Tom Gardner wrote:

I have a flow control valve like this:
http://www.grainger.com/Grainger/PAR...789?Pid=search
It controls the speed of a 2x10 cylinder and the movement of the cylinder
and mechanism is making the adjustment knob move so the cylinder gets faster
and faster. It does have a set screw that will lock the adjustment knob but
the operator needs to change it often and during the stroke. Is there a
simple way to stiffen the knob without locking it? (Yes, I could get a new
one.)

I'm guessing the knob has very little friction and
turns easily.

You could put a stiff compression spring under the knob to
add some friction. There may be a packing nut under the knob
that can be tightened which will also add friction.