Common Maintenance Events on the Allison 250 C20
By Jim Taylor
his article discusses some of the most common maintenance events on each of the three major components that make up an Allison 250 C20. The compressor, the turbine, and the gearbox.
Evidence of oil spraying out of the bleed valve of an Allison 250 C20 engine should lead you to suspect an oil leak from the carbon seal for the No. 1 bearing at the front of the compressor.
To access the carbon seal area, first the compressor front support must be removed. Remove the nut in the front of the bullet nose, along with the split line bolts and nuts, that go around the aft flange. To help reduce the grip the case halves have on the pilot diameter of the front support, loosen up at least two of the split line bolts that run axially along the case halves. To lift off the support, gently tap the bullet nose with a plastic mallet to over come the o-ring friction. Do this while applying a steady pulling force and maintaining a tight grip on the support. This will prevent the support from injuring you or falling on the floor.
Once the front support is off, it's a good practice to pressure check the oil transfer tubes. Remember that these tubes will give you the same symptoms as a leaking No. 1 carbon seal. Many people don't perform this step until after they've changed the carbon seal 2 or 3 times.
You can do a quick check quite easily by supplying about 15 lbs. of shop air to one of the oil fittings on the front support. Then block the corresponding hole on the inside of the bullet nose with your finger.
Submerge the whole apparatus in a bucket of water and look for air bubbles coming out of the anti-ice air slots found on the trailing edges of the inlet guide vanes of the front support. (Hint: make sure the air nozzle is not on a fitting for the anti-ice air.) A broken transfer tube inside the front support is cause to reject the front support.
At that point you should be looking for another front support. Contact your preferred supplier and perhaps they can sell you an overhauled or used serviceable front support. If not, you'll have to buy a new one.
To remove that hard-to-get circlip for the No. 1 bearing housing, your best bet is to use a good set of needle nose or duck bill pliers. A word of caution: if you're going into the No. 1 bearing area because of a suspected bearing failure, take steps to ensure bearing debris does not fall down inside the compressor assembly when you take off the bearing housing.
You should find a spring and a cup washer sitting on top of the No. 1 bearing. The 9/16 inch nut sitting on top of the No. 1 bearing is a 100 percent replacement item; it should be used only once and then discarded. The dust cap on this nut prevents oil from getting inside the compressor rotor. When removing the No. 1 bearing nut, test the depth of your socket and make sure your socket can't touch the brass ball retainer of the bearing. Technicians in the past, have accidentally damaged the retainer when removing the nut, then reinstalled the damaged bearing, only to have it fail in a few short hours.
To remove the bearing, first establish if you have a puller groove on the forward side of inner race. If you do, you should use puller P/N 23005023 as per CEB 1171. Use the older style puller P/N 6796925 if there isn't a puller groove on the bearing itself. With the old style puller you lift off the bearing with the seal follower. Again, if you think you've damaged the bearing in this process, replace the bearing — Only P/N 23054538 No. 1 bearing nuts are available, and they are to be used with the new style bearings that have the integral puller groove on the forward end. So if your compressor has an old style bearing without the puller groove, you'll need to order a new bearing to go with the new nut. If you just want to change the bearing, you should also change both the follower and the carbon seal assembly.
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