Evaluation of the Foaming Tendencies of Automatic Transmission Fluids 630438
Past experience indicates that the method for evaluating the foaming tendencies of automatic transmission fluids, shown in the “Specification for Automatic Transmission Fluid, Type A, Suffix A,” is adequate for the qualification of fluids for transmission operation. Its usefulness, however, for evaluating field failures and for monitoring the shelf-life stability of the foam inhibitor is severely limited by the fact that the procedure requires a sample size of at least 6 qt.
The purpose of this paper is to present a new test procedure for the evaluation of foaming tendencies that utilizes a small sample size (500 ml) and shows good correlation with the results obtained during qualification.
The paper also presents the results of studies which were necessary for the development of the proposed test procedure. These studies were concerned with the mechanism of foam formation and collapse, with the conditions affecting the equilibrium between the rates of formation and collapse, with air entrainment and retention in the fluid, and with the effect of siloxane additives on the foaming characteristics of the fluids.