Design of Worm Gearing

Introduction:

In designing a worm and worm gear, the quantities like the power transmitted, speed, velocity ratio and the centre distance between the shafts are usually given and the quantities such as lead angle, lead and number of threads on the worm are to be determined.

Design of Worm Gearing

In order to determine the satisfactory combination of lead angle, lead and centre distance, the following method may be used:

the centre distance,  

The centre distance may be expressed in terms of the axial lead (l), lead angle (λ) and velocity
ratio (V.R.), as follows :

Since the velocity ratio (V.R.) is usually given, therefore the equation (i) contains three variables i.e. x, l_N and λ. The right hand side of the above expression may be calculated for various values of velocity ratios and the curves are plotted.

The lowest point on each of the curves gives the lead angle which corresponds to the minimum value of x / l_N. This minimum value represents the minimum centre distance that can be used with a given lead or inversely the maximum lead that can be used with a given centre distance

Worm gear design curves

Note : The lowest point on the curve may be determined mathematically by differentiating the equation (i) with respect to λ and equating to zero, i.e.