Limit switch
A mechanical switch operated by a moving part reaching a position — often for end-of-travel and safety.
What it is
A limit switch is a mechanical switch operated when a moving machine part physically pushes its lever, plunger, or roller. It tells the control system that something has reached a position.
How it works
The actuator (lever/roller/plunger) is pressed by a cam or the moving load, operating the internal contacts (NO/NC). Remove the pressure and a spring returns it to rest.
Because they're mechanical, they wear: a stuck plunger, bent actuator, or worn cam can make them operate early, late, or stick made. On overtravel and safety duties they must never be bypassed.
Where it's used
End-of-travel stops on hoists, doors, and conveyors; position confirmation; and safety interlocks (guards). Often paired with slowdown/stop limits for controlled positioning.
What it looks like when it fails
Mechanical wear does most of the damage: sloppy levers that operate late or not at all, actuators bent by over-travel, plungers gummed with product, springs that no longer return. Contacts corrode in washdown areas and go intermittent long before they go open.
Cable failure at the gland is the other regular — anything that vibrates fatigues the tail, so the switch tests perfectly while the break hides inside the sheath.
How to test it
With the machine isolated so nothing can respond, meter across the contacts and operate the lever by hand: you want a clean NO↔NC changeover at a sensible point in the travel. Then compare where it operates against where the machine actually pushes it — a switch that works in your hand but sits 5 mm shy on the machine is an adjustment fault, not an electrical one.
Wiggle the cable at the gland while watching the meter. Intermittent there means the tail, not the switch.
Gotchas
Safety and end-of-travel duties must be wired through the NC contact, so a broken wire reads as 'stop'. Wired through the NO contact, a snapped conductor silently removes the protection — if you find one like that, it's a defect to report, not a preference.
Lever angle and roller orientation are part of the setting: a replacement fitted '10 degrees off the old one' moves the trip point. And product build-up shifts trip points slowly — clean before you adjust.
Safety first
Never defeat a safety or overtravel limit switch — it prevents crushing and overtravel. Confirm true position before moving anything.
Isolate, lock out / tag out, and prove dead before working unless a live test is specifically required, authorised, and carried out under proper supervision. Always follow local regulations, your site procedures, and the equipment manufacturer's documentation.
Related faults
Limit switch or proximity sensor not being detected
A limit switch or proximity/photo sensor isn't registering — the machine doesn't stop at position, the input never makes, or the sensor LED looks wrong for the target's position.
Limit switch stuck made (won't release)
A mechanical limit switch stays operated even after the actuator leaves it — the input stays made, so the machine thinks it's still at that limit.
Travel limit stops a direction too early
The drive stops short in one direction — a travel limit is operating before the load reaches the proper end position, cutting the movement off early.
Related definitions
Proximity sensor
Detects the presence of a target without contact — inductive, capacitive, or photoelectric.
Interlocks
Logic that prevents an unsafe or impossible combination of states — like two contactors closing together.
Forward / reverse circuit
Two contactors run a motor in either direction; reverse swaps two phases, and an interlock prevents both closing at once.