Thermal Oxidation Stability of Gear Lubricants

This test is used to determine the deterioration of lubricants, particularly EP gear oils, under severe oxidation conditions. The sample  is  placed in a gear case in which a spur gear  set and a test  bearing  operate under load  while  heat  is applied and  air  is  bubbled  through the oil. The temperature of the  lubricant is  maintained at  325 degree F during  the test. The  above apparatus  and  method were  developed at the  Ordnance Fuels and Lubricants Research  Laboratory, Southwest  Research  Institute, and a report  by Meckel^10  summarizes the  contents as follows: “A description  is given of the  apparatus  recommended  and a list of Purchase Materials, Instructions for Cleaning, Assembling and  Disassembling the  Gearcase. Also  Test Procedure  and  Detail Drawings of the  Assembly are included.”

                                                 

Variations in test  conditions and  their effect  on  oxidation of gear  lubricants, using this spur gear thermal oxidation apparatus, were  reported by Meckel and  Quillian^9 and should  be  consulted  if a  program  of  tests on  the  subject are  to  be made. The test in question is particularly valuable  for  use  in developing  oil additive combinations  which  will  retard  oxidation in  service.

During this test, viscosity of the  oil are  determined at  intervals of  10  hours and  the test  is  concluded  when  the  increase  in  viscosity  reaches  a point  called for in the  specifications.

Specification MIL-L-2105B calls  for a test  time of 50  hours, at  which  time  the  increase  in  viscosity shall  be  a  maximum  of 100 per cent. Also, at the  end  of this  period, the  n-pentane  insolubles must  not  be  more  than  3 per cent  by  weight and  the  benzene insolubles  not  more  than  2 per cent  by weight. Further, the  test  method  requires  inspection of the  various  parts, that  is gears, bearings  and  catalyst,  for the  amount  and  type  of  deposits. The  gear  teeth are  inspected  and any  abnormalities of the  surfaces  noted. After  all  deposits are removed  from the  catalyst, this is  weighed  to  determine  the  copper activity  of the  lubricant. The  test  bearing wear is also  determined  with a  special  fixture and  dial  indicator.

                                                                            

Various Fluids as Gear Lubricants

While a  number of  viscous fluids have no  doubt  been used  as  gear lubricants, most of these, other than the  petroleum or synthetic  oils, are  deficient in  desirable  characteristics. However, glycerol has been   suggested as a lubricant and carrier for molybdenum   disulfide. The  specific  application in this case  was  on  small gear sets of  the  worm  and  hypoid  types. Also, molasses was used as a transmission gear lubricant in France during World War II.

The London Transport Company has experimented with   an inhibited castor oil in the axles of certain buses. The thought behind this use was to reduce the fuel consumption of the vehicles. Low viscosity oils or  synthetic fluids were  previously tried but  with  such  lubricants the  necessary  damping  effect was  absent so that  proper shifting was  not  possible. Caution  is given that  this  application of  castor oil is only  possible  under  the  stop and go  operation of busses and  would not  be possible  in over the  road  vehicles. The axle in this case consists of a worm drive with a bronze worm wheel.

Gear pumps depend upon the fluid being handled as the lubricants, and this is sometimes water. Also, water has been employed as lubricant in the case of some nonmetallic gears but has its limitations. First, the temperature range over which water can be used is limited. Next, it will contribute to rusting of the ferrous parts with which it comes in contact unless inhibitors are present. Further, water affords little protection against wear.