How parts of line share the load

The number of parts is the number of rope legs supporting the load. Four parts means four legs share the load and, in theory, each carries a quarter. The words in theory matter: the share is not even in practice, and sheave efficiency changes the tension from leg to leg.

Static equilibrium is the basic tool. Draw the load and the tensions of all legs as a free-body diagram, sum the vertical components to zero, and the relationship between the tensions appears. The two sides of a moving sheave do not carry equal tension; the difference is the sheave efficiency.

More parts reduce the tension per leg and allow a smaller rope, but they add legs, complicate reeving, add sheaves and lower efficiency, so the total cost is not necessarily lower.

Sheave efficiency losses

As rope passes over a sheave, bearing friction and the rope's own bending resistance absorb part of the force. Each sheave is typically ninety-six to ninety-eight percent efficient, depending on bearing type, lubrication and rope stiffness.

The losses accumulate. A rope passing three sheaves has a total efficiency equal to the product of the three, not their sum. In systems with many parts and many sheaves, the dead-end and drum tensions differ noticeably.

Efficiency also varies with load. At light load bearing friction is a larger proportion and efficiency is lower; at heavy load it settles. Calculate with the load the machine actually sees, not the rated load.

Lead-line and dead-end forces

The dead end is the fixed end of the rope. Its tension is the lowest in the run, because every sheave takes a share. The tension is low, but the anchorage must still be designed for it, and never from the average tension.

A guide sheave carries the resultant of the tensions on both sides, acting along the bisector of the angle between them. The smaller the included angle, the larger the resultant, so the bracket and pin are designed for the resultant, not for one side.

These forces reach the drum and the frame as well. The drum sees the tension at the entry side, not the load divided by the number of parts. With many parts, the difference between drum tension and average tension grows.

The run from the last guide sheave to the drum is the lead line. It should be as close to perpendicular to the drum axis as possible; otherwise the rope winds diagonally, and a large fleet angle causes uneven spooling and crushing between layers.

Drawing the reeving path in words

The reeving diagram does not need to be elegant, but every leg must be traceable. Start at the drum, list the sheaves in order, count how often the rope passes the moving block, and show where it is fixed.

A common error is to count fixed and moving sheaves together. Only sheaves that travel with the load are moving sheaves; fixed sheaves change direction and add no parts.

The simplest way to count parts is to count the legs that support the load, including the dead end. If the dead end is fixed to the hook block it carries load; if it is fixed to the boom tip or the frame it does not, and the count changes. For how angles change force, see sling angle

What it means for rope and drum selection

Rope is selected on the highest single-leg tension, not on the total load. More parts lower the leg tension and allow a smaller rope, but the rope bends more often, so fatigue life has to be recalculated.

Drum capacity grows with the number of parts. A four-part system has four times the rope length to wind, so drum size, layer count and spooling complexity all rise.

Drum torque follows the actual tension. With many parts the drum tension is low, but drum speed must rise to keep the same lifting speed, so motor power does not necessarily fall. For drum sizing, see drum design

Lay direction and reeving also affect load rotation. When several legs load in the same direction the load may rotate as a whole, so the hook block needs a swivel or the rope needs a rotation-resistant construction.

Equalisation and block inspection

Uneven tension between legs is a common problem in blocks. Different sheave diameters, rope diameter tolerance and different lubrication all make one leg carry more than its share.

An equaliser sheave lets the leg tensions even out, but it wears too. Check that it turns freely and that the rope sits centred in its groove.

Record broken wires and wear on each leg. If one leg is clearly worse than the others, the load is not shared evenly, and the sheaves and reeving path should be inspected rather than the rope simply replaced.

Common mistakes

The first mistake is to size the rope on total load divided by parts, ignoring efficiency and dead-end tension. The second is to miscount the parts by treating a fixed sheave as moving.

The third is to design the dead-end anchorage on average tension when dynamic load and vibration amplify what the static figure suggests. The fourth is to ignore the extra bending cycles the reeving adds, which leaves the rope short of fatigue life.

Another is to change the reeving on site, for example from four parts to three, without recalculating drum pull and braking capacity.

One complete calculation logic

Fix the load including self-weight, dynamic factor and wind; fix the parts and the number of sheaves; work the tension leg by leg through the efficiencies to get the drum and dead-end tensions; then multiply the highest tension by the safety factor and select the rope from the breaking force table.

As an example, take a hundred kilonewtons on four parts through three sheaves at ninety-seven percent each. The drum tension works out at about twenty-six point one kilonewtons rather than the simple twenty-five. Insert the actual sheave count and efficiency for real numbers.

The difference looks small, but after the safety factor it affects the diameter choice, and it grows with more parts and more sheaves. Published breaking forces are in wire rope breaking strength

A selection tool can carry out this step, but the inputs have to be right. See the wire rope selector

One-line rule

Parts set the nominal share and efficiency sets the real one. Draw the reeving path, calculate each leg, and select on the highest single-leg tension.

Never divide the load by the number of parts and order, and never ignore the effect of a reeving change on the drum and the brake.