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Diagram showing lap and lead and their relation to valve travel. When on the move, a steam locomotive requires steam to enter the cylinder at precise times relative to the piston's position. [3] This entails controlling the admission and exhaust of steam to and from the cylinders with a valve linked to the motion of the piston. [3]
The connection between piston and main rod is a crosshead, which slides on a horizontal bar behind the cylinder. [2] [5] [3]: 55 Piston rod Connects the piston to the cross-head. [2] [3]: 61 Piston Produces the motion for the locomotive from expansion of the steam.
The cylinder is made pressure-tight with end covers and a piston; a valve distributes the steam to the ends of the cylinder. Cylinders were initially cast iron , but later made of steel . The cylinder casting includes other features such as (in the case of Stephenson's Rocket ) valve ports and mounting feet. [ 1 ]
The main moving parts of a steam locomotive with Walschaerts valve gear: 1 - Link, 2 - Eccentric crank, 3 - Radius rod, 4 - Lap/lead lever, 5 - Crosshead, 6 - Valve, 7 - Cylinder, 8 - Reach rod. A detailed animation of the exterior working parts of a steam locomotive. This animation is from the commons, and is already a featured picture there..
The valve opens to admit steam to the cylinder just before the start of a piston stroke. The pressure of this steam provides the driving force. Soon before the space on one side of the piston starts to contract, the valve starts to release steam from that space to the atmosphere, so as not to impede the movement of the piston.
Indicator diagram showing steam pressure with piston movement. An expansion valve is a secondary valve within a steam engine. They represent an intermediate step between steam engines with non-expansive working and later valve gears that could provide for expansion by controlling the motion of a single valve.
The steam needs to be fed into the end of the cylinder at just the right time in the cycle to push the piston in the correct direction. In the other direction, the steam needs to be allowed to escape from the cylinder. As the crankshaft rotates, the piston rod moves up and down (or side to side in the case of a vertical cylinder) as well as in ...
In 1933, a FS Class 685 2-6-2 locomotive was the object of a curious experiment, in which the piston engine was removed and a turbine fitted in its place, leaving the locomotive otherwise completely unchanged. Tests run were however a failure, as its performance proved to be well below that of a normal 685; the turbine soon broke up, and that ...