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.

I'm not an engineer, nor do I play one on the internet, but my guess is
that the shallow slit has something to do with compressional force
distribution. Since we cannot make the hole smaller, the 'grip' of the
collar comes from deforming the shape of the hole around the shaft.

Without a slit on the far side, all the compressional force from
tightening the screw is concentrated at the screw end. The round hole
basically deforms most on that side, and the very least on the far
side, making a teardrop shape.

But with the narrow slit, the 'teardrop shaped' deformation takes place
at the base of the slit, in the center of the collar: not enough to
close the slit down, but enough to distribute the deformation along the
sides of the hole and effectively squeeze the collar evenly on both
sides. Instead of a teardrop shape, it closes down like a clamshell
with semicircular halves.

--humunculus