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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]
Piston Produces the motion for the locomotive from expansion of the steam. Driven backward and forward within the cylinder by steam delivered alternately, in front and behind, by the valve. [1] [3]: 61 Cylinder Chamber that receives steam from the steam pipe. [2] [3]: 23 Valve Controls the supply of steam to the cylinders.
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 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.
The steam locomotive, as commonly employed, has its pistons directly attached to cranks on the driving wheels; thus, there is no gearing, one revolution of the driving wheels is equivalent to one revolution of the crank and thus two power strokes per piston (steam locomotives are almost universally double-acting, unlike the more familiar internal combustion engine).
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..
In a steam engine, cutoff is the point in the piston stroke at which the inlet valve is closed. On a steam locomotive, the cutoff is controlled by the reversing gear.. The point at which the inlet valve closes and stops the entry of steam into the cylinder from the boiler plays a crucial role in the control of a steam engine.
Steam enters via the steam port SP, and is admitted by the slide valve SV through the upper passage S to push down the piston P. At the same time, exhaust steam from below the piston passes back up the lower passage S, via the valve cavity, to exhaust E. As the piston descends, the valve moves upwards to admit steam below the piston and release ...