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How to Select Limit Switches?



A limit switch is a device that uses the collision of machine moving parts to activate the contacts, allowing the control circuit to be turned on or off for a specific control purpose. It works together with other devices to create more complex industrial automation equipment. As a result, selecting more appropriate limit switches will improve the overall efficiency and logic of the automatic control system.



Here are some key points for choosing limit switches:


1. SHELL : The material of the limit switch shell varies depending on the load. For light and medium loads, the shell is typically a type of plastic with or without fiber glass mixture. The shell material for large and heavy weights are usually aluminium alloys. Additionally, we have special types of shells made with materials to resist difficult environments such as high corrosion or high heat temperatures. As a result, selecting the suitable shell is necessary.

Click to learn more about: Corrosion Resist, High Temp. Resist products.


2. ACTUATOR : Actuator: Different acturatos are selected according to the shape and movement direction of the detection target. Common types of actuators include roller plunger, top plunger, roller lever, adjustable roller lever, simulated roller lever, adjustable rod lever, fork lever, coil spring, and more.


3. ELECTRICAL VALUES : Select the limit switches series according to the rated voltage and rated current of the control circuit.


4. CONTACT POINTS & FORMS : According to different needs, select different number of contact points and forms. Commonly found contact forms are: Fom A, Form B, Form C and Form Z. Each may be configured for singular or multiple points; thus creating many Poles and Throws choices. The most common are: SPST, SPDT, DPST, and DPDT.

Click to learn more about: Electrical Forms, Poles and Throws.


5. INGRESS PROTECTION : Choose the protection mode based on the installation environment: open type or enclosed type. Environmental considerations such as ambient temperature, vibration and shock resistance, and protection class are also taken into account when selecting a limit switch. The waterproof limit switch, for example, can be utilized in industrial applications that require waterproofing, oil resistance, and dust resistance.



Additionally to above key points, the following details are recommended for consideration as well:


1. OPERATING FREQUENCY : This means the maximum allowed triggering frequency. If this speed is exceeded, the limit switch may have malfunction or be damaged due to excessive force.


2. CABLE INLET/OUTLET SIZES : There are several types of cable through-holes that comply with different standards. Most common are EU, Japanese, and USA standards; with some sizes overlapping to being the same. Commonly produced are:

- Strain reliefs with dimension for SJT18/4 18AWG cables,

- Through hole: PF1/2” and,

- Adapters with M20 threads


3. OPERATING FORCE : The force required for triggering. Commonly denoted as "OF".

Heavier duty switches will have operating forces in the kilograms. Whereas small microswitches will have forces in the low tens of grams only. Using a microswitch in a heavy duty enviornment will destroy the microswitch.


4. SERVICE LIFE : The service life may be divided into machinery working life and electrical working life.


5. INSULATION : Electrical insulation must be considered for special occasions such as installing two electrical circuits in a single switch. Proper insulation is a must have to avoid damage.

Moujen's MJ-7 series is a good example for such application.

Additionally, the MJ2-1 series outer switch enclosure is made of Phenolic material which is an great electrical insulator.


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