Procedures for the Evaluation of Automatic Transmission Fluids Walter K. Fuelberth, Harold L. Sharp, and Robert A. Wilkins Chevrolet Motor Div., General Motors Corp. RECENT TRANSMISSION fleet evaluations recording trouble- free service in excess of 100,000 miles can be attributed to a proper additive balance in the transmission fluid. This additive balance can be accomplished either through a sys­ tematic oil change program or by taking advantage of re­ cent progress by the oil industry in the formulation and blend­ ing of transmission fluids. To help understand the fluid requirements for evaluating fluid performance, it may be useful to first mention the areas of the ratio change which the transmission engineer can control by design, and also those areas in which he must depend upon the lubrication engineer to provide a desirable speed-friction relationship. TRANSMISSION CHARACTERISTICS Current practice in the design of passenger car automatic transmissions is to use a torque converter followed by either a two or a three speed gear train. The ratio change of these transmissions is accomplished through the combined use of clutches, bands, and overrunning clutches. Every designer attempts to control the use of these elements in such a man­ ner as to minimize any unpleasantness created by the ac­ celeration or deceleration of the vehicle during the interac­ tion of the engaging elements. In the case of the Powerglide transmission (Fig. 1), the shift from low to high is controlled by the release of a band and the engagement of a clutch. The transition from the band to the clutch is important to the designer, for it affects both durability and customer acceptance. Ideally, the shift should be made at the intersection of the tractive effort curves (Fig. 2) as explained in an earlier paper prepared by F. J. Winchell and W. D. Route (1).* Here the output torque before the shift equals the output torque after the shift as shown in the following equation: A disturbance or change in acceleration during the transi­ tion is unavoidable even when the above relationship exists. The output shaft torque diagram in Fig. 3 would be typical of a driveline disturbance experienced during an upshift un­ der power. The decrease in torque level from A to B is necessary to accomplish an upshift, but it is unpleasant to the driver because he feels the deceleration, which is opposite to that which is expected. It should be emphasized that a change in output torque due to a ratio change and its characteristic nature is inescapable, regardless of the choice or arrange­ ment of friction elements. The degree of awareness of the *Numbers in parentheses designate References at end of paper. ABSTRACT The development of more powerful engines with higher operating temperatures, the trend toward lower axle ratios, and increased use of passenger cars for trailer hauling have imposed greater demands on the durability of transmission fluid. What had been reported in the past as clutch failures might have been more accurately recognized as oil failures due to lack of friction retention. This paper describes Chev­ rolet's efforts to define desirable fluid characteristics and to develop test procedures to evaluate transmission fluids. Downloaded from SAE International by Imperial College London, Sunday, September 09, 2018driver to this disturbance is controlled by the time rate at which the transmission engineer permits the basic phases of the torque change to occur. Therefore, while we will talk about the ratio change torque disturbance in a Power- glide, the discussion is perfectly general and applicable to any power or ratio change. A speed change cannot occur until the reaction torque is reduced to zero, as shown in Fig. 4. This allows the re­ action member (clutch drum) to begin to rotate in the same direction as the input and output members. The decrease in reaction torque is accompanied by an increase in cl