The Lap Engine is a beam engine designed by James Watt, built by

Boulton and Watt Boulton & Watt was an early British engineering and manufacturing firm in the business of designing and making marine and stationary steam engines. Founded in the English West Midlands around Birmingham in 1775 as a partnership between the Eng ...

in 1788. It is now preserved at the Science Museum, London. 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 Matthew Boulton (; 3 September 172817 August 1809) was an English manufacturer and business partner of Scottish engineer James Watt. In the final quarter of the 18th century, the partnership installed hundreds of Boulton & Watt steam engin ...

's Soho Manufactory, where it was used to drive a line of 43 polishing or

lapping Lapping is a machining process in which two surfaces are rubbed together with an abrasive between them, by hand movement or using a machine. Lapping often follows other subtractive processes with more aggressive material removal as a first ste ...

machines, used for the production of buttons and buckles.

Innovations

Thinktank Birmingham - object 1950S00022(1)

Watt did not invent the steam engine and there is no single '

Watt steam engine The Watt steam engine design became synonymous with steam engines, and it was many years before significantly new designs began to replace the basic Watt design. The first steam engines, introduced by Thomas Newcomen in 1712, were of the "a ...

' 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 The Whitbread Engine preserved in the Powerhouse Museum in Sydney, Australia, built in 1785, is one of the first rotative steam engines ever built, and is the oldest surviving. A rotative engine is a type of beam engine where the reciprocating mo ...

(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 A beam engine is a type of steam engine where a pivoted overhead beam is used to apply the force from a vertical piston to a vertical connecting rod. This configuration, with the engine directly driving a pump, was first used by Thomas Newc ...

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 r ...

of the piston and beam to

rotary motion Rotation around a fixed axis is a special case of rotational motion. The fixed-axis hypothesis excludes the possibility of an axis changing its orientation and cannot describe such phenomena as wobbling or precession. According to Euler's rot ...

. 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 bucket ...

s. However its use for a steam engine was covered by

James Pickard James Pickard was an English inventor. He modified the Newcomen engine in a manner that it could deliver a rotary motion. His solution, which he patented in 1780, involved the combined use of a crank and a flywheel. James Watt's company Boul ...

's patent at this time. Watt was unwilling to pay a license fee to use the crank and so sought an alternative. The sun and planet gear was invented by another Scottish engineer,

William Murdoch William Murdoch (sometimes spelled Murdock) (21 August 1754 – 15 November 1839) was a Scottish engineer and inventor. Murdoch was employed by the firm of Boulton & Watt and worked for them in Cornwall, as a steam engine erector for ten yea ...

, an employee of

. Watt patented it in October 1781. The sun and planet gear is a simple

epicyclic gear An epicyclic gear train (also known as a planetary gearset) consists of two gears mounted so that the center of one gear revolves around the center of the other. A carrier connects the centers of the two gears and rotates the planet and sun gea ...

. 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 con ...

Innovations

Parallel motion

Rotative beam engines

Sun and planet gear

Centrifugal governor

History

Preservation

Notes

References

Further reading