore, just do em on the floor-- tighten until I feel resistance then back off a quarter turn. The Genesis busses are all wheelchair equipped in the rear. I don't feel there is any more heat produce from braking than would be normal. I can adjust them up and the very next PM I have to do it again-- on the majority of them. I would be interested in knowing how one could screw up installing a slack. Seems pretty idiot proof to me.
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