About slide valves

Hi I have been following plans to make a steam engine, and the slide valve size is 15mm long, this is with lead, without lead it would be 12mm long, can someone tell me how this size was calculated.

Thank you for any information.

Have you got a bit more information so we can flesh out the request a little? Was this the aforementioned Jenny?

I think you are mixing up lead with lap for this question? Lead is the amount a valve will be open to steam when the piston is on dead centre. Ie lead introduces steam to the cylinder before the piston goes over centre, there are a number of reasons for this one being it provides a cushioning effect to the piston. Lead is determined mainly by the setting of the valve gear, in Stephenson's link motion this will be the angle of advance of the eccentric at full gear. Lap on the other hand is the amount the valve overlaps the outside edge of the steam ports. For example if the measurement across the outside edges of your steam ports is 12mm and you make the valve 15mm then it has 1.5mm lap (overlaps the outside edges of the ports by 1.5mm each end when the valve is set mid travel). Lap also needs to be considered when timing the valve gear. To get a real understanding of lap and lead you really need to study valve gear and the implications of valve events and the different types of valve gear available and the idea of variable or constant lead.

Paul.

Hi this is not the Jenny , in Engineering In Miniature there's an article on making a Double Acting Stationary Engine, I am not making this engine, but the size of the piston and cylinder is what I need for another project, the slide valve will be operated from the crank by an eccentric rod. The ports on the cylinder at 2mm wide x 2mm space x 4mm wide x 2mm space x 2mm wide, total 12mm but the valve is 15mm long.

I hope this helps .

Thank for your time , Tony s.