A Guide to Limit Switches
Limit switches are switches used to automatically detect the presence of an object or to monitor and indicate whether the movement limits of that object have been exceeded. Their original use, as the name suggests, was to define the limit or endpoint over which an object could travel before being stopped. Once the endpoint was reached, the switch would be engaged to control the limit of travel.
Standard limit switches used in industrial applications are electromechanical devices that consist of a mechanical actuator linked to a series of electrical contacts. When an object, sometimes called the target, comes into contact with the actuator, the actuator’s plunger moves, causing the electrical contacts within the switch to either close or open their electrical connection. Limit switches utilize the mechanical movement of an actuator plunger to control or alter the state of an electrical switch. As limit switches require contact with an object, they are considered contact switches. Most limit switches operate mechanically and contain heavy-duty contacts capable of switching higher currents than those of proximity sensors.
A limit switch consists of an actuator with an operating head, the switch body mechanism, and a series of electrical terminals used to connect the switch to the electrical circuit it is controlling. The operating head is the part of the limit switch that comes into contact with the target. The actuator is connected to the operating head, the motion of which is translated by the actuator to open or close the switch. Within the switch body is the switch mechanism, whose state is controlled by the actuator. The electrical terminals are connected to the switch contacts, enabling wires to be joined to the switch through terminal screws.
Industrial machinery used for automatic operations often require control switches that activate based on the movements involved in the machine’s performance. For repeat usage, the accuracy of electrical switches must be reliable and their response rate must be prompt. Because of the mechanical specifications and performance parameters of various machines, factors including size, operational force, mounting method, and stroke rate are important characteristics in the installation and maintenance of limit switches. Furthermore, to avoid instrument failure, the electrical rating of the limit switch should be matched to the mechanical system loads that it will be controlling.
In most cases, a limit switch begins operating once a moving machine or moving component of a machine comes into contact with an actuator or operating lever that activates the switch. The limit switch then regulates the electrical circuit that controls the machine and its moving parts. Limit switches can also be used as pilot devices for magnetic starter control circuits, allowing them to start, stop, slow, or accelerate the function of an electric motor. Limit switches can be installed into machinery as control instruments for standard operations or as emergency devices to prevent machinery malfunction. Most switches are either maintained contact or momentary contact models.
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