Two-Part Hoist Line With a Snatch Block on an Angled Pull
A 4,000 lb pump skid is to be dragged up a 15-degree ramp and then lifted clear. The crew plans to use a snatch block on a structural column to redirect a two-part hoist line. The hoist is rated 5,000 lb on single line and 10,000 lb on two-part. The block's shackle is rated 6,000 lb; the column anchor is rated 8,000 lb. The block is rigged so the incoming and outgoing line segments form a 90-degree included angle. A tag line is planned but no tagger has been assigned. The ramp surface is steel with rust scale and the skid has no runners.
1. Calculate the load on the snatch block and the anchor for the two-part configuration and compare to ratings.
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For a two-part line through a block, the block load equals 2T x cos(theta/2), where theta is the included angle between the two line segments and T is line tension. If the hoist pulls 4,000 lb total (ignoring friction and ramp resistance for the moment), each part carries 2,000 lb.
- Block load = 2 x 2,000 x cos(45 deg) = 4,000 x 0.707 = 2,828 lb
- Anchor load equals the resultant on the block, so also about 2,828 lb
Both are within the 6,000 lb shackle and 8,000 lb anchor ratings. But the real pull is not 4,000 lb: on a 15-degree ramp, the gravity component along the slope is 4,000 x sin(15 deg) = 1,035 lb, and rusted-steel-on-steel friction can add 0.3 to 0.5 of the normal force (4,000 x cos 15 deg = 3,864 lb), i.e., 1,160 to 1,930 lb. Total required pull could reach roughly 2,200 to 3,000 lb, so each part carries 1,100 to 1,500 lb and the block load rises to about 1,550 to 2,120 lb. Still within ratings, but the margin narrows and the hoist's 10,000 lb two-part rating is not the governing limit; the block and anchor are.
Judgement: acceptable only after confirming the actual pull with a dynamometer or by calculation with a defensible friction coefficient, and after verifying the column anchor for the resultant direction, not just magnitude.
2. Identify the operational hazards in this setup and the controls you would require before the move.
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Hazards and controls:
- Shock loading from breakaway: rusted steel scale can cause the skid to stick then release, producing dynamic loads well above static estimates. Control: pre-tension slowly, use a load cell, keep personnel out of the line-of-fire, and set a pull limit below the lowest rated component (the 6,000 lb shackle).
- Block orientation and side loading: a snatch block must be loaded in its plane; the 90-degree redirect must be rigged so the shackle and pin take the resultant correctly and the block cannot turn and side-load the pin. Control: verify block orientation, use a properly sized shackle, and inspect the block for a free-turning sheave.
- No tagger: an uncontrolled skid on a ramp can swing or tip. Control: assign a trained tagger with a tag line, and establish a exclusion zone.
- Ramp friction and tipping: no runners means high friction and possible digging or tipping. Control: add runners or rollers, or use a skid shoe, and check stability against the pull line.
- Two-part hoist rating: the 10,000 lb rating assumes a straight two-part line; the 90-degree redirect does not change line tension but does load the block. Control: confirm the hoist's line pull and brake capacity, and do not exceed the lowest-rated component.
- Communication: use a designated signal person and a pre-lift meeting; stop the pull if the load shifts unexpectedly.
Judgement: do not proceed until the tagger is assigned, the block is verified for orientation and rating, and a controlled pull with monitoring is in place.