Common maintenance events on the Allison 250 C20

You can save a bit of money if you buy the kit to replace the carbon element in the seal housing. You can also save a bit of money by purchasing an overhauled seal follower. The No. 1 bearing is seated by torquing the No. 1 bearing nut.

The No. 1 bearing is an one-way thrust bearing that accepts a light forward thrust load from the compressor rotor. Other than analyzing the bearing and figuring out where the thrust shoulders are, the easiest way to install this bearing correctly is to make sure the puller groove is up or on the forward side during installation.

Again, when torquing the nut, be careful not to touch the brass ball retainer with your socket. If you think you've damaged it, replace the bearing. The mating of the bearing housing with the circlip on the outside of the carbon seal is a two-person job. One person should hold the carbon seal up with the aid of two screwdrivers lightly leveraged under the seal and against the balance rim on the first stage wheel. The second person, with one hand, lowers the housing onto the seal and bearing assemblies. The person also makes sure that the spring and cup washer are in place. With the other hand the person squeezes the circlip on the carbon seal with a good pair of needle nose or duckbill pliers.

With one person pushing down on the housing and the other resisting that force with the two screwdrivers, there's a fair amount of force involved. However, with a little bit of finagling, the snap ring will pop into place. Check to see that the circlip is fully seated in its groove by trying to move one of the tangs of the circlip. If it moves relatively easily, and the other tang follows along, the circlip is in the right place. It is common for the circlip to hang up in one place. If this is the case, a concentration of some leverage from a screwdriver, lightly pried up against the bottom of the seal, will seat the ring.

When locating the front support to the case halves, put the scavenge fitting at or about the 6 o'clock position. The final seating of the front support to the case halves is done using hand pressure. Don't draw the front support up to the case halves with the splitline bolts. Push the support into place by hand, hold it there and then tighten up two opposite split line bolts until they are almost snug. Pulling the front support in the bolts can damage the pilot diameter on the aft side of the front support.

Once the pilot is damaged, the support won't sit right. The misalignment will cause vibrations, rubs, and maybe even a bearing failure.

Common Maintenance Events on the Allison 250 C20

By Jim Taylor

February 1998

Inspection and replacement of the case halves
Refer to the maintenance manual for specifics on the inspection criteria and for the proper sequencing of tightening the splitline bolts when removing or replacing the case halves. The torquing procedure can be confusing, and it helps if you get another person to call out the sequence while you do the tightening and loosening of the split line bolts.

The intent of the inspection criteria for the case halves is to make sure they are removed from service when certain conditions exist. If the plastic looks like it is lifting, replace the case halves. A good place to look for lifting plastic is along the split line cross section. Here you can observe how the plastic is maintaining its grip on the parent steel material of the case halves. The plastic usually starts lifting from the bleed band slot. Sometimes little pieces of plastic break out. That's OK provided the pieces aren't coming from the rotor blade path area, and as long as the hole left in the plastic is acceptable per the limits in the maintenance manual.

The key to analyzing eroded plastic is to look for any exposed base metal — if you see any metal exposed around the base or root of the vane, remove the case halves from service.

The plastic that covers the root of the stator vanes serves two purposes: one is to smooth the airflow through the compressor, the other is to protect the root of the stator vane from erosion.

The root of the vane is the most highly stressed area of the vane. Any material missing at the base of the vane could lead to a vane failure. Certain types of rubs in the plastic are permitted as long as they are light and there is no evidence that the plastic is lifting.