Rotary Limit Switches

Rotary limit switches use physically-actuated levers to rotate a shaft and operate the contacts in a switch, making them change state. There are three basic types of rotary limit switches. Momentary or spring return switches return to their normal state as soon as the actuator passes its release point. By contrast, maintained contact switches remain in the triggered position after the actuator is released and are reset only by further mechanical action of the operating head. Positive opening switches remain in the activated position even in the event of mechanical failure such as the breakage of a spring or contact weld. Most linear limit switches feature a rugged design and are suitable for a variety of industrial applications; however, because these devices contain mechanical parts, they wear over time. Typically, rotary limit switches are slower than noncontact, electrical devices such as proximity sensors and photoelectric sensors. Common actuation methods include standard rollers, offset rollers, adjustable rollers, fork lever rollers, wide rollers, rods or wire, loops, and wobble-type springs. Top-mounted devices position the actuator on the top of the limit switch. Side-mounted devices locate the actuation mechanism on the side.

Selecting rotary limit switches requires an analysis of pole-and-throw specifications. Single pole, single throw (SPST) devices make or break the connection of a single conductor in a single branch circuit. They have two terminals and are commonly referred to as “single pole” switches. Single pole, double throw (SPDT) switches make or break the connection of a single conductor with either of two other single conductors. Often called “three-way switches”, SPDT devices have three terminals and are used in pairs. Double pole, single throw (DPST) switches make or break the connection of two circuit conductors in a single branch circuit and usually have four terminals. Double pole, double throw (DPDT) switches make or break the connection of two conductors in two separate circuits. Most DPDT switches have six terminals and are available in either momentary or maintained contact versions. Special throw types with more than two poles are designed to split loads into separate circuits.

Rotary limit switches vary in terms of electrical specifications, movement specifications, mechanical specifications, connection options, actuator direction, and normal state. Electrical specifications include maximum current rating, maximum AC voltage rating, maximum DC voltage rating, and transistor-transistor logic (TTL) compatibility. Pre-travel distance, total travel, movement differential, operating force, minimum return force, and repeatability are important movement specifications. Mechanical specifications include minimum mechanical life and minimum electrical life. Both amounts are measured as measured in number of cycles. Connection options for rotary limit switches consist of integral cables, solder terminals, screw terminals, and spade terminals. Actuators move in either a clockwise or counterclockwise direction. Electromechanical and solid-state switches are commonly available. Normally open (NO) contacts close when actuated. Normally closed (NC) contacts open when actuated. 

Rotary limit switches are available with a variety of features. For example, some devices are rated for hostile environments or enclosed inside a NEMA-rated housing that can be accessed via a conduit. NEMA, an acronym for the National Electrical Manufacturers’ Association, is a trade organization which defines and recommends safety standards for electrical equipment. Organizations such as Underwriters Laboratories (UL) and the Canadian Standards Association (CSA) provide certifications and approvals. Other features for rotary limit switches include self-cleaning switch contacts, safety pulls or guards, and rotating heads.

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Part Numbers for Rotary Limit Switches