We analyze static grasp stability of multiple planar objects. In our previous papers, we considered that each finger is replaced with an orthogonal virtual spring model. This means that each finger is constructed with prismatic joints. Human hands and the most developing mechanical hands are constructed with revolute joints. In this paper, we investigate the effect of fingertip rotation and revolute joints for grasping multiple objects. The stability is analyzed from the potential energy method. A grasp stiffness matrix is derived by considering not only frictional rolling contact but also frictionless sliding contact. The gravity effect of the objects is also considered. The stability is evaluated by the eigenvalues and eigenvectors of the grasp stiffness matrix. The effectiveness of our method is demonstrated through numerical examples.
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