|Speedy Builder5||26/05/2021 12:50:26|
|2388 forum posts|
I am under way in making one of LBSC's injectors for a 5" SPEEDY loco.
It seemed a simpler idea once the body was made to make what was described as a "Slotted Cone" for the combining cone. as detailed in the attached drawing.
Can someone explain what the slots in the sides of the cone do and should those slots pierce the Venturi which passes through the cone?
|Jon Lawes||26/05/2021 13:17:15|
635 forum posts
I don't wish to drag your attention away from this fine forum but this has been discussed in incredible length (with research and experimentation) on the Model Engineering Clearing House forum (MECH). It might be worth having a look there.
If this post is not allowed then I completely understand if it gets removed.
|Speedy Builder5||26/05/2021 17:51:36|
|2388 forum posts|
Thank you John, apparently my question was too simple! After scouring the MECH site and reading many pages, I still didn't find the answer to my question. Part of the answer is obvious in that the slots pierce the Venturi to allow overflow water to pass via the clack to the overflow pipe, but its not clear (to me) how much material should be left between the 2 x 1/32" wide slots in the combining cone.
Also missing from LBSC's text and drawings is the bore and length of the 5/32" diameter overflow pipe.
122 forum posts
|280 forum posts|
All my jubilee injectors have only one cone in the centre and they work fine , in place of both the small centre ones. In addition it should be noted they also supply a lot of the trade.
PS. what is 1/32” bare? It’s not on my ruler or micrometer. Is it -5 thou? -10 thou? or whatever. so why not just give the actual dimension
Edited By Zan on 26/05/2021 18:16:37
|Jon Lawes||26/05/2021 18:23:13|
635 forum posts
Because LBSC used language that would be understood by the common man with limited measurement tools available. Hence he used terms like "the thickness of a tram ticket".
|Nigel Graham 2||26/05/2021 22:15:35|
|1666 forum posts|
You are right: the slot of parting does cut into the cone itself. It provides the water's escape to the non-return valve that helps the injector to start.
The overflow valve is theoretically not needed but in practice was found very early on to make an injector self re-starting after temporary perturbations in the water flow to it. It also helps us by showing water is reaching the device at the start, and allowing running cold water through it if a slight steam leak has heated it.
Some full-size injectors have the combining cone split lengthways with one half hinged to act as a flap-valve, or use a sliding cone, but they do the same thing as that little ball-valve. I don't know the design Zan cites: is the overflow gap the annulus between the combining cone outlet and delivery cone inlet?
The drawing as shown offers alternatives: cut the cone into two parts or have two slots separated by narrow bridges that keep the cone in one piece. The latter needs that waist turning in the already very fragile cone.
If I was to make an injector to that design I think I would match the two slots with a shallow annular groove in the body instead, with a compensating weeny thickening on the exterior.
As Jon says, LBSC used terms like 'bare' and ticket thicknesses - and the adjective 'weeny' - to help 'the common man'; but I have always read 'bare' to mean 'as close as you can make it but not over'. In his era most model-engineers, including presumably him, had far more basic workshop equipment than we think the norm now.
7473 forum posts
I agree 'bare' means ''as close as you can make it but not over' too. Ambiguous though! In the sense of 'bare minimum' it could mean the opposite - 'as close as you can make it but not under'. Yuk.
I wonder if the term was well understood when LBSC was a lad and chaps routinely worked to inch fractions, and work was fitted. (Later dimensions were toleranced, interchangeable parts came straight off the machine and all the fitters were sacked!)
Are there other words to indicate 'slightly over' etc? I only recall words being used in connection with shaft fits, 'Force', 'Push', 'Slide', 'Shrink', but not linear dimensions, but I don't have any Victorian books on workshop practice.
Terms like 'bare' may also highlight a problem using fractions in engineering when precision is required. The design calls for a bare ¹⁄₃₂". Next finest fractional graduation is ¹⁄₆₄", which I suspect is much too small. Going down to 128ths doesn't help either. Assuming LBSC meant no bigger than 0.029":
Dumping fractions in favour of thou and tenths provides a better system when calculation and accuracy combine The drawing office no doubt saw the advantage in short order. I guess practical men found decimal easier to adopt when industry moved on: otherwise micrometers would be graduated in ¹⁄₁₀₂₄"!
Cone dimensions and spacings look more tightly specified to me than anything else in the plans. Are injectors the most critically dimensioned part of a model locomotive?
|1329 forum posts|
My apprenticeship was flowered with "A shy 1/16th" and "A proud 1/32nd" etc. but the guys dishing it out certainly knew their job.
|Speedy Builder5||27/05/2021 12:13:32|
|2388 forum posts|
I have seen the terms 'bare' and 'full'
Bare is SIZE or just less and full is SIZE and a bit more. Looking at the injector here, the cone has to be a stiff push fit so that it doesn't move out of position (a force fit would be too tight for the "slotted cone".
SOD - If I understand correctly, injectors are a little "finicky" and those tight dimensions must be adhered to. having said that, those measurements were by eye from a rule !!
|Stewart Hart||27/05/2021 14:19:12|
668 forum posts
Don't waist your time and hair trying to build LBSC injector only with a good wind and a bucket full of luck will you get one to work. If you spend time reading LBSC build series you will see that his injectors keep changing especially the cone proportions subsequent work by others came up with far better proportions. Also his instructions on how to make them have got a lot to be desired he just says make them exactly to drawing there are some critical features that need a more detailed description on how to achieve them.
Just splash out and buy a couple from the trade.
I've made my own injectors and believe me they are not easy to make you really need some one to coach you how to do it
|stephen goodbody||27/05/2021 21:05:53|
|70 forum posts|
I applaud your willingness to attempt injector construction which is certainly an interesting exercise. That said, I would hate you to become disillusioned if/when that injector doesn't work.
Unfortunately, as Stewart suggests, I'm not sure that you will achieve success with LBSC's injector designs no matter how careful you are in their manufacture. While LBSC was undoubtedly a gifted author and prolific and talented builder, I believe his level of knowledge and understanding of injector design wasn't quite up to the job.
While I am sure that there is someone out there who has succeeded in getting an LBSC injector to work reliably, I have never yet met one of those people. Perhaps one of them will respond to this thread! Conversely, I know of many, many folks that gave up in frustration, including one of my early mentors - another Bob - who was an excellent machinist. I still have a small drawer full of Bob's discarded LBSC injector bodies and cones.
As a practical alternative, I can heartily recommend DAG Brown's book "Miniature Injectors Inside and Out". I've made three injectors to Mr. Brown's designs and they all worked first time - much to my own surprise. If you're interested and willing to put in the effort to make your own injector then I think you will stand a far better chance of success if you follow this route.
|Jan B||28/05/2021 07:00:13|
40 forum posts
I made a LBSC injector when I built my first locomotive TICH in 1978. It was made following Curlys instructions in his book Shop Shed & Road. It is of the Sellers type, combining cone is in one-piece whit a slot in the middle 1/32” wide. It always picks up quickly and with such a small boiler it only needs to feed for about 15 seconds. The only drawback is that it doesn’t run dry, there is always some water coming out from the overflow, but as long as the main part goes into the boiler I can live with that.
|280 forum posts|
So once again, how much is just less or a bit more? Just as bad as full and bare 5 thou? 10 thou? 15 thou? 1 thou? I think dimensioning in this way goes back to the time of a combination of rulers, odd legs and callipers, and has no place in the modern workshop full of cheap digital equipment . It’s why our creations are so so much better in both fit, detail and interval between maintenance than in the past
|duncan webster||28/05/2021 20:26:16|
|3447 forum posts|
We have a chap in our club who can make injectors, and can often get other people's injectors to work by making new cones. He also lives in Metropolis, wears his underpants on the outside, has X ray vision and can fly.
I bought mine from Bob Bramson, work first time every time (except when I got a bit of coal dust in), and come with a test cert. Unfortunately he has given up making them
|Nigel Graham 2||28/05/2021 22:53:24|
|1666 forum posts|
Stephen Goodbody cites D.A.G. Brown's book on building injectors - I have a copy lest I be tempted into the arcanities, but it does seem the key is careful matching to working pressure and very careful, accurate work.
More salient though, and very usefully, is that Mr. Brown devotes a chapter to tracing and correcting injector faults - not all of them the injector's fault at all.
(Something I believed for years was a sign of a miniature injector working properly, is it chirruping like a guinea-pig on heat. It was really only a few years ago I discovered it should really be almost silent - even a full-size one emits only a low rushing sound of fast-moving water.)
|Speedy Builder5||29/05/2021 08:10:58|
|2388 forum posts|
Anyone got a copy of DAG Brown's book they want to sell? I bought a copy a few years ago and lost it (I think that it was left in the aeroplane whilst whiling away 23 hours on a flight to Oz!!).
|bernard towers||29/05/2021 11:37:44|
|275 forum posts|
+1 for Dag Browns book, well written all processes done step by step and the reasons why. Really interesting project and yes the work.
|julian atkins||30/05/2021 22:59:09|
1239 forum posts
As presumably one of the persons quoted on the other forum mentioned, you cannot make a LBSC injector to work except by luck.
There are lots and lots of issues with all of the LBSC designs. He doesn't specify in print how to get the correct annular gap between the steam cone nozzle and start of the combining cone.
You would be better off copying one of DAG Brown's injectors from his book. He includes a table/graph for the annular gap.
|Nigel Graham 2||30/05/2021 23:44:48|
|1666 forum posts|
Speedy Builder -
Going right back up the thread, I don't think anyone answered your second question about the overflow pipe. Apologies to anyone who did for my not spotting it.
The drawing suggests 5/32" (ish) bore in 3/16" od (set by the swg thickness), but it may not be too critical as long as the wall is thick enough for a 40tpi thread. I am surprised it's not specified properly, though, even in the text.
Probably best to make it from brass, as easier than copper tube to thread.
The length does not matter. It is commonly a straight stub about 1/2 inch or so long, and its role is simply to direct the water as a steady stream away from the metalwork, and readily visible to the driver. It would not matter if it is longer provided the water can still fall easily out of the end.
Jan B shows a neat finishing touch on his 'Tich' injector: a partial bend cut off vertically. It directs the water further from the loco and renders it more readily visible. Typically the cut end would be a touch inboard of the footplate valance, the injector itself a little deeper under cover. I think this was common practice in full-size.
Please login to post a reply.
Want the latest issue of Model Engineer or Model Engineers' Workshop? Use our magazine locator links to find your nearest stockist!
You can contact us by phone, mail or email about the magazines including becoming a contributor, submitting reader's letters or making queries about articles. You can also get in touch about this website, advertising or other general issues.
Click THIS LINK for full contact details.
For subscription issues please see THIS LINK.