Electric motors made up of sliding bearings may have up to three marks on the shaft end, whereas the center mark is the indication of the magnetic center, the other two external marks registering the maximum axial movement limits that are tolerated for the rotor . The electric motor must be properly coupled so that the arrow, which is normally fixed to the bearing housing, is positioned on the center mark at the moment the engine is running.
During the starting or even in operation, the rotor can move freely between the two grooves found on the outer side, in case the driven machine exerts some extra axial effort on the motor shaft.
However, under no circumstances can the motor act with constant axial force on the bearing. The sliding bearings used by most manufacturers, are not produced for them to be able to withstand continuous axial stress. It is important to note that continuous operation of the machine within its axial clearance limits is not recommended.
When analyzing the coupling, consider the maximum axial play of the bearing, since the axial clearances of the driven machine and the coupling can influence the maximum play of the bearing.