Safety functions
In short: Overload protection prevents a crane from lifting loads above its rated capacity. The load is measured by a sensor, a pre-warning is given and hoisting is cut off when the limit is exceeded. Correct sensor selection, correct settings and regular testing make it effective.
Why is overload dangerous?
Every part of a crane is designed to carry the rated load with a safety margin. Overload consumes this margin: rope and hook can be permanently damaged, the girder can deflect permanently, brakes may not hold, gear teeth can be damaged. The damage is often not noticed immediately and turns into an unexpected failure later. Estimates are misleading: dies and semi-finished parts can be heavier than expected, and trying to lift a load stuck to the floor creates very high forces.
When is overload protection required?
Machinery safety regulations require devices that warn the operator and prevent dangerous movements on lifting machinery with a maximum working load of 1,000 kg or more. In practice this covers most industrial overhead cranes; the risk assessment may require it for smaller capacities too.
How is the load measured? Sensor types
| Sensor | Where | Advantage | Caution |
|---|---|---|---|
| Load pin | Equalising sheave or connecting pin | Direct, accurate, robust | Mounting designed for the crane |
| Rope-mounted sensor | Dead end of the rope | Easy retrofit | Calibration for rope type |
| Below-the-hook dynamometer | Between hook and load | Portable, ideal for tests | Reduces hook height |
| Estimation from motor current | In the panel | No extra sensor | Low accuracy; may be insufficient alone |
The sensor signal is read by an overload control unit or PLC and can be shown to the operator on a weighing indicator.
Pre-warning and cut-off settings
- Pre-warning: as the load approaches the rated value (often around 90 %) the operator is warned audibly or visually.
- Cut-off: above the limit, upward hoisting is stopped while lowering remains possible. The cut-off is set only slightly above the rated load; the relevant standards limit this margin (usually to at most 110 %).
- Dynamic effects: the signal is filtered so short force peaks at start-up do not cause nuisance trips, but the filter must never hide a real overload.
Calibration and testing
- Calibrate with known test loads or electronically.
- Test pre-warning and cut-off with known loads: free lifting at rated load, cut-off above the limit.
- Record results; repeat after sensor, rope or hook changes.
- During regular maintenance verify the displayed value with a known load.
At commissioning cranes undergo static and dynamic load tests with test loads above the rated load; these should not be confused with the overload setting. During tests the protection is managed in a controlled way under the supervision of a competent person.
Nuisance trips and missed trips: field problems
| Symptom | Possible cause | Solution |
|---|---|---|
| Trips below rated load | Calibration drift, wrong sensor mounting, dynamic effects | Recalibrate with known load, adjust filter, check mounting |
| Display does not return to zero | Sensor cable, moisture, sensor damage | Check cable and connector, test sensor |
| No cut-off on overload | Cut-off bridged, relay faulty, setting raised | Take out of use; check circuit and setting |
| Value changes with height | Rope weight or sheave friction not compensated | Suitable sensor location and correction |
Why sensor location matters
The sensor must “see” the whole load. In multi-fall reeving a sensor at the rope dead end measures one fall only, and the value is multiplied by the reeving factor. Because sheave friction and rope weight change with height, calibration should be verified at different hook heights. Load pins in the equalising sheave pin usually give more stable results.
Other crane safety functions
| Function | What it does |
|---|---|
| Emergency stop | Stops all motions and drops the main contactor; on the control station and on the crane where needed. |
| Main contactor and isolator | Safely disconnects the crane; lockable for maintenance. |
| Phase sequence and phase failure protection | Prevents motions in the wrong direction and motor damage on phase loss. |
| Upper, lower and emergency limits | Keep hook and crane within safe limits (see the brake and limit guide). |
| Anti-collision | Measures distance between cranes or obstacles with laser/radar and slows down and stops. |
| Zone limiting | Prohibits or slows motion in defined areas (e.g. personnel zones). |
| Slack rope detection | Stops motion to prevent rope slack or mis-spooling. |
| Radio remote safety | Stops motions on loss of communication; emergency stop always has priority. |
| Warning light and siren | Warn people around while the crane moves. |
The value of records
Overload systems can record how often overload was attempted, the highest load, the total number of lifts and the load distribution. This supports operator training and reveals the real load spectrum, so the duty class can be verified and remaining life calculated more accurately.
Retrofitting overload protection
- Inspect reeving, rope dead end and equalising sheave; choose sensor type and location.
- Install the control unit in the panel and connect the cut-off relay to the hoist circuit, leaving lowering free.
- Set pre-warning and cut-off, calibrate and test with known loads.
- Fit a warning light or display visible to the operator.
- Record results and brief the operators.
Smart systems
PLC-based control combines these functions and records loads, overload attempts and operating hours, supporting accident prevention and remaining-life monitoring. See crane automation and Aylen Smart Control.
Need support?
If you need help on site with the topics in this guide, the Aylen team is available 24/7. We inspect your crane, take measurements and provide recommendations in a written report. Quotation and service request form
FAQ
Is overload protection mandatory on cranes?
Lifting machinery with a maximum working load of 1,000 kg or more must have devices that prevent overloading; this covers most industrial overhead cranes.
At what value does overload protection cut off?
A pre-warning is usually given around 90 %; the cut-off is set slightly above the rated load and the relevant standards limit this margin (usually to at most 110 %).
Can overload protection be added to an existing crane?
Yes. A load pin or rope sensor is fitted, connected via a control unit or PLC to the hoist circuit, then calibrated and tested.