don young piston valves

[RRMBKParticipant @rrmbk]

Hello all.

I have often seen reference to a description by Don Young of sealing slide valves by drifting them through the bores. Can anyone please tell me which engine this relates to and where I might get copies of the articles please. Was it in ME in which case I possibly have a copy or in LLAS Dons own publication.

I want to try out his method and would like to read it through fully including the port machining and finish

Many thanks

[RRMBKParticipant @rrmbk]

sealing of piston valves.

[Nick Clarke 3Participant @nickclarke3]

I have recently read this somewhere (could be in a recent ME!) but basically turn piston valves to the same diameter as the bore and using rosalex anti scuff paste press through repeatedly until it goes through smoothly.

The only caution I see to pass on the comment in a PM I was given when DY's name was mentioned – that his experience writing about model engineering was far greater than his experience actually doing it!

[Simon CollierParticipant @simoncollier74340]

That is my understanding about DY too. Even if you got them to seal, they would start wearing out from day one. This might not matter depending on the intended use for the engine. For regular heavy passenger hauling, you want piston rings. With gunmetal cylinders, Teflon rings can be used and there is a detailed thread on MECH Forum about this. The general belief at my club is that plain bobbins wear and start to blow very quickly. However there was one engine known that sealed perfectly long term, so perhaps the DY method can work if executed perfectly.
Hopefully someone with Don’s magazines might come along.

[John BaguleyParticipant @johnbaguley78655]

It was described in Don's Black Five build which was published in LLAS. I tried the method described when I was experimenting with the piston valves for my Helen Long with limited success. I eventually fitted PTFE heads to the valves and never looked back.

Don's method was to use Molybdenum Disulphide powder, coat the valve bobbins with it and then drive the tightly fitting bobbin backwards and forwards in the valve liner until it became free. He suggested using Rocol Anti Scuffing grease if you didn't have any of the powder. That is what I used. The idea was that the ridges from machining the bobbin and the liner would be worn down smooth to give a mirror finish that would seal perfectly and last a long time. Also,the Molybdenum Disulphide would embed itself in the surfaces and provide good long term lubrication.

The problem with just turning the bobbins and the liners is that no matter how good a finish you think you have got, the surfaces will still have ridges that will eventually wear down and make the bobbins a poor fit.

I have a copy of the relevant article that I can send to you if you want to pursue Don's method.

John

[BazParticipant @baz89810]

Totally agree with Nick Clarke 3, Don Young wrote and drew a lot but built very little.

[julian atkinsParticipant @julianatkins58923]

Hi Brian,

John is correct.

The first description by Don may be earlier (I haven't checked). See his 3.5"g 'County Carlow' in ME 1969, and "Fabricated cylinders" in ME 1973. The latter describes the non ferrous piston valve cylinders for his 5"g LNER K1/1. His incomplete K1/1 was only steamed and ran a few times when no one else was present apart from Gordon Chiverton whose 'Maid of Kent' tender it borrowed for these occasions. The boiler never had a club boiler test!

Don's personal experience of piston valves 'in miniature' was limited, and he relied on feedback from others.

Note also that at the time and for very many years Don had only a secondhand Myford 7.

It was not until John Mercer (in ME 1987) described the use of fluorosint rings for piston valves that a decent solution to piston valves (and the liners) wearing out and 'blowing' was 'found' for bronze/gunmetal liners and piston valves.

LBSC advocated precision ground stainless for the solid piston valve bobbins/valves with gunmetal cyklinders on his later designs which will also 'blow' after a bit of use or an interruption in the supply of steam oil.

Cheers,

Julian

[SillyOldDufferModerator @sillyoldduffer]

An armchair view for what it's worth!

Industrial experience has shown fitting sliding surfaces by forcing one over the other gives poor results.

The idea is that driving the two parts removes the exact amount of metal in all the right places needed to get a good match. This happens, but it doesn't leave the metal in good condition. As neither of the two parts is a cutter, metal is removed by gouging and galling whilst smoothness is achieved by smearing peaks into the valleys and infilling them with debris. The resulting surface is full of microcracks and weaknesses, and galling causes deep damage.

At first a valve or bearing made this way will perform well, lasting long enough to fool the workman into thinking he's done a good job! Unfortunately, the damaged surfaces wear rapidly; bearings slop, and valves leak.

Depending on the application it may not matter: the bearings and slide valves in model locomotives aren't subjected the thousands of hours of hard running, and it might take years until a slide valve made by driving has to be replaced. The inferior method doesn't matter at all if the engine is tested a few times on compressed air and then displayed in a glass cabinet! But it wouldn't do for a hard working engine intended to give reliable service.

In contrast, lapping takes a multitude of tiny cuts with a fine abrasive, removing metal cleanly without much smearing, and the process leaves a high-polish on a mostly undamaged surface. Although lapping increases the working life of bearings and engines dramatically, it too causes enough damage for industry to routinely apply even finer finishing methods.

Remember 'Running In Please Pass' signs on new cars? Necessary because the bores and bearings were imperfectly lapped and gently knocking off the tiny remaining scars and changing the oil after the first 500 miles greatly increased the life of the engine. Still pays to run a new car gently at first, but it's no longer essential to keep under 20mph for a few months!

Dave

 

Edited By SillyOldDuffer on 26/06/2022 08:38:56

[HopperParticipant @hopper]

You might even be better to do something like they do with valve guide liners in full sized IC engines these days and force an oversized hardened steel ball down through the bore to burnish it to an exact size, all nice and smooth and round, then fit a precision ground sliding element (valve) into that hole. In the home workshop, emery cloth substitutes for cylindrical grinding, or even external lapping.

But I reckon whatever you do, a metal to metal match is always going to wear eventually. Piston rings expand to take up that wear. Teflon packing etc likewise.

Edited By Hopper on 26/06/2022 09:12:43

[JAParticipant @ja]

I have started to build a 5" loco that uses piston valves.

Fits and limits would suggest that the fit should be sliding, that is a clearance of between 0.2 to 1.0 thou. on an inch diameter. Obviously this is when hot and since the materials are likely to be different thermal expansion may have to be taken into account. If metal rings are used they will travel across steam ports so some form of pegging may be needed.

Now the question: Martin Evans, in his book "The Model Steam Locomotive", suggests turning a number of grooves (about 1/32" wide and deep) on the piston valve as an alternative. These, he reasons, will help hold the required oil for sealing. I also recognise this as a very crude, ineffecient, labyrinth seal (better than nothing). Has anyone had any experience using such grooves?

JA

[Simon CollierParticipant @simoncollier74340]

No experience but they don’t address the fundamental problem. Instead of large slots for steam ports, use radial holes around the liner which are suitable for rings.

[John RutzenParticipant @johnrutzen76569]

hi, I'm building a 31/2 gauge Schools to more or less the Jackson design and want to fit it with piston valves. Partly this is because I've never made an engine with piston valves but also because it only looks right with walschaerts valve gear. I'm fabricating the cylinders because I want to minimize the weight at the front end. I have the castings for the slide valve cylinders but they are SO heavy. I've been using Don Young's articles about fabricating cylinders and have read the references to fitting the valves. I had already decided to try Fred Cottam's PTFE valve heads and have bought some for the purpose. He says the heads should be loose on the spindles because they will expand and also made about 0.005 small for the same reason. Has anyone any experience with PTFE bobbins please? On a design note I am pushed for space for the valve diameters because the Schools has a flat on the side of the steam chest so what would be the smallest diameter for the valves? I had been trying to get 5/8th but don't think I can manage it.

[Martin Johnson 1Participant @martinjohnson1]

Piston valve diameter –

Steam velocity and hence pressure losses (AKA wire drawing) are tiny in models. If you wanted to create full size steam velocities in a model, you would need to run at full size speed – 70 mph on 5" gauge anyone?

That being so, a bit of compromise on bobbin diameter will not make a scrap of difference.

Nartin

[John RutzenParticipant @johnrutzen76569]

Hi again, I'm still hoping to get some information on how to use PTFE for piston valve heads. There seems to be a scarcity of information out there. I've heard of filled PTFE, the carbon filled material isn't much more expensive and may be better. Also flouroscint in mentioned but I don't know of a source where I can buy some small diameter. Anybody help please?

[Anonymous]

I don't know about piston valves, but have machined a lot of PTFE. It's horrible stuff to machine; deflects away from the cutting tool and creeps badly. It is also poor at resisting loads. It is difficult to hold any sort of tolerance.

I have also machined a lot of glass filled PTFE. This is much easier to machine, much less deflection and creep so easy to hold tolerances. I made the wing valves for my traction engine water pumps from glass filled PTFE, the white objects towards bottom left:

Andrew

[John RutzenParticipant @johnrutzen76569]

Thanks Andrew, I found a place in the uk that sells 25% carbon filled PTFE for a reasonable price [about £21 per metre] but I don't know what they charge for shipping. Does that machine better? A thread about making piston rings from it say it does. The difficulty with all these is getting the small quantities we need.

Edited By John Rutzen on 22/09/2022 11:02:36

[Anonymous]

I've never machined carbon filled, but I would expect it to machine similarly to glass filled. I normally buy plastics from EDPlastics as they are local and will deliver for a nominal fee. I assume they will ship, but never tried it.

Andrew

EDPlastics

[Martin KyteParticipant @martinkyte99762]

Just scanning through EDP's website (they have comprehensive materials properties data online) FLUOROSINT could be a contender, it's PTFE filled with Mica. It's worth giving them a ring they do make stuff and not just supply material so they should be able to give you some info machinability etc.

regards Martin

[John RutzenParticipant @johnrutzen76569]

John Baguley, I was interested in your comment about your use of ptfe piston valve heads. I am making fabricated cylinders according to Don Young's articles and want to use ptfe piston heads. Please could you tell me how much clearance you gave them? I understand that they will expand and jam without clearance.

[bernard towersParticipant @bernardtowers37738]

Try Noggin End for flouroscint, I’ve had a piece from him in the past.

[John RutzenParticipant @johnrutzen76569]

Yes, £25 an inch. It's a bit much, I've got a long length of PTFE so that would be my first choice.

[BufferParticipant @buffer]

Sorry if this is a dumb question but with all this talk about wear on valves and them leaking etc. Why aren't they built in a similar way to a model aero engine cylinder and piston? I've had engines doing 10,000 rpm plus for hours on end without any noticeable drop in performance so they cant have worn that much. They also get a lot hotter than a steam engine cylinder. Or why aren't they made like a car engine with iron bores and piston rings, they seem to last pretty well? Thanks.

[John RutzenParticipant @johnrutzen76569]

Hi, they do put piston rings on iron valves and liners though some seem to go over the top. A design in Henry Greenly's book from way back uses 14 rings on each valve head! I haven't been able to find any other designs, they seem to have been seldom used, especially in smaller gauges. However mine are bronze and the received wisdom is that you can't lap bronze cylinders because the grit tends to imbed in in the metal and continue wearing it. Also they say you can't make rings from bronze but I haven't been able to find out why. The reason for using bronze was that it lends itself to fabrication, I haven't heard of anyone trying to fabricate cast iron cylinders though apparently it can be silver soldered.

[John BaguleyParticipant @johnbaguley78655]

Hi John, sorry for the late reply. I've not been on the forum for a bit.

I make the PTFE valves a push fit in the bores. They will sort themselves out once they see steam.

It helps if you can make the PTFE heads as thin as possible as that helps negate the problems with the PTFE expanding. Those on my Helen Long are only 0.0625" thick and they still seal perfectly after all these years.

I've never used anything but the plain PTFE for valves or piston rings , indeed, never found the need to. The plain PTFE works fine for me.

John

[John RutzenParticipant @johnrutzen76569]

Thanks John , that's great to hear from someone who has actually made them and succeded. I will make them as you say, my valves are 15 mm diameter. I will keep the thickness of the PTFE down to 1/16 th. Did you put any grooves in them as some have suggested?

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