Transaxle Fluids

While a single lubricant for both transmissions and rear axles of certain automobiles is desirable, Hunsted et al.^28 ,when reviewing the situation with regard  to such fluids, concluded that as of January 1960 considerable  work  remained  to be done on transaxle fluids to achieve optimum results. Up to that  time  some  42 mineral  oil base and  synthetic base  fluids, with  various  additives, had  been  evaluated and only  three showed  any  promise  for  future  development. Since  it is  impractical  to  suggest even  tentative  formulations, all that  can be  done is to  indicate  the  requirements and  the handicaps.

A  transaxle  fluid  should  have  all  the  desirable  characteristics  of ATF and  in  addition  should  have  sufficient  EP value  to  lubricate  hypoid  gears. The above statement should be modified in that the desirable characteristics of ATF should also be maintained while in service. Thus, when  the  fluid is subjected to the  shear  effect of mating  gears, polymer type  additives may  lose  a large  part  of their  effectiveness  due  to  breakdown  of the  polymers to  lower  molecular weight  products. Therefore  base  fluids with  inherent  high  V.I. or  polymers of  initially  lower molecular  weight  are  dictated  in the  formulations.

EP agents  used  should  have no  adverse  effect  on any  of the  components of the  automatic transmission such  as  clutch  materials, various   metals, synthetic seals, etc. Of  course the most desirable  transaxle fluid  would  be  one  which  could  be  sealed  in a  case  and serve the  life  of the  car  but  first  a  fluid which  will perform  satisfactorily when  changed  periodically  will serve.

According to  Celanese  a  trimethylolpropane  ester  with  an EP additive  offers promise  for use  as a transaxle  fluid. Another suggestion for the purpose is methylchlorophenyl containing about 3 per cent chloride.

Stringiness Agents

Where it  is desirable  to  increase  the  adhesiveness or  tackiness of gear  oils, high  molecular  weight  hydrocarbon  polymers or  copolymers, such  as Paratac  are  added. As little as 0.01 per cent of this compound is noticeable. Solutions of natural  crepe  rubber in oil give much the same  effect but  lose  their  tacky character upon  storage.

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.  

Viscosity Temperature Characteristics of Gear Lubricants

Since gear and transmission oils are subjected to widely varying temperatures, particularly in vehicles, it is desirable that such lubricants have as little change in viscosity with temperature changes as is possible. In other words, high V.I. oils are desirable in many gear oil applications and absolutely necessary in certain cases. An illustration of the latter is automatic transmission fluid. Base oils with V.I. values of 90 to 95 are readily available when needed. Where it is desired to increase the V.I. of oil, additives, known as V.I. improvers, are sometimes used. These agents are generally  polymers  which act by either coiling  up or  becoming less  soluble  at  low temperatures but reverse  this action as temperatures  increase,  so as to  contribute increasing body at higher temperatures. In normal gear  operation the shearing  effect  tends to break  down such polymers  into those  of lower molecular weight  which  are  less  effective  as V.I. improvers. However, these agents do have a place in services such as ATF where they maintain their effectiveness.

Certain  high molecular weight  petroleum resins  have been found to improve  the V.I. of some lubricating  oils  and  not  have  the defect  of  breakdown with  shear.

Shear Stability of Gear and Transmission Lubricants

While most mechanisms containing gears will tolerate a considerable variation in viscosity of the gear oil used for lubrication, a radical change in viscosity at a stated temperature, while in use is not desirable. Further, if such a change is due to a partial breakdown of an additive the purpose of the agent may be defeated. Such changes may occur due to shear while in service. The components most often affected are polymers such as V.I. improvers.

At present, products used in vehicles are the lubricants most often influenced by shear. Such changes will become increasingly important as a single fluid is used for several purposes such as a hydraulic fluid, for ATF, and perhaps as an axle lubricant. Further, a multirange gear and transmission oil has advantage in cars and trucks. That is, oil which will cover two or three SAE viscosity ranges.

Under present formulations some of the lubricants offered for the above services contain polymers as V.I. improvers. However, the action of gears or even pumps tends to change the polymers by shear. The shearing action causes either a chemical or mechanical breakdown of the large polymer molecules so that their value is largely lost in the oil. In some cases a viscosity decrease in service is temporary and in such instances there may be an alignment of the polymer molecules at high rates of shear. Of course some polymers or additives are more resistant to break down with shear than others but unfortunately those of high molecular weight, which prove the most effective V.I. improvers, are also most susceptible to loss of viscosity with shear.

Where high V.I. is necessary or desirable in gear oils, tests should be made to determine the viscosity after shear tests. This is most often done by using a test where a pump forces the lubricant through a sharp edge orifice for a stated time at a given temperature. A similar breakdown of polymers occurs with sonic shear, and a method using this procedure has also been used to evaluate the shear resistance of V.I. improvers.

Klaus and Fenske^34 tested fluids containing about 7 per cent of polymer for their permanent decrease in viscosity due to shear. After 5000 cycles in a pump at 100 degree (F) and a pressure drop of 800 psi, decreases of 25.5 to 30.5 per cent occurred. At a pressure drop of 1500 psi, the decreases were 38.5 and 40.5 per cent. The time required to stabilize viscosity will vary both with the mechanism and the fluid used.