The following info is intended as an introduction for fellow do it yourselfers who understand the value of making their own repairs and the independence and piece of mind it brings, © March 15, 2002. If you don't believe in personal responsibility, please do not attempt a repair of any kind.... call the Repairman.
Most of my tear down experience is around the ST10. At the moment I have a 12 and 15 in my work shop as well. They appear identical in construction; where the 10 and 12 share the same size shaft (42mm), and the same exterior dimensions. The ST15 has a larger diameter shaft (48mm) and the generator body is larger.
Bearing inspection and replacement are straight forward in this head.
Tools Required for inspection: Medium sized rubber hammer, 10mm & 14mm box end wrenches.
Additional tools for bearing replacement: Bearing puller, properly sized pipe to seat the bearing.
Start by removing the slip ring inspection cover. If it is trapped under the instrument housing, loosen the screws inside the housing and remove the inspection cover completely. Next... carefully pull the brushes from their seats and trap the brush where it no longer makes contact with the ring.
Remove the three 10mm (head size) bolts located around the shaft on the slip ring end. These hold the slip ring cover which doubles as a rigid mount for the slip rings. Once the bolts have been completely removed, back the cover off the bearing being careful NOT to apply force to the brush rigging. Move the cover and rigging inboard along the shaft away from the end bell.
Remove the four 14mm bolts securing the bell housing to the rotor housing. You'll notice there's generous amounts of room to swing a rubber hammer to back the bell housing (end housing) off the 'stator' or main housing. Go from side to side and slowly force the bell housing off the shaft, then remove it completely. The bearing could have remained on the shaft, but most likely it is still in the bell housing. Put a piece of clean paper over the bearing and set it aside where the bearing and grease will not become contaminated.
WARNING... take note, the rotor weights 72 pounds on the ST10 and more for the larger heads! ST heads have pretty tight air gaps and very fine laminations. Be careful so you don't scrape the rotor and stator together.
Now it's time to remove the screens on each side of the shaft end bell housing, Grab your 14mm box end wrench and remove the four bolts that hold the bell housing in place. Use your rubber hammer to drive loose the shaft end bell housing. Once it starts backing off, go side to side and gently move the bell housing off the shaft completely. The shaft side bearing is most likely still on the shaft, at this point, you have access to both bearings for inspection and greasing.
If you find it necessary to replace the large bearing, it is best to remove the rotor, to do this get help! Prepare a soft place to lay the rotor once removed, I placed mine on a pile of rags on an old padded chair I have in the shop.
Now, remove the slip ring brush rigging and tie them out of the way. Position a person on each end of the rotor. Carefully lift the rotor and remove it through the slip ring end. This will give the person on the shaft end more to hold onto as the rotor passes through the stator. Go slow and try to eliminate as much contact as possible.
Once the rotor is out, get a decent bearing puller. I used one with three hooks. The screw goes in the end of the shaft and you tighten it up till the bearing moves. This is not always easy unless you know a few tricks. Spray some WD40 between the shaft and bearing. Tighten up the extractor bolt a bunch, if the bearing doesn't move hit it from the side, if this doesn't allow it to move, get your propane torch out and carefully add some heat to the inner race, tap on the end of the extractor bolt until the bearing starts to move.
When you replace the new bearing, it is very important not to apply force anywhere other than the inner race. If you don't have proper tools to drive on the new bearing try the following.
The seating surface for the bearing is larger than the shaft, it's also just wide enough for the bearing. This allows you to use the old bearing as a tool to drive on the new bearing.....
place the new bearing on the shaft, put the old one on top of it and find a piece of pipe that just fits over the shaft, the pipe must rest on the inner race of the old bearing for driving!
It is best to place the rotor shaft on a piece of wood and have someone hold it vertical while you drive on the new bearing. You'll feel the bearing seat against the shoulder on the shaft.
Once this operation is complete, reverse the procedure to put it back together and you're on your way.