What criteria determines a "press fit"?

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n2xlr8n
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What criteria determines a "press fit"?

Post by n2xlr8n » Thu Mar 15, 2018 7:57 am

Say I have a 1.4995" O.D. diam RC20 steel shaft through a pillow bearing (RC60) and I wanted it to juuuust make it without a press-

The pillow bearing I.D. is 1.5000".

That would seem like a press fit to me. What say you all? How large must the clearance dimension be without requiring a press?
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Re: What criteria determines a "press fit"?

Post by Dave Koehler » Thu Mar 15, 2018 8:48 am

As stated you have .0005 clearance now. (1/2 thou)
This is not unlike an floating wrist pin in an OEM application.
It should slide through but you have to hit it straight on without any nicks or blems impeding the way.
Measuring this close can be tough.
If it does not go through then the hole needs a touch hone or the shaft a bit of a polish.

If you are assuming the ID of the pillow bearing is as the mfg described versus measuring that is likely where things are not computing.
I suspect that is a "nominal" size. Could be a thou either way.
More measuring is required.
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Re: What criteria determines a "press fit"?

Post by n2xlr8n » Thu Mar 15, 2018 9:22 am

Dave Koehler wrote:
Thu Mar 15, 2018 8:48 am
As stated you have .0005 clearance now. (1/2 thou)
This is not unlike an floating wrist pin in an OEM application.
It should slide through but you have to hit it straight on without any nicks or blems impeding the way.
Measuring this close can be tough.
If it does not go through then the hole needs a touch hone or the shaft a bit of a polish.

If you are assuming the ID of the pillow bearing is as the mfg described versus measuring that is likely where things are not computing.
I suspect that is a "nominal" size. Could be a thou either way.
More measuring is required.

Excellent, Dave- thank you.

Out of curiosity, would one say that a "press fit" is measured in tenths with steel alloys?
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Re: What criteria determines a "press fit"?

Post by Dave Koehler » Thu Mar 15, 2018 9:38 am

You are not dealing with press fit. You are dealing with clearance. You have to get your terminology straight.
Press fit would be something like the hole being .500 and the shaft being .501. See the diff?
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Re: What criteria determines a "press fit"?

Post by Roundybout » Thu Mar 15, 2018 10:12 am

Excellent site dealing with what your looking to do.
If you want that shaft to be removable I wouldn't do a press fit. You're looking for a clearance fit. It all depends on the materials used and what you're trying to do.
Check this site out.

https://www.engineersedge.com/general_tolerances.htm

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Re: What criteria determines a "press fit"?

Post by n2xlr8n » Thu Mar 15, 2018 10:43 am

Dave Koehler wrote:
Thu Mar 15, 2018 9:38 am
You are not dealing with press fit. You are dealing with clearance. You have to get your terminology straight.
Press fit would be something like the hole being .500 and the shaft being .501. See the diff?
Ah. Yes, I do :lol: Didn't know that- thank you.

Great site, Roundybout- thanks to you, as well.
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Re: What criteria determines a "press fit"?

Post by PackardV8 » Thu Mar 15, 2018 12:23 pm

Diameter hasn't been mentioned yet. What makes a press fit versus a push fit is the ratio of shaft diameter to interference. On a fractional diameter shaft just a tenth over hole size can be a press. On large diameter shaft, the same tenth can be a push fit.

FWIW, we have redesigned and occasionally remanufacture Packard V8 oil pumps for customers who insist on OEM. The new shafts are .500" and the gear ID .0005" under is a strong press fit. If the ID of the gear isn't sized spot-on, i.e. more toward .001" under, the shaft will distort from the effort of pressing it in.

There are formula for determining the desired interference, pressure required to assemble/disassemble and these include diameter, but that's beyond the scope of this question.

There's also heat assembly versus pressure. Back in the bad old days, SBC et al "pressed" pins were sometimes pressed into the rods as opposed to the more common heated assembly today. As part of our oil pump redesign investigations, we pressed a shaft out of an oil pump gear, heated the same gear and dropped the shaft in (think rod/piston pin assembly) When the assembly cooled, the shaft could no longer be pressed out of the gear. It bent the same jig which had pressed out the OEM shaft. The shrink changed the equation and the new fit was so tight, it effectively locked them together. Thus, beginning dimensions have to take into consideration how the interference assembly will be produced.
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Re: What criteria determines a "press fit"?

Post by n2xlr8n » Thu Mar 15, 2018 1:54 pm

Thanks, Jack.

That is good info.

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Re: What criteria determines a "press fit"?

Post by hoodeng » Fri Mar 16, 2018 11:52 pm

This is the fits and tolerances chart that sits on the lathe top for quick reference , i use it for every turning job that requires fitting to another part. The chart is in metric, but written in pen in the shaft /hole selection range is the conversion from um to inches . 5um=.0002" , 10um=.0004" ,25um=.001" ,these three conversions will cover pretty much anything you are turning or boring ,closer tolerance accuracy will require grinding or honing . um is microns ,1000um=1mm.
I work between the two measurement systems in my job ,its not hard just annoying at times.

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Re: What criteria determines a "press fit"?

Post by n2xlr8n » Tue Mar 20, 2018 9:42 am

Thank you, sir.

That was very helpful as well.

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Re: What criteria determines a "press fit"?

Post by pdq67 » Tue Aug 21, 2018 4:36 am

When I was making ballpoint pen assembly tooling at WA Schaeffer Pen, Ft. Madison, IA years ago, an old Toolmaker said that we can feel .0002" with our thumbnail and he proved it to me.. And then he went on to say that a pressed, "highlite"/"wrinkle", in a pen barrel with a threaded bushing in it was generally .0002"!! And again he proved it to me.

His set of, "mic's" would measure down this close!

pdq67

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Re: What criteria determines a "press fit"?

Post by Monza Harry » Sat Sep 08, 2018 2:53 pm

n2xlr8n, you should get an edition of "The Machinery Handbook" [older one will be fine as not many of these sort of things change(d) over time, differences will be cutting speeds and feeds as improvements in cutter tech has evolved] there are many charts dealing with "fits" [no charts on "Hissy $#!+ though :lol: ] press fits are diameter and engagement length (maybe material been a long time since I needed to look at any of that), even in the realm of standard auto sizes, like: Diff ring gears, starter ring gears, etc. sizes are going to be substantially smaller than you might first think. I don't have my "Bible" here right now [no on-line version I trusted to open either] or I would throw up a couple of examples for your entertainment. Harry

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Re: What criteria determines a "press fit"?

Post by Kevin Johnson » Sun Sep 09, 2018 1:44 am

http://theminimachineshop.com/Technical ... ndbook.pdf Full text; over 2600 pages. Year 2000 edition.

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Re: What criteria determines a "press fit"?

Post by In-Tech » Sun Sep 09, 2018 2:11 am

Wow, thanks Kevin
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Re: What criteria determines a "press fit"?

Post by Monza Harry » Tue Sep 11, 2018 9:48 pm

Yeah it is just a little light reading, :lol: I have read whole books faster than finding some things in it, but there is knowledge there that motor heads, builders, and fixers of things, could take two or more lifetimes to just touch on. Harry

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