In the design of American standard gears, steel, Grade2, hrc58-64 (AGMA), case carbonized steel and case hardened gear materials similar to 20mncr5 are also selected. In the American Standard, the contact stress σ hlim = 1551n · mm – 2 is required for the test gear of this material. Different from the international standard ISO, the number of cycles under this contact strength is 1 × 107.
In the American Standard, the S-N curve ofis not given directly, but the life coefficient curve. According to the life coefficient curve, the S-N curve of contact life can be obtained, as shown in the figure.