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.

Forced Feed Application of Gear Oils

Forced feed application of gear oils may take the form of circulating systems which deliver the lubricant as a stream running over the gearing or to jets or sprays of some type. Such methods not only provide lubrication but also utilize the gear oil as a cooling medium. Brewer^5 mentions three types of circulating systems, namely:

(1)    Wet sump, in which the oil is pumped under pressure directly from the sump to the lubricating points. Failure of the pump on such a system will cause almost immediate starved condition of lubrication.             

(2)    Dry sump, where a scavenging pump draws the excess oil from a sump and delivers it to a reservoir located above the gearing. A circulating pump then draws oil from this reservoir and delivers it to the point requiring lubrication. Since the scavenging pump has greater capacity than the circulating pump, the sump remains practically dry. A  pump failure  on  such a system  does not result  in as star rapid vation as in the  wet system  since some oil can feed by  gravity  from the reservoir  through  an idle pump.

(3)    Gravity  feed  which the sump  pump delivers  the oil, dripping  from the gearing, to a reservoir  set  high enough to give the desired  pressure. From this tank which may be thirty feet above the gear sets, gravity feed through regulating valves and perhaps sight glasses provides the lubrication.

Any of the above systems may also include oil filters, coolers, and even alarm devices to warm against malfunction of pumps. Gear oils used in paper mills, steel mills, and other locations are frequently contaminated with water, mill scale etc. Such lubricants may have to be reclaimed by settling, filtering and centrifuging before reuse. The oils used in forced feed applications are generally those which are quite fluid at ambient temperature and not of the residual type. However, if warmed, the latter type of lubricant can be handled in circulating systems.

                            Barring a pump or line failure, circulating oil systems provide a reliable flow of clean oil to gears. If necessary, this oil can serve additional functions, such as a hydraulic fluid in addition to lubricating practically any mechanism necessary. Such methods flush contaminants continually out of the lubricating areas and in most cases sufficient oil is in the system so that a portion can be withdrawn for a purification step during the cycle. Most of the heat  absorbed by  the oil  passing  through the  operating  area  is released  in the storage  or by coolers before  the return  cycle.

The lower the viscosity of a gear oil, the more satisfactory it will be for removal of heat from gear sets. In circulating systems, Dudley^16 considers that about a gallon of oil per minute will remove the heat developed when 400 hp are transmitted through a gear set. With larger installations considerably less than this ratio circulating oil to power is required.

Foam inhibition in gear oils

The rapid movements of gears tend to aerate oils and produce foam. This tendency may be aggravated by the presence of some additives, such as EP agents. Also, the higher the viscosity of the oil the more permanent the foam as a rule. Cases have been noted where foam became so great that it filled the gear case to overflow and long before this the gear teeth failed to obtain sufficient lubricant. While  foaming of gear oils might  result  from vaporization of entrained  water or driving of air  out  of solution, the general  cause in gear sets is churning  of air  into  the oil  by  agitation.

Little trouble is experienced from foaming of gear oils in service because the use of foam inhibitors in such lubricants is almost universal. It has been suggested that there is a difference between a foam inhibitor and a foam depressant, the latter being an insoluble material. Robinson and Woods^44 use the term “antifoaming agent” to embrace all aspects of the destruction, elimination, or prevention of foams. These investigators state that a foam inhibitor may act:

“ (1) by causing  coalescence of smaller bubbles into large bubbles at or below the surface, (2) by causing the rupture  of  individual bubbles at the surface, (3) by destroying the inherent stability of the liquid  films, or (4) by causing any or all of  these actions simultaneously.”

McBain et al.^39 found that the most complete defoamers for oils are generally, but not always, insoluble. This is true of silicone fluids which have wide usage for this purpose. There is an optimum amount of antifoam agent required which is quite low. Consequently most suppliers furnish defoamers as dispersions or solutions so that low dosages will be more accurate. While Woods and Robinson^50, in testing varying proportions of  DC 200 fluid in two oils, found that 0.01 per cent gave  the greatest  foam  inhibition, actual  usage  in most  gear oils is only a fraction  of this proportion. Thus, Klaus  and Fenske^34, using two oils which foamed badly with no additive, found  that both responded to silicone  antifoam additives at concentrations as low as 0.00001 weight per cent and that maximum effectiveness  was achieved with 0.00005 per cent  or greater.