rigid joint when not rotating?

i've been googling and reading through tutorials but can't seem to find what i would like to do, hoping for some help!

it is possible to make a rigid joint or limited range joint that can move freely when the user is pressing keys? or something that can simulate it (eg. with invisible shafts).

examples i'm thinking of are human joints that become rigid when not in motion (standing), or a way to simulate how muscles in a leg work or even something mechanical like a crane.

Possibly. You need to specify exactly what you want to happen, and then we can try to work out together if it's possible.

Eg. the crane example. I think I know what you mean by this - and I think it would be possible to achieve it using forces. But you need to specify it, preferably with a diagram or capx.

i've managed to get it to work using prevent rotation, the joints seem "loose" though. when the purple arm is rotated back and forth the green arm appears to slide around and likewise the base is moved also (even with it's density at 5).

here's my mockup crane if you wouldn't mind having a look at it:

controls are left, right, up, and down arrow for the two arms and A and D for the base.

dropbox.com/s/z7phgj7n20gpx4e/crane.capx