Posted by Gordon Maney on Monday, December 20, 2004 at 1:48PM :
In Reply to: But did you check braking action each way? posted by chris case on Monday, December 20, 2004 at 11:01AM :
I don't know what you mean by a brake balance tester. I did not use that language. If you turn a rotor or drum, if you can't get the measured runout to .001 in., then you have a problem with your lathe or your setup. And, yes, in practical terms, that exceeds the performance requirements of the application, but I don't see any merit in planning on doing sloppy work. There is no benefit in that, and there is a lot of hazard.
What you are calling on-axle hubs are called hubless drums. Yes, lots of applications have hubless drums, and of course they are not turned on a hub. How could they be? They are set up on the lathe arbor using a bell, sometimes two bells [lathe manufacturer's terminology will vary....], a cone, and a spring. Different sizes for different drums. Even those can be problematic in setup. On the vehicle the drum is sandwiched between the wheel and the axle flange. In the lathe setup, there is no such compression. Setups must be tested, and variations in setup can be observed, again by the naked eye in some cases. Due to how it is that hubless drums are set up, they can still have some problems. Occasionally I have seen hubless drums that needed to be rejected due to setup problems; an inability to set up that drum, due to problems with the center [particularly sheet metal centers] of the drum.
Actually, it is that critical. It is very critical, unless we are only talking about a salvage yard truck that will only be run at low speeds in the yard. It is possible to take the attitude that it is not that critical on your vehicle.
I recall when I first worked at an International Harvester dealership, working on tractors and trailers. Some of those drums were so bad that they had a huge lip on them. They should have been turned, actually they should have been scrapped. On one job I was directed to take a body grinder, grind the lips off the drums, and put them back on. Supposedly owners did not like paying for those expensive drums. After quite a bit of discussion about the whole issue, we started replacing drums. It was way too dangerous. They realized that.
Drums and rotors are subject to having lining material getting impregnated in the cast iron. Drums become out of round and experience other cross-sectional change; barrel wear or bell mouth. A drum that is out of round causes pedal pulsations, surging braking action, and generally poorer braking action. A drum that is too thin is more subject to changing shape during brake application. A drum that is too thin can fail under hard braking action. When the crash is over and the equipment is destroyed, and the people are dead or crippled, it is too late to take a more serious attitude about the integrity of brake system parts.
Consider the clearance that is necessary at the heel and toe of the shoes. The reason for that is drum distortion under brake application. When you apply a drum brake hard, the drum goes out of round. That heel and toe clearance is to maintain full lining contact during the typical, heavy brake application.
Brakes are very critical. Any machinist will tell you that a drum set up on a hub with bearing races is far, far better than any hubless setup. Any machinist would tell you that it would be foolish to not turn the drums on hubs, and on their own hubs. Furthermore, it is so simple, simpler actually, to set them up on the hub, instead of off, and certainly very easy to keep your hubs matched with your drums, that I can't see ANY possible reason to not do so, particularly when you ensure a better, safer job. Any shop that can turn these drums would be able to set them up on hubs, setting up off the bearing races. Also, the races must be seated properly if you have just installed new races.
In another post in this thread I said I can't emphasize this enough. What I am saying here is not my opinion. It is truth recognized by any machinist, whether it be an automotive machinist or industrial machinist, by qualified automotive service personnel, and by manufacturers of automotive machine tools.