This is the Hollow Rotary Actuator model number GSN85-18K-OS with table size 85mm gear ratio 1:18 and open loop stepper motor. The OS type of GIGAGER Hollow Rotary Actuator is a line of products that combine a high rigidity hollow rotary table with a dedicated open loop stepper motor and driver package. It retains the ease of use of a stepper motor, while also allowing for highly accurate positioning of large inertia loads.