Based on D-H formula, Dowson D and Higginson GR proposed the minimum oil film thickness formula of EHL in line contact

Where Hmin is the minimum oil film thickness, mm.

The film thickness ratio and lubrication criterion of gears are as follows

Where, λ is the film thickness ratio; ra1 and ra2 are the tooth surface roughness, mm; [λ] is the allowable film thickness ratio.

It is generally considered that when [λ] ≥ 3, it is a state of complete EHL.

From the formula, it can be concluded that:

According to the simultaneous formula:

By substituting the gear parameters into the formula, the following results are obtained

From the formula, the lubrication design formula of standard spur gear transmission can be obtained

Where DL is the diameter of gear indexing circle based on D-H formula, mm.