Hi there! I wanted to provide you with a comprehensive overview of limit switches – those clever little gadgets used to control machinery movements and processes. Whether you’re an engineer looking to integrate them into a system or a budding technologist interested in how they work, my goal is to cover everything you need to know. Read on for a crash course in all things limit switches!
Let’s start from the beginning – a limit switch is an electromechanical device that controls motion and automatizes processes within preset safety limits. They contain an actuator mechanism linked to electrical contacts inside an enclosure.
When the preset limit is reached, the actuator flips the contacts to either open or close a control circuit. This sends a signal to stop the machine, open a valve, reverse direction…whatever the application requires.
Limit switches come sealed in rugged metal housings and are designed to withstand harsh environments – think heavy vibration, voltage spikes, temperature swings, water, oil, impacts, etc.
They contain silver cadmium oxide contacts renowned for their longevity and resistance to electrical arcing. Using the snap-action operating principle, the contacts cleanly transition from closed to open with ultra-fast reaction times measured in milliseconds.
While configurations vary across types, all limit switches consist of three main parts:
1. Actuator
This is the protruding lever, plunger, whisker, or roller that makes direct contact with the machine or object in motion. When displaced, it actuates the switch. Actuators come in wide varieties – rotating levers, linearly moving rods & plungers, and flexible whiskers.
2. Enclosure
The rugged housing contains the electrical control contacts, small springs, guide rails, stop pins and any mechanical linkages required. Most feature epoxy powder coating for extreme durability with ratings from IP65 to IP67 for water ingress protection. Electrical ratings typically range from 10 to 15+ amps at 115 to 690VAC.
3. Switch Contacts
These silver metal electrical contacts open or close when the actuator shifts their position. The snap-action design provides quick & consistent transition between the ON and OFF states. Contact configurations can be SPDT, DPDT, or 3PDT with up to 4 poles switching control circuits.
How Do They Work?
Limit switches automatically govern range of motion or count items by directly sensing contact with a machine component. Here’s a quick rundown:
- A machine or object contacts the limit switch actuator (lever, plunger, etc.)
- The actuator displacement mechanics sliding rails, springs, toggles, etc. to snap the electrical contacts from closed to open (or vice versa).
- These rapidly switching contacts send a control signal to actuate a relay, PLC, solenoid valve, drive motor or other output device.
- The interfaced machine receives the stop/start/reverse command per the application logic requirements.
This electromechanical process allows accurate and reliable control of any motion that needs strict limits. Now let’s look at the most common limit switch types used across industrial applications.
While many specialty configurations exist, most limit switches fall into one of these 4 categories based on their actuator type:
1. Whisker Limit Switch
Whisker limit switches contain a long metal arm that protrudes from the switch body, hence the whisker designation. The arm deflection sends the control signal.
Inventor: Whisker switches trace their origins to 1950s General Electric Toggle limits. Other early whisker switch makers were Micro Switch, Honeywell, and Allen-Bradley.
How it Works: Any item contacting the metal whisker from ANY direction bends it enough to snap internal toggle links and actuate contacts to open or closed states depending on the control logic required.
Whisker switches provide omni-directional sensing ability thanks to the flexible spring-like arm. Their long activation range also allows detection before the machine reaches the end travel limit.
Whisker Limit Switch Cutaway (Credit: AutomationDirect)
Unique Traits:
- Omni-directional sensing
- No dead zone – senses along entire arm length
- Long activation range reaches limit prematurely
- No friction – improves lifetime accuracy
Typical Applications:
- Conveyor systems
- Transfer lines
- Automated machinery
- Crane positioning
- Assembly verification
- Robotic arms
Recommended Model: Allen Bradley 802T Whirla-Switch
2. Lever Limit Switch
Lever limit switches contain an actuator arm that rotates on a central pivot pin. When the preset angle is reached, internal toggle links shift and snap contacts open/closed.
Inventor: Heimann Electric pioneered early lever limit switches in the 1940s initially using them for elevators and lifts. Allen-Bradley, GE, Square D and other US brands further popularized them in the 1950s and 60s.
How it Works: A lever actuator arm rotates via contact against a machine component or other object in motion. Upon reaching the set angle, spring-loaded toggle links instantly slide channels to open or close internal electrical contacts to send the control signal to stop motion.
Lever Limit Switch Cutaway (Credit: PCS Switches)
Unique Traits:
- Uni-directional sensing
- Adjustable lever provides customizable angles
- Super fast response time – rated down to 2 milliseconds
- Highly repeatable accuracy – 1 million cycles typical
Typical Applications:
- Machine doors & safety gates
- Stop, limit pins
- Lifting equipment
- Packaging machinery
- Movable guards
- Automatic doors
Recommended Model: Allen Bradley 802L Lever Limit Switches
3. Roller Plunger Limit Switch
These contain a spring-loaded plunger inside that depresses against a roller arm lever to actuate internal contacts.
Inventor: Developed by German switch maker Bernstein in the 1960s initially for control of presses and machine tools. Brands like Telemecanique, Honeywell and Schneider Electric now manufacture them.
How it Works: The roller transfers linear motion against its spring-loaded plunger which snaps internal toggle links to slide electrical contacts into open or closed states. The rolling action reduces friction for smooth operation.
Roller Plunger Limit Switch Cutaway (Credit: PCS Switches)
Unique Traits:
- Enclosed plunger protected from debris & liquids
- Roller handle reduces friction
- Extremely repeatable with minimal hysteresis
- Designed for rugged high cycle applications
Typical Applications:
- Pressing & stamping machinery
- Injection molding
- Lifting equipment
- Pallet stops
- Packaging equipment
Recommended Model: Telemecanique XCKJ
4. Plunger Limit Switch
These compact switches contain a spring-loaded plunger protruding from the housing. Linear plunger movement actuates the internal toggle contacts.
Inventor: Initially designed by Allen-Bradley in the 1940s for automation of American manufacturing plants supplying WWII military needs. Square D, GE, and Honeywell also released early models.
How it Works: When the plunger depresses against a machine component or moving object, inner spring forces toggle links to slide into position and snap electrical contacts into ON or OFF states per the control logic requirements.
Plunger Limit Switch Cutaway (Credit: ELN Global)
Unique Traits:
- Direct & highly repeatable actuation
- Minimal internal parts
- Compact size fits where others can’t
- Streamlined design simplifies installation
- Cost-effective workhorse solution
Typical Applications:
- Machine slides
- Presses
- Conveyors
- Lifts
- Robotic arms
- Automated doors and gates
- Assembly line stops
Recommended Model: Allen Bradley 802T
With so many models available, here are key factors to consider for your application:
Approvals
- Does it meet safety and performance standards for your country and industry? Look for UL, CSA, CE, CCC and other international quality marks.
Electrical rating
- Voltage, current, and poles must match electrical control system. NEMA 1, 3, 4, 4X, 6, 12 ratings indicate environmental protection level.
Contact type
- Slow break contacts allow higher capacitive loads while fast make/break work well for logic sensing. Consider required speed and load current.
Actuator configuration
- Is a rotating lever, enclosed plunger or omnidirectional whisker required to interface with the machine?
Environment
- Temperature, vibration, moisture and exposure to liquids/debris determine housing seal rating needed.
Quality & Lifetime
- Carefully check cycle test ratings and warranty. Limit switches often outlive the machinery they are installed on.
While electromechanical limit switches have been around for over 70 years, recent design innovations include:
Diagnostic feedback – “Smart” limit switches contain dual integrate microprocessors to collect data on the lifespan and performance – counting cycles, sensing vibration, monitoring electrical loads. Some dynamically indicate when to replace the switch before failure.
Expanded platforms – New lines integrate expanded safety capabilities like emergency stop buttons and keyed interlocks into limit switch enclosures according to latest standards.
Wireless connectivity – Recent advances eliminate home-run wiring back to the control panel and networking limits switches via Bluetooth meshes or other wireless means, vastly reducing installation costs.
Miniaturization – Nanoscaled machines and robotics have driven new micro limit switches into compact cube packages with incredibly fast response times under 1 millisecond.
So while limit switch technology has greatly advanced, their core safety function remains preventing catastrophic failures by controlling range of motion within strictly defined mechanical limits.
There are over a hundred companies producing limit switches globally. However, these brands lead worldwide market share:
Schneider Electric/Telemecanique – very popular formonitoring industrial presses, robotics arms, plastic molding machinery, conveyor systems, and lifts in Europe, Asia and North America. Known for ruggedness and precision.
Allen-Bradley – originated limit switch technology in the 1940s and still dominates US market across all industries due to advanced features and network connectivity to Rockwell Automation systems.
Omron – leading Japanese brand focused more on micro switches for electronics versus heavy machinery. Known for ultra-fast response times.
Siemens – German giant acquired Bernstein, creator of the roller plunger switch. Offers very customizable limit switch configurations.
General Electric / CRaters & Hammond – MIT partner and early pioneer now owned by GE. Models feature lifetime lubrication and left/right orientation options.
While most applications are well served by the major brands above, specialty limit switch producers focus on unique use cases:
Steute – German expert in extreme temperature and hazardous location switches certified to ATEX and IECEx standards.
FlexLink – European conveyor equipment leader with advanced control feedback modules integrated into limit switches to collect operating analytics.
Euchner – patented the first touchless electronic switch in 1976. Now focuses exclusively on industrial safety systems.
Sprecher & Schuh – Swiss company revolutionizing traditional concepts with CANopen networked smart limit switches featuring onboard microprocessors and expanded add-on modules.
I know we covered a lot of ground on the wonderful world of limit switches. Let me know if you have any other questions! Whether you’re designing a new process, improving an existing machine, or simply want to learn about these clever electromechanical gadgets, I’m always happy to chat more.
