In the gear standard design of American Standard ANSI / AGMA 2001-d04 and international standard ISO / TS 6336-2, the standard S-N curve of contact strength is generally divided into three parts, as shown in the figure.

Static load zone, considering the static load strength of the gear; 2 is the finite life zone, in which the gear can get a finite life; 3 is the long life zone, in which the gear can get a relatively long life.

American Standard and international standard have their own standard S-N curve according to the difference of gear material and heat treatment. The gear of general tractor gearbox belongs to heavy load gear. Low carbon alloy steel with hardenability is selected and carburized as the material and heat treatment method of gear.