Direct exhaust slide valve?

What you describe sounds vaguely like what I would recognise as a 'balanced slide valve' as fitted to my loco. The seal is by 'O' rings and there is a light spring to keep the parts in contact when there is no steam in the steam-chest to do so. If you want more detail as to how it works I could attempt to photocopy of the drawings of the relevant parts and post if you let me have an address off-list.

Yes.

What you have drawn is roughly the idea of a 'balanced slide valve' in which the central communicating port between the two parts of the valve has an area similar to that of the valve's port face. This means that the steam chest pressure cannot press the valve on to the port face with unnecessary force - the forces are balanced.

In a model engine the seal and the light separation force required can both be provided by lightly squeezing an O-ring between two faces. The valve rod needs to drive the valve through a yoke large enough to encompass the telescopic sleeves.

But.

You must have sufficient metal between the cylinder bore and the port face for the steam passages, and to bolt the steam chest to. That should also be enough thickness for an exhaust port. I don't think I understand what the problem is.

Good, someone has been here before :)

The problem is caused by the cylinder being round, and a slight c*ck up on my part. I meant to offset the bore, but forgot. (out with the dti, center bar in 4 jaw, make nice part, then Doh! moment) now it is bored to a nice finish and the cylinder head gland is perfectly centered i'd rather use this one, and if the design ends up a bit unusual well its not from any published drawings anyway. The Cylinder is made from 40mm dia cast iron bar, with a bore of 0.750" (good traditional mixing of units...) This leaves a wall thickness of ~10 mm. but the port face is machined directly onto the cylinder so end on it is D shaped. this reduces the wall thickness to ~6mm. Im sure i can fit the cylinder end to port face steam passages into this (although they might have to be a bit smaller than optimal?), but i am less confident about the exhaust port passage. Apart from the drilling into the edge of a 'circle' and ensuring the drill doesn't wander into the cylinder looking at some 10V's the exhaust appears to be approx

1/4" which is 6mm...

As for steam chest studs, there is 3 mm clearance between the bore (~19mm) and the edge of the port face (1" ~25mm) which has a depth of the width of the cylinder at that point, approx 32mm. If i can put the exhaust out without having another hole in the cylinder casting it also makes it less likely Ill end up having to start again on it...

I have also considered a 'cross flow' where there is an inlet slide valve and an exhaust one on the other side, however the steam pressure that normally keeps a slide valve sealed works the wrong way for the exhaust one, quite apart from having to make 2 sets of valve gear, hence the direct exhaust idea.

Dave

Can you braze a piece of material onto the cylinder? Silver solder works well for "sweating" on a bit extra.

Here is a cylinder I fabricated rather than buying the casting:

This was a practice piece I made from drawn gunmetal. It too was not a cylindrical casting in the original. Don't know if the same trick would solve your slide valve problem.

Steve

Thanks for the description and offer, hopefully I can figure it out now I know Im not in unexplored territory. I hadnt considered the need for a spring to seal when there is low / no pressure, but its obvious now.

Dave

Being fairly new to silver soldering would this work for cast Iron? The steam chest stud could go all the way through and into the original cylinder piece, there is enough material for them. maybe just a gasketed joint would work?

Might do. I hadnt thought of sticking more on... would also mean I could make the port face complete and then 'just' match it up to the cylinder.

hmm lots more to think about now, thanks :)

Dave

Silver solder seems to work with cast iron and its something I only started to use quite recently. You do need plenty of heat - a propane torch is good.

I posted "great grandad's cylinder set". This has a steam chest bolted onto the cylinder. There is a brass sheet and a paper gasket between the chest and the cylinder and the chest is bolted down using studs.

Steve

having looked more closely now that the cylinder and base are 'assemblable' it turns out that a 1/4" port face block actually lines the portface up better with where the eccentric will be :) I happen to have some 1/4" gauge plate, and having a removable portface will make making the ports simple :) win win would hardening the portface be a good or bad idea?

thanks for the ideas

Dave