Setting a 900 lb Panel on a 3 ft Pad
A 900 lb electrical panel must be set on a concrete housekeeping pad 3 ft above grade. The only access is a 12 ft aluminum ramp running from the truck bed up to the pad, and the panel will ride on 2 in. steel rollers. The crew has a 2,000 lb rated come-along, 60 ft of 5/16 in. wire rope with an 875 lb working load limit, and a chain hoist reeved with a 4-part block (mechanical advantage 4) that the rental yard rates at 80 percent efficiency. Rolling friction on the plywood ramp adds roughly 10 percent to the force needed to move the panel up the incline. The crew leader wants to know how hard the pull will be, whether the ramp is long enough to keep the pull at or under 150 lb, and whether hoisting straight up would be easier.
1. What pull is needed to move the panel up the 12 ft ramp, and does the rigging hold it?
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Ideal inclined-plane force: F = W × (rise ÷ ramp length) = 900 × (3 ÷ 12) = 225 lb. Check by work: 225 lb × 12 ft = 2,700 ft·lb, equal to 900 lb × 3 ft of lift.
Add 10 percent for rolling friction: 225 × 1.10 = 247.5 lb.
Rigging check: the come-along is rated 2,000 lb and the wire rope working load limit is 875 lb, so 247.5 lb sits at about 28 percent of the rope's WLL - acceptable, and well under the hoist rating. The binding constraint is the rope, not the come-along, so the rope's WLL is the number to watch if the ramp angle ever increases or the panel is heavier than tagged.
2. Is the 12 ft ramp long enough for a 150 lb pull limit? What length would be needed?
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No. At 12 ft the pull is 247.5 lb, well over 150 lb.
Ignoring friction, the required length is L = W × rise ÷ F = 900 × 3 ÷ 150 = 18 ft. But friction eats part of that margin: the actual pull is 1.10 × the ideal force, so 1.10 × 900 × 3 ÷ L ≤ 150 gives L ≥ 2,970 ÷ 150 = 19.8 ft, meaning a 20 ft ramp.
An 18 ft ramp would give 1.10 × 2,700 ÷ 18 = 165 lb, still above the limit. That 10 percent matters at the margin, which is the practical lesson: state whether the limit applies to the ideal force or to the measured pull before choosing a ramp.
3. Compare hoisting straight up with the 4-part block against the 12 ft ramp. Which needs less effort, and what else should drive the decision?
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Hoist line tension: load ÷ (mechanical advantage × efficiency) = 900 ÷ (4 × 0.80) = 900 ÷ 3.2 = 281.25 lb. That is within the 875 lb rope WLL and the hoist rating.
Ramp effort was 247.5 lb, so the ramp needs about 34 lb less pull. Effort is not the whole story, though. The hoist lifts vertically and needs only about 3 ft of travel, but with a 4-part block the rope must move roughly 4 × 3 = 12 ft plus reeving slack, so it is slow and it puts the load over the crew rather than on a slope. The ramp needs 12 ft of travel at 247.5 lb with the load low and controlled by rollers. Recommendation: use the ramp for effort and control, keep the hoist as the vertical option if the ramp cannot be extended, and verify both against the rope WLL before rigging.