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As its name suggests, a limit switch regulates the operations of machines that are equipped with moving parts connected to a switching action mechanism. A wide range of industrial machinery uses limit switches to control the movement of devices performing on a production line, but these switches are also found in non-industrial applications, such as electric motor operation and garage door opener units. In the case of a garage door opener, a limit switch is responsible for turning off the motor that lifts the door before the door crashes into the lifting mechanism. The switch also deactivates the motor as the door closes, preventing it from being pushed into the ground. Limit switches enable this and similar operations to work as semiautomatic processes by regulating an initiated action to keep it within performance parameters.
When installed in a machine system, a limit switch can usually start, stop, slow down, or accelerate operations, as well as activate a forward or reverse process. In order to perform these actions, limit switches are designed in a variety of shapes, sizes, and capacity ranges to accommodate differences in machine systems and production processes. A limit switch is typically composed of a series of electrical contacts coupled to an actuator that controls the mechanical device responsible for on and off functions. Limit switch instruments are employed in a broad range of applications due to their straightforward design, relatively simple installation requirements, reliability, and resilience in withstanding environmental conditions.
Types of Limit Switches
Limit switch performance depends on a number of factors. In addition to the operational parameters and mechanical specifications of a machine, these factors include the size, mounting method, and force capacity of the switch, as well as the stroke rate involved in the operating process. It is important for a limit switch’s electrical rating to match that of the system into which it is installed in order to reduce the potential for instrument failure and ensure proper functioning. The common types of limit switches used in industrial applications include:
• Heavy-Duty Precision Oil-Tight: Also known as the Type C limit switch, this device is highly reliable due to its long electrical and mechanical lifespan. It features a straightforward wiring arrangement and relatively easy installation. The Type C can be equipped with a range of different head and body styles, including a more durable design that is watertight and submersible. It is available in a standard format, as well as with specialized reed contacts.
• Heavy-Duty Oil-Tight and Foundry: When load requirements exceed the capacity range for a precision oil-tight switch, a regular heavy-duty oil-tight model, or Type T, may be needed. It can handle operating sequences unavailable on the Type C and can withstand high trip and reset forces. The heavy-duty foundry limit switch, or Type FT, is commonly used in foundries and mills where Type T operating conditions are coupled with elevated temperatures and foreign materials that may jam other types of switches.
• Miniature Enclosed Reed: This limit switch, also known as Type XA, is a smaller and less expensive device formed from die-cast zinc. It contains a contact array featuring a hermetically sealed reed, which makes it well-suited for applications that require a high level of contact reliability or involve environmental stresses. The switch is normally prewired and can be placed in smaller or harder to reach areas.
• Gravity Return: The gravity return limit switch is usually employed in production line and conveyor operations involving small, lightweight components. This type of switch relies on gravity to reset its contact switches by exerting force on a lever arm and typically functions with a low level of torque. There are several varieties of gravity return switches, including spring return, roller type, lever type, top push, and maintained contact designs.
• Snap Switches: A snap switch is designed to instantly trigger as soon as the mechanism attached to the switch has moved a predetermined distance, regardless of the speed at which the moving part travels. Snap switches are commonly used in applications that require only basic contact parameters and can work with or without an operator. They are effective in machine systems that feature short movements or a slow rate of operation.
Limit Switch Circuitry
To better understand the way a limit switch circuit operates, it may help to look at an example that illustrates contact switching principles. A limit switch with a single-station, maintained contact design will have a “Start” button that mechanically controls the contacts. Pressing the “Start” button causes the mechanism to maintain the contact sequence that closes the circuit, while pressing the “Stop” button will open the contacts and break the circuit. If a system malfunction, such as power failure or overloading, causes the switch device to deactivate, the contacts will be unaffected and the motor will automatically reset.
For illustrations and information on the symbols used for designating contact sequences, visit Fundamentals of Electrical Engineering and Electronics.
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