Steam port spacing

Take a simple model steam engine with a flat sliding "D" valve and eccentric strap. Am I safe in assuming that the distance the valve centre line travels during its movement from one end of the cylinder to the other end is the same distance between the steam port centres? Or, is there more to it than that?

I suppose its the same with a piston valve engine - yes?

I've tried googling for some simple rule of thumb advice but without much success.

ATB

Stuart

Hi Stuart,

steam engine slide valves often had some movement past the port (lead and lap). You can read a bit about this here, here or here.This will provide extra time for steam to expand and move the piston. If you have access to Model Engineer March 21. 1957 p. 421-422 E.T. Westbury discusses simple slide valve engines.

Regards

Thor

Edited By Thor on 25/01/2013 18:14:49

Edited By Thor on 25/01/2013 18:33:29

Hi Stuart,

This is a diagram of the slide valve for an engine I designed, compared with the Stuart 10V.

Click on it and it should give you enough of a guide to make functional slide valves (the port sizes are to scale)

Neil

Stuart,

the general rule is that the steam port doesnt need to open fully, but the exhaust port must. in addition with stephensons valvegear one should not aim for equal opening of both steam ports, as there is a discrepancy between front and back port caused by the geometry of the gear. the valves should be set to give equal lead rather than equal port opening, and a check on equal cut offs is no bad thing either.

i hope this helps.

cheers,

julian

Edited By julian atkins on 25/01/2013 23:49:27