Lewis Gear Strength Calculator
Input Output
Gear Pitch
Face Width (inches)
Tensile Strength of material (PSI)
Number of teeth

Maximum safe tooth load (pounds)

This Javascript application computes the maximum safe tooth load for 20 degree* pressure angle spur gears from the Lewis formula. This formula considers the gear as a cantilever beam with the full load applied to one tooth. If you enter a number other than zero for the RPM it will derate for dynamic effects of tooth impact. If exact number of teeth is not in the drop down menu, use the closest value.

Lewis Formula: W = S*F*Y / P
W = Safe tooth load
S = Safe material strength (1/3 tensile strength)
F = Face width
Y = Lewis Y factor from table based on tooth geometry
P = Pitch

* 14.5 degree pressue angle gears are about 15% weaker than 20 degree gears.

A product of Dales Homemade Robots, Copyright 2005 by Dale A. Heatherington

Material Tensile Strength Table
1020 Steel65,000
1040 Steel90,000
1080 Steel140,000
12L14 Steel60,000
4130 Steel97,000
4140 Steel Annealed98,000
4140 Steel Hardened156,000
Titanium 6AL4V130,000
303 Stainless Steel90,000
5052-H32 Aluminum33,000
6061-T6 Aluminum45,000
2024-T361 Aluminum72,000
7075-T6 Aluminum76,000
Yellow Brass71,000