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Technical GuideOverload Protection and Safety Systems
Overload limiters, limit switches, e-stops, brake systems and anti-collision on cranes: what each safety function does and which are mandatory.
The overload limiter
An overload limiter measures the load on the hook and cuts the hoisting motion once a set limit is exceeded. The measurement method varies by system: a load cell fitted at the rope anchorage, measurement through a rope pin, or indirect detection from motor current.
Direct-measuring systems are more reliable; simple current-based solutions are affected by temperature and voltage variation. When the limiter trips, hoisting stops, but lowering the load is not blocked — otherwise the load would be left stranded in the air.
The limiter's set point isn't arbitrary and is tested at the periodic inspection. A limiter disabled because it "wasn't working" is one of the most common — and most serious — non-conformities found during inspections.
Limit switches
A limit switch cuts the circuit once a motion reaches its end point. On a typical crane these are found at: the hook's upper limit (the most critical — it prevents the rope winding onto the drum and snapping), the hook's lower limit (ensures enough rope remains wound on the drum), and the end positions of the bridge and trolley.
On heavy-duty cranes, the hook's upper limit typically uses two-stage protection: the first switch stops normal operation, and a second (emergency limit) engages if the first one fails.
The brake system
Crane brakes are typically spring-applied, electromagnetically released: the brake sits closed under spring force and opens when power is applied. The reason is simple — if power is cut, the brake closes automatically and the load doesn't fall.
The hoist brake is the single most critical safety element in the lifting motion, and lining wear is measured at the periodic inspection. Travel brakes affect stopping distance and load sway.
A "the brake won't hold" complaint is often not lining wear at all but an out-of-adjustment air gap: as the lining thins the gap grows, and past a point the brake no longer closes fully.
Other safety functions
| Function | What it does | Where it's required |
|---|---|---|
| Overload limiter | Cuts hoisting above the set limit | All lifting machinery above 1,000 kg (check your local rule) |
| Hook upper limit | Prevents the rope winding onto the drum | On every crane |
| Hook lower limit | Leaves a safety wrap on the drum | On every crane |
| End-of-travel limit switch | Stops the bridge and trolley at the end of the rail | On every crane |
| Emergency stop | Cuts all motion instantly | On the control pendant/radio and the panel |
| Anti-collision | Prevents cranes on the same rail colliding | Multi-crane runways |
| Buffer | Absorbs impact at the end of the rail | At the ends of the runway |
| Phase protection | Blocks motion on incorrect phase sequence | In the panel |
Technical terms
| Term | What it means |
|---|---|
| Overload limiter | A safety system that measures the load on the hook and cuts hoisting once the limit is exceeded. |
| Load cell | A measuring element that converts load into an electrical signal. |
| Limit switch | A switch that cuts the circuit once a motion reaches its end point. |
| Air gap | The set gap between the lining and disc in an electromagnetic brake. |
| Safety wrap | The minimum rope that must remain wound on the drum when the hook is at its lowest position. |
| Buffer | An element that absorbs impact energy at the end of the rail. |
Frequently asked questions
The limiter is tripping but the load is below rated capacity — why?
The most common causes: the setting has drifted, the load cell has failed, friction has increased in the rope system, or the hook has snagged on something. Raising the setpoint isn't a fix, it's covering up a risk — the measurement should be verified first.
What's the difference between an emergency stop and a normal stop?
A normal stop cuts motion from the controls; an emergency stop cuts the power circuit directly, stopping all motion instantly and keeping the system locked out until manually reset.
Are safety functions tested at the periodic inspection?
Yes. The overload limiter, limit switches, brakes and emergency stop are standard test items at a periodic inspection. A disabled safety element is recorded as a non-conformity in the report.