What is the Worm Gear?

When it comes to the worm gear, it is usually the worm drive system that people refer to. A worm drive consists of a worm and a worm wheel. The worm is a gear that comes in the form of a threaded shaft (a screw). The worm wheel on the other hand resembles the helical gear, except the teeth are slightly curved or angled to better fit the worm thread. Unlike most gear sets that the gears share the same rotational axis, the worm drive changes the rotational axis by 90 degrees. In other words, the axis of the worm is perpendicular to that of the wheel. Based on this structure, there are several developments into the types and designs of the worm drive.

What are the Types?

The thread and the tooth design determine the type of worm drive. The worm thread is also called the start. There are variations from a single start to more than three starts. As for the tooth design, the wheel tooth and the worm thread can be either throated or non-throated. In the following sections, we will walk you through the designs of these details and how they affect the performance of the worm gear unit.

Non-throated Worm Drive

The throat is the concave design of the tooth and the thread. A non-throated worm drive has a straight (cylindrical) worm and a standard wheel. This is the most typical design of the worm gear unit. In such a configuration, only one tooth meshes with the thread completely and therefore does not allow a smooth engagement between the two components. Resultingly, there is higher wear and tear to the non-throated unit as the tooth and thread constantly collide with each other. It is a bit less quiet than the single-throated and double-throated units as well.

Single-throated Worm Drive

The single-throated, also single-enveloping unit has a straight worm and a wheel with concave teeth. The concave is a groove (the throat) cut on the teeth. This structure allows the wheel to partially wrap around the worm and offers a line instead of a point of contact for the worm and the wheel. The single-throat design improves the smoothness of the engagement and hence reduces the tear and wear. In addition, the worm drive unit can handle higher power transmission.

Double-throated Worm Drive

The double-throated worm drive has both the wheel teeth and worm thread concaved so that the worm thread envelopes the wheel as the wheel teeth envelopes the worm during the rotation. The structure of the concaved worm screw looks similar to the sandglass. The thread gets wider on the two ends. The double-throated design further increases the contact area between the two components of the unit and is capable of performing high power transmission. It renders the smoothest and quietest meshing to the unit.

Starts

The start is the thread of the worm screw. The start number of the worm is usually one, two, or three; that said, there are also worms with more than three starts. The start number affects the transmission speed and reduction ratio of the drive. For example, a single-start worm only has one thread and when it rotates 360 degrees (a full turn), it advances one wheel tooth. The two-start worm has two threads and advances two teeth at a full turn. The reduction ratio of the worm drive is the tooth number divided by the start number. Let’s say a worm drive is composed of a two-start worm and a 36-tooth wheel. The reduction ratio is 18:1.

Why Using the Worm Gear?

There are three main reasons why people adopt the worm drive system into the gearbox or other mechanical devices. Simply put, the worm gear can change the rotational direction, increase the torque, and reduce the transmission speed. This system is efficient because of its simple structure. The reduction ratio is controlled solely by the tooth and thread number. With fewer components in the system, the size of the drive is compact and suitable for installation in confined working environments.

Applications

The common applications of the worm drive include gear reducers, stairlifts, elevators, conveyor belts, gate controls, and guitars, to just name a few. Gate and door control is a gear drive application that involves the motor. The motor controls the direction of the worm to open or close the door. The other common application is the gear reducer. Gear reducers are the transmission device for motors in industrial settings. They are highly effective at adjusting the speed and torque to the desired level. Chun Yeh Gear Co. is an expert in gearbox manufacturing as well and worm gear manufacturing. Contact us right now to learn more about Chun Yeh’s services.