Development of a hollow axis swing cylinder for the elbow joint of 7 Function hydraulic manipulator

For a 7 Function hydraulic manipulator with a 270° rotary elbow joint, the rotary actuator´s axis must be hollow, so that an oil slipper ring could be integrated in it, and the 7 oil pipes for the wrist joints and jaw could be rotary connected. All the oil pipes could be hidden into the manipulator´s body safely at any rotary angle and any position. Swing cylinders are the only hydraulic actuators which can realize 270° rotary angle and hollow axis at the same time. In this paper, a hollow axis swing cylinder based on double screw pairs is put forward to decrease the surface leakage to 0, so that its rated pressure could be increased to normal level (3000 psi), doubles the manipulator´s full extension lift. Meanwhile, an extra axial area compensation ring is designed to decrease the unbalance axial force at high pressure, so that the friction loss, together with its control angle´s uncertainty due to the axial force on the bearings could also be decreased. Either analysis or simulations and experiments all show that, the efficiency of hollow axis swing cylinder based on double screw pairs with axial force compensation structure could be as high as 81%. Its application in hydraulic manipulator´s elbow joint shows that the build-in-axial oil distribution ring make the manipulator more compact, effect, and efficiency.