Pulley Systems

Part of Simple Machines

Pulleys lift heavy loads by redirecting and multiplying force through rope and wheels. A single person can raise a thousand kilograms using nothing but rope, wooden wheels, and the correct arrangement. Pulleys built the pyramids, raised cathedral walls, and rigged every sailing ship in history.

How Pulleys Work

A pulley is a wheel with a groove for rope. In its simplest form, it changes the direction of pull — you pull down on the rope and the load goes up. More sophisticated arrangements multiply force by distributing the load across multiple rope segments.

The fundamental rule: Count the number of rope segments supporting the load. That is your mechanical advantage. Two segments = 2:1 advantage (you pull with half the weight). Four segments = 4:1 (one-quarter the weight).

The Trade-Off

Like all simple machines, pulleys trade force for distance. A 4:1 pulley system means you pull with one-quarter the force, but you must pull four times the length of rope. To lift a load 1 meter with a 4:1 system, you pull 4 meters of rope. No energy is created — only redistributed.

Single Fixed Pulley

Purpose

Changes the direction of pull. No mechanical advantage — you pull with exactly the load’s weight. But pulling downward (using your body weight) is far easier than pulling upward.

Construction

1. Carve or turn the sheave (wheel) from hardwood — a disc 100-200 mm diameter with a V-groove or U-groove around the circumference sized to the rope diameter.
2. Drill a center hole for the axle — a hardwood dowel or iron pin, 10-15 mm diameter.
3. Build a frame (block) — two flat wooden plates (cheeks) bolted or pegged together with the sheave between them, spinning freely on the axle.
4. Attach a hook or becket at the top of the frame for anchoring to a support structure.
5. Thread the rope around the sheave groove.

Specification	Value
Mechanical advantage	1:1 (direction change only)
Rope pulled per meter lifted	1 meter
Best for	Hoisting from ground level, flagpoles, well buckets

Single Moveable Pulley

Purpose

A pulley attached to the load rather than a fixed point. Provides 2:1 mechanical advantage.

How It Works

1. Anchor one end of the rope to a fixed point above.
2. Run the rope down and around a pulley attached to the load.
3. Pull the free end of the rope upward.
4. The load is now supported by two rope segments — the anchored end and the pulling end — each bearing half the load.

Specification	Value
Mechanical advantage	2:1
Rope pulled per meter lifted	2 meters
Disadvantage	You pull upward, which is ergonomically awkward

Block and Tackle

The block and tackle combines fixed and moveable pulleys to provide higher mechanical advantage with the convenience of pulling downward.

Common Configurations

Configuration	Advantage	Rope Segments	Description
Single sheave block + single sheave block	2:1	2	One upper (fixed) pulley, one lower (moveable) pulley
Double sheave upper + single sheave lower	3:1	3	Two wheels in upper block, one in lower
Double sheave upper + double sheave lower	4:1	4	Two wheels in each block
Triple sheave upper + triple sheave lower	6:1	6	Three wheels in each block

Building a Block and Tackle

Materials per block:

Component	Material	Quantity
Cheek plates	Hardwood planks, 20-30 mm thick	2
Sheaves (wheels)	Hardwood or cast iron	1-3 per block
Axle pin	Iron bolt or hardwood dowel	1
Rope groove	Turned or carved in sheave	1 per sheave
Hook or becket	Forged iron	1

Assembly:

1. Shape the cheek plates — elongated ovals or rectangles, large enough to enclose all sheaves with clearance.
2. Mount sheaves between the cheeks on the axle pin. Multiple sheaves must spin independently — use spacer washers between them.
3. Bore axle holes precisely aligned in both cheeks. Misaligned holes cause binding.
4. Attach the hook at the bottom of the lower block (load attaches here) and at the top of the upper block (suspends from support structure).
5. Reeve (thread) the rope — starting from the becket (where one end of the rope is anchored to one block), weave alternately around the upper and lower block sheaves.

Reeving Direction

Always start reeving from the block with the most sheaves. If upper has 3 sheaves and lower has 2, start the rope anchored to the upper block, run down to the lower block, back up, down, up, with the free (hauling) end coming off the upper block. This keeps the blocks from twisting.

Calculating Required Rope Length

Rope length = (Number of parts x Lift height) + (Distance between blocks when unloaded) + Tail for handling

Example: 4:1 system, 10-meter lift, blocks start 1 meter apart:

• (4 x 10) + 1 + 3 meters tail = 44 meters of rope

Building High-Quality Sheaves

Wooden Sheaves

1. Select dense hardwood — lignum vitae (traditional), oak, hornbeam, or ash.
2. Turn on a lathe if available — the sheave should be perfectly round and balanced.
3. If no lathe: Cut a rough disc with a saw, then refine the roundness by mounting on an axle and grinding against a fixed abrasive.
4. Cut the rope groove — the groove should be slightly larger than the rope diameter (about 10% wider) and semicircular in profile. Too tight grips and abrades the rope; too loose allows the rope to jump out.
5. Line the axle hole with a bronze or iron bushing to reduce friction and prevent the hole from wearing oval.

Metal Sheaves

Cast iron sheaves are heavier but last far longer:

1. Cast a wheel shape in green sand mold with the groove profile built into the mold.
2. Machine the bore for the axle.
3. Mount on a steel axle pin with a bronze bushing.

Reducing Friction

Every pulley system loses some advantage to friction. A poorly made system can lose 10-15% of its mechanical advantage per sheave.

Friction Source	Reduction Method
Axle bearing	Use bronze bushings, lubricate with tallow or oil
Rope in groove	Smooth the groove surface; use slightly oversized groove
Rope-on-rope	Ensure rope segments do not cross or rub against each other
Block cheeks	Sand smooth, wax or oil surfaces

Safe Working Practices

Load Rating

The weakest component sets the system’s maximum load. Check:

1. Rope strength — a rope’s safe working load is typically 1/5 to 1/10 of its breaking strength. A 20 mm manila rope breaks at approximately 2,000 kg; safe working load is 200-400 kg.
2. Sheave axle — the axle bears the full load in the lower block. Size accordingly.
3. Hook and attachment points — forged hooks should be tested to twice the intended working load before use.
4. Anchor point — the upper block’s attachment must support the full load plus the pull force. For a 4:1 system lifting 400 kg, the anchor supports approximately 500 kg (400 kg load + 100 kg effort tension).

Rope Inspection

Inspect rope before every heavy lift. Look for: frayed or broken fibers, sections worn thin, mildew or rot (soft, dark-colored areas), kinks that weaken the structure, and cuts or abrasion marks. A rope that shows any of these defects must be retired from lifting service. Rope failure under load is catastrophic.

Operating Procedures

1. Test the system with a light load first — verify all pulleys spin freely, rope runs without jamming, and the anchor holds.
2. Lift smoothly — jerky pulls create shock loads that can exceed the system’s rating.
3. Keep clear of the load — stand to the side, never directly under a suspended load.
4. Use a tag line — a separate rope attached to the load, held by a second person to control swing and rotation.
5. Lower slowly — letting a load run freely generates heat in the rope and pulleys and can lose control.

Applications in Rebuilding

Application	System	Advantage
Well bucket	Single fixed pulley	1:1 (direction change)
Construction lifting (beams, stones)	3:1 or 4:1 block and tackle	Force multiplication
Sailing ship rigging	Multiple fixed and moveable systems	Control of heavy sails
Workshop overhead hoist	2:1 or 3:1 with ceiling mount	Convenient parts lifting
Heavy equipment positioning	5:1 or 6:1 with compound tackle	Moving multi-tonne loads
Logging — dragging felled trees	2:1 with snatch block (redirect)	Direction change around obstacles

Common Mistakes

1. Overloading the rope — rope fails without warning. Know your rope’s working load limit and never exceed it. When in doubt, use a thicker rope or more pulley advantage.
2. Sheaves too small for the rope — small sheaves force the rope into a tight bend, which weakens it and increases friction. Sheave diameter should be at least 6 times the rope diameter.
3. Crossed rope segments — rope segments rubbing against each other wear rapidly and can fuse under load, jamming the system. Ensure clean reeving with no crossovers.
4. Neglecting the anchor load — the upper anchor bears the full load plus the hauling force. Anchor failure drops everything. Over-engineer the anchor point.
5. Not securing the load after lifting — once the load reaches height, tie off the rope to a cleat or post. Never rely on someone holding the rope — fatigue, distraction, or surprise can cause a fatal drop.

Summary

Pulley Systems — At a Glance

• Mechanical advantage equals the number of rope segments supporting the load — count them to know your force multiplication
• A single fixed pulley provides direction change only (1:1); a single moveable pulley provides 2:1 advantage
• Block and tackle systems with multiple sheaves provide 2:1 through 6:1 or more — limited mainly by friction and rope length
• Sheave diameter should be at least 6x rope diameter to prevent rope damage and excessive friction
• Use bronze bushings and lubrication on axle bearings to minimize friction losses
• Inspect rope before every heavy lift — fraying, rot, or wear means immediate retirement from service
• Never stand under a suspended load; always secure the rope to a cleat after lifting
• The upper anchor must support the full system load — over-engineer this attachment point