Valve Timing

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From Frank Williams, Locomotive Valve Setting, 1944:

Valve setting is a term generally applied to the work of making the final accurate adjustments to the valves and the valve gear of a locomotive before it is placed in service. These adjustments are made in order that the positions of the valves shall at all times bear their proper relation to the positions of the pistons. Valve setting follows the erection or hanging of the valve gear by the machinist.

Dead Center

See Dead center

Valve Formulas

From Larry Koehl, Live Steam Magazine, June 1976

Valve Travel formula.gif

Valve Cut Off formula.gif

The Throw of Eccentric formula, below, applies only to locomotives using Stephenson valve gear.

StephensonEccentricThrow formula.gif

Larry suggests adding up to 0.002 inches to the Throw of Eccentric value for each pin in the valve gear (including the eccentric, too) because, even when new, there will be lost motion, possibly enough to eliminate the lead.

Larry also points out the fact that Cut-Off is always 75% when Lap equals Port Width, for all values of Port Width.

See also Valves.


Ian wrote on Yahoo Group:

When the reversing lever (Johnson Bar) is in mid gear position there WILL be movement of the valve spindle however it will be at a minimum.
With the covers off the valve/steam chest then with the lever/bar centred (mid gear) the valve should just open the ports and it should be symmetrical at top and bottom dead centre respectively.
Better to set the eccentrics at full gear so that the valve only just cracks the ports at TDC/BDC and adjust the spindle length or slide position so it is symmetrical. just as you would a non reversing engine. This adjustment can be done without taking the cover off if the boiler has some low pressure air it and tubes connected to the drain cocks are led into a jar of water. Bubbles from the tube indicates the port is just opening.

See also:

Finding top dead center and setting values for Stephenson gear on Allen Mogul:



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