In article ,
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John Martin wrote:

You're right, Nick - it's a key slot and has nothing to do with
expansion or contraction.

It's not that I want to be right, I want to understand why it is the way
it is. For that application, the key slot is quite odd, so I think the
usage is something different.

As John says, it's not a keyway, but just makes it easier
to clamp the collar on the shaft. The pictured 1/4" collar
is at the small end of the available range of sizes, so the
slot is wide relative to the clamping diameter which makes
the slot look wide enough to be a keyway, but I assure you
it's not.

I have 4 collars (3/16, 3/8, 1/2 and 1-1/2) in front of me
and the slot in all is approx .060 inch wide. The slot does
not extend to the far side of the ID on the 1-1/2" collar,
presumably because the larger diameter collar is flexible
enough without it.

It's worth getting familiar with these collars as they're
handy for all sorts of things. They're also available with
a threaded ID, in two pieces, hinged, threaded to match
bearing locknut threads, etc. Ruland and Stafford are two
mfrs.

http://www.ruland.com/collars.html
http://www.staffordmfg.com/

Here's some photos I posted a couple weeks ago of one way I
use them. They're also useful as hubs for attaching
components to a shaft that need adjustable timing - cams,
for example.

http://tinyurl.com/eymgc
http://tinyurl.com/dz8su
http://tinyurl.com/c7gzc

Ned Simmons