International Dimensions Help 1

[OhioAviator]

Hello All,

I'm not sure this is the right forum to ask this question, and if not, my apologies. I did post this question to the EU Standards Code Issues forum last week but didn't get any responses.

I have an outline General Arrangement drawing of a large electric motor that is manufactured by Alstom in South Africa. I do not know how to properly interpret the shaft dimensions. The shaft diameter is noted on the drawing like this....
"225,046-0,029"

The coupling bore is noted on the drawing like this...
"224,967+0,046"

Other than knowing that all dims are in millimeters, can someone clue me in on how to read these dimensions?

Thanks,

 

[IRstuff]

Aren't commas the decimal separator in Europe? So the diameter is about 8.8"


TTFN

 

[tinfoil]

I would speculate that the -0.029mm and +0.046mm values at the end of each value are 'worst-case' tolerances.

Are you sure that you didn't quote the values for the shaft and the bore backwards? I presume the shaft fits INTO the bore?

 

[tinfoil]

Sorry, I just reread the orgininal post. The values are not backwards.

 

[OhioAviator]

Thanks IRStuff & tinfoil (interesting handle, BTW)....

I know that the commas mean the same as decimal points here in the USA. However, when it comes to the tolerances, that's where I'm unsure. For example, I can come up with two ways to read the shaft dimension and tolerance...

(a) I could interpret it as a dimension with a max./min. tolerance range like this...
225.046mm, +0mm / -0.029 mm, or

(b) I could interpret it as a max./min. dimension like this...
225.046mm / 225.029mm

My gut feeling is that the first way (a) is correct. Suggestions anyone?

Thanks.

 

[Skogsgurra]

Hello over there!

This is how we interpret those diameters in Europe:

Both shaft and bore have equal nominal diameters: 225 mm (just under 9 inches).

The shaft has a tolerance range -29 through +46 microns (around -1 mil through +2 mils).

The bore has tolerance range -33 through +46 microns (roughly the same as above).

This means that in the worst case (maximum shaft and minimum bore) there will be a press fit (shaft 46 + 33 = 79 microns, around 3 mils, larger than bore). The opposite case (minimum shaft and maximum bore) will result in a loose fit (46 + 29 microns = 75 microns, also close to 3 mils).

There is a system with tolerance classes and this one seems to belong to class h7 (shaft) and H7 (bore). Talk to the manufacturer. He can tell you all you need to know.

 

[OhioAviator]

Hi Skogsqurra,

Thank you for your response. Much appreciated. I am confused though. Are you saying that you folks in Europe would read the dimensions like this?...

Shaft:
225,046-0,029 would be read as...
225 mm , with a tolerance range of -.029 mm to +.046mm

Coupling bore:
224,967+0,046 would be read as...
225 mm, with a tolerance range of -.033mm to +.046mm

Is this correct? If so, that doesn't make sense to me. That would mean that if the shaft was at minimum diameter and the coupling bore was at maximum diameter, then the coupling would be larger than the shaft by .075 mm. This can't be, since I know that the coupling is supposed to be an interference press fit.

Any suggestions/clarifications would be appreciated.

 

[Skogsgurra]

Aviator,

That is what I think (but I might have to rethink - after all I am more electric than mechanic). I agree that it sounds confusing. This link says a little bit more about it. I guess that you know about these things already, but this link is what I can offer:

http://www.roymech.co.uk/Useful_Tables/ISO_Tolerances/ISO_LIMITS.htm

 

[ischgl99]

Those dimensions look like a poor attempt at shorthand. I think he means 225.046 / 225.017 and 224.967 / 225.013

Metric dimensions are usually stated with a nominal diameter and then the letter/number code for the tolerance. The numbers he gives corresponds to the shaft is 225 m6 and the coupling is 225 K7 according to my fit table. That would make the dimensions:

225 mm +046 and +017 for the shaft and

225 -033 +013 for the coupling.

Mike Bensema

http://www.dutchmenservices.com

 

[UKpete]

I agree with mbensema's interpretation (sorry skogs; if it's any compensation I did just vote you a star in another post).

The format given on the drawing isn't one I've seen on any European drawings and it isn't ISO. The draughtsman may have intended to put the tolerance part in subscript/superscript form but maybe his software didn't allow it (assuming it was drawn on CAD), the result is just confusing.

 

[Skogsgurra]

I have to rethink. Definitely.

 

[ScottyUK]

Is it possible that the coupling is designed to be heated to allow a shrink fit onto the shaft? This might explain the seemingly odd tolerancing.





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If we learn from our mistakes,
I'm getting a great education!

 

[OhioAviator]

Thank you all. I did manage to contact the motor OEM (Alstom) in South Africa via email. They clarified the dimensions and tolerances with me and now we're on the same page. The dimensions, rewritten for us "yanks" are:
Shaft... 250.046 mm, +0 mm / -0.029 mm.
Coupling... 224.967 mm, +0.046 mm / -0 mm

As to whether this is the proper way to dimension and tolerance the motor shaft and coupling bore, I'll leave that to another day.

Thanks everyone. Much appreciated.

John Ruble
Cincinnati, Ohio