Common maintenance events on the Allison 250 C20
Gear spalling can also be caused by poor lubrication and contaminated oil. Gearboxes that have not been through a major repair within 3,500 hours should also have all of the gears sent in for nondestructive testing inspection requirements. This is an Allison requirement that all AMCs adhere to when performing work on high time gearboxes.
A gearbox that has been removed from service and dismantled because of metal contamination should have the oil pump taken apart and inspected for damage. Start by removing the filter housing. Once the nuts and washers have been removed, a soft-faced hammer will be required to knock the filter housing out of the gearbox housing.
The scavenge oil transfer tube to the bottom of the gearbox also has to be removed before taking out the oil pump.
Take out the oil pump attachment bolts, then carefully use a screwdriver to pry under the oil pump, to break the gasket seal. Once the seal is broken the oil pump assembly can be wiggled out by hand. On the bottom of the oil pump there are two Phillips head screws that have to be removed before splitting the oil pump.
To remove the pressure body, hold the pump upside down just above the work bench and gently tap on the N1 tach drive. This will separate the pressure body from the scavenge body.
Turn the pump over and repeat the same process on the brass shaft that is sticking up through the separator plate. This will push off the bottom of the pump, exposing the scavenge gears for the No. 1, 6, 7, and 8 bearings. What you have left is the main scavenge pump body, separator plate, and the two scavenge gears for the gearbox scavenge oil.
To get the separator plate off you will have to use a small screwdriver to walk the plate off of the dowel pins. Notice that there are tangs on either end of the castings for this purpose.
Check the gear pockets for excessive grooving and wear. A slight undercut of the gear pockets is normal provided it is relatively smooth.
The gear end-clearance is measured by using a 0 to 1 inch depth micrometer to measure the difference between the height of the gear vs. the depth of the pocket. Place the original gear in the pocket to be measured and take a drop measurement across the gear pocket to the gear.
Allison recommends this clearance to be within 0.0005 to 0.0015 inch. To reduce the clearance, the pump body has to be lapped. The pressure body is tricky to lapp squarely because the casting for the oil inlet hangs below the split line. Because of this, a proper figure eight pattern (typically desirable for good lapping) can't be done. So take your time and recheck the gear end clearance often. Avoid lapping too much as this may result in you having to scrap the pump body.
Proper gear end-clearance is also important for the scavenge pumps. To lapp the scavenge body you will first have to remove the dowel pins. First note the height of the pin then clamp the pin in a vise with rubberized jaws (unless you want to replace the pins ). Then knock the pump body off of the pin with a soft faced hammer. With the pins removed, the pump body can then be lapped using the proper figure 8 pattern.
To remove the bronze idler shafts, hold the scavenge cover plate upside down and slap it down over the edge of your work bench. Use your free hand to catch the shafts as they fall out of the cover.
You'll probably see fretting on one side of the brass idler shafts. If it's not too deep, you can salvage the shaft by rotating it 180 degrees. The shafts are installed with Loctite® No. 290. Without the Loctite the shafts can float out of their seats during operation, allowing the slots for the antirotation pins to become exposed in the gear pocket. This can degrade the performance of the pump. In the past, this has been attributed to smoking during and after engine shutdown. Do a dry run before applying the loctite. With the shafts installed in the bottom plate, hold the plate up to the light and look from below to make sure the anti rotation slots on the brass shafts are below the surface. If there is a bit of light peeking through, remove the shafts and sand the ends down a little bit and repeat the dry run.
