Linear limit switches are electromechanical devices that require physical contact between a target object and switch activator to make the contacts change state. There are two basic types of linear 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. 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, linear limit switches are slower than noncontact, electrical devices such as proximity sensors and photoelectric sensors. Common actuation methods include pushbuttons, plungers, standard levers, roller leavers, and adjustable levers. Top-mounted devices position the actuator on the top of the limit switch. Side-mounted devices locate the actuation mechanism on the side.
Selecting linear 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.
Linear limit switches vary in terms of electrical specifications, mechanical specifications, and special features. Maximum current rating, maximum AC voltage rating, and maximum DC voltage rating are important electrical specifications. Normally open (NO) contacts close when actuated. Normally closed (NC) contacts open when actuated. Mechanical specifications include actuating force, pre-travel, total travel, and operating temperature. Actuating force is the force required to move the switch actuator to the operating or trip position. Pre-travel is the distance the actuator moves from the free or rest position to the operating position. Total travel is the total physical movement of the switch from the resting position to the actuation point. Operating temperatures is the temperature range over which switches are designed to operate. In terms of features, some linear limit switches are designed for use in transistor-transistor logic (TTL) circuits. Others are enclosed inside a NEMA-rated housing and can be accessed via a conduit. The National Electrical Manufacturers’ Association (NEMA) is a trade organization which defines and recommends safety standards for electrical equipment.
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