Since the 2011 Fukushima nuclear disaster in Japan, various types of humanoid robots have been developed. Although many researchers are trying to develop the robotic technology, it is premature to deploy the robot to a real situation. The main reason is a lack of a manipulation performance. This paper suggests the new mechanism for a robot gripper. Torsion springs, an outer frame and a ballscrew are used to realize the new gripping mechanism. It is light-weight and resistant to external forces. This mechanism is useful especially for bipedal/quadrupedal transformable locomotive robots. Simulation and optimization are carried out by MATLAB 2014b. Six design parameters are chosen to minimize the required motor torque. SOLIDWORKS 2014 is used to obtain the robot hand model for the experiment. Experimental tests are done for validating the proposed robot gripper. All of details are extensively reported in the paper.