Blocks and Sheaves: The Parts, the Names, and How to Specify Them

If you have ever ordered a pulley block and received something that could not possibly do the job, the problem was probably vocabulary rather than specification. This is a corner of the trade where the same object answers to at least six names, where two quite different objects share one of those names, and where the most common question of all — what is the difference between a block and a sheave — is almost never answered on the pages that sell them.

That gap is what this page fills. It is a vocabulary and anatomy guide, not a catalogue: what the parts are called, what the names mean in different trades and countries, and the exact words to use when you specify one so that what arrives is what you meant. For the engineering — mechanical advantage, reeving arithmetic, standard requirements and discard criteria — the pulley block product page goes through it in detail.

1. A sheave is a part. A block is an assembly.

Start here, because the two words are used interchangeably in conversation and are not interchangeable in meaning.

So everything you can buy is a block. The sheave is the component inside it — and it is also the component that has to match your rope, which makes it the one that matters most when you order.

One practical note before you ask for one on the phone: sheave is pronounced “shiv”, to rhyme with give. It is not “sheeve”. Suppliers will understand either, but the pronunciation is also a useful signal that you have specified one before.

2. One object, many names

These are all names for the same family of product, and which one you use depends on trade, country and era. Knowing them matters because a supplier's catalogue may use one name while you search for another.

Two of those cause real trouble, and both are worth knowing before you send a purchase order.

Snatch block is used loosely in Europe and Australia to mean any heavy block, but in US rigging it means specifically one with an opening cheek. Order it under the wrong meaning and you receive a closed block that cannot be fitted onto a line already in service — the one job you bought it for.

Chain pulley block is the other. Some catalogues use it for a chain hoist, which lifts a load directly from its own hook. A block is a component you reeve a rope through. If a listing does not make clear which of the two it is selling, that is a question worth asking before you order rather than after.

3. The anatomy of a block: which part is called what, and which parts fail

When something goes wrong, the conversation is much shorter if you can name the part. This is the full set.

4. The reeving vocabulary

Half of a useful conversation about blocks is the words for the rope itself. These are the ones that come up:

• Standing part (dead end) — the fixed end of the rope, the one that does not move. It is anchored, often to a becket.
• Live part (hauling part, the fall) — the end you pull, or the end that runs onto a winch drum.
• Parts of line (falls) — how many rope segments support the moving block. This, not the number of sheaves, is what sets the mechanical advantage.
• Reeving — the route the rope takes through the system. The same two blocks reeved differently give completely different results.
• Standing block and running block — the fixed one and the one that travels with the load.
• Fleet angle — the angle between the rope and the plane of the sheave. Keep it small: commonly under about 4°, and under about 2° where the rope spools in multiple layers. A large fleet angle drags the rope against the sheave flange and wears both, on one side only, which is the signature to look for.

5. The types you will be quoted

• Single, double and triple sheave. More sheaves in one block lets you reeve more parts of line within a shorter assembly. It does not add capacity on its own — a two-sheave block reeved as a simple redirect gives no advantage at all.
• Opening (snatch) versus closed cheek. An opening block can be fitted onto a rope already in service; a closed block is stiffer and cheaper and is the right choice for a permanent installation where the rope can be threaded from an end.
• Fitting: hook, shackle or becket. A hook is for quick attachment, a shackle for a permanent or high-load connection, a becket where the rope's dead end terminates on the block itself.
• Sheave material. Cast iron and steel for general lifting. Nylon (polyamide) where the rope or cable must not be marked, or where the block has to insulate — cable stringing, pulling through duct. The trade-off is real: nylon has a much lower rating for the same diameter, softens with heat, and wears faster against a rough rope.
• Bearing type. Ball or roller bearings give the lowest pulling effort and suit frequent use. Bronze bushes suit heavy, slow duty and tolerate abuse better.

Our own range runs from a 1 t single-sheave block with a 100 mm sheave and a 6–8 mm rope, up to a 50 t three-sheave block on a 500 mm sheave taking 20–26 mm rope. The full table, including weights and bearing types, is on the pulley block page.

6. What the number on a block actually means

One clarification, because it is the most consequential misunderstanding in the whole subject. The rated load marked on a block applies to the block's own load-bearing fitting — the shell, axle, shackle and pin. It is not a limit on the rope tension, and it is not the weight you can lift.

A block carries more than the load: it carries the vector sum of the two rope tensions running through it. With both parts parallel that is twice the line tension; at a 90° redirect it is about 1.41 times. A snatch block used purely to change direction can therefore put close to double the line tension onto whatever it is anchored to. Anchor and shackle for that figure, not for the weight being lifted. The full derivation, with the friction allowances, is on our pulley block load ratings.

7. What has to be marked on a block

Three things, at minimum: the manufacturer's name, the rated load, and the sheave size or the rope size or sizes it accepts. That is the marking requirement in the standards that cover this hardware, and it is also the quickest way to spot a block that was never made to one.

The rope size is the one that matters most to you. Without it you cannot tell whether the groove matches your rope — and the groove is the one dimension that cannot be corrected after the fact. A 200 mm sheave may accept a 10–13 mm rope; sheave diameter and rope diameter are two different numbers, and a correct specification needs both.

8. Five mix-ups that bring the wrong part

• “Pulley” meaning a belt pulley. A transmission pulley has a flat or V-groove for a belt, no lifting rating and no design factor. If your search returns flat wheels with keyed bores, you are in the wrong aisle.
• “Snatch block” meaning any block. Covered above — and the most common transatlantic misunderstanding in this product family.
• Confusing sheave size with rope size. Both appear in a specification and they are not related by a formula you can assume. Quote both.
• Assuming more sheaves means more capacity. The reeving decides. A three-sheave block used as a redirect lifts nothing extra.
• Buying a block to lift with. A block redirects and multiplies; it does not lift. You still need a hoist or a winch in the system — see our winches and electric hoists.

9. What a block cannot do

Four limits worth stating plainly, because each of them is crossed regularly:

• It is not rated for lifting people. No configuration of a pulley block or snatch block makes it acceptable for lifting a person.
• A nylon sheave block is a pulling block, not a lifting block. When it is used to string cable, the number on it is a line-pull rating for that operation, governed by a stringing procedure — not a lifting standard. Do not carry that number across to overhead lifting of a load.
• It is not a substitute for a hoist. A block with nothing pulling the live part is a static component.
• It is not tolerant of a mismatched rope. A rope that is too large is pinched; one that is too small is unsupported. Both damage the rope, and the worn groove then damages the next rope too.

On inspection and discard criteria — groove wear, hook deformation limits, bearing play, and the inspection intervals a working rig needs — our pulley block specifications set out the measured limits. If you are specifying a reeving rather than a single block, our buyer's guide covers how the block, the hoist and the anchor have to be sized together.

Frequently Asked Questions

What is the difference between a block and a sheave?

A sheave is the grooved wheel — one component. A block is the whole assembly: the sheave or sheaves, the shell holding them, the axle and bearing, and the hook, shackle or becket that connects it to the load or anchor. In everyday speech the two are used interchangeably, but when you are ordering, the sheave is the part that has to match your rope and the block is the part that carries the rated load.

Why is it called a snatch block?

Because it can “snatch” onto a rope that is already in service. The cheek of the block opens or hinges, so the rope can be laid into the groove without threading a free end through — which is what makes it useful for a recovery pull, a redirect partway along a line, or any job where the rope is already rigged. In US rigging that opening cheek is the defining feature; elsewhere the term is often used more loosely for any heavy block, which is a good reason to confirm the design before ordering.

How do I know what rope size a block takes?

Look at the marking on the block, or the specification table for the model. A correctly marked block states the rope size or sizes it accepts, and the sheave groove is machined to suit. If the rope size is not marked, the block is not marked correctly — and since the groove cannot be altered later, that is a block to avoid rather than to work around. Our own range pairs sheave diameters from 100 mm to 500 mm with rope sizes from 6 mm to 26 mm, and both figures are quoted together for every model.

Can I use a pulley block to lift a load on its own?

No. A block changes the direction of a pull or gives mechanical advantage, but it does not generate any force itself — something has to pull the live part of the rope. In practice that means a hoist, a winch or a powered pull. What the block lets you do is lift more than that pulling device could manage alone, or pull from a more convenient direction, or both.

Is a chain pulley block the same as a chain hoist?

No, and the confusion is common enough that it is worth checking any listing that uses the phrase. A chain pulley block is a block whose sheave is grooved for chain rather than wire rope — a component in a system. A chain hoist (also called a chain block or manual hoist) is a complete lifting machine with its own brake that lifts a load directly from its hook. They are different products at different price points; our manual hoist page covers the second.