How tight is too tight a press fit?

[JRaut]

I'm working off a set of drawings for my Edwards Radial 5 build.

The crankshaft calls for a 0.313" pin (the crank pin that the big-end bearing rides on) to be press-fit into a hole specified as having a diameter between 0.3108 and 0.3113 inches.

That equates to an interference between 1.7 and 2.2 thousandths.

My understanding is that the 'rule of thumb' for press fits is about 0.001 per inch of bore.

The specified interference is more than 6 times that rule of thumb value (6.3 thou per inch of bore as opposed to 1 thou).

Is that too much interference?

I've got a 3-ton arbor press, so I ought to be able to physically press it in, but is it going to put undue stress on the web of the crankshaft or anything like that?

 

[benmychree]

It does seem like an awful lot, an engineer could tell you if the amount of interference would exceed the elastic limit of the material, which I did once with a built up crankshaft, I shrunk fit it with too much allowance and ended up with a loose fit after it cooled off, Is the pin hard? if not it may gall when forced in the hole; if you do go with the design specs, be sure to use an anti seize like white lead.

 

[jmkasunich]

Seems like an awful tight fit to me. How did you make the hole? Reamer?

I'd be trying to figure out how to use a standard 5/16" dowel pin and suitable reamer. I know there is a groove on the end of the pin...

 

[JRaut]

Seems like an awful tight fit to me. How did you make the hole? Reamer?

I'd be trying to figure out how to use a standard 5/16" dowel pin and suitable reamer. I know there is a groove on the end of the pin...

Bingo.

I have some Rockwell C60 5/16” dowel pins on order.

I had a 0.3105” reamer on hand, so that’s the hole currently in the crankshaft, giving me an interference fit of 0.002”, which is in the meat of the specified range.

But now I’m getting nervous that it’s too tight. I guess I’ll try in a test hole first and see how it goes.

 

[jmkasunich]

I like the idea of a test hole. Reamer means the test hole and real one will be the same size.

The hardness and fine finish of the pin will really help prevent galling.

If the test hole says "too tight", you could lap the dowel pin down a bit. Drill a 5/16 hole in a piece of wood, then cut it in half thru the hole with a hacksaw or thin wood saw (jewelers saw would be perfect). Put some 600 grit wet-dry paper in there, and just squeeze the two halves together with your fingers while the pin spins in the lathe. WD40 for lube. Should be able to take off tenths at a time.

 

[JRaut]

If the test hole says "too tight", you could lap the dowel pin down a bit.

That’s a really, really good idea. I was loathe to try modifying the crankshaft, as it took some painstaking effort to get the hole in the right spot and perpendicular.

Lapping in only the portion of the dowel pin to be press fit will be just the ticket if required.

Thanks!

 

[Asm109]

Is that press fit transmitting full engine torque? If so I can believe that much press is required. I would throw the web in the oven at 500 F and press it quick.

 

[Chipper5783]

There are different "Limits and Fits" for different applications. Machinery's Handbook has an excellent presentation on this. This application may very well need a "heavy press fit". There may very well be some yielding, but it will only yield down to the elastic limit (so it will reside at the maximum squeeze). You will still be left with the consideration of how to assemble - in a press and squeeze may not be the best idea.

It is very common to use a thermal shrink - a "heavy press fit" should never be assembled using a press (IMO). As Asm109 pointed out, warm the bore, chill the shaft (liquid N2 works really, really well). Be absolutely sure of your alignment (make a jig if you have to). Do the assembly in a press, the shaft should drop in easily, but it is a good practice to hold it down with the press, as it can shrink out slightly.

 

[JRaut]

Is that press fit transmitting full engine torque? If so I can believe that much press is required.

Yes, that's a good point. I certainly don't want the pin to spin in the web of the crankshaft.

There are different "Limits and Fits" for different applications. Machinery's Handbook has an excellent presentation on this. This application may very well need a "heavy press fit". There may very well be some yielding, but it will only yield down to the elastic limit (so it will reside at the maximum squeeze). You will still be left with the consideration of how to assemble - in a press and squeeze may not be the best idea.

You're right, very good information in there. For my 3/16" bore, the 0.002 interference corresponds to the very top end of the heaviest press fit (FN5) in the book. So technically it's within the range of what an engineer may specify in real life, but barely.

As Asm109 pointed out, warm the bore, chill the shaft (liquid N2 works really, really well).

By my calculations, I figure I need a total temperature differential (heat the crank and chill the pin) of 900F to negate the 0.002 interference. Or almost 1000F if the dowel pin is 2 tenths over, which it very well may be.

That's a lot of Fs.

Liquid nitrogen is down at -321F, meaning I would have to heat the crankshaft to 600F or more for the pin to just drop in.

So:
(1) Where does one acquire liquid nitrogen...?
(2) How high do I dare take the crankshaft? I made it from 4140 Prehard and would prefer not to temper it.
(3) What happens if/when I don't get them assembled in time and it jams up? Just press it home like a gorilla? Try to drill/grind out the pin and hope the crank didn't yield too much?

 

[7milesup]

I dunno, I am no expert but that seems too tight. If I were doing it, I would expect to have a broken crank. Maybe your luck (or calculations) are better than mine. I like JMK's idea of lapping the pin.