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.
