Hello I'm trying to figure out how modular arm joints are designed and what kind of bearings/shafts are used for a modular-type robotic arm. Take "UR arm" for example. I believe those 'T-shaped pipes' include both a drive and bearing system. And as you can see from second image, it can be detached easily. So I think it's not just a simple "motor shaft connecting to the member that we want to rotate" mechanism. I'm wondering which type of mechanism and bearing system is inside of those T-shaped pipes. How can i transfer rotational motion to a member without using shafts?

UR3 Robotic Arm

UR10 Robotic Arm Joint

  • $\begingroup$ My guess is there's a motor and a harmonic drive in each joint. A harmonic drive is a very compact kind of gearbox. However I don't have any info on that robot so I can't be sure. en.wikipedia.org/wiki/Harmonic_drive $\endgroup$
    – Andy
    Jan 29, 2016 at 8:08
  • $\begingroup$ Good answer @Andy, please make it a real answer so we can up-vote it. Comments should be reserved for helping to improve to questions and answers. $\endgroup$
    – Ben
    Jan 31, 2016 at 16:39
  • $\begingroup$ I voted to close this question because I feel it's not clear what you're asking. The question is titled, "How [do] modular arm joints work," then you ask what's in the joint, then you ask how to transfer rotational motion without a shaft. I don't know what you're asking. $\endgroup$
    – Chuck
    Feb 1, 2016 at 0:26
  • $\begingroup$ Welcome to robotics hsozbex, but I'm afraid that questions like this really aren't a good fit for a stack exchange site. We prefer practical, answerable questions based on actual problems that you face. Take a look at How to Ask and tour for more information on how stack exchange works. If you edit your question to fit our community guidelines, flag it for moderator attention and we can reopen it for you. Also, if you have two linked questions, it is better to ask two questions and link them rather than making a single robotics question. $\endgroup$
    – Mark Booth
    Feb 1, 2016 at 2:33
  • $\begingroup$ dear all i think i've found what i'm looking for which is 'Harmonic drive' i will make more research on that. Thank you. $\endgroup$
    – hsozbex
    Feb 2, 2016 at 9:12

1 Answer 1


Like Andy said, a Harmonic drive seems to be the most likely answer. But your final question is very vague. The other option is that it is a direct-drive system (with a clutch to protect the motor), and they are using a very high-torque motor. They certainly seem to have the diameter, as long as you aren't trying to lift something too heavy.

As for your last question, if you must avoid shafts while trying to achieve rotational motion, you're probably going to have to use a pneumatic/hydraulic system. Most companies avoid this - I know of only two that use hydraulics for rotational motion, leaks will be the bane of your existence. The other option is to start playing with some pretty advanced materials (say some electro-reactive ones, that change shape with electrical current)


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