Blast-pipe - Chimney Proportions

Thank you Howard1

It is rather nice to know where one's older possessions came from; but Goodness knows where the book was, before I found it.

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Had a day off from engineering today - now that the rules have relaxed enough for small groups to meet outdoors, the cave-finding project in which I am involved, resumed today.

Progress on the wagon in the last week or so has focussed on fitting the boiler to the chassis and discovering that the off-side one of the newly-made, Brackets, Boiler-mounting, Mark Two won't allow me to move the steering column forwards the inch I think it has to go... (it fouled the drop-arm... but if I move that too far forwards on the same alignment its ball-joint nut hits the off-side front wheel spring-hanger. And so it goes on.) So I went at it with drill, hacksaw and file, to give me an idea what Brkt, Blr-Mtg Mk3 (OS) needs look like.

I've also put the rivets in the two holes left in the smoke-box door ring by a modification I made nearly a year ago. I must have forgotten at the time and spotted them only a few days ago. They wouldn't have helped the draughting!

Stephen -

I have now started making the new choke, based on your spread-sheet. I'd broken off from that for a while to revise the boiler's mounting brackets on the chassis.

I'm making the choke in two parts, and have machined the longer, exit end, but it is only 2 inches tall and ends abruptly at about 1.5" dia, opening inside the 2.75" dia chimney. I think the theoretical continuation by that 1:6 rule is at least 9" (the outer chimney, to scale, is about 20" tall) .

I'll go with it for now and see what happens on test. If I need make an extension to the cone I'll probably have to roll it, if I can.

I've to make the converging part yet, but because the choke is in two parts I'll probably screw together via a flange on the main section, have paused to make an adjustable alignment-tool whose central stem also has a spigot turned to fit the blast-nozzle's calculated diameter.

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Two weeks ago I had a very bad fall at home, and damaged something in my leg badly enough to make me somewhat immobile. This has interrupted work on the wagon, and I did not touch it for nearly a week. As it is I cannot stand for long, such as for a long machining operation.

Hi Nigel,

Sorry to hear about your fall, and hope that you're soon back on your feet in the workshop.

If it helps, here's a sketch of how I made the sectional liner:

It seems to me that, if your choke section is short and opens immediately into a much larger diameter chimney, you run the risk that, when the engine is working harder and the exhaust jet sharpens (ie, the jet's cone angle decreases), the exhaust jet may not fill the chimney. In that case the choke probably won't provide much benefit.

While it would be an interesting experiment, if I were you I'd plan things so that you can readily extend the liner at a later date.

Best regards
Steve

Thank you Steve.

I've applied, at the surgery's advice, for physiotherapy. I planned to complete the alignment-tool today but after a very fraught and painful expedition to the shops only a couple of hundred yards away this morning, I did not go into the workshop at all today. I was booked to go on a local geological field-trip on Sunday, but that's out of the question now.

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Yes, I do have something like your drawing in mind, and I have made the choke itself so an extension can be bolted to it.

The limit on a taper length I can turn, short of making some form of taper-running attachment, is the 3" compound-slide travel on the Harrison lathe. However, using sections as you have also has the advantage of material economy by stepping up the stock diameters.

I found the best way to make the venturi is in two parts joined on the choke line, and to bore the billet through to choke diameter, then taper-bore diverging towards the chuck. That means I can see when the tool is approaching the choke itself, leaving a very small land inside it. The other end looked after itself, with due care not to run the boring-tool into the chuck of course as the diameter hidden from me increased .

I cut two wide grooves into the outside of the venturi, with a small land in the middle for its location in the chimney saddle. This gives a bolting-flange each end, and cuts the weight down. The smoke-box, a piece of 8" seamless-steel pipe, is heavy enough as it is.

One detail I am adding is making the outer chimney detachable to aid transporting and storing the vehicle; by flanges probably in prototypical way, at least to general traction-engine practice. I would have to make any liner similarly removable.

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It's occurred to me to make a long blast-nozzle threaded to screw into the pipe so its height can be adjusted. Alternatively, a simple telescopic tube locked by grub-screws.