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 Newcomen around 1705 to remove water from mines in Cornwall. The efficiency of the engines was improved by engineers including James Watt, who added a separate condenser; Jonathan Hornblower and Arthur Woolf, who compounded the cylinders; and William McNaught, who devised a method of compounding an existing engine. Beam engines were first used to pump water out of mines or into canals but could be used to pump water to supplement the flow for a waterwheel powering a mill.
The rotative beam engine is a later design of beam engine where the connecting rod drives a flywheel by means of a crank (or, historically, by means of a sun and planet gear). These beam engines could be used to directly power the line-shafting in a mill. They also could be used to power steam ships.
The first beam engines were water-powered and used to pump water from mines. A preserved example may be seen at the Straitsteps Lead Mine in Wanlockhead in Scotland.
Beam engines were extensively used to power pumps on the English canal system when it was expanded by means of locks early in the Industrial Revolution, and also to drain water from mines in the same period, and as winding engines.
The first steam-related beam engine was developed by Thomas Newcomen. This was not, strictly speaking, steam powered, as the steam introduced below the piston was condensed to create a partial vacuum thus allowing atmospheric pressure to push down the piston. It was therefore called an Atmospheric Engine. The Newcomen atmospheric engine was adopted by many mines in Cornwall and elsewhere, but it was relatively inefficient and consumed a large quantity of fuel. The engine was improved by John Smeaton but James Watt resolved the main inefficiencies of the Newcomen engine in his Watt steam engine by the addition of a separate condenser, thus allowing the cylinder to remain hot. Technically this was still an atmospheric engine until (under subsequent patents) he enclosed the upper part of the cylinder, introducing steam to also push the piston down. This made it a true steam engine and arguably confirms him as the inventor of the steam engine. He also patented the centrifugal governor and the parallel motion. the latter allowed the replacement of chains round an arch head and thus allowed its use as a rotative engine.
His patents remained in place until the start of the 19th Century and some say that this held back development. However, in reality development had been ongoing by others and at the end of the patent period there was an explosion of new ideas and improvements. Watt's beam engines were used commercially in much larger numbers and many continued to run for 100 years or more.
Watt held patents on key aspects of his engine's design, but his rotative engine was equally restricted by James Pickard's patent of the simple crank. The beam engine went on to be considerably improved and enlarged in the tin- and copper-rich areas of south west England, which enabled the draining of the deep mines that existed there. Consequently, the Cornish beam engines became world-famous, as they remain among the most massive beam engines ever constructed.
Because of the number of patents on various parts of the engines and the consequences of patent infringements, examples exist of Beam Engines with no makers name on any of the parts (Hollycombe Steam Collection).
In a rotative beam engine, the piston is mounted vertically, and the piston rod drives the beam as before. A connecting rod from the other end of the beam, rather than driving a pump rod, now drives a flywheel.
Early Watt engines used Watt's patent sun and planet gear, rather than a simple crank, as use of the latter was protected by a patent owned by James Pickard. Once the patent had expired, the simple crank was employed universally. Once rotary motion had been achieved a drive belt could be attached beside the flywheel. This transmitted the power to other drive shafts and from these other belts could then be attached to power a variety of static machinery e.g. threshing, grinding or milling machines.
The first steam-powered ships used variants of the rotative beam engine. These marine steam engines – known as side-lever, grasshopper, crosshead, or 'walking beam', among others – all varied from the original land-based machines by locating the beam or beams in different positions to take up less room on board ship.
Compounding involves two or more cylinders; low-pressure steam from the first, high-pressure, cylinder is passed to the second cylinder where it expands further and provides more drive. This is the compound effect; the waste steam from this can produce further work if it is then passed into a condenser in the normal way. The first experiment with compounding was conducted by Jonathan Hornblower, who took out a patent in 1781. His first engine was installed at Tincroft Mine, Cornwall. It had two cylinders – one 21-inch (0.53 m) diameter with 6-foot (1.8 m) stroke and one 27-inch (0.69 m) diameter with 8-foot (2.4 m) stroke – placed alongside each other at one end of the beam. The early engines showed little performance gain: the steam pressure was too low, interconnecting pipes were of small diameter and the condenser ineffective.
At this time the laws of thermodynamics were not adequately understood, particularly the concept of absolute zero. Engineers such as Arthur Woolf were trying to tackle an engineering problem with an imperfect understanding of the physics. In particular, their valve gear was cutting-in at the wrong position in the stroke, not allowing for expansive working in the cylinder. Successful Woolf compound engines were produced in 1814, for the Wheal Abraham copper mine and the Wheal Vor tin mine.
William McNaught patented a compound beam engine in 1845. On a beam engine of the standard Boulton & Watt design he placed a high-pressure cylinder, on the opposite side of the beam to the existing single cylinder, where the water pump was normally fitted. This had two important effects: it massively reduced the pressure on the beam, and the connecting steam pipe, being long, acted as an expansive receiver – the element missing in the Woolf design. This modification could be made retrospectively, and engines so modified were said to be "McNaughted". The advantages of a compound engine were not significant at pressures under 60 pounds per square inch (410 kPa), but showed at over 100 psi (690 kPa).