What is a Limit Switch?
Limit switches are types of automatic sensors that detect the position of an object through physical contact. An object moves the actuator that opens or closes a set of electrical contacts housed in the switch body, which are connected to equipment circuits by the connection terminals.
In contrast, proximity and photoelectric sensors detect the presence of objects without physically touching them. Proximity sensors detect objects by monitoring changes in inductance and capacitance (using inductors and capacitors housed in the switch body), while photoelectric sensors use LED's coupled with light-sensing transistors.
How does a limit switch work?
The main components of a limit switch are the switch body, the connection terminals, and the actuator.
The switch body, or contact block, includes the enclosure and electrical contacts.
The connection terminals are where you connect the input and output wires. In most cases, a limit switch is connected to some sort of control circuit (e.g. relays or programmable logic controllers).
The actuator physically touches the target object and will open or close the switch contacts. Most limit switch actuators are either plungers or levers, which are also called "arms," and may have roller operating heads.
Actuators fall into two general categories: momentary or maintained contact. Momentary contact, or spring-return, actuators will return to their normal unoperated position once the operating force (e.g. pushing down a plunger) has been removed. In contrast, maintained contact actuators will stay in a given position until moved by an external force.
Some actuators also come with an operating head, like a roller, that better accommodates the motion of an object as it moves. For example, an arm or plunger with a roller operating head may be better suited for some assembly lines than a spring rod actuated switch (see pictures below under "Actuator Types").
More on Actuators
Snap-action switches and the tipping-point mechanism
Since the defining feature of a limit switch is that it is a physical contact sensor, it helps to better understand what a tipping-point mechanism is, as most limit switches are snap-action.
Illustration of a snap-action plunger
The key thing to remember is that a limit switch is meant for use in physical contact object sensing/detection applications. Thus, you may use limit switches anywhere you need to remotely detect the presence, absence, or position of an object where physical contact with the target object and the sensing mechanism (i.e. the switch actuator) is allowed.
Some common limit switch applications include turning on lights when you open refrigerator or car doors, counting items on a conveyor belt, or controlling the wide variety of automated processes in manufacturing facilities.
The content we provide is meant to inform you and help support the proper selection and use of the switches we sell. As always, we recommend you consult a licensed and competent electrician to help you with the sizing and selection of parts for your particular application.
Please refer to our guide on "How to Select Electrical Switches" for more information.
|Plastic spring rod||CN0007||CN0020||CN0021||CN0022||CN0023||CN0024||CN0025|
|Flexible spring arm||CN0008||CN0026||CN0027||CN0028||CN0029||CN0030||CN0031|
|Cross roller plunger||CN0010||CN0038||CN0039||CN0040||CN0041||CN0042||CN0043|
|Fixed roller arm||CN0011||CN0044||CN0045||CN0046||CN0047||CN0048||CN0049|
|Adjustable rod arm||CN0012||CN0050||CN0051||CN0052||CN0053||CN0054||CN0055|
|Adjustable roller arm||CN0013||CN0056||CN0057||CN0058||CN0059||CN0060||CN0061|
The types of switches vary based on how the contacts are actuated.
- What object(s) do you want to detect? Consider factors like size, shape, and material.
- How will the object(s) be moving? Consider factors such as speed and direction
- Momentary or maintained contact?
- Plunger or lever? Will a roller operating head work better?
- Fixed or adjustable length?
- How far does the actuator need to move before the contacts change state? In other words, where are the trip and reset points, and what is the differential distance?
Plunger switches are operated by pushing down on the actuator. Plunger-type switches can be actuated by push buttons, pins, or rollers. Since there is no need to rotate a lever, these kind of switches are useful for assembly line operations where the space between products is small.
Flexible Spring Arm
These types of limit switches have a long flexible "whisker" that springs back to a normal unoperated position once the actuating force is removed. The advantage of using flexible arm actuators is that the arms can bend and move in any direction.
Plastic Spring Rod
Similar to the flexible spring arm, but with a non-conductive flexible plastic rod instead of a metal whisker.
A roller operating head on the end of a plunger turns horizontal motion into vertical motion that can actuate the switch.
Switches with rollers are often used with cams (i.e. rotating wheels with bumps). When a bump on the cam strikes the roller, it actuates the switch contacts.
Cross Roller Plunger
The orientation of a "cross" roller plunger is perpendicular to a standard roller plunger.
Fixed-length Roller Arm
The actuator for these switches is a roller at the end of a lever whose length is fixed.
Adjustable Rod Arm
A simple lever (no roller at the end) actuates the contacts. You can adjust the length of the lever.
Adjustable Roller Arm
A roller on the end of a lever where the length is adjustable.
How much space is available to mount the switch?
Will the switch be mounted sideways, on a surface, or be hanging from above?
Where does the actuator need to be located? For example, for a given mounting location will the switch need to detect objects above, below, or to the sides?
How many times do you need to open or close the contacts per minute? Since limit switches are often used in manufacturing applications, where there may be hundreds of products passing the switch along a conveyor belt every minute, a high switching frequency (e.g. 120 operations/minute) may be necessary.
Electrical Ratings of the Circuit Being Switched
This includes checking the voltage and current ratings of the switch contacts and whether you are switching AC or DC.
Number of Poles and Throws
Poles refer to the number of circuits you will be switching at the same time. Throws refer to the number of circuits a given pole will switch between.
Number of Normally-Open (N.O.) and Normally-Closed (N.C.) Contacts
This only applies to momentary action switches. An example would be a snap-action micro switch with 1 N.O. and 1 N.C. contact.
Be sure to check the IP or NEMA enclosure rating of the switch body to see if it is suitable for your operating environment.