Snatch blocks, winches and anchors: line pull, block loads and anchor design

Winches, tirfors (hand-operated wire rope pulling machines), tuggers and snatch blocks are used to pull loads into buildings, haul skids along tracks, tail loads during lifts and tension guys. The equipment is rated for the line pull, but the blocks and anchors that turn the line often see far more than the line pull. Most failures in hauling are at an anchor or a block that was never calculated.

1. The load on a snatch block

A sheave that turns a rope has the rope tension T on both sides. The block, its shackle or hook, and the anchor it hangs from carry the resultant of the two rope parts:

R = 2 × T × cos(θ ÷ 2)   θ = included angle between the two rope parts (0° when the parts are parallel)

Example. A 5 t winch line is turned through a snatch block with 60° between the rope parts. R = 2 × 5 × cos 30° = 8.66 t. The block, its shackle and the anchor point must each be rated for at least 8.66 t, even though the winch only pulls 5 t. If the angle closes to 0° (the line doubles back on itself), R becomes 10 t.

The resultant acts along the line that bisects the angle between the rope parts. Make sure the anchor is strong in that direction, not only in the direction of one rope.

2. Line pull through tackle and redirect sheaves

Every sheave costs some force to friction. Using a sheave efficiency e (often 0.96 for bronze-bushed blocks and 0.98 for roller bearings; use the maker’s value):

• Tackle (reeving). With n parts at the moving block and the line led off the last fixed sheave, the lead line pull is P = F × (1 − e) ÷ (1 − eⁿ), as for a crane hoist (Chapter 7).
• Redirect sheaves. Each extra sheave that only turns the line divides the pull by e on the winch side: P_winch = P ÷ eᵐ for m redirect sheaves.

Worked example: hauling a skid

A 20 t skid is to be pulled along a concrete floor on steel skid plates. Taking a sliding friction of μ = 0.30, the force to keep it moving is 0.30 × 20 = 6.0 t, and a breakout force at the start may be higher (Chapter 14).

• A two-part tackle at the skid, with e = 0.96: P = 6.0 ÷ (1 + 0.96) = 3.06 t.
• Two redirect blocks between the tackle and the winch: P_winch = 3.06 ÷ 0.96² = 3.32 t. A 5 t winch is adequate, with margin for breakout.
• Anchor for the tackle’s fixed block: it carries both parts of the tackle, about 3.06 + 2.94 = 6.0 t.
• Each redirect block: calculate from its own included angle.

3. Winch pull and rope speed on the drum

A winch is normally rated for line pull on the first layer of rope on the drum. As more layers build up, the effective drum radius grows. The torque is the same, so the line pull falls in proportion, while the rope speed rises:

P_n = P₁ × r₁ ÷ r_n   r = radius to the centre of the rope layer

Example. A winch rated 10 t on the first layer; drum core diameter 300 mm, 20 mm rope. Taking each layer as adding about one rope diameter: r₁ = 160 mm, r₂ = 180 mm, r₃ = 200 mm, r₄ = 220 mm. The line pull on the fourth layer is 10 × 160 ÷ 220 = 7.27 t. If the plan needs 8 t, keep enough rope off the drum (or use a larger winch) so that the pull is never needed on the outer layers.

Also keep at least the minimum number of dead wraps on the drum that the maker specifies (commonly two or three full turns) when the rope is paid out furthest. The rope clamp on the drum is not designed to hold the line pull.

Fleet angle

The fleet angle is the angle between the rope and a line square to the drum axis, measured at the first fixed sheave. Too large, and the rope piles up or rubs on the drum flange; too small, and it does not spool across the drum. A common guide is a maximum of about 1.5° for a plain drum and 2° for a grooved drum (follow the maker). The distance from the drum to the first sheave must be at least:

L ≥ (half the drum width) ÷ tan(maximum fleet angle)

Example. A plain drum 0.8 m wide: L ≥ 0.4 ÷ tan 1.5° = 15.3 m. For a grooved drum at 2°: 11.5 m. Place the first sheave on the centreline of the drum.

4. Anchors

Anchors are the most variable part of a hauling system. Options, from the most to the least predictable:

1. Purpose-made anchor points designed, tested and marked for the load and direction (padeyes on a foundation, embedded anchors, proprietary ground anchors with test data).
2. Structure checked by a structural engineer for the actual load, direction and height above the base. A column pulled 1 m above its base plate carries a bending moment its designers may never have considered.
3. Gravity anchors: heavy blocks or plant that resist by weight and friction. They are simple but need far more weight than people expect.
4. Deadmen: timber, steel or concrete buried in a trench, resisting by soil pressure. These need a geotechnical design and should be pull-tested.

Worked example: a concrete block anchor

A line pull T = 5 t acts on a block at α = 15° above horizontal. Friction between block and ground μ = 0.5. A factor of safety of 2.0 against sliding is wanted. The line lifts the block slightly, reducing its bearing load to W − T sin α. For sliding:

FoS × T cos α ≤ μ × (W − T sin α)   →   W ≥ T × (FoS × cos α ÷ μ + sin α)

W ≥ 5 × (2.0 × 0.966 ÷ 0.5 + 0.259) = 5 × 4.12 = 20.6 t, about 8.6 m³ of concrete, to hold a 5 t pull. Check also that the block cannot tip about its front edge, and that the ground in front of it will not heave. On wet clay or a slope, μ can be far lower than 0.5.

Testing anchors

Where an anchor’s capacity cannot be shown by calculation and certified data, proof test it before use, in the direction it will be loaded, to the multiple of the working load your procedure requires, and record the result. Mark the anchor with its tested load and direction. Never test an anchor with people in the line of the rope.

5. Safety in the bight

The bight is the area inside the angle formed by a rope running round a block. If the block, its anchor or the rope fails, the rope and block fly towards and through the bight. Keep everyone out of the bight and out of line with any rope under tension. Put the winch operator behind a guard or at a safe angle, and use rope dampers on long lines where practical. Use snatch blocks with a positive latch or locking side plate, and never a hook that can unhook when the rope goes slack.

6. Common mistakes

• Sizing a snatch block, its shackle or its anchor for the line pull instead of the resultant.
• Using the winch’s first-layer rating with the drum nearly full.
• Tying back to a column, a handrail or a pipe support without a structural check.
• Using a gravity anchor that is much too light, or one standing on mud.
• Letting the rope rub on steel edges, which reduces its strength and can cut it.
• Standing in the bight to guide the rope.

7. Field checklist

• Winch or tirfor rated for the line pull at the drum layer that will be in use; brake and controls tested.
• Every block, shackle and anchor rated for its resultant load in its direction.
• Sheave diameter suitable for the rope (D/d as the maker requires).
• Fleet angle within limits; first sheave on the drum centreline.
• Minimum dead wraps on the drum at full pay-out.
• Bight and rope lines barricaded; signaller and operator positions agreed.

8. Self-check questions

General guidance only. It does not replace the manufacturer's data, the applicable standards or the judgement of a competent person.

References

Technical references consulted for this chapter. The text and worked examples are original to LiftPlan Pro.

• Rigging Handbook, 2nd edition (Bechtel Equipment Operations, 2002).
• ASME B30.7 Winches; BS EN 14492-1 Power-driven winches and hoists; BS EN 13157 Hand-powered lifting equipment.
• ASME B30.9 Slings; ASME B30.26 Rigging Hardware; ASME B30.10 Hooks.

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