Limit Switch valves have wide use in our daily life at home as well as in our workplace.
A simple example of the application of Limit Switch is the light comes on inside the fridge whenever we open the fridge door. Here Limit Switch detects if the fridge door is opened or closed.
Another most frequently seen use of Limit Switch is in the overhead shop and garage doors where Limit Switch stops the movement of the door at its fully opened position.
What is a Limit Switch valve?
In an automatically operated machine, Limit Switches are used to convert the mechanical movement of the device into an electrical signal.
As per its name ‘Limit,’ these switches mainly define the limit or boundary of travel of an object or machinery. At the same time, these electromechanical devices indicate the presence or absence of an object.
After getting physical indications, it is easy to operate the whole circuit by converting mechanical motion into an electrical signal.
Limit Switch, Image Source: Automation forum
What is the function of Limit Switch?
Limit Switches are activated by the presence or absence of an object or by the movement of machinery.
The primary function of a Limit Switch is to open or close a contact in a circuit when a certain distance covered by a motor-operated device has been reached. Limit Switch is applied in the control circuit for different purposes like slow down, reverse, or stopping the operation of the machinery.
A Limit Switch consists of an actuator connected to an electrical switch. As soon as a moving machine or a moving machine part strikes the actuator, the operation of the Limit Switch begins and actuates the switch. As a result, the Electrical circuit controls the machine and its motion.
Limit Switches can be used either as a control device for standard operations or as emergency switches to stop the inappropriate functioning of the machine.
Components of a Limit Switch
A Limit Switch is a detection kind of switch inside a metal or resin casing. The outer case is necessary to protect the switch from dust, dirt, external forces, moisture, oil, etc.
The main components of Limit Switch are:
- Actuator: This is the part that comes in physical contact with the moving machine or machine part. It is constructed with tough material to withstand a significant amount of force and shock.
- Operating head: Sometimes, the actuator is connected with an operating head which converts the rotary, linear, or perpendicular motion of the device into an electrical signal to operate the switch.
- Switch body: It refers to the whole electrical contact mechanism.
- A series of Electrical terminals: Screws or screw/clamp assembly essential for wiring purposes.
Components of a Limit Switch, Image Source: @pub/@electrical
What triggers a Limit Switch?
Limit switches are triggered by a physical force applied to the actuator of the switch by machinery. The cams connected to the actuator shaft readily activate the switch.
When the product comes to physical contact with the actuator of a Limit Switch, this physical touch is converted into an electrical signal which activates or deactivates the electrical circuit within the switch. As the product moves away, the actuator and the switch go back to the normal position.
Different types of actuators are used in Limit Switches depending on different applications. Actuators are mainly selected considering certain factors like travel distance, shape, speed, the direction of the machine part being used to trip the Limit Switch. The main types of actuators are flexible rod, plunger, and roller lever.
Function of a Limit Switch, Image Source: omron.com
What is a valve position switch?
Valve position switches are electrical switching devices associated with a Limit Switch for giving an easy visual indication of the current position of the valve assembly.
Valve position switches inform us about the position(open, closed or in an intermediate state) of the valve in the form of continuous signal. These switches can be fixed directly on the actuator of the valve. They can be used in combination with a valve positioner.
Limit switches are activated by the physical touch experienced by the actuator of machinery. The mechanical motion experienced by the actuator plunger is converted into electrical signals which in turn change or regulate the make or break state of the circuit.
Valve position switch, Image credit : aoxactuator.com
How does a Rotary Limit switch work?
Rotary Limit Switches are generally used to control shaft revolutions or to limit movement based on the rotation angle of industrial machinery.
A Rotary Limit Switch is an assembly of gears and cams to trigger a microswitch when the preset number of revolutions has been reached. Here the working principle of a worm drive has been followed. Gears are linked to a cam mechanism that rotates entirely depending on the central gear.
In a Rotary Limit Switch, a shaft must turn a predetermined number of revolutions before the contact changes state as in cranes.
Rotary Limit Switch consists of a shaft that is connected to the shaft of a gear box through a coupling. When the shaft of the gear box rotates, the shaft of the Rotary Limit Switch also rotates. Cams are mounted concentrically with the gears, so as the gears turn, the cams rotate, and rocker arms attached to the cam lobes trip switches at set positions.
Rotary Limit Switch, Image Source: Motion control tips
Generally the input gear is a worm gear but planetary and spur gear are also used. Gear type is decided depending on the requisite gear ratio. The limit at which the switches are activated depends on design of cam lobes and on the ratio of the input gear.
Rotary Limit Switch, Image Source: ohmic-local-manufacture
At least two cams and two switches are essential for a Rotary limit Switch assembly, one for each end or upper and lower limit of travel of the machinery.
Frequently Asked Questions (FAQs)
Q.What are the applications of a Limit Switch?
Ans: Though Limit Switches are available in many forms and perform different functions but in general they are electromechanical switches which are operated by the presence or motion of an object.
Some applications of Limit Switches are as follows:
- In material handling application giving indication of passage of material from one platform to another.
- Widely used in overhead cranes
- Used in automatic machinery.
- Used in High speed equipment.
- Also used in machine tools and rusticate the travel of a machine axis.
- Used to control the liquid level in pumping system.
- Used in elevators and conveyors.
Q. What are the types of Limit Switches?
Ans: Limit switches are classified based on motion of the lever and type of actuating mechanism.
Limit switches are classified based on the actuating mechanism as:- Lever type and Plunger type. Based on the type of motion the limit switches are classified as Rotary motion type and Linear motion type.
Lever Type: A switch with lever type actuating mechanism has a lever attached to it. When the moving machinery or equipment comes in contact with the lever it pushes the lever, which in turns connects or disconnects the electrical circuit. The contact point of the lever with the machinery is normally provided with a roller. The lever is also provided with a retrace mechanism to bring back the lever to original position once the machinery/ equipment is removed from the position. It normally finds use in lifting cranes, lifts etc.
Plunger Type: A Push or plunger type Limit switch on the other hand, gets actuated when the machinery or equipment presses against the push button provided in the limit switch. This is the most common type of limit switch which are available in a car or a refrigerator door. A typical push type limit switch is shown below.
Rotary Motion Limit Switch: A rotary motion limit switch is normally attached to a rotating device such as an electric motor. It detects the rotary motion of the shaft or device connected to it. On achieving specified number of rotations or in other cases angles set for rotation, the limit switch gets activated. They are normally used for rotating service such as crane hoisting or in service where linear motion is translated to rotating motion.
Linear Motion Limit Switch: The most common type of limit switch used in the industrial application is the linear motion limit switch. They detect the linear motion of the equipment or machinery and find wide sue in packaging, manufacturing, motor control and other consumer applications.