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

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

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 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