The Lap Engine is a beam engine designed by James Watt, built by Boulton and Watt in 1788. It is now preserved at the

Science Museum, London The Science Museum is a major museum on Exhibition Road in South Kensington, London. It was founded in 1857 and is one of the city's major tourist attractions, attracting 3.3 million visitors annually in 2019. Like other publicly funde ...

. It is important as both an early example of a beam engine by Boulton and Watt, and also mainly as illustrating an important innovative step in their development for its ability to produce rotary motion. The engines name comes from its use in Matthew Boulton's Soho Manufactory, where it was used to drive a line of 43 polishing or lapping machines, used for the production of buttons and buckles.

Innovations

Thinktank Birmingham - object 1950S00022(1)

Watt did not invent the

steam engine A steam engine is a heat engine that performs mechanical work using steam as its working fluid. The steam engine uses the force produced by steam pressure to push a piston back and forth inside a cylinder. This pushing force can be ...

and there is no single ' Watt steam engine' as such. He developed a number of separate innovations, each of which improved the existing engines of the day, beginning with Newcomen's. The Lap Engine of 1788, also the Whitbread Engine (1785), represent survivors of the first engines to show all of Watt's major improvements in one.

Parallel motion

Rotative beam engines

Sun and planet gear

The rotative beam engine needs a means to convert

reciprocating motion Reciprocating motion, also called reciprocation, is a repetitive up-and-down or back-and-forth linear motion. It is found in a wide range of mechanisms, including reciprocating engines and pumps. The two opposite motions that comprise a single ...

of the piston and beam to rotary motion. The crankshaft was well known for centuries before Watt, mostly from its use in mining machinery powered by

water wheel A water wheel is a machine for converting the energy of flowing or falling water into useful forms of power, often in a watermill. A water wheel consists of a wheel (usually constructed from wood or metal), with a number of blades or buck ...

s. However its use for a steam engine was covered by James Pickard's patent at this time. Watt was unwilling to pay a license fee to use the

crank Crank may refer to: Mechanisms * Crank (mechanism), in mechanical engineering, a bent portion of an axle or shaft, or an arm keyed at right angles to the end of a shaft, by which motion is imparted to or received from it * Crankset, the compone ...

and so sought an alternative. The sun and planet gear was invented by another

Scottish Scottish usually refers to something of, from, or related to Scotland, including: *Scottish Gaelic, a Celtic Goidelic language of the Indo-European language family native to Scotland *Scottish English *Scottish national identity, the Scottish ide ...

engineer, William Murdoch, an employee of Boulton and Watt. Watt patented it in October 1781. The sun and planet gear is a simple epicyclic gear. The planet is attached rigidly to the end of the connecting rod, hung from the beam. As it rotates it applies a torque to the sun gear, just as for a crank, and so causes it to rotate. As the two gears also rotate relative to each other, like conventional gearwheels, this has the effect of giving the sun gear a further rotation. The sun, and the output crankshaft, thus rotates twice for every piston cycle of the engine, twice as fast as with a conventional crank. Beam engines were slow-moving and the output shafts driven by the Lap Engine were fast-moving, so this was an advantage.

Centrifugal governor

According to the Science Museum, it was the first steam engine to be fitted with a

centrifugal governor A centrifugal governor is a specific type of governor with a feedback system that controls the speed of an engine by regulating the flow of fuel or working fluid, so as to maintain a near-constant speed. It uses the principle of proportional c ...

Innovations

Parallel motion

Rotative beam engines

Sun and planet gear

Centrifugal governor

History

Preservation

Notes

References

Further reading