Clarifying LC Locational clearance fit H7/h6

[TorsionalStress]

The “Machinery’s Handbook” indicates that an H7/h6 is a good fit for spigots, which allows parts to be freely assembled & disassembled.

H7/h6 tolerances are as follows; “Hole diameter size” -.000” +.003”, “Shaft diameter size” +.000” -.002”

If the hole is manufactured at Hole diameter size -.000” & the shaft is manufactured at Shaft diameter size +.000”, how can there be clearance when the difference is .000” between parts?

Any response will be greatly appreciated!

 

[juergenwt]

TorsionalStress - H on h is the best fit you can get. The chance of getting a hole +000 and a shaft -000 are nil. Somewhere - even after 10 decimals or more there has to be a + for the hole (+0.0000000001) otherwise your hole would be out of tol.. Same for the shaft. Sooner or later there would have to be a - (-0.0000000001) or the shaft would be out of tol..
So much for theory. Now in practice that would not be a good idea if you want the parts to be freely assembled and disassembled. If you had a shaft 11.999 and a hole 12.001 you could have a problem with the two parts sticking therefor not "freely" moving. But they would go together!

H7/h6 is considered a "Sliding fit -fine".
For "free" assembly a G7/h6 or H7/g6 would be a better choice.

By the way - where did you get the +0.003"/-0.003" with an H/h fit?
Those would have to be some real big dia. parts. 400-500mm in dia? You sure?

 

[TorsionalStress]

Juergenwt, the tolerance values are in the “American National Standard Clearance Locational Fits ANSI B4.1-1967(R1999)” table. Nominal size range (15.75-19.69) for H7/h6. My parts are doubled in size but I haven’t seen any tables for larger sizes.

 

[KENAT]

Line fit is a clearance, since by definition it's not an interference. As juergenwt points out in practice because of anular misalignments etc. very close fits often stick a bit but the individual hole/shaft, even if line to line, is a clearance.

As to the shafts/fits etc, those shown in common tables are often just the most commonly used subset. I thought you asked a similar question before, did you not get the answer?

Posting guidelines faq731-376

http://eng-tips.com/market.cfm?

(probably not aimed specifically at you)
What is Engineering anyway: faq1088-1484​

 

[TorsionalStress]

KENAT, I tend to disagree, even if the machining is perfect, I think a line fit (size on size) is a press fit.

My question is which class to use because there’s a variance. Juergenwt indicates G7/h6 or H7/g6 but I’ve seen elsewhere to use H8/f7 for spigots? I think it’s a difficult decision since my parts are over 40 inches & the Machinery’s Handbook stops at 20 inches.

Any response will be greatly appreciated!

 

[imcjoek]

I tend to disagree, even if the machining is perfect, I think a line fit (size on size) is a press fit.

In my experience a line to line fit will be the opposite of whatever is intended.

If it was supposed to press. It will fall right through.
If it was supposed to slide, it will require hydraulic press to remove.