Re: The rear axel blues!

Posted by DavidPackard On 2016/6/8 19:17:05

Steve-52/200
Check the topic "Rear Wheel Bearing Replacement' posted by Packard 1948 . . . lots of shimming information.
Below I present an alternate explanation on what the service technician is striving to accomplish while following the service manual bearing shimming procedure.
Consider the thrust block and both axles, complete with bearings, sitting on your workbench on "V' blocks. Your mental picture should have the splined ends of the axles facing each other with the thrust block between just as they are assembled in the car. Consider the distance between the out-board faces of the bearing cups with zero internal bearing clearance. To that number add the Packard recommended bearing clearance of 0.004 - 0.007 inch. This dimension represents the distance that must be achieved between the inner surfaces of the backing plates of the rear brakes. Here I'm talking about the area where the bearing race touches the backing plate. Shim stock is used to accurately adjust the position of the backing plates. That's the high level view, and mental exercise of what is trying to be achieved.
Now let's consider the bearing cups. The outer diameter of the bearing fits into the 'rear-end' housing bore with a minimum radial clearance, but after sixty to seventy years the fit could have lost a bit of its precision. Since forces can only be transmitted across a bearing at right angles to the roller track, the forces ( that would be the vehicle weight and cornering loads) would tend to push the bearing cup 'hard' onto the backing plate. I doubt whether the cup ever leaves contact with the backing plate during the life of the car, but if it does it's not for long.
About the shims; they are stamped from steel shim stock in 3 different thicknesses, namely 0.005, 0.007, and 0.020 inches. They have 6 total pierced holes, 4 for the bolts that hold the backing plate to the axle housing, 1 for a grease/oil relief passage (so the grease or oil from a leaking seal does not contaminate the friction surfaces of the brake shoes), and finally the large hole in the middle. This center hole is large enough for the bearing race to pass through. The fit of the center hole to the outer diameter of the bearing cup is quite precise on the OEM shims I've encountered. I believe the design intent is for the bearing to touch the face of the backing plate, and the shim will never be contacted by the face of the bearing cup. I equally believe the Packard design engineers expected each rear axle assembly would be selectively shimmed and designed (and exploited) a minimum and maximum expected amount of shims in each unit by controlling manufacturing tolerances of the component parts. By accepting a shimming process during assembly, the tolerances of the individual piece stack-up does not need to be controlled to the 0.003 suggested by the bearing clearance tolerance (wider acceptable tolerances at the piece part level should be easier/lower cost).
I do not believe the procedure presented in the service manual was followed production floor during assembly of the 'rear-end' assembly. One alternate method would be to directly measure the axle detail, and then measure the housing and scribble some arithmetic for the necessary shim stack. However, the most probable alternate method would be to push one bearing cup to a known shim position, which easily could be zero ( no shims using fixtures not backing plates), and then measure how much the other bearing cup is sticking out of the housing. All of the values, including the desired clearance would be combined and the specific shim stack selected, and then divided in two for installation. Once the vehicle has left the production assembly line the service manual procedure is quite adequate, but a bit cumbersome for production. If you have both sides disassembled you could easily get into the right ballpark by using the second suggested alternate procedure.
Initially I was taken by the fact that there are only 3 different shim thicknesses. However by selecting various shims you can find a solution (lots of solutions beyond 0.014) that lies within the tolerance of the 'target' bearing clearance. Try it, quite clever those 'old-timers', and an affordable computer was decades away!
Now on to the 'funny' noises: When the bearing is rolling on the outer race no matter how the backing plate was shimmed the relative position of the backing plate and rotating drum are just as the design engineers expected (assumes nothing is worn-out). Now let's go into a turn and push hard until a bearing reacts out the cornering forces. One bearing is 'taking the load' and all of the brake drum to brake shoe dimensions are OK on that side. On the other side however the tire/wheel/hub/brake drum keeps moving inboard until the axle touches the thrust block and stops only when the thrust block encounters the other axle (which is hard on the bearing). Too much motion of that axle looking for the thrust block can result in the brake drum operating closer to the backing plate (shoes, cylinder, adjuster, etc.) than expected. Extra shims would make the condition worse.
With both rear wheels off the floor, and the brake drag adjusted out, have a 'helper' pull 'their' wheel trying to pull the tire off the car. All the while you're on the other side trying to push your wheel toward the center of the car. While you're pushing and they're pulling try to rotate your side. If the noise is a result of not shimming the bearings correctly you should be able to tell by using this procedure. Reverse your roles (you pull, they push and rotate) . . . the other side is also likely to make noise because the 'shimming' clearance is the same on both sides. With both wheels off the floor and your helper pulling on their tire you should not be able to feel a lot of bearing clearance when you push and pull on your tire, 0.004-0.007 inch is pretty hard to feel. If you're really off on the high side with the amount of shims you'll feel the 'slop'.
Reference the diagram Owen included in his reply. If the noise is a result of excessive 'shimming' I don't know which part might be contacted first but you might see witness marks on the drum's 'diaphragm between the intended friction surface and the hub, (rubbing the metal edge of the shoe, or the side of the friction material into a wear lip in the drum ). I have no idea what the design clearance would be between the stationary parts and the spinning ones inside a drum brake, but we do know the full bearing clearance would need to be accommodated.
I sympathize that modern mechanics don't not study or have experience with historic designs. It would take them some amount of time 'studying' the photos and 'cut-away' drawings to understand the intent.
Hope that helps.
dp