Petticoat pipe help needed.

[manofkent]

Hello.

County Carlow is progressing well, and I am now fitting the smokebox. Don Young refers to making a square brass plate to secure it in the smokebox, but his description of how this is done is (in my mind) bewildering.  This will be the first smokebox I have made. 
Can anyone gently tell me how the Petticoat pipe is held in place please.

Many thanks 

JOHN

[Deleted]

Hi John

From my understanding of Don's words for Doncaster the petticoat is a tight fit in the smokebox tube hole (mine is currently just held by this fit) I think the idea of the plate is to give somewhere for a sealant of some type to seal this opening, between plate and inside of tube, I can't remember of Don relies on a tight fit also for the plate to the petticoat of whether it needs to be silver soldered on. My own plan is to permanently fix the plate to the petticoat and transfer the chimney mounting holes through it so that the chimney mounting bolts go through the smokebox and into the plate which will be threaded to accept the bolts, a small bead of sealant between should do the job nicely....

At least that's the current plan...


Pete

[jma1009]

Hi John,

The description of how to make the petticoat pipe is on page 126 of ME 6th February 1970. It is a clear and detailed description.

I would only differ from it in a few respects. I would roll the 1 1/2" square of brass before boring the hole in it. I would set it up in the 4 jaw to bore the hole to a tight fit of the OD of the petticoat pipe.

I would than chamfer the concave side of the plate lightly. The you silver solder the two together. The plate can always be attacked with pliers or something more brutal to get it so that the petticoat pipe is perfectly concentric along it's length with the blast pipe when fixed to the smokebox by screws. Ive only ever fitted 2 screws fore and aft, not 4 as shown by Don.

Note do not make the tapped holes in the plate as shown on the drawing. Make them closer together otherwise the screw heads will not be covered up by the chimney base.

I would bore the internal taper on the petticoat pipe 2 degrees and not 3 degrees as stated by Don. The taper on the internal bore of the chimney should match perfectly with no step or change of taper.

Cheers,
Julian

[manofkent]

Thanks both.
That makes more sense now.
Pete - that's a really neat solution.
Julian - I was reading from the model engineer page you quote! I often find I have to read these things a good few times to work it out. (That's what comes of being an accountant by training, not an engineer by any sensible measure).

Julian - why 2 degrees not the 3 that Don suggests?

Regards

John

[jma1009]

Hi John,

2 degrees is what Sam Ell used, and is what Jos Koopmans advocates.

Incidentally, the chimney shown on the smokebox drawings is the early Churchward plain tapered cast iron chimney. These chimneys were later replaced by a larger chimny with parallel sides and the traditional copper capped top. I much prefer the later type of chimney.

If you have not yet made the steam pipes to the steam chests, and the exhaust pipes and blast pipe, then there are a few modifications you might like to incorporate.

Cheers,
Julian

[kipford]

Personally I would not worry about a 1 degree difference in diffuser angle. You will never notice the difference in performance if you made a back to back comparison on the track, you would be lucky to see the difference even set up on a dedicated test rig. Julian is correct that the bore should be smooth and continuous, however if you do have to end up with some sort of step make it a step out of wind, this type of discontinuity will have a significantly lower effect on diffuser efficiency than a step into wind.
Regards
Dave

[jma1009]

I entirely agree with Dave that the odd 1 degree either side will not make a noticeable difference, but if the experts decided upon 2 degrees either side then why depart from their conclusions when faced with a casting to machine up?

In a diffuser type arrangement Jos was drawn into concluding in his book a larger taper was permissable, but this is not a diffusser type arrangement.

Don was quite good in sticking to the Ell formula with a few exceptions, one of those being his 5"g Doncaster design. He agonised in print over reducing the taper to 1.5 degrees on his 'Aspinall' and 'Lanky' 5"g designs from 2 degrees. Why do we find 3 degrees on County Carlow? It was probably no more than to use standard Reeves castings they had at the time. That can hardly be a good reason to depart from Ell principles for the sake of commercial conveniance!

There are pages and pages in Jos's lengthy book on the velocity profile of the exhaust, and his conclusion was that as per Ell on a standard type single chimney 2 degrees either side gave the best results.

I would add for John that if the petticoat pipe casting allows, leave it as long as possible and longer than on the drawings. If you are making the petticoat pipe up as a fabrication from thick walled tube make the petticoat pipe 1/4 lower, or the blast pipe top 1/4" higher.

The blast pipe also needs increasing in OD by at least 20%, and an inverted 'Y' branch pipe of the exhaust pipes will give a much better exhaust. Most of Don's piston valve designs were much better in this respect, but County Carlow was an early design of his.

Cheers,
Julian

[manofkent]

Thanks Julian.
Yes I will be making the Petticoat pipe from thick walled tube so will go for 1/4 longer as you suggest.
I did think the exhaust arrangements were unusual, and had thought that by having an H arrangement as drawn it would restrict the exhaust flow, and possibly blow into the opposite cylinder. Your suggestion of an inverted Y is welcome. Just to be clear is it a Y left to right or front to back?
The blast pipe vent is a N0 10 drill size, although Don suggests in his words that it could be smaller. A 20% increase would take it to around 0.24" - is that what you Meant?.

Thanks Julian. Your suggestions are greatly appreciated.

John

[jma1009]

Hi John,

The inverted 'Y' I was thinking of would be in the middle front to back, though ideally adding as well the 2 further 'Y's you mention would be better still.

The 20% increase would be in respect of the blast pipe ID, not the blast nozzle itself.

I would also site the snifting valve at the front of the smokebox base rather than as shown. As drawn it will be impossible to get a spanner on the snifting valve or the pipe union.

Cheers,
Julian

[manofkent]

Thanks Julian. That all makes good sense.
I had noticed the awkward placing of the snifting valve as you mention. I will move it to the front of the smokebox.
I will see how get on making the Y joints, I might keep the horizontal bars between the exhaust ports (but blank) just to hold the arrangement in place.

Where would I find details of the larger chimney you mention?

Many thanks for your help as always.

John

[jma1009]

Hi John,

The later larger chimney with copper cap is incidentally exactly the same as fitted many years later to what would become the LBSC 'Speedy' design as per Roger on here, using a drawing kindly supplied by Bob Youldon as the LBSC drawing in incorrect.

Here is virtually the same drawing Bob has. You will need to get out your calculator and work out all the dimensions for 3/4" scale!

(The Reeves Don Young 'Saint Christopher' chimney casting should suit, unless you want to turn the bits up from odds and ends of brass and a copper or hard drawn phos bronze cap. I find it much easier to make chimneys from cored odds and ends than use castings).

Cheers,
Julian

[manofkent]

Julian.

Thank you. This is very helpful.

John

[jma1009]

Hi John,

Don Young's version of the large chimney is on page 190 of ME 20th February 1970. I havent done a comparison with the above posted drawing.

Cheers,
Julian

[manofkent]

Thanks Julian.

I have that copy of Model Engineer thankfully.

I am going to try and cnc turn the chimney from a big lump of steel on my homegrown cnc mill. The mill is an Amolco mill, and my Myford chuck fits it. So I spent an afternoon getting the profile of the chimney taken from your posting as close as possible in CamBam.
Transferring it to the mill means using the vertical mill feed as the lathe spindle, with the tool bolted to the table ( about 2" high obviously to clear the stock).
I haven't quite completed the CamBam bit yet, and will probably machine it first out of some soft wood with my hand hovering over the emergency stop just to make sure it works and the mill survives!

John