Below is an expanding collet that I designed and had machined. As the nut on top is tightened, the O-rings expand outward and apply an even outward force that is well suited for use with thin wall tubes. This is a fixturing technique used in machining, but it was perfect for this application as the rubber becomes very stiff at high pressures and while there is theoretically some compliance, there should be virtually no backlash. The lack of backlash was critical as this was used in a servo controlled system. The difficulty in designing this part was that it could not damage the tube it was inserted in and it needed to use hardware from a limited catalog of parts and the axial loading elements needed to be visually inspectable while still installed.
