Load carrying Capacity SAE Test Apparatus

As previously mentioned, this machine is a device in which two Timken test cups, no. T-48651, are rotated  in line  contact  with  each  other  and  in  opposite  directions, with  provision  for  controlling  the  speed  of rotation, the  slipping  velocity  and  the  rate  of  applying  pressure  at the  line  contact  between  the  rotating  cylinders  or  cups. To obtain reproducible  result, it is  essential  that  the  surface  finish  of the  cups  used  be  uniform  and  that  the  shafts  on  which these  test  pieces  are  carried be in  perfect  alignment. Both points are covered in details of the test methods. The  total  variation  in surface  finish  shall  not  exceed 10  micro inches and  the  inside  and  outside  surfaces  of the  test  cups  shall be  concentric  within  0.0005 inch.

With the  lubricant  tester  in good  mechanical  condition, test  shafts  true, and  the  alignment properly  adjusted, the  test  cups of  specified  surface  finish  and  concentrically  are  placed  on the upper and lower  shafts. With the oil box overflowing, an initial load of 15 to 20 pounds is applied. The  machine  is  then  started and  at the  end of a  30 second  period   the  automatic  loading  device, at  the desired  rate  of loading , is  started. The test cups are then observed for signs of scoring. This  is most  readily  detected  by  observing  the  lower  test  cup  on the  trailing  side  at a  position  approximately ½  inch  from  the  contact  line. When  signs of  scoring  are  detected, the  drive  motors  are  stopped  and the load  removed. After  thorough  cleaning  and  with  new  test  pieces and  fresh  oil  the test  can  be  repeated.

The suggested conditions of test are a main shaft speed of 1000 rpm and a rubbing ratio of 14:6:1. The  load should  not exceed 450 pounds and  if no  scoring  occurs  at this point  the test  is stopped to  prevent  over heating  of the  shafts, etc.

This test  has been  primarily  used for evaluation of automotive  gear  lubricants  but has not  replaced  tests  with gears.    

                                                                                                                                                                                                              

Freedom from separation in gear oils

Precipitation or settling of some components in gear oils is sometimes noted. This most often occurs in mixtures containing EP additives. The separation may be due to lack of solubility or to reaction of ingredients resulting in formation of sludge. Since such additives are often present in concentrations of 9 per cent or more, the base oil must keep a high proportion of heavy chemical compounds in dispersion or suspension. Both additive manufacturers and oil blenders select ingredients which will keep any separation of such agents at a minimum.  

Fletcher^23 selected three SAE 90 hypoid gear lubricants and three multipurpose SAE 90 oils meeting MIL-L-002105A specification. By precipitation tests, the first three oils showed some sludge in the unused oil which increased after a service test in two of the lubricants. In the multi-purpose oils there was only a trace of sludge before use but measurable amounts up to 7 per cent after the tests.

Of course, settling or sludge formation in gear lubricants results in loss from the action zone of valuable active ingredients, but the greatest concern is the effect sludge may have on operating mechanisms. Thus, Fletcher^23 mentions that precipitation of sludge out of oil due to centrifuging in transmission cases may result in carbon like deposits in pocket bearing positions, internal clutch teeth, and in some cases in oil grooves  and synchronizer grooves. It is conceivable that such deposits could adversely affect the operation of the unit. This fact was probably recognized by one tractor manufacturer who specified that oils used in their equipment should be filterable, thus, indicating that sludge should not separate during  normal operation.

Where sludge is formed in EP gear oils the action is accelerated by increasing temperature. It is therefore probably a result of reaction of the chemical compounds which constitute the EP additives. Detergent agents do not seem to be a correction for such sludge separation, and any improvement in the condition probably lies in selection of the EP additives.

Designation and recommendation of gear lubricants

While  industrial  gear  lubricants  are  often  marketed  under  trade  names, information  is  always  available  as  to  how  these  products  conform  to  AGMA  numbers . Therefore, little   difficulty occurs   with the designation of this time of gear oils.

However, in  the  case  of automotive  transmission  and  axle  types  of  oils the  marketing  designation  has  not  always  been  clear  and is, in fact, sometimes  misleading. The SAE  Handbook, in  about  1924, started  to  include SAE  viscosity  numbers  for  such  lubricants, and when , in 1951, API approved  definitions  for ‘Regular-Type Gear  Lubricant,’’ ‘Mild  Type  EP  Gear  Lubricant,’’ and  ‘Multipurpose Type  Gear  Lubricant’’ these  terms  were  suggested  for  use  by  SAE. With the  availability  of   improved  EP  gear  oils, as  mentioned  above,  the  API   Lubricating  Committee, in 1957,agreed  that  such  products  should  be  known  as  ‘Multipurpose  Type  Gear  Lubricants (API  Service  GL 4).” Such oils were described as follows:

“This  term  designates  Lubricants  which  have  adequate  load  carrying  ability  and  other   required  properties  to protect  hypoid  gears  in  sustained  high   speed  and /or  high  torque  service  in  modern  high  powered  passenger  cars  and  trucks.  Also  suitable   for  use  in  spiral   bevel  gears, many  transmissions, and  worm  gears  in  some  types  of  service.  Such Lubricants are identified as meeting ‘A.P.I. Service GL4’.’’

The  consumer   of  gear  lubricants  should  bear  in  mind  that   such  products  are  like  other  commodities, in  that , while  a  conscious  organization  may  set  tolerances  for  material   to  be  supplied  under  a  given  designation, some  products  offered  will  barely  meet  the  required  specifications. Therefore, the  purchaser  or  user  of  gear  oils  should   deal  only  with  reputable  firms.