1.5mm thread pitch for screw type regulator?

[Greg]

Hi,

I'm making a regulator for my  3 1/2" 264T loco. Unfortunately the tubing that'll house the regulator can't be larger than 1/2", so I'm quite limited in my design. The plan is to go with a screw type regulator. I've just enough size to fit a 10mmx1.5mm thread in or I could cut my own thread and I could then go a bit coarser e.g. 3/8" x 12TPI.

The outlet hole will be 1/4" diameter connecting to a 3/8" pipe that goes to the front tube plate.

It would be easier for me to do it with standard 10x1.5mm thread.

Does anyone know if with a 1.5mm thread pitch how far I'd have to open the regulator to get enough flow going?

I'm thinking with a small 1/4" outlet hole I might need to go with a coarser thread?

Thanks for your thoughts,

Greg

[lmsbuilder]

Hi Greg can I ask are you building this to a published design, I ask this because I am also building a 3 1/2" 2-6-4 tank and the main regulator body is 3/4" diameter which gives a lot more room for the internal workings.

Cheers John

[Greg]

Hi John, no unfortunately not. I'm in Aus so had to design it to the Australian code.

If the hole was 3/4" the backhead flange would have had to be a lot bigger. So it didn't look terrible I reduced the hole size to 1/2"

[lmsbuilder]

Ok no problem, on a previous build I used a screw type regulator. To get a reasonable movement I opted for a 3/8" dia two start, can't remember the pitch so need to look when I am in the workshop later.
I machined the screw in stainless and the tap in silver steel, the "nut" was PB, I know this is not ideal as the screw is the easiest to replace, but I considered that there would not be that much wear.

Cheers John

[Greg]

Ok thanks for looking.

I believe a simplex has a 1/2"x12TPI thread, but the outlet hole would be bigger so I'm sure would flow more for the same rotation.

The regulator is one of my last jobs. I think it's still not done because I'm unsure on the thread to get a reasonable flow without too much movement of the handle.

[lmsbuilder]

The other consideration is the angle of the tapered end, not sure what I used but possibly 45 degrees.

Cheers John

[flyingfox]

Greetings Greg, I think the thread that you proposed in metric size would be OK to use, but I think you would be better off with a shallower angle say 25 or 30 degrees which would a bigger "opening" into the wet header feed pipe.

Regards
Brian B

[lmsbuilder]

The screw thread dimensions I used were 8mm.OD. with a 3mm pitch two start, standard metric thread angle 60 Deg. Obviously because I made it two start the thread depth was as the standard for 1.5mm pitch. Using this approach I obtained a reasonable axial movement which together with a 120 Deg cone end gives a reasonable opening for an acceptable rotation of the lever. Hope this has given you some food for thought obviously others far more knowledgeable than me will have alternative solutions, but this I considered was within my skill and machine capabilities.

Cheers John

[borderer]

Recently I made a screw down regulator for a 3.5" Juliet - perhaps a little smaller than the OP's engine but in the same ball park. I used a plain 1/4" x 20 Whitworth thread, and the exit hole from the regulator to the steam pipe is 3/16" diameter. The cone end is 120 degrees.

This has been very satisfactory. When starting on say 60psi it does not require more than 90 degrees movement of the handle, and considerably less when starting on the full boiler pressure of 80psi. It is very rarely open as much as 180 degrees - only when struggling along with no steam! This echoes similar experiences with other screw down regulators I have used.

It is not difficult to work out the actual opening area. But a lot of steam will go through quite a small hole, so don't over-think it.

[Greg]

Thanks everyone for all the good information.

I'll have a go at the two start 3mm pitch and see how I go with it. If I can't manage the internal thread I'll just go with the 10x1.5.