To pick up the blue key on Doom 64 level 5, TECH CENTER:

You can't grab the blue key and escape before the crusher
drops, and there's no way to deactivate it ahead of time.

The trick is: The crusher is hollow, like a thick, square
pipe.  As the crusher is rising, quickly but carefully step
into the square where the blue key sits.  Be sure to stop
in the exact spot where the key was.

The crusher will come down around you, then rise up again.
As it starts rising, quickly get back out the way you came.

Watch out for an ambush, via a panel that will open behind
you, when you pick up the key.

Hope this helps. :)                                      

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