Strength of Worm Gear Teeth

Introduction:

In finding the tooth size and strength, it is safe to assume that the teeth of worm gear are always weaker than the threads of the worm.

Strength of Worm Gear Teeth

In worm gearing, two or more teeth are usually in contact, but due to uncertainty of load distribution among themselves it is assumed that the load is transmitted by one tooth only. We know that according to Lewis equation,

W_T = (σ_o . C_v) b. π m . y

where

W_T = Permissible tangential tooth load or beam strength of gear tooth,
σ_o = Allowable static stress,
C_v = Velocity factor,
b = Face width,
m = Module, and
y = Tooth form factor or Lewis factor.

Notes :

1. The velocity factor is given by

where v is the peripheral velocity of the worm gear in m/s.

2. The tooth form factor or Lewis factor (y) may be obtained in the similar manner as discussed in spur gears

3. The dynamic tooth load on the worm gear is given by

where W_T = Actual tangential load on the tooth.

The dynamic load need not to be calculated because it is not so severe due to the sliding action between the worm and worm gear.

4. The static tooth load or endurance strength of the tooth (WS) may also be obtained in the similar manner as discussed in spur gears, i.e.

W_S = σ_e.b π m.y

where σ_e = Flexural endurance limit. Its value may be taken as 84 MPa for cast iron and 168 MPa for phosphor bronze gears.