The second recommendation was that a gripper system be installed (see #Gripper rail). The mechanical design consists of an end effector, manipulator, and gripper. The manipulator allows the gripper and end effector to navigate through their environment. The retraction of the propel cylinders repositions the gripper assembly for the next boring cycle. One of the most common type of end effector is a gripper assembly. End effectors may consist of a gripper or a tool. There is a silicone gripper which holds the foreskin in place. This provides tubing of varying sizes to be used without changing the gripper blocks. For smaller radii a circular movement of the mould and gripper stage is necessary. To advance, the machine uses a gripper system that pushes against the tunnel walls. The most known mechanical gripper can be of two, three or even five fingers. Clearly, the robot should not continue the next move until the gripper is completely closed. A gripper system was also installed on the rack railway. Most of the gripper carriages were subsequently converted to electric traction in 1935.