USAT cracked/split axle problems

Wheels slipping on "axles"

USAT diesels have a common problem with wheels slipping on the axles. Looking inside, the axle assembly consists of 2 "half shafts" pressed into a hollow plastic casting that has the gear. Each half axle is a metal shaft pressed into a wheel.

Again this plastic casting is a tube that has the worm gear cast as part of it, and the two wheel/axle "half axles" press into this casting.

When the "wheels slip", it's really because one or more axles are spinning in the tubular casting, which has split and lets the "wheels slip".

Either the plastic is too brittle, or the clearance between the knurled end of the axles and the nylon is too close or whatever. Does not matter, for as long as I have been in the hobby, they split, even just sitting brand new in the box on a shelf.

USA claims to have fixed the problem a while ago, and modified the knurling on the axle ends so as not to split them. This has reduced the problem, but not eliminated it, I have locos with the newer knurling with "split gears" also. 

Call USA and they will send you new axles if your loco is still under warranty.

Do I have the problem?:

Put the loco on it's back, then put one thumb on each wheel of the SAME axle, and try to rotate the wheels in the opposite direction. if either wheel is loose in the gear casting (the casting on that side is split), it will turn. Use firm pressure, (but don't go all sasquatch on it). Remember that these locos put out a lot of torque by themselves, so firm pressure approximates the actual torque the motor will apply.

To repair:

I put brass sleeves over the ends of the gear casting, and this fixes the problem easily and quickly. Other people have wound nylon fishing line or wire on the axles, epoxied it in place (while held tightly) and reinserted the axles. Too much work for me, and waiting for the epoxy to cure.

The brass sleeves I have (and are shown below) have a length of 0.3", and an inside diameter of 0.3535".

You can also find some K&S brass tubing that is close enough in size. The K&S stock number is 9828.

Note well: Using thicker wall tubing can cause a short between the tubing and the metal rods that project into the motor block. This can cause weird shorts. Also too thick tubing can interfere with the housing on the smaller 2 axle blocks. Inspect carefully.

I use an inexpensive press from Harbor Freight to press the axles in, but you can do it with a vise, if you take your time.

See THIS page to see the press, scroll down. It makes it very easy and safe to press wheels on and off.

After applying the collars to each split part of the gear, be sure to press the axles in evenly. The last illustration where you press the entire assembly gives you no control over each axle. If you use a couple of metal bars (or flat files) under the gear and you will be able only to press one half shaft at a time, thus controlling your assembly.

The picture below shows a typical split / crack.


 First remove the 2 half axles from the gear casting, normally you can pull them apart by hand, if you use tools be careful not to crack it further.


You may want to put on a slight bevel on the end when applying the tubing, it's not always necessary, a file or coarse sandpaper will work:


 Now press your tubing onto the gear casting. I found a nice press at Harbor Freight to do this, but you can do it with a hammer or drill press or vise if you are careful.


You may also want to put a bevel on the inner side, I don't normally need to do this, but if you are not using a press, doing this will help the axle enter. I used a 45 degree countersinking tool in a drill press, but you can do it with a sharp hobby knife also.


Now the most critical part, pressing the axles back in. I recommend a real press, if not, try a drill press. If you aren't careful, your wheels will not run true. Also, be careful to press them in to the right back to back distance, pulling them back apart without a special wheel puller is a pain.


Here is the final pressing using a drill press. Get everything straight and true. Have your wheel gauge or calipers handy, check as you go


 (Photos courtesy of Tony Walsham).

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