Slide valve... simple question.

[rlukens]

In making a steam chest, does one rely on steam pressure to keep the seal between the valve and cylinder surface?

I'll sneak another question in here...
I notice that many cranks are designed with a counter weight opposite the rod connection. Considering the relative size of the crank versus the rotational mass of the flywheel, is this at all advantageous?

TIA Russ

 

[mcostello]

Have to balance the rod and piston somehow.

 

[JCSteam]

Depends on the engine, but in most cases the slide valve is a machined fit, and connected on a rod which protrudes past the valve, and is supported on the other side through an bearing. The valve rod usually has a screw fitting around the valve so that a gland can adjust the valve forward or backwards, to adjust the opening of the valve, (or the valve timing as it were).

Think of it as a threaded rod with the outer portions of thread removed, leaving the rod free to move forward and backward through a bearing, (machined hole), the valve sits over the threaded portion of the rod, and is adjusted by nuts to the front and back of the valve.

The space between the valve and the top of the valve chest, is for exhaust steam, which is at lower pressure than the steam coming into the cylinder through the middle of the three holes under the valve. So steam pressure can't be relied upon to form a seal. The valve face, and it's seat over the valve needs to be a polished finish, with as many machining marks removed as possible, this is achieved by rubbing on a flat surface (piece of glass) on fine emery paper and light oil.

Hope this helps, I have an example of an external valve arrangement, which relies on spring pressure to hold the valve seated. A video is on my YouTube channel. If your interested I'll post a link here, though you'll have to forgive the engine as the eccentric has worn to the point of useless!

 

[fcheslop]

The steam chest is at incoming pressure.This pressure pushes the valve onto the port face
The valve should be a free fit on its buckle no slog just able to fall free
The ports are usually smaller inlet with a larger central exhaust
The valve is set up to give equal amounts of port opening top and bottom and to time the engine rotate the crank until the piston is just at or just after TDC then turn the eccentric in the same direction until you just see a slight opening of the top port.
This only applies for slide valve engines Im sure there is some good utube videos showing how it works.
Iv not gone into lap and leed or types of valve gear but if its of interest have a look for the dockstar(i think) prog on the web

 

[rlukens]

The steam chest is at incoming pressure.This pressure pushes the valve onto the port face
The valve should be a free fit on its buckle no slog just able to fall free
The ports are usually smaller inlet with a larger central exhaust
The valve is set up to give equal amounts of port opening top and bottom and to time the engine rotate the crank until the piston is just at or just after TDC then turn the eccentric in the same direction until you just see a slight opening of the top port.
This only applies for slide valve engines Im sure there is some good utube videos showing how it works.
Iv not gone into lap and leed or types of valve gear but if its of interest have a look for the dockstar(i think) prog on the web

Further searching leads me to agree with you... steam chest at incoming pressure.
Thanks

 

[Charles Lamont]

The space between the valve and the top of the valve chest, is for exhaust steam, which is at lower pressure than the steam coming into the cylinder through the middle of the three holes under the valve. So steam pressure can't be relied upon to form a seal.

This is, of course, completely wrong. Steam is admitted to the steam chest. There is a clue in the name. The middle port is the exhaust port. Steam pressure does press the valve onto the face.

There are 'balanced' slide valves which seek to reduce the pressure between the valve and face and so reduce the power lost in driving the valve, but at the expense of considerable complication.

Piston valves are most commonly arranged the other way round, with inside admission. This means the gland has only to seal exhaust pressure (and temperature).

 

[pickleford75]

Charles is correct... the steam does in fact hold the valve against the surface... most importantly for a "D" valve to function correctly it cannot be attached to the valve rod... they "float" the first picture is from my gauge 1 locomotive i built and the second is from a stationary engine i built.... hope these help

 

[rlukens]

Charles is correct... the steam does in fact hold the valve against the surface... most importantly for a "D" valve to function correctly it cannot be attached to the valve rod... they "float" the first picture is from my gauge 1 locomotive i built and the second is from a stationary engine i built.... hope these help

Perfect example.
Thanks, Russ