Bearing replacement

Discussion in 'SuperAmp Generators' started by Mark Zinn, Apr 19, 2014.

  1. DIY bearing replacement for your SuperAmp PMA.

    First, obtain the bearing number for your particular PMA. The seller can tell you what it is but they may not be aware of small bearing model changes during production. The best way is to take off the front end bell by removing the thru-bolts and ever so gently tapping around the edge with a small wooden block and small rubber mallet to remove the end bell. Take your time with this because the bearing is seated in the end bell. It will be somewhat tight and you don't want to damage the machined aluminum seat or the machined bell edge. Also, the end bells are cast aluminum and you can break off a thru-bolt ear easily by pounding on it too hard.
    The number is stamped on the front bearing seal. For my SuperAmp it is 6205RZ Both are the same, however the back bearing stamp may be on the back side, and not visible until removed. Replacements are relatively cheap at around $6, and widely available.

    Remove the back end bell the same way, making sure not to put any stress on the 3 output wires. I removed the grommet to make this easier. Make accurate measurements preferably with a digital caliper, from each bearing face to some repeatable fixed point, like the shaft tip. Record these measurements for future reference when the new bearings are installed. You will need the new bearing measurements to be the same.

    The bearings must be pulled off. You'll need a bearing puller with a long reach for the front bearing because of the shaft length. The front shaft is 4 7/8" from the tip to the bearing seat, behind the bearing. So your puller needs a reach of 5" minimum, measured between the face of the dog and the screw that presses against the shaft tip.

    There's also not much room between the edge of the bearing and the stator windings for the puller dogs, and you DO NOT want to damage the windings. The puller needs dogs no more than 1" long, so they will fit between the stator windings and the shaft bearing seat (under the bearing) without touching the stator windings. You can rent a puller from an auto parts store, or purchase one. I purchased an OTC 4532, larger than needed puller for a future project and the dogs just barely fit. I got lucky.
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    The dogs only need to slide under the outer bearing ring. The pull pressure should only be against the outer ring while any pressure needed to press the bearing on, should only be applied to the inner ring. If done correctly you will not need to press on the new bearings. Installation will be a drop-in procedure. Also, they should not be tapped on with a hammer and a length of pipe. The race track could be damaged by brinelling (shock contact of bearing balls with the inside of the track) which will reduce the life of the bearing and make them noisy.

    I suggest pulling the back bearing first. It's a bit easier to remove and will give you valuable experience for the longer front bearing pull. Check the bearing number after removal just to make sure both bearings are the same.

    Put two layers of blue painters tape over the stator windings where the puller dogs go under the bearing, to protect the windings. Take your time attaching the puller. It will produce extreme pressure and may fly apart if not assembled and lined up correctly. Also, if the bearing isn't pulled straight, it can gouge the shaft and make a burr that will have to be filed off, changing the rotor balance slightly. Wear eye protection, follow all puller instructions, and then remove the bearings.

    After both bearings have been successfully removed, inspect and clean the shaft. Do not use anything abrasive unless absolutely necessary to remove a burr, rust or other deposit that may impede bearing installation.
    The shaft diameter is only .040mm larger then the inside diameter of the bearing. If too much shaft material is removed, the new bearing will not seize the shaft properly and/or may creep during temperature normalization. The shaft may have to be replaced.

    Finish cleaning everything with denatured alcohol (Wal-Mart paint dept.) including your hands because they have oils on them. Clean the new bearings with a clean rag and a small amount of denatured alcohol, being carefull not to get any alcohol on the seals. Set them aside and don't touch them again until you're ready to install them. The shaft and bearings must be absolutely clean and dry.

    Blow out the entire interior of the PMA thoroughly with at least 100psi. compressed air. If just one speck of loose debris remains, I guarantee it will become lodged between the stator and rotor and stop the PMA cold.

    Cool down the PMA any way you can. I placed mine on the floor of my truck and turned on the AC for about 20 minutes. This will cool It to about 40F. It doesn't need to be any colder. This makes metal shrink slightly and dries the inside of the PMA completely. It needs to be absolutely clean and dry inside when it's re-assembled.

    Position the PMA front shaft side up and stabilize it. Don't rest it on the stator windings. Use three wood blocks placed around the edge.

    Lay one new bearing on a non-flamable surface with the bearing number facing up. Using a heat gun with a 3/8" or 1/2" reduction nozzle, set to mid range heat setting, heat the inner surface in a circular motion to a point where it is hot, but you can still pick it up. You could also use a hair dryer or place it on top of a 60w light bulb, etc. Use a glove if you get it too hot. It will only take a minute with a heat gun. You should not have to heat it over 125F.

    You can practice this by heating the old bearing and trying it, to see how much heat is actually needed to seat the bearing. Remove it quickly or it will seize the shaft.

    Be ready to make a quick measurement, slide a new heated bearing all the way onto the shaft until it bottoms out on the bearing stop. Make sure it is fully seated. Immediately make a measurement to assure it has the same measurement as the old bearing before you removed it. You only have a few seconds to remove it, if something isn't right. Leave the PMA upright so the weight of the bearing will hold it in place and let the temperature return to normal naturally. The bearing will shrink and the shaft will expand. The new bearing will seize in place.

    This method is better than pressing the bearing in place, in my opinion. No damage is possible, unless you get it too hot and boil the high temp. grease inside.

    After the PMA has returned to ambient temperature, repeat the procedure for the back bearing. I had to increase the bearing temp. slightly for the back bearing. Again make sure the measurement is correct.

    Let the temperature return to normal naturally, then re-measure to make sure everything is the same as before you started. This is important. I have heard that a bearing can creep during this process, especially if the bearing seat has been sanded or filed out of tolerances. The shaft is not tapered where the bearing seats, so it shouldn't be an issue.

    Replace the front end bell, back end bell including the thrust washer, thru-bolts, lock washers and nuts. Make sure the end bells are positioned properly, tighten each bolt slightly, tap around the end bell gently. tighten slightly again, and so on, until the end bells are seated properly and snug, then finish tightening the thru-bolts in small steps to about 30fp. Do not tap too hard or over-tighten.

    Make sure the shaft spins freely. While spinning the shaft, check ac voltage between any two output wires. You should be able to spin up 6-8 volts by hand. Check between all three wires, two at a time. All pairs should be about the same voltage.

    Line up all three output wires (not overlapping) and replace the rubber grommet into the hole in the back end bell. This will be difficult and the grommet can easily be damaged, but it can be done. Use a medium flat blade screwdriver, press in and down on the tabs one at a time. Then press down on the grommet, squeeze and turn slightly to seat all four tabs. Put a little silicone sealer around the base of the grommet if necessary. If you damage the grommet, a standard grommet and some silicone will work just fine.

    larry george likes this.

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