Machine Design

Limit Switches and Centrifugal Switches

Limit switches: A mechanical limit switch interlocks a mechanical motion or position with an electrical circuit. A good starting point for limit-switch selection is contact arrangement. The most common limit switch is the single-pole contact block with one NO and one NC set of contacts; however, switches are available with up to four poles.

Limit switches also are available with time-delayed contact transfer. This type is useful in detecting jams that cause the switch to remain actuated beyond a predetermined time interval.

Other contact arrangements include neutral-position and two-step. The neutral-position or center-off type transfers one set of contacts with movement of the lever in one direction. Lever movement in the opposite direction transfers the other set of contacts. With the two-step arrangement, a small movement of the lever transfers one set of contacts, and further lever movement in the same direction transfers the other set of contacts.

Maintained-contact switches require a second definite reset motion. They are used primarily with reciprocating actuators, or where position memory or manual reset is required. Spring-return types automatically reset when actuating force is removed.

Centrifugal switches: A centrifugal switch is actuated by speed only. Simple types consist of speed-sensing units that mount directly on a rotating shaft and a stationary-contact switch assembly. The basic control element is a conical-spring steel disc that has centrifugal weights fastened to the outer edge of its circular base. Fingers on the spring are attached to an insulating spool that rides free of the shaft and actuates the movable switch contact. As the rotating sensing unit reaches switching speed, the centrifugal force of the calibrated weights overcomes spring force, resulting in an instantaneous axial displacement of the spring and the contact-actuating spool.

The contacts switch at one speed as speed increases from zero to operating speed, and at a lower speed as rotation slows from operating speed toward zero. The spring decreasingly opposes centrifugal force as rotational speed increases from standstill until the snap-over point is reached. Then, spring force adds to centrifugal force to axially snap the spool and actuate the contacts. As rotational speed decreases from operating speed, spring force overcomes the centrifugal force of the weights at a lower speed where snapback begins.

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