I'm planning to build a table tennis robot using an Arduino controller for the location/placement logistics and control, but I'm stuck on one physical detail and don't know where else to ask. I want to be able to rotate the end of the pipe independently of the base, but I can't find any fixtures that would allow for this. To clarify what I'm looking for, think of joining two pipes together axially - typically the joint will be fixed and they won't be able to rotate in the coupling. I'm looking for a coupling that will allow one end to rotate independently of the other end with minimal friction so a small servo can perform the rotation (obviously being water tight is not a requirement :-) ). Anybody know what the terminology for such a coupling is, or an exact example of one? I'm looking for an opening that can fit a ping pong ball, so 2" inner diameter should be plenty. Thanks for any insight!

  • \$\begingroup\$ A picture or sketch would be enormously helpful! \$\endgroup\$
    – RQDQ
    Mar 11, 2011 at 19:18
  • \$\begingroup\$ did you complete the robot ? i am also thinking of making one and would like to save time, if you have already made significant progress \$\endgroup\$
    – user8868
    Mar 26, 2012 at 2:34
  • \$\begingroup\$ we once covered robotics here, but it's now considered off-topic \$\endgroup\$
    – stevenvh
    Mar 26, 2012 at 5:14
  • \$\begingroup\$ It's certainly interesting, but is it electronics or EE related? \$\endgroup\$
    – JRobert
    Mar 26, 2012 at 14:02
  • \$\begingroup\$ @r-v, I'm not done but I've made significant progress; it does work though :-) I ended up purchasing a 3D printer and then making my own outer bearing race.. kinda hard to describe, but similar in principle to how a BMX gyro/detangler. \$\endgroup\$
    – Dan
    Mar 26, 2012 at 17:35

2 Answers 2


Sintered bronze bearings (often oil impregnated) can be a useful technology, they are both thinner and cheaper than ball bearings.

Sometimes a piece of metal or plastic pipe can work as a sleeve bearing.

Depending on load and life cycle, a variety of plastics or even hardwood can be used for do it yourself bearings.

For an old-school approach, if you have a polished shaft, pouring a babbit metal around it may be an option. A newer take on that idea is epoxy-teflon composites, such as Moglice.

If you are playing around with mechanics, you should have have catalogs from the likes of small parts, msc industrial, mcmaster (probably online only), northern hydraulics, and some of the radio control model outlets kicking around for light reading - it's useful to learn about the range of items that must be manufactured to enable an industrial society, and what sub-varieties thereof are affordable from various sources. And make a point to browse your discount auto-part store and the dusty corners of anything still resembling a true hardware store in your area.

  • \$\begingroup\$ Over the weekend I came up with an idea of using two bolts on the outside of the pipes going along the entire length. On the back end of the pipe (doesn't rotate), it would go through two holes. On the front end, it would go through slotted holes, allowing the pipe to rotate on the rods. This morning I got to thinking about your suggestion of Moglice and then Delrin. Now I'm thinking of using two PVC tubes coupled together. Do you know if any of these compounds bond to PVC? (don't want them to). Is it possible to melt down Delrin and pour it into a gap? (Such that it creates a bushing?) \$\endgroup\$
    – Dan
    Mar 14, 2011 at 13:48
  • \$\begingroup\$ After doing more research I realized that PVC can have a lower melting point than Acetal/Delrin. I ended up purchasing a sintered bronze bushing from McMaster Carr - hopefully it will work well :-) Thanks for the input. \$\endgroup\$
    – Dan
    Mar 16, 2011 at 12:37
  • \$\begingroup\$ Probably you don't want to run a bushing on the threads of a bolt, but if there isn't much mechanical load you will get away with it. \$\endgroup\$ Mar 17, 2011 at 4:55
  • \$\begingroup\$ I just did a rough assembly last night and it seems to be working. The setup is 2" PVC->2" PVC coupling->Bushing->1.5" PVC. There is a little bit of slop, but not enough to concern me - I might see what other techniques I can utilize to reduce the slop later on in the project. \$\endgroup\$
    – Dan
    Mar 19, 2011 at 19:04

When you say joining two pipes, do you mean at right angles, or axially?

You could use a ball bearing, with the outer race of the bearing press-fit inside one pipe, and the second pipe press fit inside the inner race. A large linear bearing would work too (but cost a lot).

Do you have access to a lathe? A slip fit between two pieces of delrin or metal and delrin might work---all the gearing and linear bearings in some modern printers and scanners work much like this.

  • \$\begingroup\$ Axially (I updated the question to reflect that). I thought about something like a large ball bearing but I was having trouble finding the right sizes and cost was getting up there for those as well. I don't have direct access to a lathe, but I might be able to work something out (my wife went to the local engineering college)... still open for other ideas but I'll look into that a bit. \$\endgroup\$
    – Dan
    Mar 11, 2011 at 12:23

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