Solids as Gear Lubricants

Dry solids, such as graphite or molybdenum disulfide, have had very limited use as lubricants for gears. More often such solids are used in conjunction with fluids for gear lubrication. Such use is not general and data as to their value is meager.                                                     

                                                                                     
A large gear on a radar  installation was  said  to  be  difficult to  move  until  molybdenum  disulfide was  mixed  with  the oil used. Vago^50 cites  gears operating  in excess  of 200 degree (F)  where  both automotive gear oil and  a molybdenum disulfide  fortified  lubricant were tested. After three years, inspection of the gear teeth indicated no advantage of the latter lubricant over “the chemically fortified gear oils”.

If  solids  are  included  in gear oils, the particle size of the additives  is  of  importance and should  preferably  be of  colloidal size. Thus, Kyropoulos^28 in citing  claims  for a 20 per cent increase in efficiency  with  a  worm gear  unit after  adding  colloidal  graphite, also stated  that powdered  graphite was not  effective. While settling of the solid occurred in this case there was said to be no  clogging of oil ducts.

No mention is made of the proportion of solids  desirable, but one  per  cent  of a  colloidal  suspension, which  in turn  contains 10 per cent  of  solids, is a  normal dosage. This holds true for either graphite or molybdenum disulfide. The latter  material  has  been  recommended where  there  is  impact  loading, or  oscillating or  reversing motion.

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.