A structural arrangement for detecting improper positioning between a hub. The structural arrangements utilize a bore which axially penetrates a stepped-down end portion of the shaft to receive a fastening means by which the hub may be secured to the shaft. A blocking bore extends radially through the stepped-down end portion of the shaft to communicate between the bore and the exterior of the shaft. A blocking member is received within the blocking bore and is movable radially therealong. A cam flange extends radially inwardly from the hub member and presents a blocking surface and a releasing surface on the radially inner extent thereof. The blocking surface is disposed in radial opposition to the blocking bore when the hub member is improperly positioned with respect to the shaft and the releasing surface is disposed in radial opposition to the blocking bore when the hub member is properly disposed with respect to the shaft. The blocking member has a radial dimension such that the blocking member will be forced to extend within the axially oriented bore so long as the hub member is improperly positioned with respect to the shaft in order to preclude the fastening means from being secured within the bore and yet the blocking member will retract to clear the bore when the releasing surface is radially opposed to the blocking bore, as occurs when the hub is properly disposed on the shaft in order to permit the fastening means to enter the bore.