Trolley Drift, Brake Chatter, and a Storm Thirty Hours Out
17:40, and the tail of a typhoon is forecast to bring signal no. 2 winds within 30 hours; it is already blowing 14 m/s. During the last three hoists the trolley drifted about 300 mm after the operator released the joystick, and the slew brake has begun to chatter when the crane stops. Once, while lifting 1.8 t at 22 m against a chart value of 5.0 t, the overload indicator flickered for about a second and cleared. The operator says the drift is just the wind pushing the jib. The site engineer wants one more pick - a 2 t bundle of rebar for tomorrow's pour - before the crew goes home. The out-of-service tag book is in the site office and has never been filled in. The manufacturer's storm procedure says: free-slew, trolley to 12 m, hook block raised, and no obstruction within the slewing circle.
1. The extra rebar pick: yes or no? Base the answer on the three symptoms, not the schedule.
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No. Three symptoms, and any one of them would be enough on its own.
Trolley drift of 300 mm after the joystick returns to neutral is a brake that is not holding. It is not the wind. Wind moves a load on the hook; it does not drive an unloaded trolley along the jib after the command has been released. A slipping trolley brake means the load can travel outboard without a command, which increases the radius and therefore the load moment, precisely when the operator is not expecting it and is not compensating.
Slew brake chatter belongs to the same class. A brake that chatters has friction surfaces or an adjustment that is not right, and a slew brake that lets go lets the jib run - with a bundle of rebar on the hook and 14 m/s of wind on the jib.
The flickering overload indicator on a lift at 36 per cent of chart means the operator has no trustworthy indication of load, and therefore no trustworthy protection.
The pour is not a criterion. Fill in the out-of-service book tonight with time, symptoms and who was told, and put the crane out of service for lifting.
2. Describe the storm shutdown you would carry out and what you check before leaving.
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Follow the manufacturer's procedure exactly. It exists because the crane is designed to weathervane in high wind, so that the jib aligns itself with the wind instead of presenting its full side to it.
The sequence: land the load and land the hook block; slew the jib to the position the manual specifies and put the slewing mechanism into free-slew, releasing the slew brake as the manual requires; park the trolley at the stated radius - 12 m in this manual, but read the figure for this jib rather than assuming it; raise the hook block to its stowed position and secure the hook; remove or secure anything on the walkways, the cab roof and the jib that could become a missile at 100 km/h; confirm the slewing circle is clear of adjacent cranes, scaffold, stacks and anything the jib could strike as it swings; check that the foundation recess drains and that the earthing is intact.
Check the anemometer is working, because when the storm passes it is the first reading you will take. Log the shutdown with time, wind speed and jib position.
Then the part that gets skipped: nobody restarts the crane until a post-storm inspection of the mast, bolts, jib, ropes, brakes and foundation has been carried out by a competent person and signed off.
3. How do you handle the flickering overload indicator - what does it rule out, what does it point to, and what must happen before the crane lifts again?
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First, what it rules out. It is not an overload. At 22 m on a 5.0 t chart value, 1.8 t is 36 per cent, and no serviceable overload device should be anywhere near its trip point. So the indicator is reporting something about itself or its sensor, not about the load - and that is worse, because a protection device that cries wolf gets ignored, and then it gets ignored on the day it is right.
What it points to, in probable order: the load cell or tension link and its cable; the connectors and junction boxes in that circuit; then calibration. A loose or corroded connection produces exactly this signature - a brief spike cleared by vibration. Moisture ingress after a wet night does the same.
What must happen: the crane is out of service for lifting until a competent technician has traced the circuit, corrected the fault, and then proved the load moment limiter against known test loads at known radii, with the result recorded and signed. A device that has been adjusted but not proved is not a device you can rely on.
And the operator does not 'lift carefully' with a suspect limiter in the meantime. Suspect instrumentation is a stop condition, not a caution.