Why is Limit Switch Supplier Better?

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What are Limit Switches?

Limit switches are used to automatically detect or sense the presence of an object or to monitor and indicate whether the movement limits of that object have been exceeded. The original use for limit switches, as implied by their name, was to define the limit, or endpoint, over which an object could travel before being stopped. It was at this point that the switch was engaged to control the limit of travel.

How Does a Limit Switch Work?

A standard limit switch used in industrial applications is an electromechanical device that consists of a mechanical actuator linked to a series of electrical contacts. When an object (sometimes called the target) comes in physical contact with the actuator, the actuator's consequential movement results in the electrical contacts within the switch to either close (for a normally open circuit) or open (for a normally closed circuit) their electrical connection. Limit switches use the mechanical movement of the actuator plunger to control or change the electrical switch's state. Similar devices, such as inductive or capacitive proximity sensors, or photoelectric sensors, can accomplish the same result without requiring contact with the object. Hence, limit switches are contact sensors in contrast to these other types of proximity sensing devices. Most limit switches are mechanical in their operation and contain heavy-duty contacts capable of switching higher currents than those of alternative proximity sensors.

Components of a Limit Switch

Limit switches consist of an actuator with an operating head, a switch body mechanism, and a series of electrical terminals that are used to connect the switch to the electrical circuit that it is controlling. The actuator is the part of the limit switch that comes in contact with the target. The actuator is connected to the operating head. The operating head allows the actuator's linear, perpendicular, or rotary motion to be translated into a motion that allows the switch to be opened or closed. The switch body contains the switch contact mechanism whose state is controlled by the actuator. The electrical terminals are connected to the switch contacts and enable wires to be joined to the switch through terminal screws.

Industrial machinery that undergoes automatic operations usually requires control switches that activate according to the movements involved in a machine's performance. For repeat usage, the accuracy of the electrical switches needs to be reliable and their response rate should be prompt. Due to the mechanical specifications and performance parameters of different machines, factors such as size, operational force, mounting method, and stroke rate are important characteristics in the installation and maintenance of limit switches. In addition, a limit switch's electrical rating should be matched to the mechanical system loads that it will be controlling in order to avoid instrument failure.

Limit Switch Uses and Operation

In most cases, a limit switch begins operating when a moving machine, or a moving component of a machine, makes 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. These switches can be used as pilot devices for magnetic starter control circuits, allowing them to start, stop, slow down, or accelerate the functions 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.

Limit Switch Contacts

Control schematics for limit switches usually display a limit switch symbol to indicate the state of the switch contacts. The most common contact symbols show whether the device has normally open or normally closed limit switch contacts. The symbol for a 'normally open held closed' state indicates that the contact has been wired as a normally open contact, but when the circuit is put into its normal off state, part of the machine keeps the contact closed. Likewise, a limit switch that is designated as 'normally closed held open' will have a closed wiring design but be held open. Other types of contacts, such as those used in pressure and flow switches, can be configured in a similar way.

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Micro Limit Switches

The micro limit switch, or micro switch, is another type of limit switch commonly found on control circuits. These switches are much smaller than their standard counterparts, allowing them to be installed in narrow or cramped spaces that would normally be inaccessible to other switches. Micro switches usually have an actuating plunger that only has to travel a small distance in order to trigger the contact sequence. The actuating plunger is often found at the top of the micro switch and must be depressed a predetermined amount before it activates. A small degree of movement is able to change the contact positions due to a spring-loading mechanism that causes movable contacts to snap between alternating positions. Micro switches can be designed with a range of different activating arms, and have contacts with electrical ratings that are usually around 250 volts for alternating current and between 10 to 15 amperes (amps).

Much like micro limit switches, subminiature micro switches are intended for use in applications requiring compact designs and limited space availability. They have contact arrangements with spring-loading mechanisms similar to those of micro switches, but tend to be from one-half to one-quarter the size of regular micro switches. Depending on the particular model, subminiature switches have contacts with electrical ratings ranging from about 1 to 7 amperes due to the reduced size of the switches themselves.

Advantages and Limitations of Limit Switches

Limit switches offer several advantages intrinsic to their design:

• The designs are generally simple and straightforward
• They work well in almost any industrial setting
• They exhibit high accuracy and repeatability
• They are low power consumption devices
• They can switch high-inductance loads
• They can be used to switch multiple loads
• They are simple to install
• They are rugged and reliable
• They typically have heavy-duty electrical contacts meaning they can be used to switch higher levels of current directly without the need to utilize secondary relay control

Limit switches also have several limitations, which means they may not be suited for every application:

• Because they rely on mechanical action, they generally are used in equipment that operates at relatively low speeds
• They are contact sensors, meaning they must make physical contact with the target for them to operate
• The nature of their mechanical design means that the devices are subject to mechanical wear or fatigue over time and will need eventual replacement

Key Limit Switch Terminology

There are several key terms that are associated with the design of limit switches. Here is a brief summary of those terms for reference:

• Pre-travel ' represents the distance or angle through which the actuator on the limit switch must travel before it trips the switch contacts
• Operating Point ' represents the actuator's position when the switch contacts move into the operating position
• Release Point ' represents the actuator's position when the contacts return to their original state
• Differential ' represents the distance or angular displacement (degrees) between the operating and release points (i.e. between when the contacts trip and when they reset)
• Overtravel ' represents any motion of the actuator component past the trip point of the switch
• Initial Position ' represents the position of the switch's actuator when it not subjected to any external forces
• Operating force (torque) ' represents the magnitude of the force (or the torque for angular movement) that is needed to produce actuator movement.
• Minimum return force (torque) ' represents the magnitude of the force (or the torque for angular movement) that is required to return the switch actuator to its initial position
• Total Travel ' the maximum distance the actuating element is capable of traveling during its operational cycle
• Repeat Accuracy ' represents a measure of the degree to which the limit switch is able to repeat its characteristics during repeated (successive) operations.

Summary

This article presented a brief overview of limit switches including their operation, components, advantages, and definitions of key terminology.  For information on other topics, consult our additional guides or visit the Thomas Supplier Discovery Platform where you can locate potential sources of supply for over 70,000 different product and service categories, including over 500 suppliers of limit switches.

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