Some of the different holding mechanisms of the most common type of adapter:
1) The top one just squeezes the tube and has no additional help.
2) The top middle one features a screw that can be turned after the tube has been mounted and has a good grip. The small edge on the head of the mandrel is another means of holding the tube.
3) The lower middle one utilizes small conical and serrated ends to keep the tube firmly set.
4) The bottom one is a needle, which simply holds the tube by simple friction against the inside of the tube.