Detent

Detent

[dih-tent]
Detent is the term for a method, as well as the actual device, used to mechanically resist or arrest the rotation of a wheel, axle or spindle.

A detent can be used to intentionally divide a rotation into discrete increments, or as in perhaps its original concept and most rudimentary form, to simply arrest rotation in one direction.

Mechanics

Arresting Movement

To arrest movement, the method commonly employs a small gravity or spring-actuated lever paired with a notched wheel. The lever is mounted on a pivot point in proximity to the wheel (so that the end cannot swing completely around) and comes into contact with it at a tangential angle less than 90 degrees.

The vertical angle of the sides of the notches that face the direction that rotation is desired is generally acute (45 degrees or less), so that as the wheel rotates in that direction, the end of the lever is easily lifted or pushed out and over the top of a notch. Following this, the lever drops into the next notch and the next et cetera as the wheel or shaft continues to spin.

The angle of the backside of the notch is severe (usually 90 degrees or greater to the end of the lever) so that the lever cannot be pushed up or out of the notch if wheel attempts to turn in the opposite direction. The lever is jammed between the back of the notch and its pivot point, stopping movement in that direction against any force that the materials used can withstand. The wheel has little resistance moving in the direction desired, other than than required to lift or push the lever over the next notch.

Resisting Movement

To resist movement (or when creating incremental steps), methods are employed which include a spring-loaded ball bearing that locates in small incremental depressions, or a piece of spring steel that snaps into position on flat surfaces or shallow notches milled into the shaft or wheel.

Examples

A well-known example of a detent can be seen on the popular game show "Wheel Of Fortune", which employs rubber "fingers" to stop the wheel at valid points after the wheel is spun by a contestant. Other common examples include:

  • A balance control on a piece of stereo equipment which seems to "click" or "snap" into the center position of its rotation, indicating the point where the volumes of the left and right channels are equal or "balanced", or volume controls with a separate detent to match each of the digits on the control knob (typically 10).
  • Rotary switches typically employ detents to keep the control shaft properly aligned with the appropriate contact.
  • Any spring-powered wind-up toy employs one, in order to disallow unwinding of the spring.
  • The ratchet wrench, which interestingly is employed to intentionally use force against the detent and comes in increasing variety of types. It was designed to allow one to keep the wrench engaged with the bolt or nut which it is turning, in an area where the swing arc of the wrench is limited, while being able to continue to turn it in one direction by simply pulling the handle back and letting the detent reposition itself. The repositioning allows the wrench to be forcibly turned again.
  • The scroll wheels on many computer mice, employ detents to divide scrolling into discrete steps.

See also

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