Work out capacity before diameter

Winch rope selection is often done in the wrong order. The right order is to fix the length to be wound, calculate how many layers the drum can hold, and only then choose rope diameter and construction. Picking the diameter first easily ends with a drum that will not hold the rope or too many layers.

A rough capacity check divides the effective drum width by the rope diameter to get turns per layer, uses the drum and rope diameters to get the length per turn, and adds layer by layer. An accurate calculation allows for spooling clearance and rope diameter tolerance.

Leave margin on the calculated capacity. A rope changes diameter slightly under load and spooling is never perfectly tight, so a figure taken to the last metre usually falls short on site.

Capacity calculation must also deduct the length taken by the anchorage. Wedge and clamp fittings both occupy part of the drum width, and using the full width overstates capacity, sometimes by several turns per layer.

How layers change diameter and pull

By the second or third layer the effective drum diameter has grown. For the same torque the outer layer produces less pull; put the other way, the drum needs more torque to hold the same pull. Size the motor and brake on the pull at the maximum layer.

An outer layer carries less crushing than the bottom layer, but the bottom layer supports the weight and pressure of every layer above it. The more layers, the worse the deformation and the shorter the life of that bottom rope.

Many winch ratings are stated at the first layer. Used at the third layer, the available pull falls noticeably. If the duty needs full pull throughout, put that requirement in the specification.

Spooling and fleet angle

The rope leaves the drum to the first guide sheave at a fleet angle. Too large an angle makes the rope wind diagonally, cross between layers and cut into itself. Keep the angle within about one and a half to two degrees unless the equipment data says otherwise.

The fleet angle varies across the drum width. It is greatest when the rope is near a flange and smallest at the centre, so the guide sheave is placed to suit the worst position rather than the drum centre.

A spooler lays the rope turn by turn. Its pitch must match the rope diameter: too wide and gaps are left, too narrow and the rope is squeezed. With a large diameter tolerance, a fixed-pitch spooler may not fit.

Small winches without a spooler rely on the operator or on tension to guide the rope. Operator habit then directly affects rope life, so the procedure should say how the rope is to be laid.

The groove of the first guide sheave matters as much as the drum. A groove that is too narrow pinches the rope, a worn groove lets it sit too deep, and both shorten life. For groove choice, see sheave grooves

Back tension and winding quality

The rope needs some back tension on the drum to lay tightly. Too little and upper layers sink into the gaps below, crossing and flattening; too much and wear on the rope and load on the drum both rise.

Back tension usually comes from the load itself or from a tensioning device. Slack laying is most likely when hauling in empty, and repeated slack layers can tighten suddenly under load and cause a shock.

Wind a new rope slowly the first time and keep tension on it. A well-laid drum spools more reliably afterwards and the rope lasts longer. For the drum and groove match, see drum design

What construction suits multi-layer winding

A rope for multi-layer winding must resist crushing between layers and keep its round shape. A fibre core flattens easily, the diameter shrinks and spooling becomes erratic, so it is generally less suitable than a steel core.

Compacted and sealed strands resist crushing well but have harder surfaces and higher friction between layers. A conventional round-strand rope with a steel core is a common compromise between crushing resistance and cost.

Rotation-resistant rope is not suited to multi-layer winding. Its construction deforms under crushing, and the rotation resistance it offers is not needed on a drum.

Rope lay must match the winding direction. If the two oppose each other, the rope tends to open up on the drum and spools less evenly, so lay direction and payout direction are chosen together.

Drums and grooves

A plain drum relies on rope-to-rope friction to lay the turns and suits multi-layer winding; a grooved drum suits single or few layers, with the pitch matched to the rope diameter. Too wide a pitch lets the rope wander, too narrow a pitch squeezes it.

The drum flange must be high enough. With many layers the outermost rope can rise above the flange and jump out. Flange height is normally set from the top of the outermost layer plus an allowance.

The drum-to-rope diameter ratio sets the bending stress. A small drum bends the rope sharply and shortens its life, so read it together with the sheave diameters. For diameter against load, see sizing rope by load

Common mistakes

The first is to choose the rope on rated pull without checking the actual pull at the maximum layer. The second is to calculate capacity too tightly, so the site has to shorten the rope or fit a smaller one.

The third is to ignore fleet angle and place the guide sheave at random, so the rope winds diagonally. The fourth is to haul in a new rope at speed without tension, leaving slack layers.

A further mistake is to treat a hoisting winch and a pulling winch as the same duty. Hoisting has to brake and hold the load, pulling usually does not, and the safety factors and construction requirements differ.

Ordering and acceptance

State the construction, diameter, steel grade, core, lay direction, surface, length and terminations. A winch rope normally has a termination at each end, one anchored to the drum and one to the hook or pulling attachment.

The termination must match the drum anchorage. Clamp, wedge socket and swaged fittings make different demands on the rope end, so attach a sketch of the anchorage to the inquiry.

At acceptance, check diameter, length, terminations and certificates, and look for transport damage. Technical figures are in the steel wire rope data

One-line rule

The order for winch rope is: rope length sets capacity, capacity sets layers, layers set pull, and pull sets diameter and construction.

Finish that sequence before discussing price, or the site will discover that the rope does not fit or will not pull.