How can multiple rotational joints be locked with one mechanism?

Question

3D Positioner

Recently I used a "3D positioner" made by PMK Mess. It is pretty awesome -- basically you move it to any position, turn the locking knob, and it stays rock solid.

How might it work internally?

I want to know how it works. I would like to be able to answer the following questions.

• How can the joints be locked in place so well using one knob in the center?
• Are there canonical mechanisms that solve this sort of issue?
• What are some good resources or references for designing something like this?

It consists of 2 ball joints and 1 DOF rotational joint. Pictures of each joint are shown below. I would take it apart, but it's not mine.

Pictures of the joints

Bottom ball joint

Center 1 DOF rotational joint

Top ball joint

Answer 1

They could be locked by friction with part of the socket (circled) pressed against the ball. That force could be applied via a push rod by a cam on the knob's shaft.

The force might also be applied by a toggle mechanism, which is two linkages that snap into a straight line, producing a very high axial force from a relatively small lateral force.

The same force that locks the ball joints could also lock (by friction) the each arm onto the knob's shaft simply because that's where the reaction force to it is.