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.

                                                                            

Types of gears to be lubricated

Where   gears  are  on  parallel  axes, either  spur  or  helical  gears  are  generally  employed. Either type can be used as external or internal drives.   The  herringbone  gear  is  similar  to  two  helical   gears  having  reversed   directions   of   spiral, placed  side  by   side  so  that  the  teeth  come  together   to  form  a  chevron   pattern. The rack and pinion, used to convert rotary motion to reciprocating, generally   uses a spur gear.

For   intersecting   axes either straight   bevel    or   spiral   bevel   gears   are used   as a  rule. The   latter type may be used   on angle   drives where   the   shafts    do not    intersect at full   90 degrees. The contact   of the   teeth in such gears gives   a rolling motion. With  non intersecting  and   nonparallel  axes  the  types   of   gears  used  are  crossed   helical, single  enveloping  worm, double  enveloping    worm, or  hypoid. Here the  contact  of  the  teeth  gives  a  sliding  as  well  as  a  rolling   motion. In  most  cases  a  gear  set  will  be  used  to  change  speed, and  in  such  cases  the  smaller   gear  is  designated  as  the pinion. Both  the  number   of  teeth  on a  pinion  and  the  ratio    of  the  teeth  on  the  driving   and  driven   member   may  vary ,   but  with  bevel  gears  there  is  seldom  less  than  12 teeth  to  a  pinion.

While   some  spur  and  straight  bevel  gears  are  still   made  of  cast  iron, the  tendency  in  all   types   of  gearing  is  for  the  use of  steel. Exceptions   will  be found  to  such  practice, for  example,  in the   use  of  bronze  for   one  member   of  worm   gears. Some small gears and even   larger pinions are   made   of plastics, such as ‘Delrin,’’ ‘Nylon.’’ ‘Teflon,’’ etc. Pinions  have been and   may still be  made  of  rawhide, pressed  paper,  etc, but  our  concern  is  primarily   with   lubrication  of  metal  gears.