Aircraft Gearing - Analysis Of Test And Service Data

Author:  Almen, J. O. and Straub, J. C.
Source:  American Gear Manufacturers Assoc., June 4, 1948
Doc ID:  1948000
Year of Publication:  1948
Abstract:  
Since we rarely know the magnitude of loads that are applied to gears in service or the extent to which the stresses from the applied loads are concentrated by elastic deflections, thermal changes of dimensions, or by dimensional errors; and since we do not know the strength of our materials in their operating environments, or the effect of processing upon metal strength--to mention some of the more important unknowns and unknowables--any prediction of performance based upon assumed knowledge of such variables must necessarily be wide of the mark. When gears must be designed in accordance with textbook procedures, which procedures are based upon many assumptions and misconceptions, they will inevitably be overdesigned and unbalanced with respect to the several kinds of failures that afflict gears. Most aircraft gears are greatly overdesigned against tooth breakage, with the result that overcaution against one kind of failure actually produces inferior gears because of their proneness to failures from causes not considered by the designer. The only known way to produce well-balanced designs of machine parts, where intensive use of material is essential, is by the construction of empirical formulas from service records of large statistical samples. To accomplish this objective for aircraft engine gears, the Research Laboratories Division of the General Motors Corporation asked and obtained the cooperation of the major aircraft engine manufacturers during the war in the accumulating of suitable data. The data are taken from service reports of many thousands of engines used in military and airline operations. They are concerned only with gears or gear combinations that failed from any cause attributable to design, material, or workmanship. It should be understood that "failure," as here used, does not necessarily mean chronic failure or catastrophic failure, but includes also occasional damage occurring to a few gears at some time during the service operation of the engine. Suitable data were obtained on seventy-three gear combinations run under a variety of loads, speeds, and temperatures. External as well as internal gears were included. The torque ranged from two pound-feet to 6500 pound-feet; the speeds ranged from 1260 rpm to 28,000 rpm; and pitch line velocity ranged from 785 feet per minute to 19,100 feet per minute.