Gear Box Pressure T53

High gearbox pressure starting with the T53-L-13B resulted from an accessory gearbox housing change that was first introduced in the T53-L-11 being built by KHD (Kloechner Humboldt Deutz) in Germany. An unproven cost reduction removed the bolted in lower power pinion bearing steel retainer in the accessory gearbox. A cast in place mounting for a bearing liner lowered the cost of the accessory gearbox. The gearbox requiring the bolted in Bearing retainer was introduced in 1958 in the T53-L-3 and T53-L-5 engine models. With the exception of the T53-L-1, all of the gears in the accessory gearbox of all of the T53 engine models are common. However with the new casting introduction a spare pad was removed.

No one expected the high gearbox pressure problem that was unwittingly introduced. The excess material that was added in the area of the lower power pinion bearing restricted the oil from getting out of the gearbox. The gears running in oil created foaming of the oil which resulted in the engine scavenge pump not being able to pump the oil and foam from the engine and the breather tube from the air/oil separator in the accessory gearbox would “spit oil and air) out of the gearbox vent. Oil scavenge temperatures would rise from approximately 250 degrees F as high as 300 degrees F. Local area in the vicinity of the air/oil separator gear would exceed 400 degrees F and the paint would discolor and blister. Instead of 0 gearbox pressure, the gearbox pressure would increase as high as 2 psi. The higher engine oil flows like 2900 #/hr would result in even higher gearbox pressure.

The higher gear box pressure causes many problems for the engine. The higher oil temperature causes oil to go acidic with fewer operating hours on the oil. Acidic oil reduces the pressure required to squeeze out the oil film between the teeth of the main reduction gears resulting in heavy gear wear. The intershaft oil seal is designed to release some air in the main reduction gear area raising the pressure in the RGB approximately one psi. It is that pressure that forces the oil from the RGB into the accessory gearbox. The excess material in the gearbox creates flooding in the main reduction (RGB) gear box and the gear losses resulting from the gears running in oil results in a 1% power loss in the engine (a loss of 12 to 14 horsepower). The heat imparted to the oil overloads the aircraft oil cooler and creates problems on hot days.

The high gearbox pressure restricts the oil scavenging from the main bearing packages and that results in the carbon seals running in oil and carbon buildup that restricts the seal elements from sealing properly. The bearings operate at a higher temperature which decreases bearing life. The seal elements in the main shaft seals run hotter, build up carbon and leak oil. The starter shaft seal becomes hard from the increased oil temperature and with the higher pressure leaks oil into the starter generator. The magnesium gearbox cover expands from the increased heat and the cover in line with the washers under the nuts yields. The yielding results in lower torque and oil leakage between the cover and gearbox housing.

Honeywell designed an improved starter shaft seal that requires gearbox pressure to seal. It drastically decreases the starter shaft oil seal leakage problem but does not fix any other engine problems resulting from the high gearbox pressure. The proper fix is to remove the excess material in the gearbox housing. Over the years, there has been at least four different manufacturers of the accessory gearbox housing. All of them have excess material that blocks the oil flow coming through the 6 o,clock strut in the inlet housing and into the gooseneck in the accessory gearbox. Each manufacturer has made the problem worse by taking liberties to beef up the gearbox making it stronger but actually decreasing the life of the engine.

ATE supplied Honeywell with ATE Engineering Orders (approved by the FAA) and Honeywell issued SB 0175 to repair housings having excess material.

There are two other reworks that are required to assure continued operation of the accessory gearbox. The upper flange of the gearbox housing that mates to the inlet housing contains the seat for the bearing retainer that holds the upper power pinion gear bearing. The groove in the housing just below the seat is for the 0’Ring seal. Without the O’Ring in place, the steel bearing retainer should be above the mating flange of the gearbox (to the inlet housing) a minimum of 0.0005 inch and a maximum of 0.0015 inch to assure proper clamping of the power pinion bearing retainer.

Apparently many gearboxes have had the upper flange resurfaced and nothing done to remove the interference when the gearbox is bolted to the inlet housing. The excess load can cause one of the two extensions for the two aft bolts to crack.

Another problem over time is corrosion of the upper seat for the power pinion bearing retainer. The corrosion does not occur equally and has caused cocking of the upper power pinion bearing. The depth and the flatness of the seat is critical. There is a repair that machines the seat flat and parallel to the upper flange. After machining the seat, the flange mating with the inlet housing has to be machined to maintain the bearing retainer seat (0,119 inch). The O’Ring groove has to be machined to it’s proper depth (0.070 to 0.080 inch) to assure that the clamping force of the bolts for holding the gearbox to the inlet housing seats the bearing retainer in it’s seat. An inadequate O’Ring groove depth could prevent the power pinion gear to be in it’s proper location.

Anyone wanting to see a partially reworked gearbox with one half of the gearbox reworked and one half unreworked can visit the Air Technology Engine Booth at the HAI Convention in Dallas.

The picture shows the accessory gearbox housing surface that mates with the inlet housing. The flange, O’Ring groove and the retainer groove for the power pinion bearing retainer. On the left is the reworks and the unreworked portion is on the right.

The 2nd picture shows the inside of the accessory gearbox looking forward. The polished area in the center of the gearbox is where the excessive material was removed. Opening the passageway eliminated the gearbox pressure, stopped the foaming and overheating of the oil, stopped the flooding of the main reduction gear cavity, lowered the main bearing temperatures, improved the main shaft and starter shaft seal lives and allows the oil cooler in the aircraft to operate within it’s design limits.