sealing of piston valves.
|154 forum posts|
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
|Nick Clarke 3||25/06/2022 21:45:40|
1475 forum posts
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 Collier||25/06/2022 22:49:42|
480 forum posts
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.
|John Baguley||26/06/2022 00:41:43|
507 forum posts
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.
|756 forum posts|
Totally agree with Nick Clarke 3, Don Young wrote and drew a lot but built very little.
|julian atkins||26/06/2022 08:27:58|
1258 forum posts
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.
8898 forum posts
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!
Edited By SillyOldDuffer on 26/06/2022 08:38:56
6690 forum posts
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
1405 forum posts
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?
|Simon Collier||27/06/2022 10:54:30|
480 forum posts
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.
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