I am trying to design robotic joints for a SCARA arm, I am mainly missing information about the shaft and bearings components.

Design would be similar to MakerArm / Dobot M1 kickstarter projects.

Maker arm

So Z translation would be built using a ball screw, the others joint would be more "classic" (I want to avoid belt/pulley).

And so here are the multiple questions pending in my head:

  1. Why do we have motors with cylindrical shaft and others with a D shaped one? Are there other shapes? How do you chose the appropriate one?

  2. Can you confirm that I should avoid mounting parts directly on the motor shaft, and instead use a 2nd shaft? For the Z-axis, I believe the ball screw can be considered as the 2nd shaft.

  3. Concerning that 2nd shaft (if needed) which formula or abacus can I use to determine shape/diameter and whether it is hollow (how much?) or not.

    Looking at universal robots joints in this video, Inside a Universal Robots Axis, it seems they use a hollow shaft while it seems harder/longer and more expensive to manufacture, so there may be a reason? Is it related to inertia?

  4. Should I use a coupler between the two shaft that will act as a fuse?

    Because in the video linked before I don't see any couplers. Gearbox (harmonic drive in their case seems) to be the only part between motor and output shaft. If needed which coupler would you recommend (keeping backlash low), oldham or another ?

Finally about bearings

  1. For the Z joint (ball screw based) am I right, when saying that mounting methods have been "standardized" by CNC industry and available choices are between the following 4 "patterns", with standard "supporting" parts that includes the bearing?

    Ball screw mounting methods

  2. For the motor part of these linear actuator they seem to often use "basic" DC brushed (or brushless?) motors and I saw 2 different mounting designs, one where the motor is in line with the ball screw

    Linear actuator with inline motor

    The 2nd one where the motor is parallel with pulley/belt or gears to transmit torque

    Linear actuator

    Is it only a matter of space available or is one design more efficient than the other?

  3. For the other joints, how do I know how many bearings, their size/type (depends on load type?), how to mount them and where (I already guessed it goes between the motor shaft and the part to rotate).

Thanks for reading/answering (even hints would be appreciated).

  • $\begingroup$ About 3/ I found part of the answer, as I was thinking hollow shaft is more efficient for the same weight, but generally more expensive to manufacture, so it is a matter of compromise. Also found that formula (if correct) to determine diameter of the shaft : Torque(Nm) = Weight(kg) × 9.81 (N/m2) × D(Dia. Of shaft)/2 $\endgroup$
    – LiohAu
    Jan 26, 2023 at 14:04
  • 1
    $\begingroup$ Please post only one question per post, otherwise you will get partial answers and it messes with the simple Q&A mechanism of SE - which answer will be the accepted answer if multiple answers address different parts of your question..? Your question will probably be closed for being too broad. Also, any additional info should go in the question and not the comments - that is not what the comment section is for. $\endgroup$ Jan 27, 2023 at 8:44
  • $\begingroup$ Ok sorry, too late to change. I will open another post for the questions without answers. $\endgroup$
    – LiohAu
    Jan 27, 2023 at 13:01

1 Answer 1


1/ Why do we have motors with cylindrical shaft and others with a D shaped one ? Are there other shapes ? How do you chose the appropriate one ?

I find shafts with D profile are better to transfer the force and it greatly helps reduce slipping since matting a D shaft (on the motor side) with a D bore (on a pulley or wheel hub) creates a geometric lock between the matting components.

All high-torque motors come with such profiles in general. Of course, there are exceptions to everything. There are other profiles too, like a shaft with keyways and tapper wedge inserts as keys. Criteria to select the profile varies based on the application under consideration.

D-shaft is the most simplest one to manufacture and easy to assemble and disassemble with its counterparts.

2/ Can you confirm that I should avoid mounting parts directly on the motor shaft, and instead use a 2nd shaft ? For the Z-axis, I believe the ball screw can be considered as the 2nd shaft.

Mounting directly or indirectly depends on the load-bearing capacity of the motor shaft. So, once you figure out the limitations of your motor shaft that should narrow down your query. The type of coupling can also be a deciding factor. Eg: A shaft coupling using a sleeve and grub screw may be adequate if it is a low torque application. If the shaft material is weaker than the coupling material, say you are using a grub screw to lock the shaft and its counterparts, under high torque the screw can eat the shaft away resulting in a lot of backlashes or total failure of shaft-to-shaft coupling.

Is it only a matter of space available or is one design more efficient than the other ?

Both are two very different motors used for two very different applications, the first one is a stepper motor which has better-holding torque & precise control, and the second one is a high-speed in-runner brushed motor (hence all the step-down/ low gear setup to convert that high speed to high torque). For your use case, I believe you need decent torque and good precision so a servo motor or stepper should be adequate.

Hope this helps :)

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    $\begingroup$ Also, I suggest breaking down your questions and addressing them in order. It can get difficult to answer all the questions in a single post. If possible break them into two separate posts $\endgroup$
    – S10
    Jan 27, 2023 at 6:50
  • $\begingroup$ teknic.com/securing-mechanics-motor-shafts This is a good read to understand the pros and cons of different shaft locking mechanisms $\endgroup$
    – S10
    Jan 27, 2023 at 7:01
  • $\begingroup$ Thanks for this great article, basically they say coupling OR loctite, but they don't elaborate on coupling types (oldham, ...). $\endgroup$
    – LiohAu
    Jan 27, 2023 at 12:53
  • $\begingroup$ About the motors, I was thinking to use CubeMars AK series which are brushless DC motors with low backlash planetary gears, for all the joints except for the ballscrew used for Z translation. For that Z axis, motor depends on design ("direct drive" or parallel motor with belt/pulley or gears), What do you think ? $\endgroup$
    – LiohAu
    Jan 27, 2023 at 12:59
  • $\begingroup$ Oldham coupling only helps with parallel misalignment so you can check these two links to get an idea of which coupling would help you. 1. linquip.com/blog/types-of-shaft-couplings 2. ksb.com/en-global/centrifugal-pump-lexicon/article/… $\endgroup$
    – S10
    Jan 27, 2023 at 17:49

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