Common maintenance events on the Allison 250 C20

Common Maintenance Events on the Allison 250 C20 By Jim Taylor February 1998 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...

Turn the cover over again and push the pinion gear and the bearings out. If there were ball bearings in the 3 and 4 positions, there might be grooves worn into the cage due to the outer races of the bearings spinning. This will make it difficult for the gear to come out. In this case, a little bit of force might be required. Be careful not to damage the No. 2 1/2 bearing in this process. Plan to replace the No. 2 1/2 bearing if you suspect that it's received any amount of direct force while removing the pinion gear.

If there are roller bearings installed, the pinion gear should come out more easily. However, be careful not to have the roller bearings fall apart in your hands! Turn the cover over again and remove the N1 gear train. The special pliers that Allison designed (P/N 6796966) are really helpful when removing and installing the fuel control drive gear.

A good technique for this gear is to stand the cover on its side with the fuel control gear at the top. With the special pliers in your left hand, you can actually see the tangs of the circlip, so it's easier to get the pliers on them. Also, with the cover on its side you don't have to overcome the weight of the gear, and it won't slip back into place before you let go of the pliers.

Remember that you only have to use the pliers until the gear has moved forward enough so that the circlip is away from the circlip groove. Keep in mind that the bearings on the FCU gear could have worn grooves into the bearing cage from spinning. This will mean that a bit of force might be required to remove the gear.

With all of the gears removed from the cover, carefully inspect each ball of each bearing. The torquemeter roller bearing can come apart, so hold the outer race and gently tap the assembly on the bench to encourage the rollers to fall inwards as much as possible.

Once separated, make sure the rollers don't lose their position or orientation. All of the bearings can be given a good visual inspection while installed on their shafts.

Common Maintenance Events on the Allison 250 C20

By Jim Taylor

February 1998

To remove bearings, it's always best to use the correct Allison special tooling. Used properly, the Allison tools will reduce the risk of damaging the bearings or the gearshafts that they're installed on. Most AMCs will rent or lend out special tools to their customers. Should you find yourself stuck, you can get by with a standard clam-shell puller and two pieces of angle iron. Where the gear diameters are larger than the bearing, or the clamshell puller, use the angle iron and some blocks to build a support bridge around the gear while pressing off the bearing.

It's most important to make sure the puller is only contacting the inner race of the bearing to be removed. Scrap the bearing if you think any force has been inadvertently put on the outer race or the balls of the bearing during its removal.

Avoid hydraulic presses. A hand arbor press should provide all of the force required to remove any gearbox bearing on an Allison 250 series engine. On the fuel control, fuel pump, starter generator, and spare accessory drive, remember to remove the spiral circlip, before trying to press off the bearings. Slide the tail of the circlip around to the cut out section, then lift it up with a pick, and pull out the circlip with needle nose pliers.

To inspect gears, you need a 10X power magnifying glass and a good light. Look for indications that the case hardening has been cracked or chipped. Case hardening is a process that gears go through during manufacturing to make the outer case of the gear a lot harder than the inside. Gear spalling happens when a localized wear spot has worn or broken through the hardened outer shell. Once the outer shell has been penetrated, wear will accelerate dramatically and produce tremendous amounts of metal — usually in the form of ragged gray pieces of ferrous metal.

Spalling is usually a result of the gears being misaligned. This results in the gear teeth not meshing properly. Look at the wear patter, on the gear teeth — ideally there should be even wear all across the contact surface of each tooth. Heavier wear or a larger wear spot at one end of the gear teeth is an indication that the gears are misaligned. To correct misalignment, the gearbox cover and housing should be sent to an authorized AMC to have an alignment check done on the bearing bores.


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