Dual-Crane Lift with Unequal Load Share
Two 10 t overhead bridge cranes share a runway in a workshop. Each has its own pendant control, and there is no anti-collision or load-sharing system. A 14 t fabricated steel vessel must be lifted and set onto a foundation 6 m away. The rigging supervisor proposes using both cranes on a single spreader beam, with each crane hook connected to an end of the beam. The vessel's centre of gravity is offset 400 mm toward one end because of an attached internal tray. The lift plan is being written by a first-year engineer who has never done a dual-crane lift. The workshop supervisor wants the lift done during the day shift while the plant is quiet.
1. What load-share problem arises from the offset centre of gravity, and how should the lift plan address it before any lift occurs?
Read the worked answer
An offset CoG means the reaction at each hook is not equal. Taking moments about the vessel's CoG, the hook nearer the CoG carries more than half the load. With 14 t split across two hooks and a 400 mm offset on a beam span of, say, 4 m, the near hook could carry roughly 14 x (2.0 + 0.4)/4 = 8.4 t and the far hook 5.6 t. That is a 60/40 split, not 50/50, and it changes with any tilt, wind or drift during the lift.
The plan must therefore:
- calculate the actual hook loads from the geometry, including the spreader beam and rigging weight, not just divide by two;
- confirm each crane and its runway/trolley is rated for its calculated share with margin;
- specify the CoG position on the vessel by physical marking and verify it by trial lift of 100-150 mm;
- require a competent rigger to monitor the beam for level and to stop if either hook load or beam tilt departs from plan;
- set a maximum tilt/skew limit and a communication protocol with one person in charge.
The engineer must not sign the plan alone; a competent person with dual-crane experience should review and approve it.
2. What operational controls should be in place during the shared lift, given there is no load-sharing system?
Read the worked answer
Without a load-sharing or anti-collision system, the cranes can fight each other and load transfer can swing rapidly. Controls:
- Single controller. One person, the lift supervisor, directs both operators by radio or agreed hand signals. Operators act only on that person's commands.
- Trial lift. Lift 100-150 mm, hold, check beam level, hook loads, rigging and vessel CoG. Adjust slinging before committing.
- Slow, synchronised travel. Both cranes travel and traverse together at reduced speed; no simultaneous hoist and travel unless the plan allows it. One crane leads and the other follows the load, never pushes it.
- Exclusion zone. Barricade the full travel path and landing area; no personnel under the load or between the cranes.
- Monitoring. A rigger watches beam level and hook approach; another watches the vessel for tilt. Any unexpected tilt, slack rope or unusual noise stops the lift.
- Environmental limits. Set a wind limit for the vessel's sail area and stop if wind gusts approach it.
- Emergency plan. Agreed procedure for lowering the load to a safe resting place if one crane faults; never leave a shared load suspended unattended.
3. The workshop supervisor asks whether the lift can proceed on day shift while the plant is quiet. What factors should govern the timing decision?
Read the worked answer
Quiet plant reduces people in the area but does not by itself make the lift safer. Decide on:
- Exclusion zone practicality. Can the full travel path be barricaded without blocking emergency egress or fire doors? Quiet shifts can make this easier, but check egress routes remain clear.
- Emergency response. Is first aid, fire response and a competent rigger available on the proposed shift? Night or quiet shifts may have reduced emergency cover, which argues against, not for, the lift.
- Supervision. The lift supervisor and a dual-crane-competent person must be present. If they are day-shift staff, the quiet shift may lack them.
- Power and services. Confirm no planned electrical isolation or service interruption to either crane during the window.
- Lighting and visibility. If the shift is at night, confirm adequate task lighting on the beam, hooks and landing point.
- Weather. For an outdoor or partially open gantry, wind and rain forecasts matter more than plant activity.
The supervisor's preference for a quiet shift is a scheduling input, not a safety control. The lift should run when the controls above can all be met.