tools
I'm having trouble identifying this type of screw: it has a small hexagonal hole inset 2-3mm inside the barrel. Will a small hex key suffice or do I need something more specific?
(The view from above)
(The view from the side)
Apologies for the poor quality of my photos! For reference, this is on the camshaft of the motor of a Barnstead Vari-Mix Rocker.
Ratchets can be rather bulky as you've found. Combined with a deep socket in the wrong place, you aren't going to remove the fastener. Using a shallow socket and and proper length extension moves the ratchet out of the way to where it belongs.
Straight Drive Extension
Wobble Drive Extension
The wobble drive allows for some misalignment between what the socket is removing and the ratchet drive handle, probably what you're looking for to take care of the fact that gear is in the way.
Or you use this drive handle to convert the socket to a simple nut driver
Then there's the time honored nut driver
Similar tools are also available for automotive trim removal. They are often called "automotive trim removal tool" (go figure, eh...)
Best Answer
Just looks like a hex set screw. An appropriately sized hex key should work.