Allchin slide valve help please

Hi
I am building an Allchin at 3” scale and now trying to set up the slide valve but there seems to be some discrepancy, I would very much appreciate it if anyone can clarify whether I have some error.

Can anyone clarify the required width of the slide valve in relation to the inlet ports in the cylinder.

I have made everything to the 1.5" scale drawings (x2). Referring to the 3 staem ports milled in the cylinder, the outside dimension across the 2 inlet ports is 31.75mm. (1.25" )

The dimension across the slide valve is 40.48mm (1.59375" ) thus overlapping the inlet ports 4.365mm per side when the slide valve is in a central position.

However, the Allchin book says “Set the die block to the centre of the radial slot, and turn the flywheel slowly, watching the valve. At dead centre at each end, the edge of the inlet port should show a thin black line. In other words, it should be open to steam at either end just before the crank reaches dead centre.”

This indicates to me that the slide valve should be 31.75mm wide (less about 0.2mm) and not 40.48mm wide as per the drawings.

Am I missing something here, or do these very long established drawings really have such an error?

If the drawings are in error then this also questions what width the steam cavity on the underside of the slide valve should be in relation to the cylindre inlet/exhaust ports?

As this is the very first time I have built a steam cylinder valve arrangement I don’t have the experience to be sure what’s correct. I am in dis-belief that the drawings are wrong and sure there must be something I am not seeing.

Thanks for any guidance.

Nigel

Edited By tractionengine42 on 10/08/2013 17:32:30

Edited By tractionengine42 on 10/08/2013 17:33:39

Edited By JasonB on 10/08/2013 18:08:59

Hi agian

For some reason some silly smilley face things have appeared in my post which when I edit the post I can't get rid of.

Not to worry, from bitter experience I pasted from note ad to avoid loosing everything and having to start again.

Thanks

Nigel

Edited By tractionengine42 on 10/08/2013 17:39:20

Because the valve leads the crank by 90deg plus about another 7deg it will have started to move by the time the crank gets to TDC and that is why you should see the slight crack as it has started to open.

Have a look at chapter 3 here it gives an idea of wher the valve should be in relation to where the crank is at various times during one revolution

When the valve is central (not the crank) it should cover both inlet ports, that is why it is wider than the gap between the two outer edges

Edited By JasonB on 10/08/2013 18:08:20

Hi Nigel,

The drawings are correct. The valve should be wider than the outside of the steam ports by an amount equal to the 'lap'. The 'lap' enables the steam to be used expansively and more efficiently. The position of the valve eccentrics relative to the crankpin determine the position of the valve edge at piston dead centres, not it's physical dimensions.

John

edit - Jason posted whilst I was writing

Edited By John Baguley on 10/08/2013 18:11:12

Hi Jason and John

Thanks for your rapid reply and thanks Jason for that link.

Your information has reallyhelped me get my head around this, seems I mis-understood the wording in the book and got the wrong end of the stick. It's a relief to know the drawings and therefore the physical parts are OK.

So I am heading back to the workshop with confidence to finish setting up and checking the value gear.

Cheers

Nigel

I don't know if a third explanation will help, but...

The valve needs to be set to start opening the valve just before the cylinder gets to top (or bottom) dead centre. This means it starts moving back before the piston reaches its furthest point. The extra length on the valve is correct and the extra length combined with the setting process described is what makes sure the eccentric is properly advanced.

A side effect of this is that the valve is in a different position at TDC to BDC, so you need to turn the engine between the two points when setting. Note that TDC and BDC are VERY difficult to find accurately as there is a dead spot around them in even the best made engine. Better to measure the piston position eiether side of the dead centre and find two places where it is the same distance from the end of its travel. The dead centre will be exactly half way between these two points.

Neil

Hi everyone

Thanks Neil for your additional post.

At the moment I am doing a relatively rough check and I am along way from the published information.

According to the book the eccentrics are set at 82.5 deg and with the die block central as the crank is rotated I should see the thin black line. For this to happen the slide valve would need to move about 4.5mm but only moves 1mm. Furthermore when I put the valve gear in forward or reverse the inlet port does not open until the crank has gone about 45 deg.

These pics show this set up.

crank set at approx TDC, dieblock approx mid position.

Cr

Crank through approx 90 deg.

Slide valve moves only 1mm as crank is rotated with the die block in mid position.

I tried playing about with the eccentrics and, with the valve gear in forward or reverse position, and the eccentrics set at approx 70 to 75 deg I am some where near the inlet port starting to open as the piston moves away from top or bottom dead centre. This is a significant deviation from the published information.

I have decided to leave it for now and study some more about this valve gear arrangement.

In the mean time appreciate your feedback/thoughts.

Many Thanks

Nigel

Hi Nigel,

Does the valve only move 1mm over a complete rotation of the crankshaft or just the 90 degrees?

John

Hi John

From the TDC position through to 90 deg ithe slide valve moves 1mm, moving on to BDC it reverses back 1mm.

Just been having another look, set at about 75 deg, it all looks to work OK, only approximately at the moment but not far off however, that's a lot different from the published 82.5 deg.

Many thanks

Nigel

Hi Nigel,

That's strange, you should be getting something like 9mm travel on the valve when the crank moves from front dead centre to back dead centre. If the eccentric throws are correct (0.75", 19.05mm) then the only thing I can think of is that the eccentrics are not set correctly. I can't see anything obvious from your photos though

John

Edited By John Baguley on 11/08/2013 01:22:14

With your expansion link in the mid position as shown in the photos you won't get much movement.

Put it into a full gear position eg up or down and you will get more movement

Hi John / Jason

I originally set the eccentrics accurately to 82.5 deg as per the Allchin book (it does say somewhere between 80 and 85 deg) The book says that with the die block in a central position in the reversing link (I guess this is mid gear?) rotation of the crank should move the slide valve so that you see a thin black line on each inlet port. This would require the slide valve to move 4.365 mm each way ,8.73mm total (+ a few thou). As Jason says the slide valve movement is actually very small, only 1mm. When in say forward gear, the inlet port does not open until the crank is well past the 0 deg which is not correct.

Referring to the info Jason pointed me to, I have played around with the eccentric positions. With the eccentrics set at 65 deg I am very close to the valve events described in the info Jason showed me and the Allchin book. At this angle, when with the die block is in the centre of the reversing link I do get 8.73mm (+ a few thou) movement of the slide valve and see the thin black lines at the inlet ports. Also in say forward gear the inlet port in beginning to open when at 0 deg., when the crank gets to about 100 deg the inlet port is shut (allowing steam expansion).

Ok, this is very much by eye at the moment so the 65 deg I mentioned above may be a few deg out but what I do know is that all seems to work as it should but with the eccentrics at an angle very much less than the 82.5 deg specified.

I guess I just need to do some accurate adjustment of the eccentrics to get the thin black line described, (this would appear to give the correct valve events in forward and reverse gear) and run it on air to see how it performs.

Frustratingly, work commitments and MOT time are going to divert my attention so it will be a week or so before I can get back to this.

Thanks very much for your help and info.

Nigel

NIgel,

Read Jason's short post carefully. In the photo the reversing link is set near to mid travel.

I suggest you read up on how 'stephenson valve gear' works. Essentially to move between forward and reverse you move the link up or down so that the motion of one or other eccentric is passed to the valve. At points in between the valve motion is a mixture of the two movements. Exactly on mid travel valve movement will be zero.

This allows you to control speed and direction without using the regulator, and is much more economical of steam usage when running with the valve travel redudced somewhat ("notched up".

You need to have the die block at one end to set the valve. Use the direction for forwards motion. At this position you will effectively only be dealing with the position of one eccentric.

When you have that eccentric correct, move into reverse and set the other. You may need to repeat the exercise a couple of times as there is actually a small interaction between the two eccentrics.

I made a small (and rather odd) stationary steam engine with stevenson gear, just to find out more about how it works. Simple in action, complex in effect.

Neil

Hi Neil

Thanks for your info.

Yes, I will do some reading as you suggest, the info Jason showed me has already been very helpful. I would like to do a 3d model on the cad and have a play, first I will see how i get on today for an hour or so, then work beckons.

Set as per the book 82.5 deg., in full forward and reverse positions, reversing link positioned up or down accordingly ,I get full slide valve movement, everything looks fine except the inlet port does not open until the crank is well past 0 deg. As mentioned the slide valve moves little when the die block is in mid position which is what Jason and your good self suggest and what I had assumed would be the case. However, the Allchin book says different by saying that, when the die block is in the mid position rotation, of the crank should move the die block such that you see the thin black line (inlet ports barely opening) thus needing 4.375 mm movement of the slide valve either way. This is not what I had assumed to be the case but it's there in print but not happening in practice.

I am going to spend a little time today setting up more accurately and do as you suggest, I will set the eccentric with the reversing gear lowered and raised (full forward and reverse post ions) and see what happens. I will set them so that at TDC and BDC I just see the thin black line, making sure the crank is turning in the appropriate direction, then see how everything works out. This makes more sense to me now, more sense than that described in the Allchin book..

Thanks again

Nigel

Hi all,

Even with the die block in the central position (Mid Gear) you should still get enough valve travel to just crack the ports. The expansion link does not merely pivot about the mid position - it physically moves backwards and forwards due to the difference in the positions of the two eccentrics. Moving the gear towards full gear just increases the amount that the port opens. Have a look at any of the valve gear simulators to check this out.

John

Edited By John Baguley on 11/08/2013 12:03:11

Hi Nigel, when you say the inlet ports doesn't open until the crank is well past 0-degrees, what are you using as the reference point for zero degrees? Zero should be taken as top dead centre which is actually with the crank more or less HORIZONTAL and pointing FORWARDS as it means with the piston as far into the bore as it can go.

If you are measuring from when the crank is vertical, then the valve won't crack open until nearly a quarter of a rotation past this point.

Neil

John you are right, but I would not expect as much as 4.3mm travel at miid gear?

HI Nigel,

Right, I've put your dimensions into a valve gear simulator and you are quite correct - the valve will not crack the ports with the eccentric settings given by Hughes. The angle needs to be something like 65° for that to happen. The other alternative is to shorten the valve to reduce the lap but the question is:

Is the 82.5° angle correct and the valve dimension wrong, or

Is the valve dimension correct and the angle wrong?

John

Hi Neil / John

Neil, my reference point is TDC crank horizontal, with the eccentric at 82.5 deg the inlet port does not open until the crank ha smoved 40 deg or more.

John, thanks so much for checking the dimensions on simulator, I hadn't thought of looking for such a thing.

I set up the eccentrics by adjusting them in full gear so that a thin line appeared. As I rotate the crank the inlet port shuts of at about 110 deg (by eye) which seems Ok. Once everything was set I removed the crank and eccentric assembly from the engine and set up on the face plate to check the angle of the eccentric which measured 65 deg,

In mid gear the inlet ports remain covered but only just, so I probably need to reduce the width of the slide valve a few thou. For now I am leaving everything as is until I am able to run it in air.

I am delighted the 65 deg agrees with John's simulator calculation so I am now confident everything is OK .

I think the Allchin book has lead me a stray, I am sure many successful models must have been built to the drawings so the drawings must be OK.

Thanks very much for all your help,

Nigel

This is interesting. There are so many small differneces that could affect the opening at mid gear (which all comes from imperfections in the gear geometry as with perfect gear it wouldn't move) I can't see why this would be used for setting up.

The normal way is to set the eccentrics in full gear to open the port by a specified amount (e.g. a tram ticket) at dead centre.

Neil

P.S. I should say that the quality of your work looks fantastic.

And don't forget you will have to adjust it all again as the boiler heats up and stretches, though that should only need an adjustment of the valve rod nuts.