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Bearing running clearance - rear axle

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John Reid 117/01/2011 00:16:54
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This topic may have been covered elsewhere, but I am wondering what the running clearance should be between the rear axle diameter and the bronze bush bearings.
 
I am building a 4" Little Samson which has an axle diameter of 1.375". The bearing material I plan to use is Hidurel 5, also known as Colisbro. There are two bearings at opposite ends of the cast iron axle tube. I have read of using 1 thou per inch of diameter, but when I discussed this a while ago on the LS forum, it was suggested that this was not enough because of the temperature rise.
 
Any guidance on what clearance I should machine to would be much appreciated.
 
John R
Andrew Johnston17/01/2011 09:35:06
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Hi John,
 
I'll be interested to read the replies you get. I was advised not to bother with separate bronze bearings in my rear axle shaft. Given the price of bronze I eagerly accepted the advice! I'll be running steel (EN8) directly in cast iron. Given the relatively slow speeds (see my recent thread asking about this) I don't suppose the axle/bearings will get particularly hot, or even warm.
 
Regards,
 
Andrew
 
PS: Must drop over and have a nose round your workshop sometime!
Ian S C17/01/2011 10:01:04
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    Andrew, a steel shaft running on cast iron should make a good bearing. Ian S C
JasonB17/01/2011 13:41:45
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Andrew its not the heat from bearing friction you have to think about, it the fact that the axle passes along the back of the firebox and can get quite hot.
 
I'm not sure of these bearing materials but for SA660 and that size I would think around 0.004" would be about right.
 
This has been discusses a couple of times on TT so you may want to search there. Edward is a member as is Dave who has written the "a dream come true" books
 
Jason
John Reid 117/01/2011 14:45:21
4 forum posts
Gentlemen,
 
Thanks for helpful replies.
 
Yes, my concern was the expansion due to firebox heat.
Will have a search on TT - I am also a member and  should have thought of that! I also have both Edward's and Dave's books.
 
Andrew - will contact you offline to arrange a visit.
 
John
michael cole18/01/2011 21:13:58
166 forum posts
Hi John
I am also building a little samson in 4 inch scale.From what I can gather any thing from it is best too loose than too tight. so I would use around 10 thou.  My bearings are currently to much on the tight side .
 
Mike
Ian S C19/01/2011 08:57:02
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On another site cast iron is recomended over softer bearings, unless the bronze, or oillite bushes are run on a hardend shaft. The person recomending cast iron says that he has run his locomotives for 30 (I think) years without changing bearings.
     Cast iron will be less effected by expansion that bronze. I agree with Mike, but would perhaps go a bit tigher than .010", maybe half that.  Ian S C

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