Don Young Combined Vacuum/Steam Brake Valve - Problems in achieving a tight fit between valve spindle and the steam brake valve

Satisfactory working of the Don Young combined Vacuum/Steam brake as shown on Sheet 11 for his 5”G Black Five is dependent upon achieving a tight fit between the valve spindle and disc making up the steam brake valve. As drawn, this is achieved by means of a 5BA screw thread (see drawing) but I am having great difficulty in achieving a tight fit into the threaded disc. One option might be to use a high temperature/steam resisting grade of Loctite or something similar or, to resort to putting in a few indentations from the reverse side between the spindle and disc using a centre pop However, before doing so I wondered if there might be any other possible alternative solutions to the problem.

To me that does look like a loctite job.

I'd try making the stem and disc in one piece out of a stainless steel bar a bit bigger than max OD of the disc. No joint means high strength and no chance of a leak between stem and disc. Free cutting stainless would be best.

I thought the same at first Jeff but milling that little crescent shaped slot will be a pain with the shaft sticking up so far

Given that the valve assembly will be in contact with hp steam, which grade of Loctite would the team suggest?

I think 272 has the highest rating. I've never tried it, but I have used Permabond HM162 with great results. It's more of an adhesive than a Threadlock though.

Jason - I did think about that, and a) I would EDM that slot in my shop, as I have an EDM machine I built. Or b) rearrange the design a bit to have just holes on the disc and move the slot to the fixed valve body. Many ways to skin a cat (as usual).

Silver Solder?

High-temperature soft-solder would probably seal the thread perfectly well, but another sealant you could use is Steam Seal, or Foliac - graphite-loaded sealing paste made for, well, sealing steam-pipe joints!

I forget where I bought my tin but it would have been one of "our" suppliers.

Silver solder it. All the Loctites will soften at the temperature of pressurised steam, despite the temperature claims, and slowly fail.

Plus one for silver soldering.

I've made at least 3, and silver solder.

Lap with Brasso, making sure the Brasso remains wet, and only turn the disc the movement it makes in service

Possibly one for Julian but I am open to all suggestions. Having made up the brake valve to the DY drawing I have been puzzled by the fact that when the valve is in the closed position, the steam port from the ‘Limit Valve’ is left slightly open thereby admitting steam into the end cap when I would have thought it should be left shut off. A dimensional check reveals that the radius to the outer edge of the steam port = 7/32 + 1/32 = 1/4 = 0.250” whereas the radius of the steam valve = 15/64 = 0.234” which results in a 0.016 overlap all of which seems to be borne out by the drawing.

could machine it with the shaft sticking down

Greensands -

Without knowing the specific design here, is that slight opening to steam the "lap", or running, position, allowing a thread of steam though the brake cylinder to keep it hot?

I think some miniature steam-brake valves have this so when the steam-brake is applied, it can work immediately and does not simply fill with condensate that would hamper both application and release. I have seen at least one commercially-made, model brake-valve with three positions: (Brake) ON, OFF and beyond that, LAP; but trying to verify this through the suppliers' catalogues proved fruitless.

The official handbook for operating full-size locomotives, with their much more complicated fittings, makes no mention of this so possibly it was not necessary there. The steam-brake control was either ON or OFF!

Hi Reg,

I haven't made this particular design, and indeed I think we discussed your particular valve some time ago either on here or modelengproboards.

The limit valve limits the vacuum. It should never have steam admitted to it. It is part of the vacuum brake system in effect a part of the vacuum brake pipe albeit here as part of the valve. I cannot see how you would be able to accurately make this limit valve to work. You would be better off making a separate limit valve as per the Hughes/Reeves design. Leave yours as a non working dummy.

Additionally, something must be wrong somewhere if the valve disc is admitting steam to the vacuum part of the valve via the port for the vacuum system.

I think you would be better off having a good look at Don Young's later 'E.S. Cox' and 'Doncaster' combined steam and vacuum brake valves. Don's 'Black 5' brake valve is a bit of a 'dog's dinner'.

Cheers,

Julian

Hi Julian – Very pleased you have been able to pick up once again on this one. As I understand it, Dons' arrangement for the steam side of the combined brake valve, the so called ‘Limit Valve’ is a dummy fitting with blind perforations and is simply used as a convenient means of getting steam into the brake valve as described in LLAS No.55, Pages 33/34. It does however leave unanswered the question raised in my posting of the 10 Aug relating to the partial uncovering of the steam port when the valve is in the Off position. I need to have another look at his design for E.S.Cox but am under the impression that they were essentially the same design.