Rotational Displacement Apparatus With Ultra-low Torque And High Thrust Load Capability

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An improved rotational displacement apparatus is provided for measuring accurately the twist induced in an extension-twist coupled specimen in response to application of an axial load. The apparatus has an internal cylinder within which a piston rides. A piston rod extends from the piston through a pair of radial ball bearings in the base of the cylinder block to a free end outside the cylinder block. The piston is sized to be slightly smaller in diameter than the cylinder to provide a small space between the piston and the cylinder walls. A part for supplying compressed air to the region of the cylinder below the piston and a part for venting air from the region of the cylinder above the piston are provided. An optical encoder is coupled to measure rotational motion of the piston within the cylinder. In use, the apparatus is mounted in a testing machine and a specimen to be tested is secured at one end to the free end of the piston rod and at its other end to the mounting block of the testing machine. Compressed air is supplied to the cylinder causing the piston to ride up in the cylinder to apply an axial load to the test specimen. The piston rides on a cushion of compressed air and is free to rotate virtually friction free. Thus, any twist induced in the specimen as a result of the axial load causes the piston to rotate and this rotation is measured by the optical encoder.
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