Parts of line formula
Parts of line are the lines of hoist rope running between the boom head (or upper block) and the hook block. With the hook block hanging still, the parts are the only upward forces on it, so vertical equilibrium gives n × T = W, where W is the total load on the hook and T is the tension in each part. Setting T to the allowable single-line pull P gives n = W ÷ P, rounded up to the next whole part.
W must include everything below the boom head: W = load + hook block + rigging (slings, shackles, spreader or lifting beam). If your procedure also counts the weight of the hoist rope below the boom head, add it to the rigging figure. This is our derivation from the equilibrium condition in OpenStax, section 12.1; it assumes every part shares the load equally.
Round up even when the ratio is only just over a whole number. 3.01 needs 4 parts, because 3 parts would put each line above the allowable pull. When the ratio is exactly a whole number, that number of parts is enough.
Worked example: 20,000 lb load, 6,000 lb line pull
Total load = 20,000 + 800 (block) + 200 (rigging) = 21,000 lb. 21,000 ÷ 6,000 = 3.5, which rounds up to 4 parts of line. Each part then carries 21,000 ÷ 4 = 5,250 lb, or 87.5% of the allowable pull. Leaving out the block and rigging would give 20,000 ÷ 6,000 = 3.33, the same 4 parts here, but with a 17,500 lb load the omission matters: 17,500 ÷ 6,000 = 2.92 rounds up to 3 parts, while the true 18,500 lb total gives 3.08 and needs 4 parts.
These fictional inputs illustrate the arithmetic; they are not taken from any crane, chart or exam question.
Checking the result against a chart's hoist limits
Line pull is only one limit. A load chart usually also lists the maximum load for each number of parts of line, and that figure can be lower than parts × line pull. The calculator checks your result against the invented chart below: single-line pull 11,000 lb, with rows from 1 to 8 parts.
- A 62,000 lb total load on the 11,000 lb line pull needs 62,000 ÷ 11,000 = 5.64, so 6 parts. The example chart allows 64,000 lb at 6 parts, so the check passes.
- A 66,000 lb total load gives exactly 6 parts by line pull, but the example chart allows only 64,000 lb at 6 parts. 7 parts (70,000 lb) satisfy both checks, so 7 governs.
- Above 70,000 lb the example chart has no row, however many parts you reeve. A larger total load needs a different crane or configuration.
The chart and its numbers are invented for practice and match no real crane. On a real lift, read the line pull, the parts-of-line limits and the gross capacity at the actual radius from that crane's own chart. Then subtract deductions with the net capacity calculator, and find the radius from boom length and angle with the crane radius and boom angle calculator.
Option: sheave efficiency
The equal-sharing model ignores friction. Real sheaves lose a little tension at each turn, so while hoisting, the lead line to the drum pulls more than W ÷ n. If each sheave passes on a fraction e of the tension, the parts carry T, T ÷ e, T ÷ e², and so on, and they add up to W. Solving that series gives the lead line pull:
lead line pull = W × (1 − e) ÷ [es × (1 − en)], where s is the number of sheaves between the last part and the drum. With e = 1 it reduces to W ÷ n. This is our own derivation, shown so you can check it; it is not quoted from a standard or a manufacturer.
Example: 21,000 lb total load, 5,400 lb allowable pull, e = 0.98 and s = 1. Without the option, 21,000 ÷ 5,400 = 3.89 gives 4 parts. With it, 4 parts give 21,000 × 0.02 ÷ (0.98 × 0.07763) = 5,520.6 lb on the lead line, over the limit, and 5 parts give 4,460.6 lb. So the option raises the answer to 5 parts. 0.98 is an example value chosen for this tool, not a published rating. Use the efficiency your crane manufacturer gives for its sheaves and bearings, or leave the option off.
Sheave losses also set a floor: however many parts you add, the lead line never drops below W × (1 − e) ÷ es. If the allowable pull is below that floor, the calculator reports that no reeving works.
Which line pull to enter
Enter the lowest of the limits that apply to this crane and rope: the hoist's rated line pull for the layer and drum in use, and the rope's allowable pull from its rating and required design factor. OSHA's rope selection rules are in 29 CFR 1926.1414, and 29 CFR 1926.1417 (paragraph (a)) requires following the manufacturer's procedures. ASME B30.5 applies to mobile cranes. No standard text, rope tables or manufacturer charts are reproduced here.
The crane manufacturer's load chart, its reeving and line pull notes, and the wire rope rating govern. Use the lower of the hoist's rated line pull and the rope's allowable pull, and never lift more than the chart allows at the actual radius.
Units and limits
Enter weights in lb, kg or metric tonnes (t); switching units converts the entered values using 1 lb = 0.45359237 kg from NIST's SI conversion factors. Line pull is entered in the same unit as weight-equivalent force. A metric tonne is 1,000 kg, not a 2,000 lb US short ton.
The model is static. It leaves out acceleration and braking, side loading, wind, rope weight beyond what you add, and drum layer effects. Reeving must also suit the block and boom head sheaves, which this tool does not check.
Preparing for a crane written exam? Try the NCCCO written practice test to see which load chart and rigging topics need work.