Since anyone who can restore a player piano is likely to be confronted
with an inoperative clock or music box, I thought I'd impart some of
my clock-repair wisdom on the manly art of mainspring wrestling.

Yep, mainsprings -- at least those big enough to drive a big music box
or a clock or especially a phonograph -- can be dangerous.  If it's an
open spring -- that is, one where you can see the coils of the spring
-- then the easiest way to deal with it is to clamp the winding key
into a let-down handle and while maintaining a firm grip release the
ratchet, allowing the spring to slowly wind down until it has expanded
enough to be large and floppy and thus harmless.

If the spring is inside an enclosure (clock people call it a 'barrel')
then you can let the spring down the same way, which is to say slowly.

None of this works if the spring is actually broken inside its barrel.
To get the pieces of the spring out you'll want to wear protective
gloves and maybe goggles.  Then you take the cap off the barrel, clamp
the barrel in a vise, and pull the spring out with a pair of long-nose
pliers.  Some spiritual preparation is recommended if you haven't done
this before, because upon release the spring will whang all over the
place, sowing destruction.

And be really careful with either big music boxes or phonographs, for
their springs are particularly long, tight, and sharp.  These big ones
can break into several pieces inside the barrel.  I have replaced one
(1) double phonograph spring in my clock-repair career, and it was an
adventure.  (Worse, they're packed in graphite grease.)

Mark Kinsler