Parts of line explained: why more falls lift more

What a "part" is

Wire rope leaves the hoist drum, goes up over a sheave at the boom tip and comes down to the hook block. If it runs down to the block and stops there, one strand supports the block. If it passes through a sheave in the block and goes back up to the boom tip, two strands support it. Keep looping and you get three, four and more.

Count the strands that go down to the block and carry its weight. That count is your parts of line. The words "reeving" and "falls" mean the same arrangement. Reeving is the act of threading the rope, and a "fall" is one supporting strand.

Reeving is why a crane can have a single main line for light work and a heavily reeved block for heavy work. The same rope, drum and engine can do either job, as long as the configuration matches the lift.

Why more parts lift more

Think of the load as shared. If a load hangs from four equal strands, each strand carries about a quarter of it. Ignoring friction, the pull on each strand is the load divided by the number of parts.

We made up the numbers below for teaching. They are not from a manufacturer chart.

Parts of line	Load	Share on each part
1	12,000 lb	12,000 lb
2	12,000 lb	6,000 lb
4	12,000 lb	3,000 lb
6	12,000 lb	2,000 lb

Now suppose our invented rope can safely pull 5,000 lb on one strand. On a single part it cannot lift the 12,000 lb load. On four parts, each strand sees 3,000 lb, so the rope has margin. That is the whole idea: more parts spread the load so the rope is not the weak link.

Real ropes also lose a little pull to friction in the sheaves, and the rope's rating and the hoist's line pull both matter. Your crane's manual and chart give the actual limits. For the exam, learn the direction of each effect and the simple division.

The speed trade-off

Nothing is free. To raise the hook one foot on a four-part block, each of the four strands must shorten by one foot, so the drum has to take in four feet of rope. More parts means a slower hook for the same drum speed.

Parts of line	Rope taken in to raise the hook 10 ft
1	10 ft
2	20 ft
4	40 ft
6	60 ft

This is also why you do not simply reeve every lift with the maximum parts. If the load is light, fewer parts finish the job faster, with less rope on the move.

How it connects to the load chart

Many capacity charts list a number of parts of line needed to reach a given capacity, or a limit on what a certain number of parts can lift. The chart's number comes first. A rope that could lift a heavy load on paper is no use if the crane is reeved with too few parts for that chart cell.

That makes parts of line a second check, after the capacity lookup. Our how to read a crane load chart guide covers the lookup and the net capacity steps. A good habit is to add one line to your routine:

1. Find the gross capacity for your radius and boom length.
2. Subtract the deductions to get net capacity.
3. Check the parts of line the chart requires for that capacity, and compare it with how the crane is reeved.

The parts of line calculator lets you test the division and the speed trade-off with your own numbers once you have worked a few by hand. In the Core outline this topic sits in Operations task 2.6, and chart skills in task 4.4, so it pays in two domains. Our Core study guide lists the reference to open for each.

Mistakes to avoid

Counting the wrong strands. Count only the strands that support the block, not every line you can see on the boom. The dead-end strand that anchors the rope counts only if it carries load.

Forgetting the load includes more than the hook load. The load on the rope includes the block and any rigging hanging below the boom tip. That belongs to the net capacity step, so check your deductions before dividing.

Reversing the trade-off. More parts means more capacity and less speed. A choice that offers more parts as a way to lift faster is wrong.

Ignoring friction and limits. The division is a teaching tool. The chart and the manufacturer's limits decide what is allowed.

Practice

Try these invented problems by hand, then check the answers against the figures above.

1. A 9,000 lb load hangs from a three-part block. About how much pull is on each strand, ignoring friction?
2. Our invented rope can pull 5,000 lb per strand. What is the fewest parts needed for a 12,000 lb load?
3. You change from two parts to four. How does the line speed to the hook change?

Answers: (1) about 3,000 lb. (2) Three parts gives 4,000 lb per strand, which is within the 5,000 lb limit, and two parts gives 6,000 lb, which is over it. So three. (3) The hook rises about half as fast for the same drum speed.

When you are done, take the free 10-question check. It shows exactly which areas to study first, including whether Load Charts or Operations needs the next session. You can also read range diagrams and working-area charts, explained for the other drawings that sit beside the capacity table.

FAQ

What is parts of line?

It is the number of rope strands supporting the hook block. Each strand carries a share of the load, so more parts means less pull per strand.

Why do more parts of line lift more?

The load is divided among the strands. With the same rope, a higher part count means each strand carries less, so the rope stays within its limit at a heavier load. Friction and the hoist's own limits still apply.

Does more parts of line make the hook slower?

Yes. Each strand must shorten when the hook rises, so the drum takes in rope equal to the lift multiplied by the number of parts. Same drum speed, more parts, slower hook.

What is reeving?

Reeving is threading the wire rope through the sheaves in the boom tip and the hook block. The way you reeve it sets the parts of line.