Deposit Forming Tendencies of Aircraft Turbine Lubricants

This test method describes  a procedure for  determining the  deposit  and  sludge  forming  tendencies  of  aircraft gas  turbine  lubricants when  a sample  of the  oil is circulated  under  controlled conditions for a  prescribed  period of time  through an  aerated test  chamber  containing  an  aluminum  tube  held at a constant  temperature.

The coking tube is held at 590 degree F while oil  heated  to  300 degree F  is circulated by  a pump  from  the  chamber through  a  cooler and a line  filter and  back  into  the chamber. The oil flow is regulated to 300 ml per minute while air flow is the same amount.

At  the  end  of the test, the  weight of solid  decomposition  products  on  the  heated tube  and  in the  line after  are  determined. Also,  changes  in the  viscosity and  neutralization  number  of the oil can be  determined  if  desired.

Corrosion fog cabinet

A number of test  methods, of which  this is one, make  use  of  bench  tests to  indicate  how  well  a lubricant will  protect steel  from rusting. These  methods  are  most  often  used  for  comparatively  low viscosity oils,  such  as  turbine or  aircraft oils.

Cold - rolled sheet steel panels 2 x 4 x 1/8 inch, surface ground to a finish of approximately 20 micro –inches as measured by  a  Brush analyzer, are  used  as  specimens. These are  coated with  the  oil by  dipping  after  which  they  are  held  in a  rotating  table in a  cabinet  into  which  water  is  atomized. The  cabinet  is capable  of  regulation  from  110  to 160  degree F, but  most  tests are  run  at the lower  temperature  for a given  number  of days  or  hours.

The  specimens  are  observed  through  a window  at the  end of the  first  24  hours  and  each  subsequent  24  hours  increment of  exposure. The  time  of  failure  of a  specimen  is  recorded  as  the  day  on which  at least  3 rust  sports 1 millimeter  in  diameter  forms  on the  front  surface  of  the  specimen  in the  central  area  which  excludes  ¼ -inch zones  adjacent  to  the  top  and  sides  and a ½ - inch zone  at the  bottom. Three specimens are coated with particular oil. At  least  two  of  these  must  last  the  specified time  without  failure.

Aircraft Gear lubricating problems

The oils used  to  lubricate  aircraft  turbine  gears  are  almost  without  exception ones  which  conform  to  specifications of military  agencies  of  various nations or  those  issued  by  engine manufacturers. Even though  the oils  have been  approved for use, this  does not necessarily  eliminate  the  chance  for lubricating  problems but  does  reduce the  possibility. If the lubricant is actually at fault, in case of trouble with aircraft gears, the first correction is to use oil from another lot. The oil in question may have oxidized, lost its load carrying ability, or become contaminated. Contamination may arise from unexpected sources. Thus, failure of aircraft engine pinion sets is cited in Lubrication^32 where: “At one engine overhaul shop, engine pinion gear set failure began to occur during green runs”. Finally, the trouble was traced to abrasive material used to clean parts by blasting. Such abrasives were being “built into” the engines at overhaul. Later these abrasives were picked up by the circulating oil on test stands.

The steps by which this problem was solved illustrate the investigation sometimes necessary before the reasons for gear trouble can be pinpointed. To  determine if the  failures were  due  to  overhaul  practice   or  to test stand  operation, pinion  gear  set  which had  operated  satisfactorily  in flight  service for  1,400 hours was  installed in a  freshly overhauled  engine. On the test stand this gear set promptly failed. Abrasive scoring was found on both the gear and the bushings. Therefore, the deposit from the oil screen was examined under a microscope followed by photomicrographs and emission spectrographic analysis. All  of this  pointed  to  alpha aluminum  oxide  as  the  primary  offender, this being  the  material  used to  clean  parts  by  blasting.

     

Fog or Mist Application of Gear Oils

Airborne oil mist or fog is being used to advantage in the lubrication of gear sets. The fog, consisting of oil atomized by compressed air into particles in the order of 2 microns in size, can be carried in pipes for long distances. At the point of application these will have to be reclassified to larger particles so that they will wet a metal surface. The air carrier must be vented from the lubricated part and is said to carry away heat. However, perhaps the heat dissipated will not be as great as in cases where gear oils splash or flow over gear sets.

The oil is used on a once through basis; consequently, clean, unoxidized oil is applied continuously. The amount of lubricant used is quite low, and the gears expend no energy in overcoming fluid friction of gear oils. With such a system, gear boxes are under some pressure; hence, no contaminants, such as dust, will enter.

Fog lubrication is only practical where sufficient points for lubrication justify a centralized system or where compressed air is available for use with a few points. The latter reason, therefore, probably rules out such systems in the case of aircraft and vehicles. If properly  adjusted, definite amounts  of oil  can be applied by  fog, and the system  requires  little  attention,  provided  there  are no air failures.