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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:

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?

  • Can the axle be modified by adding additional parts? If you control both sides of the interface, the addition of other parts (bricklink.com/v2/catalog/catalogitem.page?P=3941#T=C and another one I can't find right now) could result in a compact joint that can transmit reasonable torques but still can be pulled apart without any resistance. – zovits Jan 29 at 13:00
  • The other piece is the palm tree top, which can be inserted into the 2x2 round brick and will turn with it thanks to the grooves around its central "trunk" holding against the studs on the 2x2. By building two of this assembly and putting it on the ends of axles, the four "fingers" on the "palm tree top" pieces can be meshed with each other to transmit rotation, but they don't have any friction lengthwise. – zovits Jan 30 at 9:21
  • Additional parts can be used, but the size constraints limit what could be used. The 2566 palm tree top is an interesting idea but would be prone to slip under load, because there is only the 3.2mm shaft connection. Looks like there would be some backlash too. – frasertweedale Jan 30 at 23:40
  • There is indeed some backlash (I'd say about 15°), but the inclusion of a 2x2 round brick eliminates the slippage problem via the four studs. – zovits Jan 31 at 6:41
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One possibility is the Technic, Gear 8 Tooth without Friction. It was specificially designed to allow axles to slide through it. It can be held by 4 other 8-tooth gears, one of which could allow for the mechanical connection:

 o 
o+* 
 o

+ This is the frictionless 8-tooth gear your axle can be inserted into
* This gear is used to transmit the torque via another, fixed axle
o These are gears pinned to the frame but connected only to the central one

The above solution does violate the 2 studs width requirement, but it still might be possible to use the gear with the fixed axle inserted by half a stud and the removable axle occupying the remaining half stud. This would require a way to fix the gear in place, since both axles would have the same amount of clutch, and depending on your definition of "moderate torque", the interface along the half stud length of gear and axle might prove insufficient.

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Another solution, which, depending on your definition of "moderate torque", could be sufficient.

Note the 1 stud long bush not being in click, in order to increase friction between the blue pin with axle and the harpoon piece, and thus the transmitted torque before the clutch power is overcome. enter image description here enter image description here This setup is quite long, a bit more than 6 studs between the ends of the axle joiners, but fits in a 1x1 cross-section. It needs constant pressure in the axial direction though, or else the harpoon head will strip the axle joiner like a Philips screwdriver.

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