Combined Machines

Part of Simple Machines

How to combine levers, pulleys, gears, and screws to build compound mechanisms with high mechanical advantage.

Why This Matters

Each simple machine provides a specific kind of mechanical advantage. A lever trades force for distance. A pulley changes direction. A screw converts rotation to linear motion. But the real power of simple machines comes when you combine them. A windlass combines a wheel-and-axle with a ratchet (a modified lever). A screw press combines a screw with a lever handle. A crane combines a boom (lever), a block and tackle (pulley), and a windlass (wheel and axle). Each combination multiplies the advantages of its component machines.

Understanding how to combine simple machines is the foundation of all mechanical engineering. Every machine ever built — from a Roman ballista to a steam engine to a modern automobile — is a combination of simple machines. When you can identify the simple machines within a complex mechanism and understand how they interact, you can design new machines, diagnose failures, and improve performance.

This article explains the principles of machine combination and walks through several historically important combined machines that a rebuilding community can actually build.

Principles of Combination

Multiplying mechanical advantage: When two machines are combined in series (the output of one becomes the input of the next), their mechanical advantages multiply.

Example: A lever with 5:1 MA driving a block and tackle with 4:1 MA gives a combined MA of 5 × 4 = 20:1. You push with 5 kg and the far end of the system lifts 100 kg.

Multiplying disadvantages: The multiplication works both ways. Each machine in series also multiplies the distance loss. A 20:1 combined machine requires 20 meters of input motion to produce 1 meter of output. And friction losses multiply too — two machines, each with 80% efficiency, have combined efficiency of 0.8 × 0.8 = 64%.

The practical limit: Adding more machines to a combination always provides more mechanical advantage but also increases friction, adds complexity (more parts to fail), and requires more input movement. There is always a practical optimum beyond which adding more machines hurts more than it helps.

The Winch (Wheel-and-Axle + Ratchet)

The winch is the simplest and most useful combined machine. It multiplies force by turning a large-radius handle (the wheel) to wind rope onto a smaller-radius drum (the axle), and a ratchet mechanism holds the load when you stop pulling.

Components:

• Drum: a cylinder of wood or iron, typically 5-10 cm radius
• Handle: a crank arm extending 30-60 cm from the drum axis
• Pawl and ratchet: a pivoting tooth (pawl) that catches on notches cut around a disc on the drum shaft

Mechanical advantage: MA = Handle radius / Drum radius

A 50 cm handle arm (radius) on a 7 cm drum (radius) gives MA = 50/7 = 7:1. You pull with 15 kg of force and reel in rope under a 100 kg load.

Building a basic winch:

1. Turn or carve a cylindrical drum from hardwood, 10-15 cm diameter and 30-40 cm long
2. Mount the drum on an axle between two upright timber posts
3. Drill a hole through one end of the drum for the crank pin
4. Attach a crank arm — a bent iron rod or a shaped hardwood arm — to the crank pin
5. Cut a ratchet wheel from hardwood or iron plate — evenly spaced notches around the circumference
6. Mount the ratchet wheel on the drum shaft
7. Pivot a hardwood or iron pawl to engage the ratchet teeth

Ratchet pawl design: The pawl must be:

• Pivoted at one end, free to swing at the other
• Spring-loaded toward the ratchet (a bent piece of springy wood or a simple leaf spring keeps it engaged)
• Shaped so that it allows rotation in one direction (loading/winding) but locks against backward rotation (unloading)

The Screw Press (Screw + Lever)

The screw press combines a screw (enormous mechanical advantage for linear force) with a lever handle (additional force multiplication and ergonomic operation). Uses: pressing cider, grapes, olives; compressing hay; making paper; forming metal; extracting oil.

MA calculation:

• Screw MA = 2π × handle radius / thread pitch
• A lever handle of radius 80 cm and a 10 mm pitch screw: MA = 2π × 80 / 1.0 = 503:1

This is why a screw press can generate enormous forces — thousands of kilograms of pressing force from one person’s effort.

Building a timber-frame screw press:

1. Cut a heavy timber frame: two upright posts (15 × 15 cm) joined by a top beam and a base
2. The base is the lower platen — the surface the material rests on
3. The top beam has a round hole bored vertically through it, threaded to match the screw
4. Cut or buy a wooden screw, 5-8 cm diameter with 10-15 mm thread pitch — this requires either a thread-cutting tool (a screw box and tap) or a skilled turner
5. Thread the screw through the top beam hole
6. Attach a horizontal lever bar through a hole near the top of the screw
7. The lower end of the screw carries a pressing plate

Threading the screw hole: If you cannot cut threads, an alternative is to drive wedge pegs into the hole walls to act as thread followers. This is cruder but functional for low-precision pressing applications.

The Crane (Lever + Pulley + Winch)

A crane combines:

• A boom (inclined lever) that extends the reach beyond the base of the support
• A block and tackle (pulley) that multiplies lifting force
• A winch (wheel and axle) that applies continuous force to the pulley rope

Combined MA: Crane boom provides reach but not MA. The block and tackle provides 4-6:1 MA. The winch provides 6-8:1 MA. Combined: 24-48:1 MA — one person can lift several tons.

Derrick crane (simplest practical crane):

Components:

1. A mast: a vertical timber, 4-6 m tall, stepped in a heavy socket on the ground
2. A boom: an inclined timber, 3-4 m long, attached to the mast at the base with a pivot fitting
3. A topping lift: a rope from the boom tip to the mast top, adjusting boom angle
4. A block and tackle: attached between the boom tip and the load hook
5. A winch: on the ground, pulls the tackle rope

Construction notes:

• The mast must be guyed (held with wire or rope stays) to prevent tipping under the lateral forces created by the outreach of the boom
• Minimum three guys, arranged 120° apart horizontally around the mast
• The boom pivot must carry the full compressive load in the boom (which equals the load times a factor based on geometry — expect the boom to carry 1.5-2× the load weight in compression)
• The winch frame must be anchored firmly to prevent it moving when under load

The Capstan (Wheel-and-Axle for Large Forces)

A capstan is a vertical-axis winch used for pulling large loads horizontally — dragging heavy stones, hauling ships, pulling stumps.

Principle: A heavy timber post set vertically in the ground, with holes at the top for capstan bars (long levers). Rope is wrapped around the post. Multiple workers push on the capstan bars, walking in a circle.

MA: Each capstan bar adds force equal to (bar length / post radius). With four capstan bars each 2.5 m long on a post of 15 cm radius: MA per bar = 2500/15 = 167:1. With four workers: 4 × 167 = approximately 600:1 theoretical MA.

Building a capstan:

1. Set a post (20-30 cm diameter, well-seasoned hardwood or oak) vertically in the ground to a depth of 1 meter
2. Cut mortise holes at the top of the post for four capstan bars, at 90° to each other
3. Insert and pin the capstan bars (2-3 m long poles)
4. Wrap the tow rope around the post 2-3 times (friction holds the load as workers push)
5. One person holds and manages the tow rope end (paying it out or securing it)

Historical use: Ancient Egyptians used this principle to move the stones of the pyramids. Medieval construction used capstans to raise cathedral stones. Naval vessels used permanent iron capstans to anchor chains and haul cargo.

Design Rules for Combined Machines

Rule 1: Identify the weakest component. The combined MA cannot exceed what the weakest component can handle. A 20:1 combined machine built with rope rated for 100 kg working load cannot safely lift more than 100 kg despite the high MA. Calculate the forces at every component.

Rule 2: Friction multiplies at every stage. If each component is 85% efficient, three in series gives 0.85³ = 61% overall efficiency. Add enough stages and the friction absorbs most of your effort. For practical combined machines, aim for 3-4 component stages maximum.

Rule 3: Build in holding mechanisms at every stage. Every stage that can run backward under load (pulleys, winches) needs a ratchet or brake. A combined machine with no intermediate holding points can unwind catastrophically if you release the input.

Rule 4: Design for maintenance access. Combined machines have more parts, more bearings, more wear points. Design so that you can reach every lubricating point, replace every worn component, and inspect every joint without disassembling the entire machine.