I wonder why most robotic arms use large J2 (shoulder) motors rather than counterweights. What sort of complexity do they add to the design beside increase in overall weight? In fact, rather than using extra weight as counterweight, the J3 (elbow) motor could be shifted down past J2 and use J3 motor as counterweight, but why isn't it done?

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    $\begingroup$ the weight is probably not the issue, but the mass probably is ... i would think that the designer would want to limit inertia as much as possible $\endgroup$
    – jsotola
    Commented Jun 29, 2020 at 3:32
  • $\begingroup$ @jsotola But if the elbow motor was shifted past the shoulder motor, there won't be an increase in inertia, right? $\endgroup$
    – John M.
    Commented Jun 29, 2020 at 4:16
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    $\begingroup$ Look at the parallelogram designs, such as the Fanuc S400. Or the American Robots Merlin robots. They both took advantage of the “weight vs torque” design concept. $\endgroup$
    – SteveO
    Commented Jun 29, 2020 at 4:20

1 Answer 1


The weight of the motors is less then the weight of the gears and bearings in the joint. So moving the motors only not that advantageous. Moving the gearboxes can be done, but bearings still need to be left at the joint. If motors and gearboxes are moved, but bearings are not, then additional bearings are required for the "moved" gearboxes and some kind of transmission shaft or mechanism will also be needed, which adds further weight.

Many large robots include a hydro-pneumatic "spring" at J2 instead of a mass. This has more flexibility than a mess, since the force of this spring can be more easily adjusted (by increasing/decreasing the pressure). Usually this is set up based on the intended load of the robot.

J3 is sometimes co-axial with J2, but this is more common for older robot models or currently for palatalizing robots. enter image description here enter image description here

As for the J3 axis, moving J4-6 behind J3 is currently more popular then moving J3 behind or next to J2. Some manufacturers favour this approach, but even if all 3 motors are placed behind J3 a counterweight might still be needed, as this image shows. The black element behind the 3 motors next to J3 is a counterweight. this counterweight is optional, based on the intended load. (Please also note the hydro-pneumatic spring next to J2).

enter image description here

  • $\begingroup$ "The weight of the motors is less than the weight of the gears and bearings in the joint" - I'm not sure this is true. If you take a look at harmonic drive actuators, for example, I think you will find that the motor and gear box weigh about the same. $\endgroup$
    – guero64
    Commented Mar 4, 2022 at 15:53

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