ANSI Standard Force and Shrink Fits

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EOsteam
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ANSI Standard Force and Shrink Fits

Post by EOsteam »

My question involves how to interpret the Table for Standard Force and Shrink Fits in the 28th edition of Machinery's Handbook, Table 11, page 645.

I'm looking at a Class FN 1 fit for my steel axle to cast iron wheel. The bore in the cast iron wheel is 11/16" (.6875").

The table lists an entry for .56 - .71" and then we get some numbers that I'm not quite sure how to do the math. I get that they are allowances for acceptable clearances but how do you apply the math to the .6875" bore? 3 of my wheels came out at exactly .6875" and one is .687" I'm ready to machine the axle ends so what should I be shooting for? I'm trying to figure this table out so that I won't have to ask these basic questions here.

Thanks as always!

HJ
Carm
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Re: ANSI Standard Force and Shrink Fits

Post by Carm »

The book math is only a guideline in my experience.
Experience is a very good but stern teacher.
From your description, and from your other post, it appears the parts roll only and do not drive.
If you could make the axle and bore EXACTLY the same size (0.0000) they would not go together without thermal difference, nor would they come apart once so assembled without use of force.
If the bore is minus and the axle plus past the yield then one component upsets or cracks.
A general thumb rule is .001"/1" for interference. So a 1" axle into a .999 bore.
Your tolerance is between 0 and .000X
Yes, an X. I would be on the side of "X" closest to zero. Small numbers to measure.
The axle should be finished smooth, i.e. "polished", best if the bore is too.
Chamfer the lead of the axle. A shoulder is VERY helpful, leave the stub long if outer face is dressed flush.
Warming the wheels a couple hundred F makes assembly easier.
Rich_Carlstedt
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Re: ANSI Standard Force and Shrink Fits

Post by Rich_Carlstedt »

Steel and Cast Iron expand /contract at "5 ball 63"
That means a thermal expansion rate of .0000063 per inch, per degree F
Think of 100 degree increments which means multiplying the bore by .00063
( .00063 x .687= .0004") to get the amount of change per 100 degrees
So 500 degrees ( difference !) will get you about 2 thousandths of growth.
Remember that size for size , a thermal fit is twice as strong as a press fit

Rich
EOsteam
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Re: ANSI Standard Force and Shrink Fits

Post by EOsteam »

So .001/1 = X/.6875
Cross multiply X*1 = .0006875
Divide.0006875/1 and get
X= .0006875

Then rounding up to .0007
My goal for the axle should be no greater than .6882"
Would you recommend 400 or 500 degrees for heating for a thermal fit?
CaptonZap
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Re: ANSI Standard Force and Shrink Fits

Post by CaptonZap »

If you are going to use a heat/ cool joint, a word of advice. Make sure that the cooled axle can be pressed home with one quick motion. As soon as the two get together, heat transfer takes place, and you end up with the axle about half way in and it wont go any further without using a press.
BTDT.
The cast is weak in tension, so don't overdo the interference fit, or you may crack the wheel.

The cast iron can withstand the higher temperature, so do what ever you can handle. Literally :wink:

CZ
John Hasler
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Re: ANSI Standard Force and Shrink Fits

Post by John Hasler »

CaptonZap wrote:If you are going to use a heat/ cool joint, a word of advice. Make sure that the cooled axle can be pressed home with one quick motion. As soon as the two get together, heat transfer takes place, and you end up with the axle about half way in and it wont go any further without using a press.

CZ
It can be a good idea to use a press even though the axle should slide right in. That way if the axle heats enough to start to interfere a little bit part way in you can still get it in place as a light press fit before the axle heats up fully.
Rich_Carlstedt
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Re: ANSI Standard Force and Shrink Fits

Post by Rich_Carlstedt »

What John said is right, you want to be ready in case the fit is too tight and the axle freezes in the fit too early.
I always found it was smart to have a "persuader " available in the form of a mallet or hammer . Sometimes, a Arbor press is too slow.
Important that you have a stop on the axle ! Having a shoulder is excellent as a stop.

So put the axle in the freezer and get it to Zero degrees
Now if you heat the wheel to 500, you will have 2 thou LESS the .0007 over sized axle or .0013 clearance
That is a little close, so i would go to 700-800 on the wheel as you will lose a bit of heat in the assembly process so you have at least .002"
You will not hurt the Cast Iron even going higher
Rich
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NP317
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Re: ANSI Standard Force and Shrink Fits

Post by NP317 »

It strikes me that a more modern approach might be easier to machine and require less support equipment (freezers/ovens/presses):

Machine for a light press fit (or even a tight slide fit), lube the pieces with appropriate LocTite, then position together!
Clean up the excess LocTite, and set aside to cure.
This is a relatively forgiving method, that should work for the life of the items being joined.

There is a red "permanent" LocTite, and of course I do not remember the product number...
I have successfully used this method on steam locomotive drive wheels, which have stayed solid for the past 14 years, and hundreds of 7.5" gauge track miles..
~RN
shootnride
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Re: ANSI Standard Force and Shrink Fits

Post by shootnride »

NP317 wrote:It strikes me that a more modern approach might be easier to machine and require less support equipment (freezers/ovens/presses):

Machine for a light press fit (or even a tight slide fit), lube the pieces with appropriate LocTite, then position together!
Clean up the excess LocTite, and set aside to cure.
This is a relatively forgiving method, that should work for the life of the items being joined.

There is a red "permanent" LocTite, and of course I do not remember the product number...
I have successfully used this method on steam locomotive drive wheels, which have stayed solid for the past 14 years, and hundreds of 7.5" gauge track miles..
~RN
Loctite 609 is made specifically for this purpose.

Ted
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liveaboard
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Re: ANSI Standard Force and Shrink Fits

Post by liveaboard »

I use this web based calculator; http://www.amesweb.info/InterferenceFit ... ceFit.aspx
Or if it's not small and not critical I just go by feel for cold press fits.
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RichD
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Re: ANSI Standard Force and Shrink Fits

Post by RichD »

I did the shrink fit for iron wheels recently. Calculating the coefficient of expansion I chose 300*F for 1" dia.
Didn't work. The heat transfer is VERY fast. 500* gave enough clearance vs time to get'er in there to the shoulder. :D
RichD
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John Hasler
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Re: ANSI Standard Force and Shrink Fits

Post by John Hasler »

RichD wrote:I did the shrink fit for iron wheels recently. Calculating the coefficient of expansion I chose 300*F for 1" dia.
Didn't work. The heat transfer is VERY fast. 500* gave enough clearance vs time to get'er in there to the shoulder. :D
RichD
I once made the mistake of trying to shrink-fit a bronze bushing into a cast iron wheel. Not good. The thermal coefficient of expansion of bronze is larger than that of iron so the thin bushing heated up very quickly while the thick iron didn't cool much at all. As a result before I got the bushing in more than a quarter of the way it had heated so much that the interference was greater than it would have been at room temperature.
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