Boiler bushes

[modeng2000]

I am wondering why phosphor bronze or gunmetal is used for boiler bushes. With a copper boiler, why not use copper for the bushes? I have scanned all my books like The Model Engineers Handbook and nowhere is it suggested that copper might be used. The only thing I have come up with is that it could be too soft for a reliable thread.

John

[greasemonkey]

Hi John
Hi John
I think youv'e hit the nail on the head. Copper is very soft, more so when it has been anealed which is effectively what happens to it when it gets heated for silver soldering. Its also a right pig to machine.

Andy

[havoc]

In the club someone made a completely copper boiler (5"), bushes included. But he used an inox thread insert like is used for repairing ripped threads in automotive practise. (so you make a larger thread than needed and srew the inox piece in the larger thread, itself forming the smaller thread) This has not given problms so far, and he is commited to continue that idea.

[greasemonkey]

Intresting idea but whats the 'inox' insert made from? Wouldnt Steel corrode and if I remeber correctly copper and stainless have a high electrolitic action between each other?
Could be a good way of reparing damaged bushes. I have a boiler were the previous owner filled and refiolled by removing the saftey valve. The thread is now getting very loose.

Andy

[modeng2000]

I guess I had better make my bushes from the usual materials as experience is the best teacher.

Thanks, John

[ron]

Andy
I've used thread inserts [Helicoils] during classic car restoration, the quality ones are stainless, they are very easy to install and give a better thread in soft metal than the original, haven't had any electrolitic problems with aluminium alloys but I've no experience of them in copper.
Ron

[greasemonkey]

Hi Ron
I agree they are very good, just hadnt thought about using them in a boiler and would like to find a bit more about the issues before using them.

Andy

[waggy]

I use one of my safety valves to fill the boiler on my engine, no thread wear!

To avoid wear, wrap PTFE tape around the thread every time the valve is refitted. The tape wears, not the thread and you don't have to tighten the valve against a washer or other type of joint face, the tape seals the union.

Waggy.

[alanstepney]

Gunmetal or PB has been found to be the best over many years.
Threads in copper tend to tear, particularly so with sharp form threads, (Whit or Metric form) although BA isnt as bad.

GM or PB thread nicely, last well, and, well, do the job.
They also silver solder to copper easily.

[havoc]

I've used thread inserts [Helicoils]

Those are the ones, couldn't remember the name.

[modeng2000]

There seems to be generally available three types of phosphor bronze, PB 102, Colphos 90 and SAE 660.

Which one would be best for my boiler bushes please?

As SAE 660 is leaded I suppose it might not be best when silver soldering because of the lead content.

John

[greasemonkey]

HI John
I used SAE 660 when I made my Simplex boiler recently with no problems at all which is what I believe all the proffesional boiler makers use.

Andy

[modeng2000]

Andy,

Thanks for that.
Being fairly new to this game I am learning as I go.

John

[ukjimoo]

Yes i believe all professional boiler makers use SAE660 and i also used it for my boiler bushes.
Since then i have managed to achieve some extremely weak silver solder joints using SAE660, which worries me somewhat. I think its because the lead gets boiled out to the surface?
So it beats me as to why this material is the norm for bushes.
Incidently, i was using easyflo flux, could this be the problem?
To go off track slightly, I recently found out that when silversoldering steel, you get a miles stronger joint if you use Tenacity5. This is because steel requires more heat to make the silver solder flow and easyflo flux tends to boil away at these higher temperatures and you get oxidation in the joint. I have done tests to prove this and the difference is quite amazing.
I just wish i knew this a few years ago before i started!

[modeng2000]

Thanks ukjimoo.
Is SAE660 used because it is easier to machine I wonder? Although I am making a boiler for a gauge 1 loco I would like it to be right. Perhaps I should use a non leaded PB then there would not be a problem with the silver soldering.

John

[greasemonkey]

The heat required to make the solder flow is the same be it steel or copper. The reason you may have had problems is due to the copper conducting the heat away quicker. Tenacity 5 flux is better for assemblies which are going to have a prolonged heating cycle and be at elevated temperature for a long time, just as in boiler making.
The flux itself doesnt have any effect on the strength of the joint but it keeps it clean and free from oxidation, tenacity doing this for longer than easy flow flux.
If you have had problems with lead 'leaching' out of the 660 under heat then I would either question if you had the right grade material or suggest you may have over heated it! Try using a larger burner on your gas torch or surrounding the asembly with fire bricks to keep the heat in. This really makes a big diffrence and enables a shorter heating time and less chance of exhausting the flux. I cant see the pro- boiler makers getting it wrong somehow.
Modeng2000 your local to me so if you want a hand let me know.

cheers
Andy

Andy

[GWRdriver]

Hello all,
I have made many boilers, from Ga1 to 6"diam for 7+ga, and have always used 660 with no problems. Although I would say that Phos Brz is perhaps a better material for bushes it is more expensive and can often be difficult for me to find in larger diameters. Although this is usually not a consideration, or shouldn't be, PB can also be very tough to machine and will turn a drill blue in short order.

The American designation for the PB material I use is C510 and I have found that depending upon the supplier some of it can be very tough machining and some of it turns very easily.

660 continuous cast bar on the other hand machines like butter and is available locally in small quantities with no quantity penalty in diameters up to larger than I would ever need for bushes. The lead content is so low (typically 6-7%) that it presents no problems with leaching, solder flow, or joint strength so far as I can tell.

I checked with several mfgs of 660 bar about the leaching and they said that these products were used widely in industry for pressure fittings in all manner of atmospheres including steam with no evidence of weakening due to lead content. So far as I can tell, when properly prepared and fluxed, it takes and holds silver solder as awell as anything else.

On account of cost and availability of stock I currently use PB for bushes of 1/2" diameter and smaller, mostly for Ga1 boilers, and 660 for everything else.

[havoc]

Used PB for my boiler bushes. Searched for the lowed Pb content.

[modeng2000]

Hi Andy,

Thanks for your offer of help. If I get stuck I'll email you and perhaps you will be able to rescue me!

I am sorting out my ideas for the boiler while making other parts of the loco. I think I have had enough guidance from fellow modelers to make my mind up about what to use for the bushes.

Regards, John

[Nexuas]

Sorry to come to the party a bit late, but have only just found this board.

When it is mentioned above that Gunmetal is suitable for bushes in a copper boiler does that mean LG2? Please tell me this is acceptable as I have complete some fairly complicated sections out of this, then had a bit of a panic that it is not the corret stuff.

It is for 5.5" Dia X 2' long Boiler, Working pressure 90psi...

Cheers