Standard tolerancing on stainless keys and keyways 1

[CristianF]

Hello all,

I am looking for some feedback regarding a request from a supplier.
One from our supplier is requesting that shaft keys and keyways be toleranced on our prints in such a way that the keyways would *always* be bigger than the keys. For instance, if standard tolerancing on stainless keys is, say, 0.250 +0.000 / -0.004, then the shaft keyway should be 0.251 +0.xx / -0.000 so that there is no possibility that the key and keyway would be the same. (We were shown a drawing where a key was 0.250 +0.000/-0.004 with a keyway of 0.250 +0.xx/-0.000 so it’s possible that *both* could be 0.250, still be in tolerance, but not fit.)

My argument is that the standard (I believe is ANSI B17.1) should be followed as we have now on our prints. Is anyone else having this issue?

Regards,
C

 

[Cockroach]

Well...think about it. The keyway must be slightly larger than the key itself if the key needs to fit into the pocket.

I typically use +0.002/-0.000 on the width and ends of the keyway, +0.000/-0.002 on the key. That gives me a total clearance of 0.004/0.000 inches on width. That way the key doesn't rattle around in the pocket when the piece is in motion.

This isn't rocket science. It is very logical and practical, which is what your vendor is suggesting.

Regards,
Cockroach

 

[CristianF]

Hi Cockroach and thank you for quick response.

Just to understand this better, do you dimension the key as 0.250 +0.000/-0.002 and keyway 0.251 +0.002/-0.000 ?

Regards,
C

 

[nickelkid]

A class 1 fit, as you have specified, has a line-to-line to .004 clearances. That has been a standard for over 40 years - nothing magic.
My questions are: Is this the only supplier making this request? Is it possible that they have keys that are out of spec?

 

[boo1]

Per USAS 517.1 - 1967
"KEY-KEYSEAT ASSEMBLIES" tolerances are a funtion on class fit, type key and size.
Class 1 - A relatively free fit applies only to parallel keys.
Class 2 - A side fit (possible interference or clearance)
Class 3 - An interference side fit.

Side to side and top to bottom fit
for up to Classs 1 square key 1/2" width
Width Tolerance
Key +0/-0.002
Seat +0.002/-0

Depth Tolerance
Key +0/-0.002
Shaft keyset +0/-0.015
Hub keyset +0.010/-0

 

[CristianF]

Hello all!

RobinHandy and boo1
From what I was told, the supplier claims that our tolerance are based on carbon steel keys and not on stainless steel keys. So far, this is the only vendor asking for this change, but as you mentioned, there is a standard for keys and keyways and what he's asking is not quite making sense... His claiming that if his going with the standard dimensions and tolerances (0.250 +0.000/-0.002 on the key and 0.250 +0.002/-0.000 on the keyway) at the max key and min keyway you have a problem and the components are not going to fit and that is why his proposing 0.250 for key but 0.251 for keyway.

 

[ewh]

I can understand his problem, as a line-to-line fit can still be an interference, albeit slight, depending on how precisely they are measured (.250005 key and .249995 slot).

“Know the rules well, so you can break them effectively.”
-Dalai Lama XIV

 

[boo1]

Class 1, 0-1/2" key width, CL of width would be +0/-0.004"
This tolerance is independent of material
O CL is a perfect fit, chill the key in dry ice

 

[hydtools]

To concur with boo1, tolerance is material independent.

Ted

 

[nickelkid]

A line-to-line fit is not an issue. If it comes across as ewh described a few passes with a flat file on the key can take care of it. Isn't the key the less costly component to match?

NEMA motor shafts are typically shipped with a pre-fit key. I am sure, that within the ASME standard, a key or two may be too tight, but a shaft will eventually come around to match if one is not will to perform fundamental mechanic/millwright tasks.

 

[Cockroach]

I think the supplier is full of it. Stainless is easy to machine, buddy needs to lighten up on his final two cuts, which is experience. So what is he saying, they want to maximize production of keys by making one pass and pulling the piece out of the machine? Absolutely ludicrous.

The print doesn't tell them how to manufacture the piece. It tells them what is needed. Up to them to get compliance to your ISO system level of inspection. Lay that one on them. You need 0.250 +0.002/-0.000 inch on the Key and 0.255 +0.000/-0.002 inch on the keyway. Done deal.

If you inspect and there is a failure to comply, bound the piece or find a different supplier who knows how to read a print and hold a tolerance. You need to be tough on these jokers, OMG, stainless steel has a different tolerance than carbon steel alloy. Now I heard it all!

Regards,
Cockroach

 

[hydtools]

Unless there is something special about the key size, buy key stock and just cut to length.

Ted

 

[geesamand]

Are they concerned about the interference fit being ruined by galling action?

David

 

[nickelkid]

geesamand brings a valid concern, especially with SS on SS. Have been around a few problems - primarily with threads - that have been a nightmare without proper lube: SS/SS and SS/DI.

 

[Cockroach]

Yeah, but this is old hat, stainless steel threads. We're actually talking about keys and keyway interfaces here. So there is little metal-to-metal transfer, galling due to high face loads or mechanical interferences. This is purely a geometrical argument and whether the vendor can supply a finished part that meets a partiular set of tolerance criteria. When the supplier suggests it is a material issue and tolerance cannot be met, then that is purely a manufacturing issue. I suspect very junior machinists who don't have a feel for material or at least are inexperienced and running speeds and feeds matching carbon steel alloys.

Regards,
Cockroach

 

[hydtools]

I don't think it matters, clearance or line-to-line. If loose, fretting or galling may occur due to movement after time in service.

Ted

 

[desertfox]

Why not ask your customer his reasoning behind the request for none standard key/keyways. Then the discussion here can be more meaningful.

Desertfox

 

[boo1]

Gates said:
"The key is responsible for preventing rotation between the shaft and the bore, and carries a portion of the torque load. Improperly fitted keys and keyways -either too tight or too loose- can result in mechanical failures. Therefore, to ensure appropriate fit, the width and height dimensions of standard key and keyways must be held to recommended tolerances."

Addition reference:

http://www.mpta.org/

MPTA B1c-2010

 

[Tmoose]

Depending on the application, the key will need help preventing micromotions between the hub and shaft. If varying side loads from belt drives, or driven by an internal combustion engine probably a LOT of help, via friction, from powerful axial clamping against a shoulder, or at the very least strong radial push from one or more aggressively tightened carefully oriented non-opposing setscrews.

disclaimer - The following links lead to images that some viewers may find disturbing. Intended for mature audiences only.
Viewer discretion advised.

http://i9.photobucket.com/albums/a53/whitfimb/ZuIFS/Crank1.jpg

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http://i417.photobucket.com/albums/pp254/whhrsfan/B-80002.jpg

 

[desertfox]

The link below shows stainless steel key stock material so its not like it isn't available but supports the contention that key tolerances are independant of material grade as stated by others.

http://www.keysandpins.com/products/keysteel

If you can find out what the clients exact cocern about the keyways is, why can maybe give better advice, also what is the application of the keys/keyways.