Worm Gear Design: Gear Parameter and Description

Worm Gear Design: Gear Parameter and Description - In the previous article we have explained some of the gear transmission systems such as spur gear, helical gear and bevel gear. If we will connect two parallel shafts, we can use a spur gear or a helical gear. (Read more: spur gear design) (Read more: helical gear design) If we will connect two mutually perpendicular shafts (right angle), we can use a bevel gear. (Read more: bevel gear design)

In this article we will explain about the worm gear. Worm gear is always paired with a worm screw in the transmission system. We can use worm screw and worm gear, when shaft are at right angle and need more reduction in speed between the driving and driven shafts. Couple worm gear is consisting of a worm has an external threaded and a worm gear associated with worms.

What is a worm screw and worm gear?

A worm screw is a cylinder on which is cut single or multiple start acme-type thread. The angle of this thread ranges from a 14,5 to 30o pressure angle. As the angle of the worm increases, the greater of pressure angle must on the side of the thread.

The teeth on a worm gear are machined on a peripheral groove which has a radius equal to half the root diameter of the worm. The drive ratio between a worm and worm gear assembly is one to the number of teeth in the worm gear. The worm which meshes with the worm gear may be a single or multiple start thread. A worm with a double start thread will revolve the worm gear twice as fast as a worm with a single start thread and the same pitch. Thus if worm gear had 50 teeth, the ratio would be 50:1, providing the worm had a single start thread. If it had a double start thread, the ratio would be 50:2 or 25:1.

Based on the above understanding, we can know the advantages and disadvantages of worm gear transmission. Among its advantages are very prominent on his worm gear is smooth and almost without sound, as well as allowing large transmission ratio. Reduction ratio can reach 1/100. However its disadvantages, we can not reverse the direction of transmission of the worm gear to increase the rotation of the worm wheel to the worm screw. This sort of thing will 'lock itself' because the worm screw will stop rotation turn of the worm gear. Another disadvantage of the worm gear is a low efficiency, especially if the pitch angle is small.

Worm screw and worm gear parameter

The standard measure of the worm screw is outside diameter, pitch diameter, root diameter, pitch screw, pitch module, axial module and pitch angle. The standard measure of the worm gear is outside diameter, pitch diameter, throat diameter, root diameter, normal module, arch gear radius and arch gear angle. Worm screw and worm gear is an integral gear paired, so that some of its parameters must have the same dimension is normal module, addendum, dedendum and whole depth. Another important measure as a basis for gear calculation is shaft distance and clearance. This measure will decide the amount of friction between the worm screw and worm gear.

Such as generally gears, module is the main measure of gears that will decide the size of the gear. Worm screw and worm gear pairs must have the same module size. Worm gear is using the normal module that has the same module profile with the generally gears as to spur gear, helical gear and bevel gear. The worm screw is using the axial module that has a trapezoidal screw profile. The relationship between the normal module and the axial module is affected by the pitch angle. If m_n is normal module, m_s is module axial and g is pitch angle then, relations between them are:

We can also calculate the module value by shaft axial distance (a) and the teeth number of worm gear (Z) is through the formula:

In the production process of worm screw, don’t know the term of module, however using the pitch screw. Pitch screw is the distance between two peaks screw in one full turn. If the screw number of worm more than two (z> 2) then the distance between the peaks screw nearby called pitch modules. If the screw number of worm equal to one (z = 1) then pitch screw equal to the pitch module. If m_s is the axial module, g is the pitch angle, z is the screw number of worm, p_m is pitch module and p_s is a pitch screw, the equation between them are:

That is a description about Worm Gear Design: Gear Parameter and Description. If you find misconceptions in the worm gear formula, please give me correction in the comment box.

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