# What are worm gears?

Worm and worm gears are used for rotation between two intersecting shafts (the intersecting angle is generally a right angle). Usually the worm is active and the worm gear is driven, which is used for deceleration, and a larger transmission ratio can be obtained.

In the worm and worm gear drive, the most commonly used worm is the cylindrical Archimedes worm. The axial profile of this worm is straight, and the axial section is an isosceles trapezoid, similar to a trapezoidal thread. The number of teeth of the worm is called the number of heads, which is equivalent to the number of threads of the thread, and is usually single-ended or double-ended.

The worm is equivalent to a helical cylindrical gear, and its gear teeth are distributed on the annular surface, so that the gear can wrap the worm to improve the contact condition, which is a feature of the worm gear.

Calculation of main parameters and size of worm gear

1. Pitch P and modulus P

In the middle plane including the axis of the worm and perpendicular to the axis of the worm wheel (Figure 9-60), the axial pitch P of the worm should be equal to the pitch P of the end face of the worm wheel (P=P=P), so the axial modulus of the worm The number M is also equal to the end face modulus M of the worm gear (M=M=M), and is specified as the standard modulus. The worm indexing circle diameter d, the throat circle diameter d, and the tooth root circle diameter d are all measured in the middle plane.

2. Worm diameter coefficient q

The worm diameter coefficient is an important parameter unique to the worm, which is equal to the ratio of the worm's index circle diameter d to the axial modulus M, that is, q=d/m or d=mq

Corresponding to different standard modulus, the corresponding q value is specified. The purpose of introducing this coefficient is mainly to reduce the number of machining tools.  