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Part 5 - Valve and piston layout and movement

Position the mouse cursor over part of the image below to find its name and function

The valve head is ahead of the front port, allowing steam under pressure into the cylinder to push the piston back As the piston begins to move backwards, used steam from the previous stroke is exhausted through the rear port After almost half of the pistons full stroke, the valve head cuts off the steam supply. The steam continues to expand and pushes the piston back The front port begins to open ready to exhaust the used steam. The rear port is closed creating back-pressure before the next stroke begins Fresh steam enters the rear port and the process begins again

     Tto ensure the efficient working of a steam locomotive, steam has to be admitted to, and exhausted from the cylinders at the precise moment in the pistons cycle. This is achieved by valves located next to each cylinder.

     Basically, the cylinder has a port at each end, and the function of the valve is to admit fresh steam under pressure at one end, while allowing used steam to be exhausted from the other. Later locomotives featured the use of piston valves as shown, in which the cylindrical valve chest contains two heads that open and close the ports in sequence.

     The amount of valve movement past the port is known as lap. In slow moving locomotives, a long lap is desirable as the delayed opening of the exhaust port gives time for the steam trapped inside the cylinder to make the best use of its expanding energy against the piston.

     On fast running locomotives, allowing the exhaust port to open early when the valve is in mid-position helps the steam to escape faster and so reducing back-pressure. Higher speed locomotives also utilise long lead, meaning that the admission port is already open when the piston is at the end of its movement and so supplying good steam pressure immediately it begins its movement away from the end of the cylinder.

     Cut-off is the term used to denote the position of the piston in its cycle at the precise moment that the valve is closing and therefore stopping steam being admitted to the cylinder.

     When the locomotive is working slowly, a long cut-off can admit steam for most of the stroke of the piston, however as speed builds up, this overtaxes the boiler and leads to back-pressure. To prevent this, cut-off can be reduced at high speeds until steam is admitted for only 15% of the piston's stroke, its expansive properties being used to push the piston for the remainder of the stroke.

Back to Basics Menu
Part 1: Locomotive layout    Part 2: Locomotive Tenders     Part 3: Tank Locomotives    Part 4: Firebox Layout
Part 6: Valve Gear     Part 7: Compounding     Part 8: Cylinder Design     Part 9: Blastpipe Design
Part 10: Superheating     Part 11: Tyres and Profiles     Part 12: Steam Sanding     Part 13: Streamlining
Part 14: Water Quality     Part 15: Coal Quality     Part 16: GWR Headlamp Codes     Part 17: GWR Signalbox Bell Codes

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