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Posted by TGP (IL) [64.12.116.76] on Monday, December 27, 2010 at 18:23:22 :
In Reply to: Re: Brake Drum Question posted by gmharris [71.105.47.144] on Monday, December 27, 2010 at 17:21:05 :
In case you missed this article written by Gordon Maney some years ago I reposted it.
Excellently Done.
TGP
I published this article in the Power Wagon Advertiser not too long ago. Perhaps an example of content....
The braking system is a critical service area on our trucks, because we need the best brakes we can get. One component of the braking system that warrants special attention is the brake drum, because the population of good drums is relentlessly declining.
Unfortunately, there is no company reproducing our brake drums. The fact that there are people converting to disc brakes eases some of the pressure, but the fact remains that drums are in short supply. Lots of folks want their drum brakes, either because of originality issues, or because they feel it is too expensive to switch to disc brakes.
All of this together creates circumstances where our drums are a bit like gold, and should be cared for as such. Leaving them with just anyone to have them turned becomes very risky.
If you are interested in having good brakes, and also in taking the best care of your drums, then there are some things you should know about having them machined, or turned, as the procedure is often called.
The drums on many of our trucks are quite similar in design. There is a hub containing races for tapered roller bearings, and the drum attaches to that hub.
Ignoring issues of oversize diameter, let me comment on drum setup on a lathe for proper machining. Any drum that is mounted on a hub on the vehicle should be mounted on that hub when on the drum lathe. Do not take your four drums and one hub, figuring that Eli Whitney's concept of mass production ensures that all the hubs are the same, and that all the mated sets of hubs and drums will be the same. Forget all that. Each drum should be on its own hub. The hubs and drums have unique relationships; they should be maintained.
Furthermore, each hub's bearing races must be firmly seated in clean bores, emphasizing the point that, yes, there must be races in those bores. It seems needless to explain that, but I have encountered people who thought that it would be acceptable to setup on the machined counterbores in the hub when it contained no races. Do not do that, ever.
If you have replaced any bearing races, it is imperative that you carefully examined the bores into which you drove the races. It is also imperative that you make absolutely sure while installing those races that they are fully seated in those bores. I cannot emphasize this enough.
Some people decide that they are going to replace the bearings and races, but they will do that later, after the drums are turned, thinking, somehow, that it will be better for the new races. Do not take that approach.
If a business engaged in turning brake drums tells you that they can only setup your drums with the hubs out, that is the wrong shop for your precious drums. Do not leave them there to be turned in a hubless setup. What they are really saying is that they don't have the proper adapters.
When you make an initial setup on a lathe, that setup must be evaluated to determine that it is actually a good one. That comes, most often, from visual inspection of the rotating work piece. My point is that if you improperly mount the drum and then turn it, that drum will have runout when installed on the vehicle. That will translate into pedal pulsation and uneven braking action. You don't want that. You will have to turn the drum again, which will remove more material, and likely it will then be too big.
Considerable experimentation with turning old drums on one hub, then moving them to different hubs, bears out the fact that a drum should be turned on its own hub. Each drum should be mounted on its own hub, maintaining that mated relationship.
Let's move on to another aspect of brake drum machining. Consider the mentality of the typical brake drum lathe operator. He wants the finished drum to be totally free of scoring and display a nice finish. He wants to achieve this end quickly, as time is money in his world. His unstated attitude is that if the drum is too big when he finishes his work, it needs to be replaced. It is not a problem for him if he ruins your drum. He will move on to the next job. Drum replacement is your problem.
The approach he has been trained to use is to make an initial cut that will almost take out the deepest score in the drum's friction surface, and then make a finish pass to produce the final surface. Given the age and service history of your drum, the deepest score is likely beyond the maximum oversize of the drum.
I believe considerable prudence should be exercised when turning one of these drums. First, with respect to setup, and second, with respect to a strategy. I will talk to you about how I approach my truck. You need to make your own decisions, I will not make them for you.
On my truck, if I have a score — a line — or two on the face of the drum, but the rest of the drum's surface is good, I am not concerned about that. So I take an initial, shallow cut, intended to find out about the lay of the land. When I machine one of those drums, time is not the valuable thing, the drum is the thing of value. I will take a number of small cuts, being careful with my drum material.
I urge you to have a serious discussion with your machinist before he turns your drums. Otherwise, you could go back to pick them up, only to be told that all four of them are junk.
Power Wagon Advertiser
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