Block and Tackle

Part of Simple Machines

Building and using multi-pulley systems to lift heavy loads with minimal effort.

Why This Matters

A block and tackle is possibly the single most useful mechanical device a rebuilding community can construct. With a well-made four-pulley block and tackle, one person can lift a 400 kg roof beam, move a 300 kg stone block into position, or launch a 500 kg boat from a beach. None of these things are possible by hand alone, and all of them are essential tasks in building a community.

The mathematics are simple: a block and tackle with four supporting rope segments gives you 4:1 mechanical advantage, minus about 15-20% for friction. That means you pull 30 kg to lift 100 kg. Pull 75 kg (your own body weight, leaning back) and you lift 250 kg. A rope and two sets of pulleys gives you the lifting power of four strong adults.

Block and tackle systems were the dominant heavy lifting technology from ancient Egypt through the industrial revolution — used to build pyramids, raise cathedral walls, launch warships, and load cargo. Understanding how to build and operate one gives your community lifting capability far beyond what bare hands can achieve.

The Physics

Mechanical advantage from pulley count:

Count the number of rope segments supporting the movable (lower) block. That number is your mechanical advantage.

Configuration	Rope Segments on Lower Block	MA	Pull to Lift 100 kg
Single fixed pulley	1	1:1	100 kg (but direction change)
Single movable pulley	2	2:1	50 kg
Two fixed + two movable	4	4:1	25 kg
Three fixed + three movable	6	6:1	17 kg

The friction penalty: Each pulley adds friction that reduces effective mechanical advantage by approximately 5-10% per pulley. A 4:1 theoretical MA system with four pulleys loses roughly 20-30% to friction, giving approximately 3:1 real MA. For hand-built wooden pulleys, use 70% efficiency as your working estimate.

The distance tradeoff: For every unit of MA, you pull that many times the distance the load travels. Lifting a 200 kg beam 3 meters with a 4:1 tackle requires pulling 12 meters of rope.

Building the Blocks

A “block” is the wooden frame containing one or more pulley wheels. You need at least two blocks: a fixed upper block (attached to a high support point) and a movable lower block (attached to the load).

Pulley Wheels

Material: Dense hardwood — oak, maple, ash, or elm. Avoid softwoods (they wear out quickly) and green wood (it will shrink and crack).

Dimensions (for lifting loads up to 500 kg):

• Diameter: 12-18 cm
• Thickness: 3-4 cm
• Groove depth: half the rope diameter
• Groove width: slightly wider than the rope diameter

Cutting the pulley wheel:

1. From a seasoned hardwood plank, scribe a circle of the desired diameter
2. Cut the disc with a frame saw or bowsaw
3. Mark the center precisely — this is the axle hole
4. Drill the axle hole (1-2 cm diameter) precisely at center
5. Cut the groove around the circumference: first score two parallel lines around the edge, then carve out the material between them with a gouge or chisel

The groove profile: A rounded V-groove holds rope most securely. The rope sits in the groove and the groove walls prevent it from jumping off under side loads.

Axle hole clearance: The hole should be 2-3 mm larger than the axle pin diameter. This allows the wheel to spin freely on the axle. A tight fit causes the wheel to bind and can split the hub.

Building the Block Frame (Cheeks)

The frame holds the pulley wheels and provides attachment points for the hook and the rope.

For a single-pulley block:

1. Cut two identical plates (cheeks) from 2-3 cm thick hardwood: approximately 20 cm tall and 8-10 cm wide
2. At the top, drill matching holes for the crown pin (which holds the hook or strap)
3. At the position for the pulley, drill matching axle holes in both cheeks — precisely aligned
4. At the bottom of the lower block, drill matching holes for the swivel hook attachment

For a double-pulley block (4:1 system):

• The cheeks are wider (12-15 cm) to accommodate two pulley wheels side by side
• Drill two sets of axle holes, each carrying one pulley wheel
• Space the pulley holes so the rope can run over one wheel without rubbing the other

Assembling the block:

1. Insert the axle pin through one cheek, through the pulley wheel(s), and through the other cheek
2. Secure the axle with bent nails, cotter pins, or hardwood wedges on each outer face
3. The axle must be fixed — it does not rotate. The pulley wheel(s) rotate on the axle.
4. Insert the crown pin at the top and attach the hook or iron strap for attachment to the load or anchor point

Block spacers: Between the two cheeks, add wooden spacer blocks on the sides (but not between the pulley and the cheeks). These spacers set the frame width and prevent the cheeks from pinching the pulley wheel.

Rope Selection and Reeving

Rope requirements: The working load per rope segment = total load / MA. For a 4:1 system lifting 400 kg: each rope segment carries approximately 100 kg (plus friction). Use rope with a breaking strength of at least 400 kg (4x working load safety factor).

Rope material options:

Material	Strength	Durability	Notes
Twisted hemp	Good	Good	Traditional, widely available
Twisted sisal	Moderate	Moderate	Stiffer than hemp
Twisted manila	Very good	Very good	Best natural fiber option
Rawhide	Very good	Fair	Shrinks when wet
Chain (iron links)	Excellent	Excellent	Heavy, expensive to make

For hand-built blocks, use the thickest rope that fits comfortably in the pulley groove — this maximizes strength margin.

Reeving the rope (threading through the block system):

For a 4:1 double block and tackle:

1. Tie one end of the rope permanently to the fixed upper block frame (the anchor end)
2. Thread the rope down through the lower block’s first pulley wheel
3. Thread up through the upper block’s first pulley wheel
4. Thread down through the lower block’s second pulley wheel
5. Thread up through the upper block’s second pulley wheel — this end is the hauling end (where you pull)

Verify the reeving by counting rope segments between the blocks: you should count 4 segments running between upper and lower blocks. This confirms 4:1 MA.

The Anchor Point

The anchor point for the fixed block must support the full load, not just the pull force. In a 4:1 system lifting 500 kg, the anchor point sees the full 500 kg plus the hauling force (approximately 125 kg), totaling approximately 625 kg.

Suitable anchor points:

• A hardwood beam, 15+ cm diameter, securely mortised into a structure
• A living tree, 30+ cm diameter, with the load attachment below the first major branch
• A purpose-built timber tripod: three heavy poles (15+ cm diameter, 3-4 m long) lashed together at the top

Tripod construction:

1. Lay three poles parallel, tips overlapping by 60 cm
2. Lash the tips together tightly with rope (spiral lashing, at least 10 wraps plus frapping turns)
3. Stand the tripod upright — spread the feet apart to approximately 1.5 m per side
4. Spread wider for stability, narrower to allow the load to clear the ground at center

The tripod can be moved to wherever lifting is needed and is far more practical than fixed installation in most construction scenarios.

Operating Safely

Before every lift:

• Inspect all rope for cuts, fraying, or discoloration (sun damage weakens rope significantly)
• Check all block axle pins are secure
• Check all hooks and attachments
• Test the anchor point by applying body weight before applying the full load

During lifting:

• Keep all people away from directly below the suspended load
• Never wrap rope around hands or arms (if a load falls and rope moves, it can remove fingers)
• Use a cleat or tie-off point to hold the load when not actively pulling — never rely on holding the rope by hand for an extended period

Load control:

• A ratchet mechanism (a pawl that catches on notches cut into a drum or disc on the fixed block) allows holding the load without continuous pulling force
• Alternatively, a friction hitch on the hauling rope can hold the load: wrap the rope three times around a fixed post and the friction holds it
• For lowering a load, do not simply release the hauling end — control the descent by letting rope out slowly with the rope wrapped around a cleat

Shock Loads

Never allow a suspended load to drop suddenly and then stop. The shock load on the rope and blocks when a load drops and is caught can be 3-5 times the static load. A system rated for 500 kg static load may fail at 1,500 kg shock load. Lower all loads smoothly and controlled.

Maintenance

After each use:

• Dry wet ropes before storage — wet hemp or sisal rots and loses 20-30% strength
• Grease the pulley axle pins lightly with animal fat
• Inspect blocks for splits, cracking, or worn groove surfaces

Pulley wheel replacement: When the groove wears to more than twice its original depth, replace the wheel. A worn groove allows the rope to contact the cheeks rather than the wheel, dramatically increasing friction and rope wear.

Long-term rope inspection: Every 6 months, unreeeve the rope and inspect the full length for wear. The sections that run through the pulley grooves most often (the rope segments nearest the blocks) wear fastest. Trim the rope end periodically to expose fresh sections to the high-wear locations.