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Post by Deleted on May 6, 2015 11:41:08 GMT
At least one of the TR engines has the feed tube wrapped over the top of the gauge and back down the other side to the connection point Yes - the wrapped around variant was used on quite a few other engines as well . |
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Post by Deleted on May 6, 2015 12:21:22 GMT
Just a note about making small gauges for model size engines :
Bourdon tubes for small gauges designed the conventional way usually end up being very stiff or very fragile .
There is no need to stick to the 3/4 turn tubes usually used - tubes with more turns can be used - for example 1+3/4 turns .
Having more turns means that tubes are much more flexible and give much bigger deflection for a given pressure . Also can be made a bit stronger and still have more deflection .
Very tiny pressure gauges can be made this way and even tiny vacuum gauges .
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Post by Jim on May 6, 2015 22:55:04 GMT
When I was talking about this with a mate of mine his comment was, "Oh is that why they wrap the tube round the gauge, I thought it was because the tube was too long." True.
Jim
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Post by Jim on Jun 13, 2015 0:56:18 GMT
At last the cab is starting to come together after a lot of pondering as to where and how various fittings could best be placed to be workable while maintaining some resemblance to the prototype. As previously mentioned I wasn't happy with the location of the blower valve so that has been moved to the top centre of the cab where it takes the place of the original manifold shut off valve. Installing and connecting the drive shaft with its universal joints to the reverser and the lifting arm is another issue that needs to be thought through. The shaft passes through the front panel of the cab where access is very limited then via a protective tube through the running plate to the worm drive. I think I have it nutted out, well I had last night sitting by the fire a soothing ale in hand.  The photos show progress to date, I should add too that the brake stand is the original JP design, I intend remaking it to the proper standard design brake stand. 
Jim |
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Post by joanlluch on Jun 13, 2015 7:05:21 GMT
Hello Jim. Nice looking cabin. I am not yet at this stage so a question came to me after looking at your picture. I noted that your water gauges are fitted at a certain distance of the top of the boiler. It would seem to me that there may be a range of upper water levels that will not show on the gauges. The upper range of water levels are less important, but I guess showing them may be useful while you are filling the boiler when it's cold. So I wonder whether you did it that way for a particular reason. Thanks.
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Post by Jim on Jun 13, 2015 8:31:24 GMT
The location of the top of the gauges is not important it's the bottom that is.
Our AMBSC code states 'The bottom (visible) end of the glass shall be above the highest point of the crown of the inner firebox by not less than 10% of the distance from the crown to the outer wrapper'. That's the reason for the short lengths of brass tube resting on the bottom nut of the two gauges to ensure the required minimum is still covering the crown if the water level drops to that point. Of course the water level must be kept above this level at all times.
Jim
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Post by ejparrott on Jun 13, 2015 8:33:59 GMT
That's a good idea with the blower valve Jim, I like that..... May copy that when I do the Std 4...see how David's 9F works out for actually using the blower valve on the pedestal
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Post by Jim on Jun 13, 2015 8:42:39 GMT
Glad you like it Ed. I was thinking of the pedestal Ed but given I am using a hollow stay hidden behind the dummy gauges this seemed the simpler solution plus having a working valve where there should be one anyway.
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Post by joanlluch on Jun 13, 2015 9:17:23 GMT
The location of the top of the gauges is not important it's the bottom that is. Our AMBSC code states 'The bottom (visible) end of the glass shall be above the highest point of the crown of the inner firebox by not less than 10% of the distance from the crown to the outer wrapper'. That's the reason for the short lengths of brass tube resting on the bottom nut of the two gauges to ensure the required minimum is still covering the crown if the water level drops to that point. Of course the water level must be kept above this level at all times. Jim Thanks for that Jim. So the gauges would still follow the AMBSC code if they were taller, right? Thanks. |
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Post by Jim on Jun 13, 2015 10:50:59 GMT
They certainly would conform. As I said it's the bottom level that must be correct.
Jim
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Post by joanlluch on Jun 14, 2015 12:54:14 GMT
They certainly would conform. As I said it's the bottom level that must be correct. Jim Hi Jim, Maybe I didn't explain myself. I mean to get a response on the reasons why your water level is not taller. Much appreciated. |
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Post by Jim on Jun 14, 2015 13:16:59 GMT
I'm sorry John but there isn't a scientific reason for everything.
I'm trying to build a reasonable representation of the backhead of a Britannia. The gauge glasses are as they are, given the requirements of our AMBSC code and the need for me to be able to see the water level in the boiler while trying to keep it all looking reasonably like the prototype.
Jim
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Post by joanlluch on Jun 14, 2015 14:41:19 GMT
Thank you, Jim.
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pault
Elder Statesman
Posts: 1,502
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Post by pault on Jun 14, 2015 15:09:33 GMT
Generally speaking you don't want the water in the top nut or higher as you will get priming via the regulator and carry over to ancillaries which will cause them problems. In other words you only need to know if the water is at its highest desirable level. There is no need to know how far above the highest desirable level as you should not be there, hence there is no need to have the top fitting at the very highest point of the boiler.
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steam4ian
Elder Statesman
One good turn deserves another
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Post by steam4ian on Jun 15, 2015 6:31:37 GMT
Paul
Whilst I agree with you about boiler level management I don't fully hold with your comment about the location of the top of the glass. IMHO the glass length needs to be such that you can see the water/steam interface under all/most circumstances. You may be skilled enough to keep the water at the same point on the glass all, underline ALL, the time but not every driver is. Vigorous steaming on a rising grade will lift the water to the top of the glass whilst a shut regulator on a falling grade will have the water near the bottom nut. The ideal length of the glass should be such that these variation in level under traffic conditions still leave a margin at both the bottom and the top. If you can't see how high the water is under high water conditions you can have no certainty that you will have enough water as you shut off steam and pass over the crest of a grade.
We have a well built, well steaming loco at our club where the builder had to modify arrangements to get longer glasses (on the back-head) after he, an experienced enginemen, lost sight of the water sufficient to justify a boiler inspection and hydro test before its next steaming.
Ian
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Post by Cro on Jun 15, 2015 8:19:56 GMT
Ian,
You must have one hell of a change in gradient at your track then. We have the peak of our gradient somewhere in the middle of our tunnel and all our drivers know that you should have over have around 3/4 of a glass when going in and you'll come out with something around a 1/4 of a glass coming out.
I agree with Paul the top of the fitting isn't overly important if it's not right at the top of the boiler and as said if fitted too high and the loco was ran in the way you describe you'd be picking up water in the regulator and other fittings too.
I was driving yesterday on a loco where the gauges are similar to Jims Brit, it was a Std class 5, and you have to keep puttin water in to keep it quiet so it's always in the top nut but I would never have said I wanted the gauges higher to know exactly where it is.
Adam
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Post by ejparrott on Jun 15, 2015 10:58:27 GMT
NGG16 on the WHR - water always out the top.
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pault
Elder Statesman
Posts: 1,502
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Post by pault on Jun 15, 2015 11:55:03 GMT
My personal view is that once you are in the top nut or above you do not know accurately where the water is, and that is a situation you should not routinely be in,or it could be argued never in.
The top fitting can obviously be too low like wise there is little point in having a glass that is unnecessarily long. This can tempt someone unfamiliar with the loco
To run with excessively high water which is asking for priming. That can be a frightening event even for an experienced drive.
I have seen a few people people who make up for lack of boiler management skills by simply running with the water excessively high but it is damaging and not elegant
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steam4ian
Elder Statesman
One good turn deserves another
Posts: 2,069
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Post by steam4ian on Jun 15, 2015 23:52:23 GMT
Jim.
I am not suggesting there is anything wrong with you glasses or their location (pushed a bit further up your nose might help). However the fact that you have had to sleeve the bottom half to comply with the AMBSC Code may say something about the boiler design. The sleeves have taken out about 20% of useable glass length.
You can in part satisfy yourself at this time about how easy your glasses will be able to use. Find the worst gradient on any of the tracks you intend to visit then fill your boiler to mid glass then tilt it back and forward at the same gradient you measured. Most clubs do not have gradients exceeding 1 in 50 (2%) which for a boiler 24" long means a 1/2" movement in the glass. Heavy steaming will typically raise the water level 1/4" but it can be more depending on the boiler type and the water level already in the boiler; Belpaire flat tops are easier in this respect.
As others have said, there is no point having a glass too long as it can encourage running with the water too high. My contention is that a driver has to know where the water is. Is just out of sight slightly above the top nut or just about to prime?
As for priming, whilst not ideal from a management point of view, it does happen so get used to handling it; particularly with piston valves make sure the engine has adequate cylinder drain cocks and know how to operate them quickly.
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Post by Jim on Jun 16, 2015 1:04:43 GMT
Thanks Ian,
Boadicea will be fine on the track I intend using.
Jim
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