Safety functions
In short: Brakes and limit switches are the most critical safety elements of a crane. Brakes must be checked with a daily function test, air gap and lining measurement and a slip test under load; limits by testing each motion at low speed.
Why they matter
The brake holds the suspended load and stops motions; the limit switch prevents the hook and crane from going where they must not. If either fails the result is usually an accident rather than a fault: the load slips, the hook block hits the drum, the rope breaks or the crane strikes the end stops.
Brake types on cranes
- Electromagnetic disc brake: the most common type on hoist and travel motors; spring-applied, released when the coil is energised, applies automatically on power loss.
- Conical rotor motor brake: built into some hoist motors.
- Electro-hydraulic (thruster) drum brake: used on heavy duty cranes.
Brake inspection step by step
1. Daily function test (operator)
At the start of the shift each motion is started and stopped without load. If the brake releases late, scrapes, smells of burning or slips when stopping, the crane is taken out of use and reported.
2. Air gap measurement
On electromagnetic disc brakes the gap between armature and coil body is measured with feeler gauges at several points with the brake applied. As the lining wears the gap increases: the brake releases late, the coil overheats and holding torque may drop. Outside the manufacturer’s range, adjust or replace the lining.
3. Lining and disc
Lining thickness is compared with the manufacturer’s minimum. Disc and lining must be free of oil, heat marks, cracks or uneven wear. An oil-contaminated lining cannot be saved by cleaning; replace it.
4. Electrical check
Brake rectifier, terminals and cable are checked and coil voltage is measured. Hoist brakes must close quickly (e.g. DC-side switching); a brake that closes late can let the load drop a few centimetres.
5. Slip test under load
A load close to rated capacity is lifted a short height and stopped, then watched for several minutes; it must not creep down at all. When stopping during lowering the load must stop safely within a short distance.
6. Travel brakes
Travel brakes must be neither too hard (load swing, wheel slip) nor too soft (long stopping distance). With inverter drives the inverter stops the motion and the mechanical brake holds; settings must suit this arrangement.
Limit switch inspection
Upper hoist limit
Prevents the hook block from hitting the drum or trolley frame. Test without load at the lowest speed; the hook must stop at the set point with a safe clearance. On rotary limit switches one contact is usually used for slow-down and one for stop.
Emergency (second) upper limit
On heavy duty cranes and where the risk assessment requires it, an independent second limit acts if the normal limit fails, usually dropping the main contactor. Its independence must be tested separately.
Lower hoist limit
Ensures enough rope wraps remain on the drum with the hook at its lowest point (in common practice at least two wraps). If the drum empties completely, the rope anchorage has to carry the load.
Bridge and trolley travel limits
Approaching the end of travel the crane first slows down, then stops. Cross limit switches or lever limits are used; where several cranes share a runway, laser or radar anti-collision systems are added. Buffers are checked too.
From symptom to cause: brake faults
| Symptom | Possible cause | Action |
|---|---|---|
| Load creeps after stopping | Worn lining, large air gap, oily lining | Take out of use; measure, adjust or replace lining |
| Load drops briefly before lifting | Brake releases before motor produces torque (inverter setting) | Check brake release conditions and delays |
| Late release, scraping noise | Large air gap, low coil voltage, faulty rectifier | Measure gap, check voltage and rectifier |
| Brake does not release | Open coil, fuse, rectifier, cable | Electrical measurements; never run the motor against the brake |
| Brake overheats, burning smell | Constant dragging, too many starts | Adjust, review duty profile, suitable brake |
| Hard stop on travel, wheel slip | Brake too hard or inverter ramp too short | Adjust brake and ramp time |
From symptom to cause: limit faults
| Symptom | Possible cause | Action |
|---|---|---|
| Hook does not stop at upper limit | Limit setting shifted, faulty contact, rope replaced | Take out of use; adjust and test the limit |
| Crane stops for no reason | Limit contact opens with vibration, loose connection | Check and secure connections and contacts |
| Stops at end of travel without slowing down | Slow-down contact misadjusted or disabled | Readjust slow-down point |
| No motion in one direction | Limit contact stuck open | Mechanical and electrical check of the limit |
When re-adjustment is needed
- After rope replacement
- After replacing the hook block, drum or gearbox
- After motor or inverter replacement
- After a collision, overload or unexpected stop
What to record
- Brake air gap at each point and lining thickness
- Coil voltage and rectifier condition
- Slip test: load, waiting time, observed creep (if any)
- Clearance at the upper limit
- Rope wraps remaining at the lower limit
- Slow-down and stop points of travel limits
Checklist
| Check | Frequency (recommended) | By |
|---|---|---|
| Brake and limit function test without load | Start of every shift | Operator |
| Brake noise, smell, delay | Every use | Operator |
| Air gap and lining measurement | Every 1–3 months | Maintenance team |
| Slip test under load | Every 3 months and after brake work | Maintenance team |
| Upper, lower and emergency limit test | Every maintenance | Maintenance team |
| Travel limits and buffers | Every maintenance | Maintenance team |
| Documented thorough examination | At least annually | Competent person |
Shorten the intervals for heavy use; see the maintenance plan guide.
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
How often should crane brakes be checked?
Function tests at the start of every shift by the operator; air gap, lining and slip checks every 1–3 months by maintenance staff, depending on use.
The crane does not hold the load; what should I do?
Take the crane out of use immediately, lower the load safely and close off the area. Do not operate the crane again until a competent person has checked air gap, lining, rectifier and brake circuit.
Must limits be adjusted after a rope change?
Yes. Rope length and drum wraps change, so upper and lower limits must be readjusted and tested.