Axial Float in Motor

Normally, in an electric motor the drive end bearing is locked such that the axial float is zero, while the expansion of entire shaft is allowed to take place at non-driving end, hence the problem as given by you do not take place in normal circumtance. this condition is ensured when using deep groove ball bearing at drive end, while use roller bearing at non-drive end.

The axial float of 18mm is a large one. This means that the bearings are probably of journal type and having no control over the shaft axial float. If the thrust bearing is in place, at the non-driving end, than the shaft expansion will take place at the driving end and if the axial float is larger than the pin-bush type coupling flange clearance, than once again you will have same problem as before.

You have not given the motor horsepower here, so one can visualise the size of the motor and probably its construction aspects too. If the motor is of larger Horsepower than the bearings can be of journal type and with thrust bearing etc.

In the total axial floating shaft situation, like in your case (18mm), for motor when in running condition, the rotor of motor will find its own natural position because of magnetic pull on the rotor. When the motor is free without gearbox and Lobe compressor attached to it, you will be able to see the rotor's actual position.

But when you attach the gearbox and Lobe compressor to motor, this may get restricted because of relative position of each motor, gearbox and the pin-bush type coupling in between, assuming that the whole of train is mounted on a Baseplate. Also you may have to check up the situation of gearbox, Lobe compressor and the coupling in between again.