Joined: Nov 2011 Gender: Male Posts: 227 Location: Oxfordshire, UK
Cast iron vs bronze motion bearings « Thread Started on Feb 18, 2012, 10:31pm »
I've just been reading Doug Hewson's article in the latest EIM, on building the Y4 0-4-0 loco in 5" gauge. I see that once more, Doug is asserting that cast iron will outlast bronze when used for motion bearings. Now I hold Doug in high esteem and I think he is probably right, but if he is right, then all the others (LBSC/Evans/Young et al) must have been wrong for all those years.
I did a search for previous posts on this subject and it seems that a couple of contributors are also in favour of cast iron, although the comments were perhaps subjective.
If cast iron is such a superior material, we have to wonder why it wasn't favoured for full size bearings. As far as I'm aware, most full size locos had either whitemetal or phosphor bronze motion bearings (although I'm sure someone will prove me wrong). Possibly, because thin cast iron bearings would not stand stand up to the hammering in full size?
Good as it may be for models, Doug's claim that cast iron will last ten times as long as bronze does sound like a slight exaggeration! Does anyone have any hard evidence, or operating experience to offer? Also, what are the disadvantages of iron bearings? I can see that corrosion might be an issue, seeing that the motion on our locos often gets an unwanted washing down with water.
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Re: Cast iron vs bronze motion bearings « Reply #1 on Feb 18, 2012, 11:57pm »
Hi All, The biggest problem I have seen with cast iron is when it galls up. The destruction that can take place in a very short time to both the bush and pin has to be seen to be believed. As cast iron is brittle I guess the hammering that a full size loco could give a cast iron bush could cause it to break up. As for lasting 10 times longer well Doug must have been very young when he fitted the bushes that lasted that long (said tongue in cheek) That said personally I have used both and don’t have strong feelings either way but normally use bronze because of what can happen on the admittedly rare times that galling takes place. I have seen it 3 times in 30 years. Regards Paul
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Re: Cast iron vs bronze motion bearings « Reply #2 on Feb 19, 2012, 7:13am »
I have used both not by choice but by what it comes with set of castings . I haven't been around long enough to have hard evidence , both seems to work OK , I wouldn't hesitate using cast iron ,it works fine .Now to reduce the rust problems I leave cast iron axles for example in an oil container overnight to soak before assembly .
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Re: Cast iron vs bronze motion bearings « Reply #3 on Feb 19, 2012, 12:42pm »
i am guessing that in fullsize cast iron wasnt used as a bearing material because of the increased wear it might cause on journals, unless those journals could be hardened (which they couldnt). also bronze has 2 other useful benefits... it is easy to get whitemetal to adhere to it, and it has a useful heat transfer property. it can also be scraped much easier than cast iron. in our sizes there are problems tapping cast iron for small parts unless the cast iron is of the quality that used to be supplied by stuart turner. some of the bearing assemblies i have made have been quite small and fiddly. i was told years ago never to use cast steel/silver steel for journals and never use stainless steel for axles or journals. apparently stainless does peculiar things with phosphor bronze! the old clockmakers knew a thing or to about making good journals. i have on occasions burnished journal pins, though as a rule i generally use ground mild steel and caseharden it. even mild steel can get badly worn by hard phoshor bronze. in the model engineers workshop of a few years ago (LBSC etc) no one would want to use their nice reamers etc on cast iron. the only time i ream cast iron is for crankpin holes in wheels, and ive got one reamer i only use for this type of job (so all my crankpins have to be the same diameter where they fit into the wheels!)
Re: Cast iron vs bronze motion bearings « Reply #4 on Feb 19, 2012, 3:13pm »
Three potential reasons:
On full size; I believe that the babit bearing material sticks well to bronze used in friction bearings, while it will not do so to cast iron, thus any white metal bearings will have bronze (referred to as brass, but seems bronze to me). Note that the bronze is NOT used as a bearing surface, only as a holder for the white metal bearing.
Also on full size - side rod bearings need play; I wonder if the reason for bronze for rod bearings is that is is more ductile and less prone to shattering? Anyone who has been on a full sized steam locomotive running at "flank speed" with the side rods clattering will know that there'll be some collision forces at play here.
Cast iron holds oil, does it not? I know there is sintered bronze - oilite - that can hold oil, but Cast Iron does it automatically, right?
Re: Cast iron vs bronze motion bearings « Reply #5 on Feb 19, 2012, 7:09pm »
The original query refers to motion bearings, presumably not the valve gear as most of these do not rotate through 360 degrees. Cast iron has certainly been used occasionally for crank pin duties, where there are heavy loadings supplementary to pure rotation, but have found little favour.
A clue to the universally applied white metalled 'brasses' may lie in the fact that axle box journals and crank pins rarely require other than a mere polish to serve new metalling in a lifetime. Having just been inspecting journals that on top of 40 years of service have withstood the rigours of another 40 years in the salt air of Barry, it is reasonable to conclude that our railway engineers got it right. Even a breakdown of the white metal surface (easily replaced) is likely to leave the journals (a major job to replace) free from problems. Good lubrication, of course, is vital.
One could argue that model coupling rod bearings in iron might hold the oil longer, as these usually get a perfunctory squirt of the oil can before setting of on an afternoon's drive. Most of the oil will never reach the bearing surface and the bearings will not see a drop until the next session.
Cast iron holds oil, does it not? I know there is sintered bronze - oilite - that can hold oil, but Cast Iron does it automatically, right?
I can't see how cast iron can retain oil, unless of course it's sintered iron. Cast iron does have self-lubricating properties though, due to the free graphite (carbon) that is present.
I'm building two locos at present, and both have silver steel axles and crankpins. As Don says, replacing an axle or a crankpin is to be avoided at all costs. Therefore, unless anyone can convince me that there is a tangible advantage in using cast iron bushes, I will stick to SAE660 continuous cast leaded gunmetal. Better the devil you know, as they say!
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Re: Cast iron vs bronze motion bearings « Reply #7 on Feb 19, 2012, 8:52pm »
A good friend of mine built and runs a 4" scale Mclaren showmans road loco, that has case hardened little end pins and cast iron bushes in the conrods, before the overhaul 2 years ago after 10yrs use it had run in excess of 1000 miles on the road plus rally field running about and generating outside of the rally beer tents on the evenings for about 3 to 4 hrs each evening. After all of this the pins and bushes were in such good condition that they did not get changed during the overhaul. My 4" agricultural after a single seasons rallies has a knock and noticable play in the bronze bush, next winter at the latest i will be making a new case hardened pin and a cast iron bush.
Running 4" scale Foster agricultural and a 4" scale Foster 7NHP DCC road loco being researched before building, 5" gauge GWR 5101 on the back burner but for how long? Want to build 5" gauge GWR 68xx, 47xx, 43xx.
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Re: Cast iron vs bronze motion bearings « Reply #8 on Feb 19, 2012, 9:50pm »
Hi All,
My 5" Hall has been running for over 12 years with cast iron bushes and on inspection last year only found the small end to have worn a little other than that there as good as the day they were fitted.
My 5" Hall has been running for over 12 years with cast iron bushes and on inspection last year only found the small end to have worn a little other than that there as good as the day they were fitted.
That's impressive. Can I ask what your crankpins are made of, and did you find any appreciable wear on them?
Re: Cast iron vs bronze motion bearings « Reply #11 on Feb 27, 2012, 2:03pm »
I Once asked this question on another forum and was laughed at but as mentioned before it is the practice to white metal bearings in prototype now would this have any benefits for us as modellers?
I Once asked this question on another forum and was laughed at but as mentioned before it is the practice to white metal bearings in prototype now would this have any benefits for us as modellers?
I think you need to look at the reasons why whitemetal is used in full size practice before you make any decision on it's usefulness or otherwise in model engineering.
First of all whitemetal has a low coefficient of friction so it makes for a very efficient bearing. This is most critical when starting from rest when it takes a few revolutions to establish a proper oil film.
Whitemetal is a fairly weak material when unsupported and therefore it is usual to cast a thin layer of it onto a more substantial backing, usually gunmetal or leaded bronze. In full size practice a railway company would have literally thousands of locos, all with different sized bearings. A supposedly standard class (or even a single locomotive) would have variations in the size of crankpins, axles etc depending upon wear and tear and so you can imagine the logistical nightmare, never mind the expense, of trying to cater for spares. These might be needed in a rush if the loco ran hot and the problem would be compounded if it was away from its home depot.
The simple answer was whitemetal. Every depot could carry a stock which was universal in its application, no special sizes were needed and it could be fitted, if necessary, without the use of specialist machine tools. Another advantage of whitemetal is that a worn out bush can be restored to size relatively cheaply; certainly at a massive saving when compared with the cost of machining a complete new bronze bush or bearing.
Finally, if a whitemetal bearing ran hot the metal would run from the bush or bearing thus increasing the clearance. This would allow a loco to be nursed back to the shed with a minimum disruption to other traffic.
I think the advantages in the smaller sizes would be minimal personally. Machining time is increased - you have to bore the bushes twice as well as make a set of "chills" for casting the metal. On top of that you will need some whitemetal - don't use solder it's not the same! - and it's not cheap stuff to buy. I would think the frictional advantages in the smaller scales are minimal and I would not have thought the amount of wear the average miniature loco will experience would really justify the extra expense.
If you are dead keen on having a go and want some further advice let me know, I'm just whitemetalling bearings for a full sized loco at the moment.
Roy Amesbury used to whitemetal bearings and eccentric sheaves. he built some fantastic locos and is sadly missed
Don't want to appear pedantic but I think you might mean eccentric straps. The sheaves are the eccentric portion which is keyed to the axle. The straps go round the outside and hence are easily removed for re-metalling.
Roy Amesbury used to whitemetal bearings and eccentric sheaves. he built some fantastic locos and is sadly missed
Mr. Amsbury wrote an article on white metal bearings in Model Engineer No.3758 (2-15 Aug 1985). From photos I've seen of his work-he was indeed an exceptional model engineer.
Built a Tubal Cain 'Trevithick' engine (as the 'Catch Me Who Can' loco). Building 5" gauge BR Standard Class 2 tender loco,and 5" GWR 41xx tank loco. Member of Burton Upon Trent M.E.S.
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