Cataract (beam engine)

A cataract was a speed governing device used for early single-acting beam engines, particularly atmospheric engines and Cornish engines.

The cataract is distinctly different from the Watt governor, in that it does not control the speed of the engine's stroke, but rather the timing between them.

The typical installation of a house-built beam engine spanned four floors. The cylinder and the engine driver's usual working position were located in the 'bottom chamber', approximately at ground level. Above this were the 'middle chamber', with the cylinder top cover and 'top nozzle' (the upper valve chest), and above that the 'top chamber' or beam chamber. The cataracts were located in the lowest part of the engine house, in a chamber below the bottom chamber, along with the exhaust pipe. This space was awkward to access and not visited in normal operation.

The valve gear (or 'working gear') of a Newcomen or Cornish engine is based on the plug rod. This is a vertical rod, hung from the beam, and moving in parallel to the piston. Adjustable tappets are attached to this rod. These tappets strike long curved iron levers or 'horns' that are carried on three horizontal shafts or 'arbors'. Each arbor works one of the engine's valves. For the Cornish cycles, these valves are the upper steam inlet to the top of the cylinder, the equilibrium valve that links upper and lower portions of the cylinder, and the lower exhaust and condensing water injection valves, which share an arbor. Unlike most other steam engines, these engines could be run intermittently: making a single stroke before stopping and waiting for the valves to be restarted again. The speed of each power stroke or 'coming indoors' was a feature of the engine and was not easily varied, but there was no need for the engines to run continuously, stroke after stroke. This was a direct contrast to the rotative beam engine, and the rotary nature of almost all other steam engines. With the original Newcomen cycle, the speed of the return stroke varied according to the boiler pressure, although this still did not affect the strength or speed of the power stroke.

Use of a cataract could allow an engine to be operated at only a third of its ungoverned speed. When pumping load was variable, cataracts could also be connected and disconnected as required, allowing the engine to work at full speed for a period and then stopped in between.

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Wikipedia