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Post by maninshed on Nov 19, 2009 13:36:08 GMT
This section seems to always prompt some controversal comments, as far as the structure of boilers go. I'd like to ask for advice on the following, the engine i'm having a go at is Holmside which is 7 1/4" gauge. My question is some fittings on larger scale engines are bolted to the boiler not secrewed into bushes as on some other scales. My main concern is the steam dome, which is bolted to the bush in the boiler barrel, the drawing dosn't give any clues as to how many or what size of studs to use to secure the dome. Can anyone enlighten me as to the sums I need to do, to make sure I fit fixings adequate for the task. I assume these fixings would have to be able to withstand in excess of the initial 2 x pressure test, if so how much in excess? Also suitable materials for the studs, I was thinking stainless steel, but whatever I used I assume I would need to know the tensile strength? Any bright sparks out there got any clues, thanks.
martyn.
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redmog
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Post by redmog on Nov 19, 2009 18:05:10 GMT
Dome Screws. I've thought the same myself Martyn. I think the answer is that it's unlikely that the builder will have facilities to cut a large diameter thread, and to be able to run a large diameter tap into it to finish the thread crests when a boiler is complete. So screws are the answer. Yes I worked mine out. I don't think I will be far away with the sizes I've used. I've used stainless steel cap screws. The tensile strength of stainless steel is about 30,000 lb/sq inch. The core diameter of my 3mm screws is 100thou. ish. pi x r squared for core area is 0.008 sq inch. 30, 000 x 0.008sq" = 240 lbs each. I've used 8 in my dome so 8 x 240 = 1920lb The area of my dome is less than a square inch so to be able to withstand 1920lbs is well above any 8 time safety factor. The thread would strip in the bronze bush first. Anyone know how to calculate that? I stand to be corrected by the more knowledgeable. Chris 
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Post by Shawki Shlemon on Nov 21, 2009 7:49:39 GMT
Chris calculation are correct ( I think ) ,the OZ code has a table giving the number of studs/screws and their sizes for various sizes of bushes. Unfortunately my MFC is not operational at present to show the table . example -
PCD 50mm , number of studs 10 ,size 4BA ( minimum )
PCD 100mm ,number of studs 16 , size 2BA ( min )
Material SS/ Bronze/ monel .
If you tell me the PCD on you bush , I will give the info as per OZ code . Of Course for you this is only a guide as our code does not apply in UK .
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redmog
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Post by redmog on Nov 21, 2009 8:21:50 GMT
Threads Weakest.
Thanks for the offer Shawki but this isn't my thread,
it' Martyns.
He was enquiring how the tensile strength could be worked out.
I've done my bushes and used the tried and tested method, which is -
whats been done before with no trouble.
I think it's a bit of a waste of time working out the tensile strength because I think that the thread would be the weakest part.
Chris
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Post by Shawki Shlemon on Nov 21, 2009 8:36:11 GMT
You are wright Chris the weakest point is the threads and that is why the code stipulate the sizes of screws .the offer was and is for martyn as he is the starter of this thread .
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Post by havoc on Nov 21, 2009 10:44:24 GMT
IIRC from my courses too long ago -for metric bolts- they are designed that something else fails before the thread strips. So using the core diameter as base for the calculations looks fine.
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redmog
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Post by redmog on Nov 21, 2009 14:49:43 GMT
How Many Threads? IIRC from my courses too long ago -for metric bolts- they are designed that something else fails before the thread strips. So using the core diameter as base for the calculations looks fine. It's not the stainless thread that worries me. It's the thread into a thin phosphor bronze flange. Some flanges are only about 3mm deep. There must be some theory of how many turns of thread you should have for optimum strength. Someone, on this forum, who is more knowledgeable than me will know. Chris
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Post by havoc on Nov 21, 2009 16:16:41 GMT
In steel the thread should engage on the same depth as the diameter of the bolt. So for M3, you should have 3mm thread engaged. Yield strength of steel is about 250MPa, PB1 is about 170MPa. So about 1.5 times the diameter as a minimum I would say.
Checked with our code and that is what they also say: thickness of the flange > 1.5 times the diameter of the holes.
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redmog
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Post by redmog on Nov 21, 2009 17:15:02 GMT
In steel the thread should engage on the same depth as the diameter of the bolt. So for M3, you should have 3mm thread engaged. Yield strength of steel is about 250MPa, PB1 is about 170MPa. So about 1.5 times the diameter as a minimum I would say. Checked with our code and that is what they also say: thickness of the flange > 1.5 times the diameter of the holes. Great. All useful information. Can I just ask what 'Our Code' is refering to. Chris |
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Post by havoc on Nov 21, 2009 20:43:38 GMT
This is the Dutch code. In Belgium we use that one as we are too small to have our own. It is being rewritten right now, again together with Holland and a new one should be available next year. If all goes well etcetc.
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Post by singleshotman on Jun 28, 2010 18:08:33 GMT
About domes, i've bought four boilers from commericial suppliers in the past, i swear NONE WERE TESTED, as all the domes were undrilled, is it possable to plug and test without drilling and tapping? I recieved a faulty one from Norman Spink, i never orderded anything from him ever again.It would not hold water, i've seen teapots hold water better.I could not return as i'm in the USA, i guess he didn't give a rats ass.Would cost more to return than worth.
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Post by alanstepney on Jun 28, 2010 19:59:23 GMT
Yes you can test without drilling etc the dome.
I am amazed to hear of a Spink boiler having faults as he had a superb reputation, and you dont get and keep that if you keep sending out unsatisfactory products.
However, even in the best organisations, things can go wrong.
I can guarantee that had you returned it, he would have fixed or exchanged it.
A bit late now of course.
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Post by 2tenths on Jun 28, 2010 20:53:59 GMT
I am amazed to hear of a Spink boiler having faults as he had a superb reputation, and you dont get and keep that if you keep sending out unsatisfactory products. However, even in the best organisations, things can go wrong. I can guarantee that had you returned it, he would have fixed or exchanged it. A bit late now of course. I'll second that. Dealt with him for many years. I once bought the castings for Torquay Manor (about 150years ago ;D) The cylinder castings had blow holes on the valve face, I took them back and he gave me his whole stock saying"find a pair that's ok and let me have the rest back" Can't do better than that. Tony |
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denis M
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Post by denis M on Jun 28, 2010 21:33:34 GMT
In my mind Norman Spinks was a true gentleman.
He once, many years ago, sent me £250-00 worth of Easyflo without me paying and as he did not know what the P&P was told me to add the amount of the stamps onto the £250-00 and send it to him.
Denis
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Post by singleshotman on Jun 28, 2010 23:42:43 GMT
i think it would have been impractable to return a boiler 6,000 miles away.His castings for the virginia were very nice, however. Maybe he didn't make the boiler, i don't know.
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Post by drjohn on Jun 29, 2010 7:42:50 GMT
I returned a boiler from Malaysia to Australia east coast (and that's over 4,000 miles as the crow flies) and got the carriage returned - where in the states are you, 'cos it's not 6,000 miles across the pond by normal mile measurement - even LA to London is only 5,400 miles?
DJ
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Post by Shawki Shlemon on Jun 29, 2010 8:54:34 GMT
Many years ago I ordered few injectors from Spinks and asked for the cost so that I can send him the money , instead he sent me the injectors and the bill , he didn't know me and that was the first time I contacted him . I can only say that he was more than a gentleman .
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Post by peterseager on Jun 29, 2010 21:15:30 GMT
I believe the usual procedure is to soft solder a blanking cap over such holes.
Peter
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russell
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Post by russell on Jul 3, 2010 16:15:49 GMT
This is the Dutch code. In Belgium we use that one as we are too small to have our own. It is being rewritten right now, again together with Holland and a new one should be available next year. If all goes well etcetc. Isn't it time we had a European code rather than all these national ones? If I bring my engine, made and tested to the French (CAV) recommendations, to England will I be allowed to run it publicly without having it tested again (and probably failed as the rules are different) in the UK? What happens when Brits take their engines to Sinsheim? On the other hand, perhaps we shouldn't have these codes other than to give guidance. After all, if a boiler and it's associated equipment complies with the Pressure Equipment Directive, it must be allowed to be sold and used in all EU Member States. Russell (Just stirring a bit  ) |
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kwil
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Post by kwil on Jul 3, 2010 16:28:15 GMT
But I thought European codes or any thing similar are there to be ignored by most (Esp in Fr)
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