Mechanical position and electrical circuitry meet in the no-nonsense limit switch. These cost-effective input devices excel in tough environments and are as simple to install as they are to maintain.

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How it works

Limit switches convert mechanical motion into an electrical signal by using a lever to force open or closed a set of electrical contacts. They come in die-cast metal as well as rugged plastic housings, with a variety of conduit openings based on the intended machine destination. Common styles include pre-wired and connector versions (usually found on enclosed switches), some of which even incorporate indicator LEDs to simplify monitoring and diagnostics.

Because the switch's actuator is in direct physical contact with moving objects, limit switches are designed for easy servicing, especially the replacement of the actuator and internal switch. In some cases, the levers and actuator heads are adjustable, simplifying installation as well.

Versatile switches

Limit switches are used in a variety of applications, including conveyors, elevator control, counting, positioning, detecting, sequencing, and monitoring. Machine operations they initiate when tripped can include start, stop, change direction, recycle, slow down, or speed up. About 80% of all applications use a roller lever type actuator. Plunger types are the next most common, and a few use “wobble stick” rod or spring actuators.

Direct opening limit switches are typically used in failsafe applications such as slow/stop positioning for gantry cranes, elevator doors, and telescoping passenger bridges to aircraft. Enclosed switches are used on conveyors, cutting and machining tools, and a wide range of assembly equipment. Other styles include slim compact switches that facilitate close mounting and multi-element switches used for stacking.

Elements of a limit switch

Common limit switch styles include enclosed pin plunger switches, side rotary lever switches, and direct opening safety limit switches. Choice of style depends on the specific application.

Direct opening contact type limit switches, designated by a symbol of a circle with an arrow inside, avoids one of the problems (stuck contacts) sometimes encountered by other switch types. If metal deposition between mating contacts occurs on the normally closed contact side, the contacts are usually pulled apart by the shearing and tensile forces generated when the safety cam or plunger engages the movable contact blade.

Sizing savvy

Standard limit switches are designed to actuate frequently (up to 30 electrical operations per minute) without long periods of inactivity and with a steady current to prevent corrosion on the contacts. Microload limit switches handle low current applications below 0.1 A. For critical failsafe applications, choose a safety limit switch with direct opening contacts. Limit switches usually come with one normally open (NO) contact and one normally closed (NC) contact. Typical contact configurations include single-pole double throw (SPDT), SPDT double break, DPDT and DPDT double break.

Wiring wisdom

Polarity must be observed when wiring limit switches. Failure to observe polarity may result in arcing between contacts.

Connections to contactors

Using a contactor in the control circuit can help in cases where the limit switch contacts are not rated to directly handle the load.

Be sure to choose the electrical rating of the contacts carefully, staying within the proper voltage and current size according to the load and manufacturer's specifications. If the load current exceeds the contact rating, a relay, contactor, or motor starter must be used to interface the switch with the load. Also be careful not to mix ac and dc in the same circuit.

Inductive load considerations

When testing the switch, it's important to apply the actual load condition together with the true operating environment. Most limit switches are rated with an inductive load — a minimum power factor of 0.4 for ac or a maximum time constant of 7 msec for dc. Because an inductive load causes a problem in dc circuitry, it's essential to know the time constant of the load's inrush current. For example, an A600 rating refers to a “pilot-duty” load under the parameters discussed in the standards and regulations section.

Installation information

With rotary actuator limit switches, the cam or piece (dog) encountering the actuator should be shaped to prevent the roller lever from snapping back freely. Use a cam that's tapered to allow a slow release of the lever. This reduces contact bounce and switch wear, and improves repeat accuracy. In installations where the limit switch actuator encounters a fast-moving cam or piece, make sure the limit switch's lever does not receive a severe impact. Instead, taper the cam to extend the interval over which electrical contacts are engaged. These precautions assure the circuit is completed and prevent excessive switch wear.

Cam configuration

Cams must be configured according to the type of actuator as well as the type of cam being used. Following are guidelines for roller lever actuators (both non-overtravel and overtravel type cams) and plunger type actuators.

Roller Lever Actuators

Non-overtravel type cams

Velocity: less than or equal to 0.25 m/sec

If velocity (v) does not exceed 0.25 m/sec, the lever can be set vertically. As operating speed increases, the cam angle decreases.

Velocity: greater than or equal to 0.25 m/sec but less than or equal to 2 m/sec

In higher-speed operations, the lever angle (θ) may be required to change according to the cam angle (φ). Set the lever angle within a range of 45° to 75.°

Velocity: greater than or equal to 2 m/sec but less than or equal to 3 m/sec

Overtravel type cams

Velocity: less than or equal to 0.25 m/sec

If velocity (v) does not exceed 0.25 m/sec, the lever can be set vertically. As operating speed increases, the cam angle decreases. Cam stroke (y) may be up to 80% of total travel.

Velocity: greater than or equal to 0.5 m/sec

In higher-speed operations, the cam angle (φ) of the rear edge should be in a range of 15° to 30°. Otherwise, make sure the cam's acting surface is straight in order to reduce lever shaking.

Plunger Type Actuators

Cams that override the actuator may have the same shape for both forward and backward directions. Avoid cam shapes that leave the actuator abruptly. Possible reference locations are shown here.

Protecting the limit switch

When an overtravel situation may occur, such as an emergency stop application, choose a rotary actuator lever limit switch instead of a pin or roller-plunger type. Plunger type switches are not designed for use beyond the normal travel length, and will be damaged if the switch is used as a stop without protection. To use a plunger type switch, a stop plate should be added to protect the limit switch and its mounting from damage due to overtravel. A marginal overtravel (OT) value should be set. The ideal value is the rated OT value × 0.7.

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Protecting people

Operator safety should always be considered when installing limit switches. These devices are designed for mechanical activation in automatic control. Prevent accidental activation due to operator error by installing a shield to protect the switch. Also, use a non-overriding cam or dog to activate the switch.

Cost-saving strategies

The best cost-saving strategy is to select a switch that meets the physical demands of the application. Choose an actuator type with the right shape and material in order to minimize wear and reduce maintenance. Design in the switch so that no more than 70% of the overtravel rating is used. This can be accomplished by designing the cam or dog to eliminate snap-back or excessive overtravel on the switch's actuator.

Standards and regulations

Limit switches fall under the category of auxiliary devices for industrial control equipment in the Underwriters Laboratories UL508 certification process. Shorthand created by UL designates a rating code that represents the control circuit load that may be controlled by the device that combines continuous current, maximum current for make and break at various voltages, and maximum volt-amps. Standard duty switches begin with codes B and P; heavy-duty switches begin with codes A and N. The voltage rating at 50 and 60 Hz follows the prefix.

Spud switch

Misapplication of a limit switch can range from merely frustrating to tragic. Our engineers once received a call about repeated premature failures of a limit switch installed as part of a food-handling conveyor. The maintenance engineer explained that he had potatoes pouring continuously past the limit switch's thin wire actuator and into a hopper below. This pounding avalanche exceeded the switch's overtravel rating, causing failures just about every month. A design modification prevented the limit switch and actuator from direct potato contact and provided a link to a more robust actuator type, thereby eliminating the problem.

Safety ratings

In North America, most designers are familiar with both Underwriters Laboratories (UL) and Canadian Standards Association (CSA). However, as more machine builders do business in China, limit switches are now starting to show the CCC ratings as well.

Limit switches are subject to Europe's Reduction of Hazardous Substances (RoHS) regulations and each manufacturer must provide compliance information. California and other states have recently enacted similar regulations. Limit switches used on machines built for Europe and some Latin American countries must be RoHS compliant.

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Limit Switch Types & Usage

Figure 1: Limit switch in a pneumatic industry

Limit switches are devices used to detect the presence or absence of an object or to monitor and indicate whether the movement limits of that object have been exceeded. They are commonly used in industrial control systems and various mechanical applications to control machinery by turning it on or off when an object passes a certain point. This article explores the various types of limit switches and their applications.

Types of limit switches

Limit switches can be categorized based on actuation type and contact configuration.

Based on the actuation type

Mechanical

• Plunger actuated: Plunger limit switches have a plunger or push button depressed by the moving object to activate the switch. They are ideal for simple on-off position detection. Their direct contact nature requires precise alignment, and they are not suitable for applications with high cycle rates due to wear on the actuator.
• Roller lever actuated: Equipped with a roller at the end of a lever, these switches are actuated by the movement of an object that pushes the roller. The roller reduces wear and allows for smoother actuation with less friction. They're versatile, with adjustable levers for different actuation points, but they can be sensitive to misalignment, and the lever can be damaged by excessive force.
• Rod lever actuated: These switches are actuated by a rod or lever pushed by an object. They are typically more robust than roller lever switches and can withstand higher actuation forces but require more activation force.
• Cable pull: Activated by a pulled cable, these limit switches are great for perimeter guarding or monitoring large areas. They offer flexibility in switch placement relative to the moving object but require proper cable tension and can have less precise actuation than direct contact switches.
• Rotary actuated: Rotary limit switches are activated by a rotating cam or wheel that depresses the switch at certain positions. They are precise for rotational movements but require careful setup and can be complex to adjust.
• Whisker actuated: Whisker-type limit switches are sensitive tactile sensors that use a thin metal wire or "whisker" to detect the presence or position of objects through physical contact. They are valued for their high sensitivity and simple design, making them suitable for applications in industrial automation, robotics, and safety mechanisms where precise parts detection is necessary. However, they can be susceptible to environmental factors, prone to whisker fatigue and breakage, and require more maintenance.

Non-contact

These limit switches do not require physical contact with the object, which reduces wear and can be beneficial in harsh environments.

• Magnetic limit switches: Utilizing a magnetic field to detect the presence of a magnet or a ferrous object, these switches are durable and immune to dust, dirt, and moisture. However, their sensing range is limited, and they can be prone to false triggering in environments with stray magnetic fields.
• Inductive limit switches: These switches detect metallic objects through an electromagnetic field. They are robust against dust and moisture and are suitable for high-speed applications. However, they can only detect metal objects and have a relatively short sensing range.
• Capacitive limit switches: Capable of detecting both metallic and non-metallic objects by sensing changes in capacitance, these switches are versatile and can even detect materials through thin barriers. However, they can be sensitive to environmental changes like temperature and humidity.
• Optical limit switches: With a light beam, often infrared, to detect objects, these switches can offer high-speed actuation and are not affected by the object's material. They can be sensitive to alignment, dirt, and dust on the sensor, and their performance can vary with the object's surface and color.

Based on the contact configuration

1. Normally open (NO): The contacts in this configuration are open when the switch is resting and close when the switch is activated.
2. Normally closed (NC): The contacts are closed in the resting state and open when the switch is actuated.
3. Changeover or single-pole, double-throw (SPDT): These have a common terminal and two other contacts where the switch can either close the circuit with the normally open contact or open the circuit with the normally closed contact.

Other types

In addition to mechanical and non-contact actuation, specialized limit switches are designed for specific applications or environments.

• Pressure limit switches: Pressure limit switches are actuated by a change in pressure and are often used in fluid power systems. They are sensitive to pressure variations and can be used to monitor flow rates, but they require calibration and can be affected by temperature fluctuations.
• Thermal limit switches: Thermal limit switches respond to temperature changes and are used in heating and cooling systems. They provide reliable operation within certain temperature ranges but may not have the precision of dedicated temperature sensors like thermocouples or RTDs.

Choosing a limit switch

• When choosing a limit switch, consider the application's specific needs, such as actuation method, environmental conditions, required precision, and potential for wear and tear.
• Mechanical switches are typically chosen for their reliability and simplicity in applications where direct contact is acceptable.
• Non-contact switches are preferred when contact with the object is undesirable or could lead to damage or in environments where the switch might be exposed to contaminants that could affect its operation.
• Specialized limit switches are chosen based on the specific parameters they are designed to monitor, such as pressure or temperature.

Limit switch applications

Industrial use

In industries, a limit switch provides position feedback for machinery. A limit switch can be used to determine when a mechanical part has reached the end of its travel on a production line. This is crucial in automated systems where precise control of movement is necessary. For example, limit switches are installed in conveyor belt systems at the end of the path to signal when an item has reached its destination. When the product touches the actuator of the limit switch, the contacts change state, sending a signal to the control system. This signal can stop the conveyor belt, prevent items from falling off the end, or trigger the start of another process, such as sorting, packaging, or stacking.

Furnace limit switch

The furnace limit switch measures the temperature of the air passing over the heat exchanger and will shut off the furnace if the system overheats. This prevents damage to the furnace and the possible release of dangerous gasses.

A fan limit switch is a control switch in the furnace control system. Its function is discussed below:

1. Temperature sensing: Continuously monitor the air temperature within the furnace or air handler.
2. Fan activation: Activate the blower fan when the temperature reaches a preset fan-on setting (typically around 49 °C - 65 °C or 120 °F - 140 °F).
3. Fan operation: Keep the fan running until the temperature drops below a fan-off setting (usually around 32 °C - 38 °C or 90 °F - 100 °F).
4. Overheat protection: Act as a safety device, shutting off the heating system entirely if the temperature exceeds a high-limit setting (usually around 93 °C or 200 °F) to prevent overheating and potential fire hazards.

These switches are typically inside the furnace's access panel, often mounted on or near the heat exchanger or blower assembly.

Garage door limit switch

Garage door limit switches control the stopping points of the garage door during opening and closing. These switches are usually located at the track or motor unit and are adjusted to ensure the door stops accurately at the open and closed positions. They are typically located on the opener track or motor assembly. Two switches are used:

• Up limit switch: Signals the opener to stop when the door reaches its fully open position.
• Down limit switch: Signals the opener to stop when the door is closed.

HVAC limit switch

An HVAC limit switch is similar to a furnace limit switch. It monitors temperatures within heating and cooling systems and will interrupt power if temperatures exceed safe limits. This protects the system from overheating or freezing. Applications of limit switches in HVAC systems are:

• Overheat protection: Prevent overheating the furnace or heat pump and shut off the system if the temperature exceeds a safe threshold (usually around 93 °C or 200 °F).
• Blower control: Limit switches control the fan to manage airflow and temperature distribution. They combine a temperature sensor and a switch within a single housing. The limit switch activates the blower when the furnace reaches a preset temperature and continues running until the temperature drops below a lower limit.
• Damper and valve control: Roller limit switches are used for position sensing in dampers, valves, and other moving components. They are activated when a roller contacts a cam or lever, indicating a specific position has been reached.
• Condensate overflow prevention: Protect against water damage.
• Refrigerant pressure control: Pressure switches monitor the system's air or refrigerant pressure and trigger control actions to protect compressors and maintain proper system operation.

Water heater limit switch

A water heater limit switch is a safety device designed to shut off the power to the heating element if the water temperature exceeds a safe level. They are typically located near the top heating element or thermostat within the water heater's access panel.

Elevator limit switch

Elevator limit switches control the positions of the elevator car. Applications of limit switches in elevators are summarized below:

• Prevent overtravel at the top and bottom of the hoistway.
• Initiate speed reduction as the elevator approaches a floor.
• Control door opening and closing sequences.
• Restrict access to certain levels or areas.

FAQs

Where is the limit switch on a furnace?

Typically, the limit switch in a furnace is located near the burner assembly or within the blower compartment.

What is a high-temperature limit switch?

A high-temperature limit switch is a safety device that shuts off a furnace if internal temperatures exceed safe limits, preventing overheating and potential damage.

What does a proximity limit switch/sensor do?

A proximity limit switch/sensor detects the presence or absence of objects nearby without physical contact, often using inductive, capacitive, magnetic, or optical technology to trigger a response when an object enters its sensing field.

What is a pneumatic actuated limit switch?

Pneumatic limit switches rely on air pressure change caused by the approaching object, which then actuates the switch.

What is a pressure limit switch?

Pressure limit switches respond to fluid pressure. They ensure safe pressure levels in pipelines and activate alarms in the event of overpressure conditions.

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