What is a worm gearbox?

Nov 3, 2020 | Blog

Before understanding what is worm gearbox, we will first know what is a gearbox. A Gearbox is a mechanical system used to transmit the mechanical power of a motor in an automobile or in any dynamic machine. Gearbox is used to change output speed or torque of a motor as per requirement.

A worm gearbox is a type of gearbox in which the driving component is a shaft with a spiral thread. The threaded shaft meshes or engages with a toothed wheel and advance the teeth of the wheel. The spiral threaded shaft and the toothed wheel are also called the worm screw and worm wheel, respectively. The worm gearbox is sometimes referred to as the worm drive. In worm gearbox, the spiral threaded shaft (worm screw) and the toothed wheel (worm wheel) are oriented in non-parallel and non-intersecting manner, at 90° to each other.

The worm gear transfers the motion in 90 degrees because it changes the rotational movement by 90 degrees. Because of the position of the worm screw on the worm wheel, the plane of movement also changes

worm gearbox

How does a worm gear work?

A mechanical or an electrical motor drives the worm screw, or spiral threaded shaft. As the worm screw rotates against the worm wheel, it pushes one tooth off the wheel and advances the wheel. The worm wheel then rotates the load connected to it, directly or by another shaft.

Types of Worm gear:

Depending on various areas of application, there are three different types of worm gears used in gearboxes:

  • Non-throated worm gear: This has a straight worm screw, without a throat or groove positioned around the circumference of the worm wheel. In this worm gear, a single moving point provides contact tooth contact. Thus, the minimal contact makes this most vulnerable to wear and tear from high unit load.
  • Single-throated worm gear:  This type is throated with concave helical threads around the circumference of the worm wheel for continuous contact. This type has high load capacity and relatively less prone to wear and tear.
  • Double-throated or enveloping worm gear: Also known as hourglass worm gear. This type has throats or concave teeth on both the worm screw and worm wheel, which increases the contact area. This type has the highest loading capacity without excessive wear and tear.

Features of Worm gearbox:

Advantages:

  • High-ratio speed reductions can be achieved in a compact type of system, since the gear ratio is very low.
  • Can also achieve high torque multiplication ratio.
  • They can achieve precise linear motion that is used in innumerable mechanisms.
  • Smoothest and quietest type of gear with proper mounting and lubrication.
  • Worm gearboxes can transmit mechanical power at a ninety-degree angle.
  • They can produce motion in one direction and have good meshing effectiveness because of which irreversible mechanism can be achieved, but is not unidirectional. If the worm wheel is attempted to reverse, the gear will self-lock and prevent any motion at all.

Disadvantages:

  • Cost of raw material and manufacturing cost is quite high.
  • Worm gearboxes have low efficiency because of have heavy power loss
  • If the speed reduction ratio is large, there will be considerable sliding action, which will produce lots of heat, leading to low efficiency.
  • They are subjected to high friction, thus, require proper lubrication for maintaining the health of worm screw and gear for smooth working of gearbox.
  • Without proper lubrication and mounting, they produce a lot of heat.

What is a worm gearbox used? — Applications of Worm Gears

Worm gearbox is one of the main and most widespread power transmission system used in mechanical projects. They can easily transfer mechanical force and motion between two perpendicular shafts at an angle of 90 degree from each other. It is such popular because of its simple operation and mechanical advantage. Worm gearboxes have a variety of application such as in presses, rolling mills, conveying engineering, mining industry machines, on rudders, and worm drive saws. The key area of applications of worm gearboxes are:

  • Automatic door system — A unidirectional movement (right and left) and blocking the system at the end of the end of the run is required for these systems. These functions are effortlessly achieved by worm gears.
  • Conveyor belts — The conveyor belt needs to be run in one direction and secured motionless when shutdown, which can be achieved with the self-locking mechanism of the worm gears.
  • Lifts and elevators — The non-reversible feature of the worm gear makes them useful to the mechanics of the elevator. This feature also acts as a fail-safe secondary braking system for an elevator in case of accidental free fall.
  • Musical instruments — Worm gears are used in the tuning mechanism found on the head of the string instruments like guitars, violins, banjos, and so on.
  •  Vehicle rear-axle drive system — Worm gearboxes are used to produce torsion differentials for four-wheel vehicles where different amount of torque is needed to be delivered to each of the wheels.
  • Speed reducers in small motors — They are used to reducing speed of high speed small motors in small space systems.
  • Automotive steering system — Modern steering system of cars use worm gear as a basic component. In this system, the worm screw connects to the steering wheel through a steering column. For each turn of the steering wheel the car’s wheel turns by a precise angle which depends on the gear ratio of the worm gear system.
  • Worm gearboxes are widely used as an industrial speed reducers since they have significant gear ratios in small spaces and allows power transmission to be performed at a right angle.

Can a worm gear go both directions? — Direction of transmission of worm gears

The direction of transmission of worm gear cannot be reversed like ordinary gearboxes. This is because of high friction between the worm-screw and worm-wheel, which is common when a single start or one-spiral worm-screw is used. This self-locking feature can be an advantage when it is required to eliminate any possibility of output driving the input. But if multi start or multiple spiral worm-screw is used, then self-locking or braking effect of the gear system may be compromised. Whether a worm gear system is self-locking depends on three factors, the lead angle, the pressure angle and the coefficient of friction.

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