Interference fit

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ChipsAhoy
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Interference fit

Post by ChipsAhoy »

I think we have pretty much come up with a good interference fit at a one thousand per inch of shaft diameter.
Now....
If I can freeze my axles, and adequately heat my wheels & hubs... what would then be a PREFERRED interference? By doing this I have reduced the mechanical resistance to mating the parts. So am I safe to assume that tighter is a better quality fit?
In other words....
Is the one thousandth chosen as a happy medium to enable the mechanical process of pressing on, or is it the best fit for the final product in the long run?
I ask these questions not just to stir discussion, but we are looking around the corner here at assembling some larger diameter wheels on some larger diameter axles. And the more information we can gleen the better decision we can make.
Scotty
pat1027
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Re: Interference fit

Post by pat1027 »

I followed the interference recommended in Machinerys Handbook which I think was .001" per inch of diameter. I packed the axles in a cooler with dry ice and the wheels in the kitchen oven turned up as high as it would go. I was just short of them dimensionally slipping together. I had to work fast but they went together pretty well with a block of wood and medium size hammer. One locked up about 1/8" from being fully seated. A hydraulic press bumped it the rest of the way in.
Russ Hanscom
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Re: Interference fit

Post by Russ Hanscom »

At a real railroad, the C&T, we have disassembled and reassembled a number of wheel sets, some with new axles, some with new wheels, and some a mix. In the 6-8" wheel seat diameter range, an interference of .001 inch per inch works well and gives a pressing pressure in the range of 50 tons. A good lubricant is essential. No heating or cooling is allowed.
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tornitore45
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Re: Interference fit

Post by tornitore45 »

Lacking a press, I use the vise for model size work. Hammering is too slow, the part can cool and keeping the axes line up is not guaranteed.
I found a heat gun work well, just enough Temp rise for the sizes I use, around 1".
Mauro Gaetano
in Austin TX
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liveaboard
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Re: Interference fit

Post by liveaboard »

I have a pretty solid bench vise.
I had the opportunity to measure the clamping force; 5 tons without a cheater bar.
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Harold_V
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Re: Interference fit

Post by Harold_V »

I can't speak for vises in general, but not all of them have jaws that are parallel. That can be troublesome when trying to assemble pieces that must be perpendicular.
It's wise to avoid using a cheater. More than one vise has been broken that way.

H
Wise people talk because they have something to say. Fools talk because they have to say something.
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ChipsAhoy
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Re: Interference fit

Post by ChipsAhoy »

And if I can us the heat/cold method, should I then increase the amount of the interference fit?
Scotty
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Bill Shields
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Re: Interference fit

Post by Bill Shields »

I have seen cases where heating a large disc with a center hole actually makes the hole smaller.

Would suggest that you heat a wheel as a trial and see how the fit changes.

A home made plug gauge would be a way to measure, unless you are confident in your use of id bore gauges QUICKLY
Too many things going on to bother listing them.
ChipMaker4130
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Re: Interference fit

Post by ChipMaker4130 »

If the standard works, why would you want to increase it? It is possible, depending on materials, to fracture a hub with a too-tight fit. If you are having problems keeping things together you need to consider a key of some sort, or welding or soldering.
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Bill Shields
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Re: Interference fit

Post by Bill Shields »

Or loctite
Too many things going on to bother listing them.
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liveaboard
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Re: Interference fit

Post by liveaboard »

Using heat difference can be tricky.
I've had the parts seize together halfway there; I had a big hammer at the ready and got it done but I wasn't really happy about it.
If the collar is really hot it can deform during assembly.

I've had to cut a ruined collar off and do it all again.

I prefer using a press and doing it cold. plenty of time to line things up, make sure it's parallel, and ensure the parts go together all the way.
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tornitore45
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Re: Interference fit

Post by tornitore45 »

I have seen cases where heating a large disc with a center hole actually makes the hole smaller.
The operating word here is "large" Unless the disk is heated uniformly the geometry does not simply scale up. The middle of the disk expands but has nowhere to go because the periphery is cooler, so the material migrates toward the center.
In extreme case the disk can buckle, kind of the reverse effect of the potato chip.
Mauro Gaetano
in Austin TX
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