Single Pulley

Part of Simple Machines

Building and using single fixed and movable pulleys — the simplest rope-based lifting system.

Why This Matters

A single pulley seems like the most primitive lifting device — it does not even provide mechanical advantage in its simplest form. But this dismissal misses what a single pulley actually does: it changes the direction of force. This is enormously useful. Without a pulley, lifting a roof beam means everyone stands above it pulling up — impossible when the beam is going up to the top of a wall. With a fixed pulley at the top of the wall, you pull downward instead. Now you can use your body weight and the most natural pulling posture to apply force while the beam rises.

A single movable pulley — where the pulley itself travels with the load — does provide 2:1 mechanical advantage. One person can lift twice what they could lift with a direct rope. This is the entry point to the block-and-tackle system: add more movable pulleys and the advantage keeps increasing.

Understanding single pulleys — how they are built, what determines their load rating, and how they fail — is prerequisite knowledge for all more complex pulley systems.

Fixed vs. Movable Pulleys

Fixed pulley: The pulley is anchored to a fixed point (beam, tree, wall). The rope runs over the pulley, changing direction. The load hangs from one end of the rope; you pull the other end.

Effect: No mechanical advantage (MA = 1). You pull with a force equal to the load weight. But you pull downward, in the direction most natural and powerful for a standing person, while the load rises.

Movable pulley: The pulley is attached to the load (or to a hook that attaches to the load). The fixed end of the rope is anchored above; the load hangs from the pulley itself. You pull the free end of the rope upward.

Effect: MA = 2. Two rope segments support the load (the fixed end and the hauling end each carry half the load weight). You pull with half the load force.

Building a Simple Fixed Pulley

A single fixed pulley is quick to build and immediately useful for well-digging, hay lofts, and general lifting.

The Pulley Wheel

Material: Hardwood (oak, maple, ash). The wheel must resist the rope groove wearing into it.

Dimensions for typical use (loads up to 200 kg):

• Diameter: 10-15 cm
• Thickness: 3-4 cm (wider than the rope diameter by at least 50%)
• Groove width: slightly wider than the rope (the rope should sit snugly in the groove but not be pinched)
• Groove depth: half the rope diameter

Making the wheel:

1. Scribe a circle on a hardwood plank (3-4 cm thick)
2. Cut the disc with a bowsaw or frame saw
3. Find the center and drill the axle hole — 1-2 mm larger than the axle pin diameter
4. Mark the groove: two parallel lines around the circumference, spaced = groove width apart
5. Carve the groove between the lines with a gouge or chisel
6. Round the bottom of the groove with a curved file or rounded stone
7. Smooth all surfaces — rough spots abrade rope

Checking the wheel:

• Mount on a rod through the axle hole
• The wheel must spin freely without binding
• The wheel should run true — no visible wobble

The Block Frame

The block frame holds the wheel, provides an attachment point, and contains the rope.

For a simple single fixed pulley:

1. Cut two identical plates (cheeks) from 2.5 cm hardwood: approximately 15 cm tall, 8 cm wide
2. Mark and drill matching axle holes in both cheeks at the same position
3. At the top: drill a hole for the crown pin (the attachment pin for hanging or hooking)
4. Drill a matching hole to allow rope entry and exit

Assembling the block:

1. Place one cheek flat on a work surface
2. Set the pulley wheel between the cheeks, aligned with the axle holes
3. Add wooden spacers on each side of the wheel to fill the space between cheek and wheel (leave 1-2 mm clearance each side so the wheel can spin freely)
4. Insert the axle pin through the entire assembly
5. Secure the axle with bent nails, wooden wedges, or drilled holes with cotter pins
6. The crown pin at the top: a strong bolt or iron rod passing through both cheeks, from which the block hangs

Attachment options:

• Iron hook forged with a loop, threaded over the crown pin
• Rope becket (a fixed loop) tied around the crown pin
• Chain link over the crown pin

Load Rating

The load rating of the pulley is determined by the weakest component. Check each:

Rope: The rope carries the full load on the hauling side plus the full load on the load side (for a fixed pulley). Wait — for a fixed pulley, the rope carries only the load force on each side (equal, since it’s one rope). Use rope rated for at least 2× your maximum load.

Axle pin: Carries shear force equal to the load weight. For a 200 kg load, the pin must resist 200 kg of shear. An iron rod 10 mm diameter has shear strength of approximately 800-1,200 kg — adequate.

Cheeks: Carry the same load in bending. Check that the wood around the axle hole is at least 2× the axle hole diameter on each side.

Crown attachment: Carries the combined hauling force + load = 2× load for a fixed pulley (both rope ends pull down, so the attachment pulls up with the sum). Size accordingly.

Using a Fixed Pulley

Setting up:

1. Anchor the pulley block firmly at height. The attachment point must carry 2× the maximum load (since it supports both sides of the rope).
2. Thread the rope through the groove
3. Attach the load to one end
4. Check that the rope runs cleanly in the groove without touching the cheeks

Operating:

1. Pull the free end — the load rises
2. The pull direction can be vertical, horizontal, or at any angle
3. Hold the load at height by tying off the hauling rope to a cleat

Working with the Rope's Natural Bend

The rope bends around the pulley groove. At the point of contact with the groove, the rope is under the highest stress. Inspect this section of rope most carefully for wear — it is the area that frays soonest.

Building a Single Movable Pulley

A movable pulley gives 2:1 mechanical advantage — useful as a standalone device or as the first stage of a block and tackle.

Difference from fixed pulley: The block does not have a fixed attachment point for hanging. Instead, it has a swivel hook or loop at the bottom to attach to the load. The rope’s fixed end is tied to an anchor above; you pull the free end upward.

Construction:

1. Build the block exactly as for a fixed pulley
2. At the bottom: attach a swivel hook (iron forged to a loop, then straightened and re-bent as a hook shape) or a rope becket loop for direct rope attachment
3. The top of the block (where the rope enters and exits) must have rope guides (chamfered edges or a slot) to prevent the rope from slipping off

Reeving the rope:

1. Tie one end of the rope firmly to an anchor point above (the ceiling beam, a hook in a wall)
2. Thread the rope through the groove of the movable pulley
3. Bring the free end back up to where you will pull it
4. The load hook hangs from the movable pulley block

Verification: With this setup, you should count 2 rope segments running up from the movable pulley — one going to the anchor, one going to your hand. This is your 2:1 MA confirmation.

Sheaves and Multiple Rope Grooves

A sheave is a pulley wheel with two or more separate grooves, allowing multiple ropes to run over the same wheel. This is an intermediate step toward block-and-tackle construction without requiring separate blocks for each rope.

Making a two-groove sheave:

1. Cut the wheel thicker (5-7 cm instead of 3-4 cm)
2. Cut two parallel grooves, separated by a ridge of wood between them
3. Each groove handles one rope independently

Sheaves reduce the total number of blocks needed in a system, reducing friction losses (fewer axles) and making the system more compact.

Maintenance

Daily inspection:

• Check rope for fraying, especially at the groove contact area
• Check the axle pin — it should spin the wheel, not just carry it
• Listen for squealing during use — indicates insufficient lubrication

Lubrication:

• Apply a light coating of grease (tallow or lard) to the axle pin surface before each use
• Well-lubricated pulleys are noticeably easier to use — you can feel the friction difference

Groove wear:

• As the rope runs repeatedly through the groove, the groove bottom deepens
• When groove depth exceeds the rope radius (the groove is deeper than the rope center), the rope contacts the groove walls and friction increases dramatically
• Replace the pulley wheel when the groove becomes noticeably deeper than the rope