Fabrication Slide valve Cylinder blocks

[k36no4862002]

Hi,
As some of you may know I'm currently building a 7 1/4 gauge Challenger which will be a little overscale so you can sit in the tender. I have been advised by a friend who is a good machinist that we could have issues in machining the cylinder bores if I go down the casting option as he no longer has access to the heavier machines needed to deal with the proposed blocks and he in concerened about holding them and getting a good finish.

So we are looking into fabricating from steel the cylinder blocks and valve chests and then adding CI liners.

I have had a look around and found some really good pics of fabricated piston valve blocks. However I seem to be a bit short on ideas for a slide valve block.

If anybody has any advice or pictures they could share with me I would be very grateful.

My concerns are:

How thick should the pipe be to be able to weld to it without distorting the tube so that I can still press in a liner? I was think minimum 1/2"?
What to do about the face where the slide valve will rub? Cast iron on steel? or should I bolt a piece of sheet bronze ontop?
Liners - what kind of wall thickness should I be looking for on a liner - proposed finished bore will be around 3.25" with a CI piston (assumed)
Are there any pitfalls that would not be abvious? Another concern I have is my ability to weld this up and not get any steam leaks!!!!

Many Thanks in advance

Paul

[Fender]

Paul,
I'm a bit puzzled by the combination of "slide valves" and "challenger". Why not make piston valves? You'll need a different valve gear setup for outside admission (slide valves) than for inside admission (piston valves). Also, I think it will be easier to get a good seal on the valve with fabbed cylinders and piston valve liner.

[kenrinc]

Yeah, it wasn't clear if you have to design LE style "simulated piston" valves because your piston valve design is too large to machine or you were talking about just fabricated a piston valve block. I'd look at the Little Engine's design just as an example. I've seen replaceable valve faces of bronze and CI even on a standard slide valve block. Regardless, if your fabricating a block by welding, you'll have to take a "cleanup" cut before pressing in liners. There is always going to a bit of warping.

Ken-

[Bill Shields]

Paul:

good suggestions already....I wouldn't recommend doing the slide valve trick unless there is a very specific reason why you CANNOT use piston valves. If you are going to weld up sets of cylinders, it is probably easier to make piston valves than slides (been there, done that on both types in 3/4 and 1-1/2" locos).

Also;

Are you going to sit ON or IN the tender? The last 1.5" Challenger I saw was in correct scale was a MONSTER, including the tender.

Any bigger and....well it would not have fit on many of the turntables / transfer tables of the local clubs.

Where in Surry? I have a customer in Worthing, which really is not that far. I am in / out of Birmingham a couple / 3 times / year.

I have pictures of how to weld up a set of piston cylinders if you are interested.

[k36no4862002]

Gents,

Originally I had designed the valve gear etc for Slide Valve but using castings - I prefer slide valves as I have built a loco with these before and had very few issues and because of this I felt that 'stick with what you know'.

Also there is the old saying 'slide valves wear in piston valves wear out' henace I feel that long term maintenance may be reduced, obviously they will require facing from time to time.

I had not linked the change of cylinder construction to a change in valve type, although I may need to consider this.

I was hoping that some pics of built up blocks may help me to visualise how I might be able to construct them.

I'll have to put my brain into gear over the coming weeks.

Bill,
The Challenger will be in 'sit in tender' which will be produced to the same scale as the Ex Big Boy that used to run at the Dobwalls Adventure Park in Cornwall.
I'm based nr Leatherhead in Surrey if your'e ever around that way Worthing is about 1.5 hours away - and yes please any constuction pics would be very welcome.

Many Thanks
Paul

[Bill Shields]

Piston valves wearing out.....may be true in some circumstances, but in live steam terms...may take several lifetimes to come about.

It is really not something about which I would be concerned.

I would worry more about load / wear on the linkage, which is much higher for an unbalanced slide valve than on a piston valve.

Let me know if this link does not work.

http://s93.photobucket.com/user/crazyst ... /cylinders

I have more pictures, but not with me (in Manchester / B'ham area now) and would have to send them later.

This technique is not typical way of making cylinders, but for me, at the time, it worked (still does work).

All bores are original steel with cast iron rings and have run a LOT of miles. All it takes is a little wet steam and oil and you really do not have to worry.

In general, the concern about wear in piston valves is because of the weight of the valve would tend to wear the bores egg-shape.

With a valve that only weighs a few ounces and rings that are significantly thicker (by scale proportion), and lower operating pressures which reduce ring wall load, it just isn't something to really worry about unless you are 20 years old, running daily in an amusement park for the remainder of your life.

Maybe not even then..

[k36no4862002]

I just found this post I made ages ago and I thought I would share my current fabrication progress with you all.

After deciding that I would ignore all advice about slide valves - I went ahead anyway and started to fabricate my cylinders with the intention of operating them using a slide valve rather than a piston valve.

In simple terms I made a cast iron tube with a 20mm wall and 3.25" bore. This was then pressed between 2 end plates that contained an added piece of steel to hold the steam passage which will then be completed using the top flat face of the fabrication, which in turn will then be finished by a piece of 10mm cast iron plate as the final finish for the slide valve.

I'll try and keep this updated as work progresses, I'm hoping to finish these by mid July.

[ALCOSTEAM]

If you have ever messed with a slide valve engine that has even a minor bit of steam on the top of the valve, manually try and move the slide valve with your hand. It will take alot of effort, do the same thing with a piston valve and you will not know if there is pressure present or not.

You loose alot of energy moving slide valves back and forth. Piston valves is how I would go.

[JKreider]

Then of course the other thing about outside admission valves on a locomotive with outside valve gear is that the eccentric crank has to be set 90 degrees from where it would be with inside admission valves. The eccentric rod pin therefore leads the main pin instead of following it in forward motion. To me this gives the eccentric crank and eccentric rod kind of a lop-sided motion compared to the prototype and a model equipped with inside admission valves.
Martin Lewis started this because he had inexperienced builders trying to fit round valves liners in a round hole without the proper fit and thus making the liner egg shaped. You were never going to get a piston valve to seal under those conditions. His idea was that a flat slide valve was easier to make seal. His point that the valves would wear in is true but at the expense of the valve gear wearing out much quicker because, as was pointed out, the greatly increased forces required to move the unbalanced outside admission slide valve.
Also on the slide valve, the radius rod on Walchearts gear (or valve rod on Baker gear) connects below the valve crosshead pin on the combination lever instead of on top as per the prototype.