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MG Midget and Sprite Technical - one for the boffin's

Just thinking that it would make fitting a new hub to a splined shaft easier if it was heated up a bit before pressing the shaft in. How much can a rear wire wheel hub be heated up to before its ruined.
Tim Dalton

If it's as tight as all that and went on snugly when heated, would there not be a danger of never getting it off again without force?
Dominic Excell

I would assume that once its on, you wouldn't want to take it off again!

But in answer to Tim's question, when fitting a ring gear I put the ring gear in the oven at 240 degrees, and the flywheel in the freezer at around - 10. Enough for the fit to be really quite loose and give time to make sure it was aligned properly. Presumably similar temperatures would be OK for the hub.

But having offered that, I expect someone actually knows the proper answer.
Guy Weller

A rough answer to the question: how hot can you heat a piece of steel without risk of changing its material properties?

can be estimated by referring to this chart:
http://en.wikipedia.org/wiki/File:DiagrammeTTT.GIF

You can heat up to just over 200C, for as long as you want, and it will retain its original properties. When you go above that point, the change depends on how hot you got it, how long you keep it at that temperature and how you cool it.

Usually, to use that chart, you heat the steel over A3 (~740C), and then cool by various means to get the desired cooling rate (often, very hot oil).



Norm
Norm Kerr

many thanks for reply's i will try an old hub at gas mark 4 (180c approx) for an hour or two and see what sort of expansion I get.
Is there an official procedure for pressing on hubs?
Tim Dalton

The splined shaft is a TIGHT fit in the wire wheel hub. Although some have had success with an FBH, I had to use all the power of a 10 tonne press to fit the shaft! That was a new hub on an old shaft.

When looked into it at the time, general reports suggested that size press as the norm.
G Williams (Graeme)

there's a manual somewhere that states 60 ton press to remove old, and 30 ton to refit new shafts into splined hubs.
David Smith

The trouble is when you press them on it destroys the interference fit and they become loose after a few miles. Same thing happens when you remove the old hub, you can tap a new shaft into an old hub with a FBH but it won't last long. So I'm thinking along the lines Guy & Norm have suggested, shaft in the freezer for a day and the hub in the oven at less than 200c for a hour or two.
Tim Dalton

When doing this to fit a ring gear I was surprised at how slack the gear was when first dropped onto the flywheel, before it began to cool. But the narrow nature of the relatively large diameter ring would perhaps emphasise the linear expansion more than doing the same with the hub. The hole in the middle of a hub may not grow to the same extent? A clever person would be able to work out how much expansion to expect at the achievable temperatures, but given that the hub steel will expand in all directions the hole in the middle may not grow by much. But then, you don't need much, but would need to work quickly!

Fitting the ring gear was my first attempt at such a job - very satisfying when it worked so easily!
Tim, I would be interested to know how you get on.
Guy Weller

"The trouble is when you press them on it destroys the interference fit and they become loose after a few miles."
Don't understand this - how do you think the factory that made them did it?
David Smith

Exactly, I don't understand either, that's why I'm asking is there an official procedure for fitting the hubs?
Tim Dalton

This thread was discussed between 21/03/2013 and 23/03/2013

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