Steam cylinders question

[AlanM]

I have a question about how to talk about types of cylinders on a steam engine.  I am doing a kitbash that needs to replace the more modern type with the older one.  My problem: what do I call the "2 tootsie rolls tied together with sheet metal" modern type vs. the "single tootsie roll with a square box on top type"?  It is embarrassing to talk like that.  These things must have names.
     Alan

[Tim]

Alan

The modern one with the round "Tootsie roll" on top is a piston valve.

The older style is a "D" valve.  They are the same in function.

They control the steam admission and exaust to the main cylinder.

Lubrication is the reason for the piston valve replacing the "D" valve.

When they went to superheated steam the "D" valves failed, because
superheated steam contains no moisture.

The moisture in saturated steam was the primary lube along with tallow
a waxy oil made from animal fat.

It couldn't stand up to the higher temp's of superheated steam.

Tim Anders
Souderton, PA

[AlanM]

Thanks, Tim.  Just what I wanted to know.  Is the D-valve the same as a slide valve, perhaps?  I have heard that term.
     Alan

[ebtnut]

Yes, "D" valves and slide valves are the same thing.  They were knows as "D" valves because the valve port in section looks like a D lying on its straight side.