Screw Regulator dimensions

I have a vintage 21/2" Atlantic locomotive restoration that I've been working on for (far) too long and which I'd like to return to service this year. One of the issues is that although the boiler itself appears sound, I cannot get the disk-in-a-tube type regulator to seal well enough for the required hydraulic test.

I've finally decided therefore to replace the regulator with a simpler (and hopefully better) screw-down type. I'm currently doing the CAD for this and I've discovered that I know very little about the parameters involved. I remembered an article in EIM by Mike Wheelwright (July 2019) but I must admit that I'm not finding it very helpful, although that's certainly not Mikes fault (my math is a bit basic I'm afraid). I'm sure I've seen another discussion of this subject somewhere (that I followed at the time) but I haven't been able find it (it will almost certainly turn up once I don't need it)

The two cylinders are 11/16th diameter with a 1.3/8th stroke - so I'm trying to work out what combination of 'hole' with screw thread would be a good starting point as I'd much prefer to get it generally right first time around. From preference I'll use a stainless steel [metric] bolt for the screw. There was also a recent recommendation (in the G1MRA Journal) to use a 70 degree point - rather than a 90 degree one - so any thoughts on that would also help.

If anyone can point me to that article (or any other good reference - hopefully one I can understand) - or give me a simple rule of thumb, perhaps explaining some of science behind it - that would be very helpful indeed.

Regards,

IanT

Don't over-think it! Typically for 2 1/2 inch gauge, a 3/8 whit thread (16tpi), and a 3/16 spindle with a conical end seating in, or on, a 5/32 "hole." Assuming the cone makes a line contact with the seating, and it is a plain parallel hole, the angle won't make very much difference. A shallower taper (i.e. a longer point) would tend to give a smaller opening for a given angle of rotation, but that may not be what you want. I doubt if the difference between 70 and 90 degrees would be significant in practice.

Hi Ian,

My Helen Longish has a screw down regulator. The thread is 3/8" BSW, the hole in the port face is 3/16" and the angle on the end of the valve spindle is 120°. The loco has 3 off 7/8" bore cylinders and for normal running I never have to open the regulator more than 1/8 of a turn. Use a quarter turn and she takes off like a scalded cat!

The end fitting that has the port face is made from bearing grade PEEK which avoids the problem of the regulator seizing up if fitted with a metal port face and you forget to open the regulator when the boiler cools down.

John

OK thank you both.

3/8th BSW is 16tpi - so just under 1.6mm pitch. I have some M8 (1.25 pitch) stainless bolts which will be easier to fit than M10 (1.5) - so I think I will try M8 first.

John I know you like PEEK from previous comments elsewhere - can you suggest a source for a small length please?

Regards,

IanT

PS "Don't overthink it" good advice & unfortunately I do tend to do exactly that RS but sometimes I've also jumped right in and later thought maybe I should have taken that a bit more slowly..

The PEEK that I used (because I had some!) was a bearing grade PEEK that contains carbon fibre, graphite and PTFE (Ketron PEEK-HPV). I originally got a 300mm length from RS but they don't seem to have that grade anymore. They do have black PEEK that contains carbon but it's not cheap!

I then got a 1 metre length of similar bearing grade PEEK from Davis Industrial Plastics (Bunaday on Ebay). They do list the ordinary plain PEEK on Ebay. That may be suitable but I've never tried it.

The bearing grade PEEK takes threads very well which was necessary in my design as the block screws into the end of the regulator body and the steam pipe screws into the block. It also makes a perfect seal which is useful when you are doing a hydraulic test.

If you struggle to find anything suitable, I've still got some 12mm diameter left and could send you a couple of inches.

You could possibly use PTFE or maybe the carbon filled PTFE which is a bit harder?

John

Thanks John,

I've found online sources of PEEK but not PEEK-HPV - at least not without picking up the phone. It also seems that 1m lengths may be the norm from the commercial suppliers. I'll speak to some of these suppliers next week and see if I can find a reasonable (e.g. affordable) solution.

I've looked at the spec sheets and 'basic' PEEK seems to be OK to use for this application - and I have a feeling that youv'e used HPV elsewhere on your engines (presumably as a bearing material) but I haven't gone back through 'Steam Chest' to find out why I have that impression.

I'll PM you if I can't find a reasonable solution.

Thank you for your advice.

Regards,

IanT

Ian - have you tried Direct Plastics link they list PEEK in sizes from 5mm dia. and sell in lengths from 250mm. I haven't bought PEEK from them but have bought other plastic materials and been happy with their prices and service.

Hi Keith - yes thank you.

I've spent just some time looking at various UK suppliers of "PEEK" and Direct did seem the best source for a shorter length - but I've just come across Trent Plastics and they are best on price for 12 x 250mm natural PEEK rod I've found at the moment.

Trent Plastics

Regards,

IanT