Patent ID: 7753800

Claim:
An expandable shaft comprising: an inner shaft having an outer peripheral surface and having a substantially rectangular cross section, at least one raceway groove being formed on the outer peripheral surface of the inner shaft to extend in an axial direction of the inner shaft, at least first and second flat portions disposed on the outer peripheral surface of the inner shaft, the at least first and second flat portions being oriented parallel to each other and having the at least one raceway groove of the inner shaft disposed thereon; a tubular outer shaft having an inner peripheral surface, a deformation promoting portion that promotes deformation of the outer shaft, a pair of opposing portions that oppose each other in a radial direction of the outer shaft, and a pair of remaining portions disposed between the pair of opposing portions in a circumferential direction of the outer shaft, the pair of opposing portions having a circular arc shape in cross section and abutting the central axis line of the outer shaft, the outer shaft surrounding the inner shaft; at least one raceway groove being formed on the inner surface of the outer shaft to oppose the raceway groove of the inner shaft and being formed on a respective inner peripheral surface of the remaining portions, the deformation promoting portion being placed in a specific region of the outer shaft in the circumferential direction between a plane including a center of curvature of the raceway groove of the outer shaft as well as a central axis line of the outer shaft and each limiting portion; plural rolling elements pinched elastically in a space between the raceway grooves of the inner shaft and the raceway grooves of the outer shaft by an elastic restoring force of the outer shaft in response to the deformation of the outer shaft, the plural rolling elements being aligned in an array along the raceway grooves in the axial direction; flat limiting portions formed on the inner peripheral surface of one of the remaining portions and on the inner peripheral surface of another one of the remaining portions, and disposed on the inner peripheral surface of the outer shaft and oriented parallel to each other, the limiting portions limiting relative rotation between the inner shaft and the outer shaft by engaging, respectively, with the corresponding flat portions.