Described herein are systems and devices for counterbalancing a surgical robotic system using a counterbalanced Z-axis drive. The counterbalanced Z-axis drive includes a self-centering ball screw assembly having a linear actuator and a counter weight. The counter weight is configured to substantially support a load associated with an arm of the robotic system when the driving portion is engaged or disengaged. In some embodiments, the counterbalanced Z-axis drive controls the arm and a movable effector mounted on the arm. Also disclosed herein are systems and devices for direct drive actuation of rotary axes of the arm which reduce the size and weight of the arm.
