Reeving and Capacity on a 25-Ton Bridge Crane
A double-girder bridge crane has a bridge placard reading 25 tons (50,000 lb). The hoist data plate gives two reeving options: 2-part reeving, rated 25,000 lb, and 4-part reeving, rated 50,000 lb. The crane is currently reeved 2-part. The load is a machine skid with a scale ticket of 24,600 lb, a below-the-hook spreader beam of 1,900 lb, and slings and shackles totaling 300 lb. The trolley is about mid-span. The operator also finds an older cab chart that lists 20,000 lb for all lifts. The lift is a straight vertical pick and set, with no side pull.
1. Can the lift be made with the current reeving? Show the arithmetic.
Read the worked answer
Total suspended load = 24,600 + 1,900 + 300 = 26,800 lb. The below-hook gear counts because the hoist is lifting it.
Current 2-part rating = 25,000 lb. 26,800 > 25,000, so the lift is over capacity by 1,800 lb and cannot be made as rigged.
4-part rating = 50,000 lb. 26,800 lb is 53.6% of rating, so the load is acceptable once the reeving is changed.
The older cab chart (20,000 lb) conflicts with the hoist data plate. The most restrictive document controls until the conflict is resolved; here the 2-part calculation already fails, and even in 4-part the 20,000 lb chart would prohibit the lift. The chart must be reconciled with the manufacturer before the crane is used for this load.
2. What must be done before the lift, and what else must be verified after re-reeving?
Read the worked answer
Before the lift:
- Resolve the chart conflict with the manufacturer or a qualified engineer; do not pick the higher number because it is convenient.
- Change reeving properly. Going from 2-part to 4-part is not just adding falls: the hook block must have the correct number of sheaves, the dead-end link and wedge socket must be correct, and the rope must be long enough with the right fleet angle.
- Re-inspect the rope, sheaves, and hook block after re-reeving, and re-set the hoist limit to account for the changed headroom.
After re-reeving:
- Confirm the hook block and hook are rated for 26,800 lb.
- Confirm the spreader beam and slings are rated for their shares, including angle effects.
- Test the brake with a load and verify the load cell or weight indication.
- Confirm the load will clear the floor and any obstructions at the new hook height.
3. The next load is the same skid plus a 2,400 lb fixture. Can it be lifted in 4-part reeving?
Read the worked answer
Yes, with checks. New total = 26,800 + 2,400 = 29,200 lb. Against the 50,000 lb 4-part rating that is 58.4% utilization, so the hoist is not the limiting item.
The checks that now govern:
- Below-the-hook gear: the spreader, slings, and shackles must carry 29,200 lb with the actual sling angles, not just their vertical ratings.
- Hook and block: confirm their ratings cover the new total.
- Structure and runway: confirm the bridge and runway are rated for the load at that trolley position and that the floor or truck bed can take it.
- Chart conflict still open: if the 20,000 lb cab chart is confirmed as the controlling document, the lift is still prohibited regardless of reeving.
Capacity margin is not a substitute for the rigging check; the 4-part reeving only solves the hoist limit.