Rotative Beam Engine

This is a working model of the mechanism, turned by hand or a small motor — no steam, no boiler, no pressure anywhere. The linkages are the subject; the power source is not.

Build an upright frame on a baseboard tall enough to carry the beam above the cylinder. Any flex in the frame turns into slop in every linkage hanging off it.

Mount a beam 400 mm long on a free central pivot at the top of the frame. It must rock with almost no friction — a stiff beam pivot masks everything downstream.

Fit a tube as the cylinder under one end of the beam, with a rod sliding in it. In a real engine the rod must stay in line with the cylinder or it wears the bore oval and leaks.

Connect the rod straight to the beam end with one pin and rock the beam. The rod is dragged sideways because the beam end travels an arc while the rod must travel a line. Feel that binding — it is what the next step exists to fix.

Add Watt's linkage: a four-bar arrangement of the beam end, two links and a fixed anchor, proportioned so the connection point traces a near-straight line over its working range.

Clip a pencil to the connection point and let it draw on card as the beam rocks its full travel. Measure the deviation from a ruled line — a few tenths of a millimetre over a long stroke is a good result, and it is approximately straight, never exactly.

Mount a heavy disc on a free-running shaft at the far end. The flywheel carries the engine through the dead points where the beam reverses and no force is being delivered.

Fix one gear — the sun — to the flywheel shaft, and fasten a second gear of the same size — the planet — rigidly to the end of the connecting rod so that it cannot rotate on its own pin. That fixed planet is the whole trick.

Add a link between the two gear centres to hold them in mesh while the planet swings around the sun. The connecting rod pushes the planet around and the meshing teeth turn the sun.

Mark the flywheel and count its revolutions per full beam cycle. With equal gears the sun turns twice per orbit — the arrangement gives two flywheel revolutions per engine stroke, where a plain crank gives one.

Swap in a simple crank and turn it. It does the same job with fewer parts and less friction. The sun-and-planet is not the better mechanism — it existed to get around a patent, and knowing that is the point of the comparison.

Belt a small centrifugal governor off the flywheel shaft and link its sleeve to a throttle flap. Now the model has all three of Watt's mechanisms working together.

Turn the flywheel steadily and press a finger on it to simulate load. Watch the beam, the straight-line linkage and the governor respond together. That combination — rotation, guided rod, self-regulated speed — is what a mill actually needed.

The patent that forced an invention. The obvious way to turn reciprocating motion into rotation is a crank and flywheel — and James Pickard patented exactly that combination in 1780. Boulton and Watt needed a way round it. The sun-and-planet gear was invented by William Murdoch, an employee of the firm, and patented by Watt in October 1781. It had a commercial life of about twenty years and was largely dropped after Pickard's crank patent expired in 1794, when Boulton and Watt moved most new engines to plain cranks. The mechanism is a monument to patent law rather than to engineering necessity — and it does have one genuine advantage, the doubled flywheel speed.

Parallel motion. The beam end swings through an arc; the piston rod must move in a straight line. Watt's linkage traces a very close approximation to a straight line over the working stroke — approximate, always, but close enough that the rod does not bind. Watt is said to have been prouder of it than of anything else he devised, which for the man who separated the condenser is a striking judgement. Parallel motion and the sun-and-planet gear appear together on a drawing dated November 1784.

Single-acting, then double. The early rotative engines were single-acting: steam pushed the piston one way and the beam and flywheel returned it. Conversion to double action — steam admitted alternately to both sides of the piston — roughly doubled the power from the same cylinder.

Why this engine changed the shape of industry. A pumping engine has to sit next to what it drains. A rotative engine drives a shaft, and a shaft drives a whole mill through belts and gearing. That is what took manufacturing off rivers and put it wherever coal could be delivered — and it is the single mechanical change most directly behind the geography of the Industrial Revolution.