A wire rope pulley that is too small or badly grooved can wear out a good rope in weeks. These are the basics for sizing pulleys (sheaves) and winches.
Pulley, sheave and snatch block
- Sheave: the grooved wheel itself.
- Pulley or block: a sheave in a frame with a hook, eye or shackle.
- Snatch block: a block with a side plate that opens, so the rope can be put in without threading the end through.
The D/d ratio
D/d is the sheave tread diameter divided by the rope diameter. The smaller the ratio, the more sharply the rope bends and the faster it fatigues. Typical minimum ratios given in wire rope industry references are:
| Rope construction | Minimum D/d (approx.) | Minimum sheave for 12 mm rope |
|---|---|---|
| 6x7 | 42 | about 500 mm |
| 6x19 Seale | 34 | about 410 mm |
| 6x26 Warrington-Seale | 30 | about 360 mm |
| 6x36 Warrington-Seale | 23 | about 280 mm |
These are minimums; larger sheaves give longer rope life. Cranes and hoists must follow the D/d set by their design code and manufacturer.
Groove size
The groove should be slightly larger than the rope so it supports the rope around about a third of its circumference. A groove that is too tight pinches and crushes the rope; one that is too wide lets it flatten. Replace or re-machine worn sheaves, as a groove worn to an old rope's size will damage a new rope.
Fleet angle
The fleet angle is the angle between the rope and the centre line of the sheave or drum. For good spooling, keep it between about 0.5° and 1.5° on a plain drum, and up to about 2° on a grooved drum. Too large an angle makes the rope rub on the flanges; too small makes it pile up.
Load on a snatch block
A snatch block that changes the rope's direction carries more than the line pull. The load on the block is 2 × line pull × cos(half the angle between the two rope parts):
| Angle between rope parts | Load on block |
|---|---|
| 0° (rope doubles back) | 2.00 × line pull |
| 60° | 1.73 × line pull |
| 90° | 1.41 × line pull |
| 120° | 1.00 × line pull |
Size the block, its anchor and its shackle for this load, not just the winch pull.
Choosing a wire rope winch
- Pulling vs lifting: many winches are rated only for pulling. Use a winch rated for lifting, or a hoist, when the load hangs.
- Line pull falls as layers build up: the rated pull is usually on the first layer of rope on the drum.
- Drum capacity: check the rope length the drum holds at your rope diameter.
- Power: hand, electric or hydraulic, depending on site power and duty.
- A wire rope pulling hoist (Tirfor type) is a portable option that works with unlimited rope length.
We supply winches, rope pulleys and snatch blocks. Send us the line pull, rope size and application.
Frequently asked questions
What size pulley do I need for wire rope?
Use a sheave with a tread diameter at least the minimum D/d ratio for the rope: roughly 34 times the rope diameter for 6x19 Seale and 23 times for 6x36 Warrington-Seale. Larger is better for rope life.
How much load does a snatch block carry?
It carries 2 times the line pull times the cosine of half the angle between the rope parts. When the rope doubles back, that is twice the line pull.
Can a pulling winch be used for lifting?
Only if the manufacturer rates it for lifting. Many winches are designed only for pulling and lack the brakes and safety factors needed for suspended loads.