Power Steering Gear Fluids

As a base for such a fluids, a solvent refined oil of about 250 viscosity SUS at 100 degree F may be used. While , as Johnson and Mortensen state, the major requirements are shear  stability  and high  V.I., the oil should  also  be  resistant  to oxidation and  have no  adverse effects on both metals  and  seal and  hose  materials. Likewise the blended oil should have a low pour point. All of the above point to a solvent refined oil which will have the desired high V.I. and aniline number.

Chrysler MS 3590 “Hydraulic Steering Fluid with Rust Preventive”, summarizes the composition requirements as follows:

“Well refined oil with admixture of proper rust preventive additives. Small amounts of oxidation inhibitor and pour depressant may also be used to meet the details requirements. The use of   detergents and V.I. improvers is prohibited. The additives small be selected to have a minimum effect on the seal and hose materials”.

Therefore, additives may  include up to 0.5 per cent  of an  oxidation inhibitor, such as  can be  recommended  by  an  additive supplier, or  may  be  phenyl  alpha napththylamine , 2,6-ditertbutyl – 4- methyl  phenol, mixtures  of these  two  types, or  an oil-soluble  organic thiophosphate salt of certain  metals, such  as zinc  methylphenyl  dithiophosphate.

If the pour point of the oil is above -20 degree F, from 0.2 to 1 per cent of a pour depressant, such as “Acryloid  150”, “Paraflow”, or “ Santopour C”, can be added.

Suitable rust inhibitors, used in concentrations of 0.5 to 2 per cent, include petroleum or synthetic sulfonates, such as “Petronate” or barium dinonyl naphthalene sulfonate. A combination oxidation and rust inhibitor, which is usually used in a concentration of 1 per cent or less, is “Vanlube 76”. 

Additives for Straight Mineral Oil Gear Lubricants

All gear oils should contain a foam inhibitor, probably in the range of 5 to 10 parts per million. In order to arrive at approximately the correct proportion of such an additive, it is best to use a concentrate. This may be a ten per cent dispersion of a dimethy1 silicone in either kerosene or carbon tetrachloride.

Suppliers of silicone polymers offer antifoam additives, most often as concentrates and will suggest proportions to add to gear lubricants. Other compounds or  dispersions of the  same  are also  available  from  additive  suppliers who  furnish  instructions  for use.

While  most  industrial gear oils  are  not subjected  to  elevated  temperatures, the  presence  of a small  proportion of  oxidation  inhibitor is an assurance  of long life  and little deterioration of the oils. Additives such as 2, 6 – ditertiary – buty1 -4 – methyl  phenols or phenyl  alpha or  beta  naphthylamine , used  in  proportions  of 0.1 to 1.0 per cent, should be  found satisfactory for  most  straight  mineral gear  oils. The first compound or one quite similar is supplied by several concerns, often under a trade name. Other  antioxidants  will  be  noted in  various formulations to  follow and  will also  be  found in  Boner where the  source  is also shown.

For special applications other types of additives may also be found necessary in what can be classed as straight mineral oils. Thus, Watson  and Tierney  Formulated oils to be used under wet  conditions on paper  making  machines where a circulating  oil would  serve both bearings  and  gears. Since, in  this  operation, the  oil passing through the  system is  quickly  contaminated  with  water which  settles out, retention of  the rust   preventive additive  is quite important. A  suitable  formulation, which  retained the  major portion of the  additives after over a 1000 hours of  service, consists of : a  base  oil  blended from a paraffin  distillate  and a  paraffin residual  and having a  viscosity SUS at 100 degree F of 653, a pour  point of -10 degree F, and  a V.I. of  85; 2.0 per cent  by  weight  of a 50  per cent concentrate of  barium  dinonylnaphthalene   sulfonate in a light oil; 0.75 per cent of Zinc  dimethylisobutyl  carbinyl  dithiophosphate ; 0,10 per cent  of a  chlorinated  paraffin  wax naphthalene  condensation product; and 150  ppm of a 10 per cent  solution of  dimethyl  silicone  polymer in  kerosine.

Corrosion prevention by gear lubricants

Ellis et al.^22 consider that staining, tarnishing, and rusting are all  indications of corrosion. The thought is that light stain or tarnish represents the early stages of corrosion since, unfortunately these  changes do not proceed very far before pitting starts. Unreactive gear oils, which have not been subjected to excessive high temperature oxidation, have no tendency  to corrode  metals but, under moist or humid conditions or in the presence of  most salts or acids, do not offer proper protection against  rusting of ferrous  metal  surfaces such as gears. However, additives can be included in gear compounds which will provide rust prevention. Where conditions of incipient rusting prevail, the gears and other metal parts even to the inside of the gear case may require protection, particularly when idle. In such cases not only will the presence of a rust inhibitor but also the viscosity of the base oil be factors. Thus, the higher the viscosity of the gear lubricant, the slower this will drain from the metal surfaces and consequently the greater the rust prevention. Rusting may occur in different environments and various theories are offered as to the mechanisms of corrosion, but normally moisture and oxygen are the offenders

Most rust preventives are polar substances, such as long chain fatty acids, fatty amines, metal sulfonates, certain esters, oxidized petroleum fractions, etc. Such materials wet a metal surface preferentially and displace any water which may come in contact with the steel. The coating of polar substance then acts as a barrier against water reaching the metal surface.

As previously mentioned, controlled  corrosion due  to  EP  additives  is generally beneficial in  that  it  corrodes  away high  spots   on the gear  teeth after  which corrosion may decrease. With the proper selection of the chemical agents, these are not activated except under extreme conditions of load and /or temperature. Further, most of the EP additives which are  used  in  gear  lubricants will have little effect  upon  other  metals such  as bronze, copper, etc; at the bulk oil temperatures maintained  in normal  gear operation .