I bought an old model steam engine at an estate sale today, and I am
puzzled by the valve gear. There is no mechanical connection between
the crankshaft and the valve!
The steam chest has a shaft protruding from the fore and aft - these
will slide in and out. Haven't gotten it to run yet - I would like to
try to find out more about it before dis-assembling.
Can anyone fill me in on this beast?
I'm no expert on steam, but I get the strongest feeling that there is
SUPPOSED to be a link to that shaft on the steam chest. Is there
anyplace on the flywheel or the crosshead that the valve could be linked
to? Is there anyplace on the mounting board where something else once
went, like the reversing gear?
I have to agree. Your angles for your photos make it more
difficult to determine whether there is an eccentric on the flywheel
shaft -- perhaps between the flywheel and the crankpin.
It *looks* as though the shaft in the steam chest needs to move
the opposite direction of the piston in the main cylinder to switch
steam to the proper place. But what appears to be missing is anything to
You could experiment with powering it from compressed air as
being a little less likely to cause serious problems than steam would
be. I'll bet that as it sits, it will rotate to TDC or BDC and stay
there, unless you flip the valve in the steam chest at just the right
It just might be that there is some way that the main piston
valves steam to a smaller piston in the steam chest to shift the valve.
The best thing would be to find what model this was -- perhaps
find it in an *old* issue of Model Engineer (in the UK), to find out
what it was supposed to look like when complete.
But it *might* have been a patent model, which means that
finding the right patent to examine would be the only real solution.
"DoN. Nichols" wrote: (clip) But it *might* have been a patent model, which
means that finding the right patent to examine would be the only real
This engine may be a hand-built that was never finished. You may have to
invent the missing linkage. I would look at other engines to see how it is
Isn't there a steam newsgroup?
Is the shaft that appears to be attached to the valve spring-loaded? Is
it a single shaft or two that stick out opposite each other? I assume
there's no internal linkage or you'd have turned the flywheel and seen
movement at the valve chest..
I wonder about the fact that steam can't be gotten to both ends of the
cylinder (assuming double-double acting) I see what looks like a tube
going to the head opposite the shaft, but it's quite small to be feeding
steam to that end of the cylinder. Is that a tube connected to the
center of the head and the flange? If so, I could see that having
something to do with working the valve.
I can imagine a couple of methods of valving an engine like this with no
linkage, but they wouldn't be very ideal..
As Jon says, it could be missing valve gear, but I just don't see any
room on the crank for it, unless the counterweight worked as a cam, but
I just don't see that happening..
Regardless, you've got a nice little engine and I'll bet someone here is
going to know what the score is.
You can do a valve gear system by having the piston move the valve gear back
and forth. This will make an engine which is rather wasteful of steam but
it will work.
The piston approaches the end of it's stroke and hits a pin that moves the
valve from the position where the steam is entering one side of the cylinder
to the other side of the cylinder. Inertia and the steam pressure on the
bottom side of the piston allows the piston to top out on the stroke and
start back with the pressure now on the topside of the piston while the
bottom side of the piston is vented to the air.
That system doesn't make use of the expansion properties of the steam but it
does work and requires no valve gear beyond what is in the piston head
stuff. I'll also note that steam passages from the cylinder to the valve
can also do the movement of the valve.
Why isn't there an Ozone Hole at the NORTH Pole?
I think it must have originally been designed to have some sort of drive to
the valve gear other than steam. No one smart enough to design an engine,
would be dumb enough to bring the shafts through the end covers of the steam
chest if they didn't have to. Gland are a never ending source of
annoyance,so if you didn't need them , why make them?
It's getting stranger. I took the outboard head off - it has a tube in
the center that goes through a hole in the middle of the piston -
apparently to feed pressure/vacuum from the cylinder at the inboard
side of the piston. This is fed out through the top of the head and
back into the valve block.
The "shafts" protruding from fore and aft on the valve block are
actually just pins. The butt against the top of a valve, allowing it
to be pushed in with the pin.
I *know* I've read about this recently, I just can't remember where!
I don't know what kind of valve gear this engine has, if any. He says it
doesn't run, which may be enlightning.
But ... Extended, or 'balanced' piston rods were very common on steam
engines (including locomotives) at one time (up to ablout 1900 anyway).
Early crossheads, or what passed for them, were NOT good, and allowed
the pistons to tip in the cylinders. This wore both the piston and the
cylinder into elliptical shapes. NOT good. The simple solution was to
extend the piston rod through the piston and out through the cylinder
head. This, of course, added another packing gland which was a source of
different problems as you allude to.
Ok, I'm thinking the piston shaft is hollow, and the tube fits it very
If that tube goes far enough down that it's exposed to the steam in the
outboard end of the cylinder at about the far travel of the piston, then
you could make pressure differentials work the valve if you add in a
check valve; pressure in the tube (produced when the piston goes to the
end of the stroke toward the flywheel) would shift the valve to send
steam to the flywheel end (and exhaust the other end). A restriction
between the valve actuating cylinder or diaphram and the check valve
would ensure adequate pressure to shift the valve, and still allow the
pressure produced by the shaft travelling over the tube (forcing more
steam up it) to go on out. When the piston reaches the end of the stroke
where it's moving away from the flywheel and begins to go the other way
the tube starts to carry a vacuum (between decreasing loss of volume and
condensation in the loop of the tube), which would shift the valve back.
This could be improved by adding a spring on the valve to work against
the tube pressure- as soon as the tube pressure dropped to close to
atmospheric the spring would shift the valve against it, exhausting the
flywheel end and sending steam to the other.
Can you shift the valve with the pins that stick out? I'm thinking
that's how you start the engine, push a pin..