Whether it be the industrial, construction, agriculture, commercial, or transportation industry, limit switches have become an integral part of the sensing and control community. There are perhaps trillions of limit switches in use at this very moment in various types of industrial equipment, agriculture or construction machinery, and control systems around the globe. Below I explained all the technical details of limit switches.
What is a limit switch?
The limit switch is a mechanically operated switch that is controlled automatically by factors such as pressure, position, and temperature. It is a very common type of mechanically operated motor control device. A limit switch is designed to operate only when a predetermined limit is reached, and it is usually actuated by contact with an object such as a cam.
As the object makes contact with the actuator of the switch, it eventually moves the actuator to its limit where the contacts change state. Through this mechanical action, electrical contacts are either opened or closed.
What is the function of a limit switch?
Some basic functions of a limit switch are:
- Detecting presence/absence.
- Detecting range of movement.
- Detecting positioning & travel limit.
- Breaking a live circuit when unsafe conditions arise.
- Detecting speed.
Limit switch construction
Most limit switches contain the following functional parts:
The actuator is the part of the switch which physically comes in contact with the target. In some limit switches, the actuator is attached to an operating head which translates a rotary, linear, or perpendicular motion to open or close the electrical contacts of the switch.
Mechanical limit switch actuators are available in many shapes and sizes based on their functionality and application.
Momentary, or “spring return” limit switches return to their normal state as soon as the actuator is released from the object it is sensing.
Maintained limit switches will remain in the actuated position even after the actuator has been released.
The component contains the electrical contact mechanism.
The component containing the terminal screws or clamp assembly necessary for wiring purposes.
Limit switch types
The types of limit switches are:
Levers and roller levers actuate radially. They rotate on an axis. Roller levers are tipped with a roller to lessen the friction and the force required to activate the lever. Levers spring returns to neutral when the force is removed. Roller levers are the most popular option for operators.
Adjustable roller lever
An adjustable roller lever is similar to a standard roller lever. The length of the lever is adjustable from a few millimeters to up to 5 centimeters. The rollers are also available in various diameters.
Plunger heads come in many varieties and sizes. Plungers are actuated by a perpendicular force applied directly to the end of the plunger.
When a plunger switch is needed, but the force will not be applied directly at a 90º angle, a roller plunger can be used. The roller converts some of the non-perpendicular force into perpendicular force that can actuate the plunger.
Rod levers are also actuated radially like levers. They are thin and much longer. Adjustable rods can be as long as 10 or more centimeters.
A flexible rod is similar to a rod lever in its length and appearance. However, unlike levers that actuate only in one plane, flex rods activate in 360º
These switches are tipped with a lanyard to which can be attached a cable. Pulling or tightening the cable draws out the springloaded lever, which activates the switch.
These switches can be installed onto an axis (i.e., a door hinge) so that when the hinge or axis turns, the switch actuates.
Applications of limit switches
Limit switches have wide usage areas such as:
- Machine tools: Metal fabrication equipment, presses, transfer lines, and special machinery.
- Material handling equipment: Conveyors, elevators, cranes, and hoists.
- Packaging machinery and process equipment.
- Textile machinery.
- Construction machinery and equipment, vehicles, and lift trucks.
- Control cabinets.
- Garage doors.
Features of the limit switch
The limit switch is a practical solution for sensing in most situations. Some of the features of the limit switch are listed below:
- It can switch high currents up to 10A.
- It has high precision, accuracy, and repeatability.
- It is an economic sensing solution.
- It can withstand the toughest environmental conditions.
- It consumes low energy.
- It can control multiple loads.
- It has a compact size.
- It is easy to install.
- It must make physical contact with an object to actuate.
- The mechanical component can wear out.
- Generally restricted to equipment operating at relatively low speeds.
Terms about limit switches
Mechanical durability refers to the number of available switching operations on the condition that the switch is actuated to the overtravel position per operation
Electrical durability is the switching durability at the rated load (i.e., a resistive load) with an overtravel set as the reference value.
Rated operating current (Ie)
Rated current for the switch to operate.
Rated operating voltage (Ue)
The rated switch operating voltage, which must not exceed the rated insulation voltage (Ui).
Rated insulation voltage (Ui)
The maximum rated voltage at which the insulation voltage of the switch is maintained. This value is used as the parameter of the dielectric strength and creepage distance of the switch.
Contact element form
The contact element form defines the configuration of the contacts and the number of contacts within the switch. e.g.
Form Za – both contact elements have the same polarity.
Form Zb – the two contact elements are electrically separated.
Limit switch failures
Sometimes a limit switch fails. The reasons are:
- A short-circuit or contact weld due to contact migration.
- Contact weld due to an incorrectly connected power source.
- Foreign materials or oil penetrated the contact area.
- Cracks due to direct sunlight.
- The sealing part has not been tightened sufficiently.
- The wrong switch has been selected.
- Vibration or shock is beyond the rated value.
- Wear and tear of the internal mechanism.
- The operating direction of the actuator is not correct.
- Excessive stroke.
- Too-slow operating speed.
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