Ratchet (device)

This article is about mechanical ratchets. For other uses, see Ratchet (disambiguation).

A ratchet lever hoist, usually called a come-along or come-a-long colloquially but most often not in catalogs (probably to avoid real or perceived trademark genericization).

A ratchet is a device that allows continuous linear or rotary motion in only one direction while preventing motion in the opposite direction. Because most socket wrenches today use ratcheting handles, the term "ratchet" alone is often used to refer to a ratcheting wrench, and the terms "ratchet" and "socket" are closely associated in many users' minds.

Ratchet mechanisms are used in a variety of devices, including:

• Cable ties
• Capstans
• Clocks
• Hoists
• Jacks
• Roller coasters
• Spanners
• Turnstiles
• Winders

Theory of operation

Figure 1: A ratchet featuring gear (1) and pawl (2) mounted on base (3).

Figure 2: A ratchet moving in its "forward" direction.

A ratchet consists of a round gear (see Figure 1) or linear rack with teeth, and a pivoting, springloaded finger called a pawl (or click) that engages the teeth. The teeth are uniform but asymmetrical, with each tooth having a moderate slope on one edge and a much steeper slope on the other edge.

When the teeth are moving in the unrestricted (i.e., forward) direction (see Figure 2), the pawl easily slides up and over the gently sloped edges of the teeth, with a spring forcing it (often with an audible 'click') into the depression between the teeth as it passes the tip of each tooth. When the teeth move in the opposite (backward) direction, however, the pawl will catch against the steeply sloped edge of the first tooth it encounters, thereby locking it against the tooth and preventing any further motion in that direction.

Backlash

Because the ratchet can only stop backward motion at discrete points (i.e., at tooth boundaries), a ratchet does allow a limited amount of backward motion. This backward motion—which is limited to a maximum distance equal to the spacing between the teeth—is called backlash. In cases where backlash must be minimized, a smooth, toothless ratchet with a high friction surface such as rubber is sometimes used. The pawl bears against the surface at an angle so that any backward motion will cause the pawl to jam against the surface and thus prevent any further backward motion. Since the backward travel distance is primarily a function of the compressibility of the high friction surface, this mechanism can result in significantly reduced backlash.

See also

• Freewheel
• Sprag clutch

External links

• Silent Ratchet patent (a toothless ratchet) on the USPTO. The patent includes a full description of ratchet technologies, advantages and drawbacks.