Polymers as components of gear lubricants

Polymers find application in gear lubricants both as additives and as actual fractions of the base fluids. Specific types of polymers used as additives will be mentioned later. Since the fluids in gear lubricants consist  primarily of mineral  oils  or of  asphaltic materials, any polymers  added  should  be compatible  with  the  major  ingredients. Likewise, polymerization  of  the added compounds  should  be complete  so that  no  further  thickening  occurs  in  service  even  if  the  mixture  is  subjected  to  some  heat. Most  polymeric  materials will  break  down  under  shearing  action  of gear  to  products  of  lower  molecular   weight  and  hence  any  initial  bodying  effect  may  be  partly  lost  after  use.

Although the polymers may be miscible with lubricating oils, special procedures may be necessary to introduce them into gear oils. Thus the grade of  polyisobutylene  which  is used  to  give  tackiness  or stringiness to oils is about  the  nature  of, but tougher than, crepe  rubber. Consequently this polymer is dispersed by a rubber mill in lubricating oil and the solution known as paratac is available to compounders. Also  most  grades of  polyethylene  are  of  such  a  nature that  only  prolonged  heat and  agitation  will  disperse them  in  lubricating oils.

The following polymers have been  suggested or  actually  used as components of  fluids for  gear lubricants: polyisobutylene  of various molecular  weight; styrene-polyisobutylene;  polyethylene ;and resins recovered  from  mineral oils. With the continued  investigation and  production of new  or  modified polymers it  seems  reasonable that  increasing  use  of  such  materials can  be expected in gear  lubricants.  

Desirable low and high temperature qualities of gear oils

Transmission and gear oils which are used either in vehicles or in outside locations in cold climates should remain fluid at low temperatures. Low temperature is a relative term and an extreme case is that of aircraft flying at high altitudes which, therefore, encounter temperatures approaching – 70 degree (F). The gear oils used in this case are compounded from synthetic fluids.

Disregarding synthetic lubricating fluids, two general types of mineral oils are available. These are naphthenic and paraffin. The former, as a rule has the best cold test because no wax is present, but as temperatures are reduced such oils may tend to become quite viscous. This tendency will vary both with the viscosity and the source of the base oil. Most paraffin type oils contain wax which crystallizes at low temperatures and may therefore form a solid mass which will not flow or pump and may even channel in a gear case so that the gear teeth would lack a lubricant. By the addition of pour depressants, generally certain  types of  polymers, the wax crystals are coated  so  that in place  of growing to long  needles  thy deposit as fine crystals which produce a slurry  which moves at a lower temperature  than the untreated oil.

A more positive approach to securing low cold test gear oil is to choose lubricating oil which has been refined so as to have a low pour point. Low viscosity oils generally have lower cold tests than high viscosity oils of the same type. Thus, lubricating oils are available with viscosity of 70 to 80 SUS at 100 degree (F) which have pour points of -60 to -70 degree (F). Proper dewaxing will also lower pour points. In extreme cases gear oils are offered which consist of lubricating oils diluted with fractions boiling below the lubricating fractions. Likewise, in the arctic regions Kerosene has been used to dilute gear oils. This poses a  hazard to the equipment  if the diluted  lubricant  is not removed  from  the  gear  case  with the advent  of normal temperatures.

High temperature limitations of most gear oils will be due to deterioration of the oil and additives. Oils should be chosen having high flash points if operating temperatures are excessive. The flash point of oil is generally indicative of the temperature at which volatility starts. In most industrial gear applications, the bulk oil temperature seldom exceeds 125 to 150 degree (F). In vehicles such temperatures seldom exceed 300 degree (F). Therefore, for  what considered normal  gear lubrication, additives are  selected  with  the  thought  that  temperatures of  use  will  not exceed 300 degree(F)

However, Borsoff et al ^8 describes tests run at both 400 degree (F) and 600 degree(F) on a  gear test  machine in which the gear compartment and the oil were  heated while  the machine  was in operation. Conclusions drawn from this investigation were:

                                   “Due to the thermal instability of the lubricants, the operation of gears for any prolonged period of time at 600 degree (F) or higher is inadvisable; (2) no new or unusual types of failures were observed during the operations of gears at 400degree (F) and 600degree (F). Just as at normal operating temperatures, scoring and abrasion were the two primary destructive failures; (3) the load carrying capacities of oils decreased with the increase in temperature. However, at temperatures over 400degree (F) gum deposits are formed by oils. These deposits serve as a protective coating and improve scoring performance of gears and gear lubricants”. For this reason SAE 20 and SAE 30 mineral oils and “Plexol 201” showed about the same load carrying capacity at 400degree (F) as at 100 degree (F). However, 1010 grade and SAE 60 mineral oils, “Ucons LB-170” and 50-HB-170 and “DC-200” silicone fluid all scored at lower loads at 400degree (F) than at 100 degree (F).

Bowden and Tabor^11 investigated the frictional behavior of oil films on steel surfaces as they were heated and as oxidation took place. At 302 degree (F) after 30 minutes heating the friction became low. At 392 degree (F) the reduction in friction occurred after 15 minutes heating and at 572 degree (F) low friction was observed after less than 2 minutes heating. In the latter case if heating was continued for 20 minutes a thick gum was formed and  friction increased to a high value. The above action was no doubt due to the formation of acidic products due to oxidation. Such products, in time, might lead to corrosive wear. Next the oil would increase in viscosity and compounds would from which are insoluble in the oil. When and if acidic products result from heating of gear oils, metal compounds will eventually from and these in turn will act as catalysts to promote further oxidation.

In view of the fact that high temperatures cause rapid deterioration of most gear lubricants, equipment should be so designed that operating gears are subjected to only moderate temperatures if long life of gear oils is expected.

However, since the military  desire gear lubricants for  jet aircraft  and  other agencies for space vehicles which  will stand  very  high  temperatures, it can be  expected that synthetic fluids or solids will  be  available  which will withstand 700 degree (F) or even 1000 degree(F) for a period.