I want to make a no (or very low) friction inline coupling for axles. The axles must turn, transmit moderate torque, and be in phase (therefore backlash must be minimal). The axles remain inline at all times. The axles do not need to slide when in use; the no-friction requirement is just for easy (dis)assembly.
The coupling must be very compact, especially in width (not greater than 2 studs wide). So I am interested in parts with no-friction axle holes.
I am aware of the following parts:
32905 Screw Gear. This has the disadvantage of being > 1 stud wide, so cannot have beams or connectors immediately adjacent.
Are there other parts with no/low-friction axle holes that could be useful for this coupling design? Or are there alternative coupling designs that meet the stated requirements?