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.

                                                                            

Application of Lubricating Greases to Gearing

Methods of application of lubricating greases to gearing will vary with the specific mechanism. As a rule, such products are not used on fast moving gears or on those where considerable cooling is necessary. Lubricating  greases  do have a place  when  gears are used  infrequently  and receive scant  attention, as in home  washing  machines, kitchen mixers, etc. Here the lubricant may be applied during assembly.

Thinking  on the application of  lubricating  greases  to gears may  change  if  the   results reported  by  Gesdorf^21 can be  repeated. As, a trial, all points on a steel mill table were lubricated by spraying a lithium base grease once an hour. The points covered were 38 table roll and line shaft bearings and 27 on open gearing. Previous to the test, 40 pounds of gear lubricant and 2 gallons of bearing oil were used on each 8 hour shift. After installation of the new spray system, 1.6 pounds of lubricating grease were used per eight hour operating turn. Further, it was found that bearing life was increased 40 per cent and it was estimated that gear life was increased 75 per cent.

Rather than make general recommendations, suggestions for applying lubricating greases will be given when lubrication of specific equipment is discussed.

Demands made on gear and transmission lubricants

Mention  of  some  of  the  demands  made  on  gear  and  transmission   lubricants  may  well  serve  as  a  further  justification  for  the  assembly   and   publication  of   the  information   to  follow. Reviewing the problems confronting the automotive industry, Raymond^9  considers  the  hypoid   axle  to  be  the  hardest  working   and  perhaps  the  most  neglected  automotive   component. It  is  said  to  be  far  easier  to  deliver  increased  horsepower  and  torque  to  a  rear  axle   than    to  build  satisfactory  durability  into  such  a   critical  unit  which  is  confined   by  size   and  weight  limitations. Since  higher  engine  horsepower  and  automatic   transmissions  have  greatly  increased   low  gear  loading  and  higher  pinion  offset   has  produced  more  sliding   motion  between   gears, Raymond^9  con-  clues: Lubricants   thus   have  become  a  limiting  factor    in  the  load  carrying   capacity  of  American  axles.

 Automatic   transmission  fluids  offer  another  challenge  to  the  oil  industry   in  that  a  complex  combination   of  requirements   exists   for  such  fluids, and  Raymond^9  believes  that  the  future  appears  to  offer  a  still  greater  challenge  in  severity  of  automotive   transmission  operation.

In  addition  there  is  a  tendency  for  the  use  of  a  single  gear  oil  to  per  from  several  functions. For  example , a  lubricant  may  be  called  upon  not  only  to  protect  bearings  and  gears  against  wear  and   corrosion, but  also  to  act  as  a  hydraulic  medium. Likewise, an  automatic  transmission  fluid, the  primary  purpose  of  which  is  that  of  a  torque  converter, may  also be  called  upon to  lubricate hypoid  axles   in  the  future.

The  severe  conditions  under  which  some  gear  oils  operate  is  stressed   by  Hundere^5   who   states: “ The  most  difficult  function  that  a  gear  lubricant   must   perform  is  that  of  preventing  excessive  metal  to  metal  contact  in  the  region  of  maximum  sliding   velocity. When  it  is  realized   that  the  unit  loading  at  the  point   of  contact  is  as  high  as  400,000  psi  and  the  sliding  velocity   is  as  high  as  6,000   fpm, it is  amazing   that  excessive    wear, let  alone  fusion,  can  be  avoided.’’

It  is  quite   evident  from  the  above  that  constant   improvement  in  qualities  of  gear  lubricants  is  necessary  to  meet  changing   demands. In  spite  of  the  upgrading  of  such  products, they  are  universally  available  at  reasonable  costs. It  is  well  to  keep  in  mind  that, from  an  economic  standpoint, the  benefits  of  correct   lubrication, including  that  of  gear  sets,  are  mainly  due  to  reduction   in  breakdown  or  maintenance   and  consequent   decrease   in  loss  due  to  downtime   or  curtailed   production. This  holds  true  for  both  transportation  and  manufacture. Saving s   in  power  and  frictional   energy,  while   often  attributed   to  correct  lubrication, are  small   compared   with  the  first  benefits   mentioned.