PURPOSE: To increase the holding force of a core fitted to a shaft by providing a slot and a bladder in a substantially helical manner in a shaft assembly in which an expansible slender bladder is provided in a large number of slots extending in the longitudinal direction formed in the shaft under the hydraulic pressure condition.

CONSTITUTION: A large number of entangled helical slots 20 of T-shaped section are formed on an outer circumferential surface of a shaft 10 fitted with a core 12 of a sheet roll 14 having a circumferential surface 16 extending in the longitudinal direction and the axis 18 at the pitch angle 22 of about 30° or under measured from the longitudinal axis 18. An elastically expansible, slender bladder 28 of T-shaped section having a narrow portion 28a and a wide portion 28b is attached to each slot 20. The bladder 28 is integratedly provided with a void 30 extending across the wide portion 28b over both sides of the narrow portion 28a to receive the fluid such as compressed air, and when the bladder 28 is expanded by compressed air, a leaf 34 is pressed against the core 12 of the sheet roll 14 to hold it.