Standard for rounded keys 5

[Mannes Karsten]

Hi,


I'm unable to find a standard concerning rounded keys.

As seen by the image, the top part of the key is also rounded. Meaning the keyway in the hub has a rounded contour instead of a rectangular contour.

I'm reverse engineering a shaft with impellers that have these rounded keyways, but I need to know what the clearance should be between the hub and the top of the key and the rest of the standards.

Anyone got the golden tip?

 

[Jboggs]

Thank you for the images. You have an impeller mounted on a shaft. The shaft rotates and drives the impeller, right? If so, that means it is transmitting torque. I have never seen an arrangement like that. It seems very unreliable to me. Maybe that's why you have not been able to find any standards. It is non-standard.

 

[Mannes Karsten]

Main transfer of torque comes from the impellers being shrink fitted onto the shaft. I'm also starting to think that there's no standard for these types of keys.

 

[SnTMan]

Almost looks like it is intended to index the impeller to the shaft. No idea why

Regards,

Mike

The problem with sloppy work is that the supply FAR EXCEEDS the demand

 

[cdill21]

You need to figure out what the key is doing there in the first place if it is not transmitting torque. Once you know what it is for, you can select the tolerance and fit you need. It sounds like you probably have done this with rectangular keys. What makes this any different?

Judging from your scan, it looks like the edges were chamfered, perhaps done by the machinist who made the key, which also makes me think that the rounded top was a custom job to fit whatever need existed at the time. Is it possible this shaft originally used a true key, but then was re-purposed to be used with a press fit? Maybe the rounded key is just for looks.

 

[MintJulep]

Perhaps it has "evolved" over the years from an original design using a circular or "Scotch" key.

 

[Jboggs]

Is it possible this key used for synchronization? Is there some feature on the shaft or another component that must be properly aligned with the impeller?

 

[GregLocock]

The proper use of a key is to provide alignment during the assembly process. Using them to transfer torque is desperate. If you can't use friction or glue then use a spline.

Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376

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

 

[Mannes Karsten]

The thought here at the office is that indeed the keys are placed for radial positioning
only.

 

[Jboggs]

"Using them to transfer torque is desperate."
Except for the fact that they work very well in trillions of properly designed industrial applications.

 

[dvd]

I think that the rounded aspect, apparently just at the shaft surface, would prevent damage to the hub if the impeller slipped. It seems like a locational detail to me, as well.

 

[GregLocock]

JBoggs, I think you are confusing 'work' with 'work very well'. Grins.

Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376

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

 

[cdill21]

Greg, Isn't that the point though? If the key works, why spend the spend the money on splines?

Chris

 

[electricpete]

I think the tendency is to rely on the key for smaller, slower, non-critical connections and rely on the fit (rather than the key) for larger (*), faster (**), more critical (***)connections.

* ANSI / AGMA standard 9002-C14 (for flexible couplings) allows clearance fit or interference fit for coupling size up to 6.5”. but requires interference fit above that. This shows the larger part.
** The faster the machine, the larger the potential adverse effect of a clearance upon the imbalance.
*** There is higher reliability if the fit transmits the torque and the key is not relied on for torque transmission (attached example key/keyway failure). And if keyway is elminated altogether in presence of an reliable interfeence fit, the keyway stress concentrators on the shaft / hub are eliminated.

Going back to the “why” of this key design, I can see that you have a miniscule increase in torque transmission capability (compared to a key cut off at the beginning of the curve) of the key itself (not taking credit for fit), with no effect on strength of the outer item / hub (the radius of the keyway outward doesn’t increase), but there is an adverse effect on the strength of the inner item / shaft since the keyway reaches further inward. And harder to manufacture and no standards.

Sorry if I'm repeating what someone else said. I'm just summarizing my limited view.

https://files.engineering.com/getfi...fd5-418e-8fc1-110c7fc20999&file=KeyDamage.pdf

 

[Compositepro]

In my experience, a tapered shaft and hub are very robust, with high torque capacity, and relative easy to assemble.

 

[Tmoose]

"Except for the fact that they work very well in trillions of properly designed industrial applications."

"properly designed" if the power transmitting loads are reversing/variable torque very often demotes the key to a relatively minor role. Or should.
Adding a radial setscrew or two is a decidedly light duty method of "locking" the assembly against torque variations.

If the key connects a pulley/sheave to a shaft the belt pull changes direction once per rev and is very effective at worrying loose a hub with even .0005" diametral clearance. This also a big problem withe bearings "locked" to commercial slip fits on shafts with set screws or eccentric collars.