Abstract: A device for transmitting rotational motion, which is used, for example, in belt conveyors operating in a dusty atmosphere in coal production, mining or construction, includes a cylindrical body mounted on a shaft in bearing assemblies with an inner space being formed between the internal surface of the body and the external surface of the shaft, is equipped with a bladder which is deformable under the action of a difference in pressure, is arranged in the inner space and communicates with the atmosphere. The bladder preferably communicates with the atmosphere via a duct provided in the shaft. The unit may be equipped with a protective perforated box in which the bladder is arranged. The design of the device prevents dust from dropping into the internal space thereof, thus reducing wear of the parts of the device and increasing the service life thereof.