Rotary ball spline
There is a lot of torque acting on a robotic arm. A ball spline’s forte is the transfer of torque. In addition, it can even be customized so cabling can run the length of the robotic arm by using a hollow shaft ball spline.
For greater load rating without increasing the size of the machine, you can specify a ball spline with cross rollers as opposed to balls in the rotary motion.
Source: NB Corporation of America