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, and the gearbox.

Compressor Maintenance
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.

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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.

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.

Rubs between the stator vanes and the compressor rotor, however, are not allowed. If rubs are noticed in this area, pull the compressor from service immediately. Rubs on the rotor are most commonly caused either by corrosion between the stator vane band assemblies and the case halves skins which results in the stator band lifting into the rotor. They can also be caused from misalignment of the case halves during installation. Any signs of blueing associated with rubbing is quite serious.

Blueing indicates that the material affected has been stressed by the heat of the friction. The effects of this friction usually penetrates quite deeply. The slightest tint of blue on the surface can be seen on the inside of the wheel as well, and if left unchanged, can quickly develop into a hole. The result will be an under-powered engine.

Any time you have a compressor module off, give it a spin and listen to it for rubs. Any ticking, or rub sounds should be investigated by removing a case halve and having a look to see what's going on. To install the compressor, the turbine must be off. This allows you to use the 2 1/2 bearing guide. Also with the turbine off, this gives you an important opportunity to have a better look at the 2 1/2 bearing. Check each roller for spalling, dents or flat spots. If there are any, reject the bearing.

Also, carefully inspect the roller retainer. If the rollers are loose and they look as if they could fall out, replace the bearing. Prior to installing the compressor, pack the bearing with petrolatum.

Before removing the compressor it's a good idea to count the number of shims under each of the compressor mount legs. This will be a great help when it comes time to reinstall the compressor. When mating the compressor with the gearbox, keep the compressor rotor turning, and seat the assembly onto the gearbox by hand.

Don't draw it up with the mounting bolts. It's possible to bend the first stage reduction gear. This happens when the teeth of the spur adapter are stacked, instead of engaged with the spur teeth of the fuel control, oil pump, and idler gear.

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Turbine Module Maintenance
Tips A hot end inspection could be required if the turbine outlet temperature (TOT) limits are exceeded during a start, afterfire, power transient, or a one-engine inoperative event.

Before going to the Allison maintenance manual to look up the specific temperature limits, be sure to obtain the most accurate information available for the highest temperatures observed and for how long the temperature was above the suspected limit.

Vague data should force you to assume a worst case scenario. Once you've reviewed your data, check it out with the limits in the Allison maintenance manual.

In the event a hot end is required, the first items to remove for a turbine inspection are the outer combustion case and the discharge tubes. These items are "on condition." They should be given a close inspection every time they're removed.

Some of the key items to look for during the hot end inspection are:

Combustion Liner — Check it for cracks, localized bulges and warpage. Check for the incorporation of CEB1299, which calls out for the addition of weld beads all around the fuel nozzle and ignitor ferrules (which are located on the aft dome section of the combustion liner). This CEB has been successful about 98 percent of the time in stopping the formation of "carbon clinkers" which can form in the dome section of the liner. If you discover or know that you have these "carbon clinkers" or "carbon fingers" forming in your liner, do yourself a favor — take steps to stop them from forming. These carbon deposits form and break off quickly and continuously. As the deposits fly into the turbine, the impact erodes the first stage wheel causing loss of power, compressor surge, and expensive repair bills. Bulges and warpage are usually indications that there is streaking in the fuel nozzle.

Fuel Nozzle — Check it by first removing it from the combustion can. It's much easier to remove the fuel nozzle when the outer combustion can is still attached to the turbine module.

As the wrenching flats are relatively thin on the fuel nozzle, it's a good idea to grind off the lead-in chamfers from your socket or wrench so there will be less chance of rounding off the corners of the fuel nozzle.

With the fuel nozzle removed, hook up the P3 fuel line again and hang the nozzle so it will spray into a large mason jar. Make sure the ignition is off, then motor the engine with the throttle open. Observe the spray pattern. If you sea any streaks or voids, replace the fuel nozzle.

First Stage Nozzle Shield — The shield must be removed from the turbine. This is essential not only to better inspect the nozzle shield it self, but it also allows a much better view of the first stage nozzle and the first stage wheel and blade path.

It's normal to see wavy edges on the deflector which is welded on to the dome. What you don't want to see however are cracks and burnt edges. CEB-A-1341 applies to the first stage nozzle shield, and advises operators of the possibility of having an incorrectly manufactured nozzle shield made with a dome of the stainless 310 material — the wrong material. Most Allison Maintenance Centers (AMC) have an acid test kit available to confirm which type of material of the dome on your nozzle shield.

First Stage Nozzle — With the first stage nozzle shield removed, you can now have a good look at the first stage nozzle. Use good lighting and a 6-inch steel rule to help measure any of the cracks you might see. Inspect each vane and measure and record on a piece of paper the number and length of any cracks.

Check also for erosion, convergent cracks, V-type notching, and pieces missing from the both the leading and trailing edges. The two rings of material that hold the vanes in place are called the inner and outer bands. These require a detailed inspection as well.

Pay particular attention to cracks that go into the "saddle areas" which are the vanes that wrap around the support struts of the G/P ( gas producer ) support. Axial inner band cracks can sometimes extend into the diaphragm and then run circumferential. Circumferential cracks in the diaphragm can cause excessive power loss. Again, take good notes, so that when you check the limits in the maintenance manual you can make an intelligent decision on whether to change the nozzle.

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Special tools required to change the 1st stage nozzle:
1) Fixture - holds No. 8 sump nut socket,
2) No. 8 bearing puller,
3) Slide hammer for oil jet,
4) No. 8 press and adapters,
5) Scrap pea shooter and sags — to hold N1,
6) No. 8 sump socket,
7) No. 8 bearing nut socket align tool,
8) No. 8 bearing nut socket

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Blade Path Area — The first stage blade path area is an integral part of the second nozzle. In performing a good hot end inspection, this area must receive a thorough inspection for blade tip rub and evidence of metal transfer from the blade tips of the first stage turbine wheel.

Rubs in the blade path area are not allowed and are an indication that a serious overtemp has occurred or that there is misalignment between the GP & PT supports of the turbine — both of which are serious enough to justify removing the turbine and sending it to an Allison AMC for repair or overhaul.

Turbine Blades — The turbine blade tips are another important area to inspect. Blade tips that are in good shape have square sharp corners and the tops are flat, smooth, and square to the blade path. Blade tips that have a ragged edge may have rubbed as a result of an overtemp.

Blade tips that have rounded corners or a taper along the tops of the blades are showing signs of erosion and wear. These conditions will usually be accompanied with low power and maybe even compressor surge.

After completing a hot end inspection and reviewing your data with the limits in the maintenance manual, you have three options: if there is no damage found, and if the overtemp didn't exceed prescribed limits, the turbine can be returned to service. If there's an excessive amount of damage, the turbine has to be sent to an AMC for repair or overhaul. If there's damage, and it's limited to the first stage nozzle, you can change the nozzle.

Replacing the first stage nozzle — Replacement of the first stage nozzle shouldn't be attempted unless you've had specialized training or the assistance of someone who is very familiar with the procedure. Many AMCs provide on-site technical support for this very reason. Give your preferred AMC a call, and perhaps they could send out a service representative to help. Also, the AMC should lend you the special tools required to change the first stage nozzle.

Here are some points to remember while performing a nozzle change:

• AlliedSignal fuel controls have to be removed before taking off the turbine module from the engine assembly. However, CECO fuel controls can remain in place.
•As the turbine comes off the engine, take care not to damage or drop the N1 and N2 coupling shafts.
•After the turbine is removed, check the gearbox for the following:

— If the torquemeter nut on the gearbox cover is leaking, the gearbox should be removed and repaired.
— Surface "G" or turbine mounting surface shouldn't be showing any signs of lifting or missing material.
— Take a good look at the No. 4 ball bearing for nicked or spalled balls. With a flashlight look inside the pinion gear and check the No. 2 1/2 bearing to make sure all the rollers are still round, and turning smoothly when the N1 gear train is turned.

For dismantling the turbine itself here are few more pointers: if the oil pressure tube that delivers oil to the No. 8 bearing oil jet is removed, it must be reinstalled prior to installing the No. 8 bearing oil jet.

Before the No. 8 bearing oil jet is installed, the oil tube must be properly in place and then blown out before installing the oil jet. Once the oil jet is installed, pump some turbine oil through the pressure tube and oil jet to make sure it's not plugged and that it's targeted to supply a stream of oil right into the balls of the No. 8 bearing.

Following these steps will help to make sure there is no carbon blocking the entrance to the oil jet. This is very important because if this oil jet gets plugged up, the No. 8 bearing will fail shortly afterwards. Remember the nut for the No. 8 bearing has a left-hand thread.

With the aft side of the first stage wheel exposed, inspect the rim for cracks. Also, give the first stage blade path a close look, checking for lifting and or missing pieces of blade path material. Once you have the first stage nozzle in your hands, review your inspection notes and the repairable limits in the maintenance manual.

As a general rule pieces missing from more than seven vanes or cracks through the saddle area are not repairable. To avoid getting an expensive bill for a "core charge," talk to the AMC your buying the replacement nozzle from and make sure your nozzle core will accepted as an exchange unit.

Assembly Tips — The turbine goes back together much the same way it came apart. Here are a few more tips to help you out:

• Use petrolatum on the fiberglass packing to help keep them in place during assembly • Remember to safety the No. 8 bearing nut by deforming the locking ring into one of the notches of the stub shaft in the first stage wheel, (not the notches in the tie-bolt nut). Use a pin punch with the end rounded off to perform this task.
• Follow the steps in the maintenance manual for torquing the No. 8 bearing sump nut.
• To help get a good seal between the sump nut and the GP support, make sure the u-ring groove is spotlessly clean.
• Lap the sealing surface of the sump nut and use a lot of oil on the threads and sealing surfaces.

Common Maintenance Events on the Allison 250 C20

By Jim Taylor

February 1998

Gearbox maintenance tips
The purpose of this next portion is to give the reader some tips and advice on dismantling, inspecting, and reassembling an Allison 250 C20B gearbox. Again, if you don't have a maintenance manual, please call the Allison Access Center (phone number located at the end of this article) to arrange the purchase of an up-to-date manual.

Some of the reasons why you would want to split an Allison 250 C20B gearbox include:

— The torquemeter insert in the gearbox cover could be leaking.
— The threads for the oil fittings could be stripped.
— A suspect oil pump is causing low oil pressure.
— Through troubleshooting, the gearbox has been determined to be the source of metal generation.

A gearbox that is generating metal would most likely have metal on both the upper and lower chip detector. Scavenge oil from the gearbox also includes scavenge oil from the No. 5 bearing in the turbine and the No. 2 bearing in the compressor. This is important to note because before the gearbox is dismantled, the No. 5 and 2 bearings must be eliminated as possible sources of metal generation. So take your time, use a strong light and a 10-power magnifying glass, and thoroughly inspect both the No. 5 & No. 2 bearing areas.

Once the turbine and the compressor have been eliminated, the search for the source of metal can be focused on the gearbox.

To split a Allison 250 C20 gearbox, the gearbox cover or aft portion is lifted off of the housing or front half. Place the gearbox on a flat, clean work bench with a lot of room. The chip detectors should already have been removed and as much oil as possible should have been drained.

Start by removing all of the nuts and bolts on the split-line. Remove the washers because they sometimes jam in the threads of the studs and create a hang up when splitting the cover from the housing. With all the split-line hardware removed, and the gearbox lying horizontally and with the cover side up, use a soft-faced hammer from below to carefully tap upward the top mount for the turbine. At the same time, use your free hand to lift the gearbox cover from the boss for the bottom chip detector.

Tap the cover a few times at this location and then alternate the tapping on the boss for the chip detector — do this while lifting the cover from the top mount for the turbine and listen for a change in the sound of the gearbox. Once the cover has split from the housing, the sound of the knocking will go hollow.

At this point the cover should come off the rest of the way by hand force alone. If the cover does not separate completely, push down on the power turbine governor gear with the handle portion of a screwdriver. Also, peek through the gap in the split-line around half way up the gearbox on the right side near the oil pump and make sure the first reduction idler gear in the N1 gear train is not lifting off of the oil pump. If it is, carefully use a piece of wood to pry it back down. Be aware that most of the gears come off with the cover, so it will seem a bit heavy.

To dismantle the cover start with the N2 gear train, rest the cover on the gears and take some precautions to protect the gears and bearings. In the middle of the cover, on the aft side, you'll find the torquemeter nut. First, use a punch and a hammer to unlock the torquemeter lock cup by displacing the material out of the notch. With the torquemeter socket Allison P/N 6795597 and a 1/2 inch drive breaker bar, undo the torquemeter nut.

This nut is torqued to 430 inch pounds of torque at assembly. If you have an oil leak from the torquemeter nut, consider undoing the torquemeter nut with a torque wrench. If there is hardly any torque left, there is a good chance that there is considerable wear in the torquemeter bore area of the gear box cover.

Once the torquemeter nut is removed, don't turn the N2 gear train or move the gearbox cover around. A torquemeter thrust washer is located between the torquemeter roller bearing and the gearbox cover. The thrust washer has a key-way notch in it that allows the antirotation pin in the support shaft to pass through. By inadvertently spinning the gears or by moving the cover around, the thrust washer could spin and lock the support shaft into the cover.

If the torquemeter support shaft does not come out with a quick smart blow using a soft-faced hammer, chances are the trust washer has spun and you'll now have to spend some time blind fiddling to get the key-way and the pin lined up again.

Usually the support shaft comes out ant you catch it in your hand. At that point it will feel like the torquemeter gear is free and in your hand as well. Pull your hand out from underneath the cover along with the torquemeter gear and the support shaft.

Now, turn the gear box cover over, and remove the PTO gear. To remove the pinion gear turn the cover again and remove the oil nozzle for the No. 4 and 5 bearings. If there are roller bearings in the 3 & 4 positions already, you'll have to remove the circlip as well.

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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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.

Common Maintenance Events on the Allison 250 C20

By Jim Taylor

February 1998

Before flushing out the main oil gallery in the gearbox, remove the torquemeter support shaft, the two oil jets, and the return gravity check valve which is located under the filter housing. With the gearbox totally dismantled review the condition of the gearbox cover and housing to determine if you should send them out for rework, or purchase an exchange unit. Things to consider on the cover are:

• Worn 3 & 4 bearing cage, or upgrading to 3 & 4 roller bearings.
• Worn turbine mounting surface known as "surface G."
• Loose or worn bearing cages.
• Incorporation of CEB-A-1226 (through-bolt retention of the idler gears).
• Helicoils® required for the turbine oil scavenge ports.
• Leaking or worn torquemeter insert.
• Bearing bore position suspected due to poor wear pattern on gear teeth.

Items to consider on the gearbox housing are:

• Loose compressor mounting inserts. Note: more than two loose inserts requires the housing be sent to an AMC to have all of the compressor mount inserts checked for being square to the compressor axis.
• Incorporation or CEB-A-1226 (through-bolt retention of the idler gears).
• Worn compressor pilot diameter, causes misalignment of the compressor, resulting in possible high vibrations and an oil leak from the compressor packing.
• Installation of helicoils® in the oil fitting ports.
•Bearing bore position suspected due to poor wear pattern on gear teeth.

The above items should be discussed with your AMC to help you decide what is the best method of accomplishing the repairs.

Assembly tips for the gearbox housing — Keep in mind that these tips are not step-by-step instructions on how to put a gearbox together — you must still refer to the Allison maintenance manual for complete instructions.

A good maintenance practice which can give you piece of mind and help you sleep at night is to save all of the removed packings. Then take all of the empty packages for the new packings installed and do a comparison of the packings removed versus the ones installed. If they don't match, investigate the problem.

Install and lockwire the oil jets in the gearbox housing immediately while there's adequate room in the housing. Once the oil pump and filter housing are installed, it's a good idea to arrange for oil to be supplied to the oil inlet, and then turn the pump over by hand.

Listen for the return gravity check valve to pop. Then turn the housing back over and make sure there are no oil leaks from the pressure oil system.

Once the housing is fully reassembled, install the following items to aid in the mating of the gearbox housing, with the gearbox cover:

• Place the muff spline on top of the oil pump (mates with the fuel control gear).
• Place the first stage reduction N1 reduction gear on top of the oil pump.
• Install the three oil transfer tubes beside the oil pump, and the short oil transfer tube into the gearbox scavenge oil transfer tube at the bottom of the gearbox.
•Install the N2 tach & PTG drive gear into the housing.

Finally apply petrolatum to the rollers of the PTO roller bearing. This is to keep the rollers from falling inwards when installing the cover on the housing.

Assembly tips for the gearbox cover —The gearbox cover is best reassembled if you first reinstall all of the N1 gear train. Remember to install the garlock seal for the air/oil separator gear and the circlip that goes in between the seal and the air/oil separator gear.

The fuel control gearshaft is best installed with the cover lying horizontally. Use the specially designed pliers P/N 6796966. As the gear is lowered into position, bend down to the bench level and try to peek in from side to see when the circlip is at the circlip groove. To test the circlip for proper engagement, move one of the tangs from left to right. If the circlip is properly installed the other tang will follow the movement.

The pinion gear is to be the first N2 gear to be installed, followed by the PTO gear. Have the cover sit as level and as steady as possible. Then place the torquemeter thrust washer and bearing thrust plate so that the cut-outs line up with the keyway in the torquemeter insert. Use petrolatum to help keep the pieces in place.

Carefully maneuver the torquemeter gear into position while holding the torquemeter roller bearing in the gear. The idea is to place gear and the bearing into position without moving or disturbing the torquemeter thrust plate and thrust washer.

Now you can push the torquemeter shaft support through the roller bearing and into the torquemeter insert in the gearbox cover. Install the torquemeter nut and lockcup and just snug it up at this time. This will help in the assembly of the gearbox cover onto the housing — it's a little more comfortable torquing up the nut when there is a whole gearbox to hang on to.

Mating the cover and housing — It's advisable to use one of the approved sealants on the splitline to help avoid oil leaks from this area. However use it sparingly, as the excess will be squeezed into the gearbox where it will get into the oil system and eventually end up in either the scavenge oil filter or the main engine oil filter.

After reviewing your work, making sure all the safety wire has been done, and that all the pieces are in place. Place the housing in front of you so that the oil pump side is closest to you.

This is important because you'll want to use the three long studs located beside the oil pump on the splitline as guides for lining up the cover.

Lift the cover up over the housing and while looking through the holes for the three long studs, line up the studs and lower the cover on to the housing. You will have to do a bit of jiggling, however, none of the gear trains will be in a position to mesh until the cover and housing are less than a 1/4-inch apart. Also, before the gears mesh, all of the oil transfer tubes have to be engaged. So don't try turning any of the gear trains, until the tubes are in place.

Quite often you must use a hooked pick or a small screwdriver to help line up the three oil transfer tubes located beside the oil pump. Once the tubes are fully engaged, and if your packings have been well lubricated with petrolatum and oil, the cover can be pushed on the remainder of the way — be sure to do this while turning the geartrains to ensure they mesh.

Very little pounding with a soft face hammer should be required at this point. Once all of the splitline hardware has been reinstalled and torqued, proceed to installing the garlock seals.

Use only engine oil to lubricate the lip surface of a garlock seal. During installation of a garlock seal it is normal that a small ring or orange seal material be scraped off of the outer portion of the seal. This is not a cause to reject the seal.

As a final note remember that this information is meant to assist the reader in performing certain maintenance tasks on the Allison Model 250 C20 series engines. It is not intended to replace the Operation and Maintenance manual. When performing any maintenance on Allison 250 series engines always an up-to-date current manual. Compare the revision status of the manual you intend to use with the revision status of the manuals listed in every CEB index.

Should you find your manual out of date contact the Allison Access Center at 1-888-255-4766 and order a new up-to-date manual or the required revisions. Also should you discover a contradiction between this article and the Operation and Maintenance manual, use the info in the manual.

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