We have a 1500 HP 1800 rpm motor that's giving us problems with axial movement.(driving between bearing pump) We are using the Thomas (shim pack) type couplings.Motor was solo'd for mag. center (scribed) then coupling moved to correct spacing.(aligned). When motor stops it's approx. less than 1/16 from mag. center. While running motor bounces erratically about 1/16 in and out. I know when the motor is uncoupled and stopped it tends to drift inward. I'm thinking the small flex built into this coupling gives a little this is why when coupled it stops with the scribe slightly in. Question is 1.)How much is too much from mag. center. 2.) How much force is created at 1/16, 1/18 etc from mag.center. 3.) Can uneven current through each motor leg cause motor to hunt. (a little) 4.) Is there a standard for 900, 1200. 1800, 3600 rpm motors as to where how much you can be off mag. center. ????? Any info. would be greatly appreciated.

Sorry bad data. rpm is 1200.

Coupling on magnetic center is not a big deal imo. The magnetic centering forces are relatively low. The important thing is coupling up the motor so that you are close enough to mechanical center to avoid contact with motor thrust shoulder.

NEMA MG-1 provides some specs (for motors purchased to that spec which is typical in US) - for your size motor (above 450hp), endplay should be at least 1/2" and endfloat limited < 3/32. IMO you should be at most 1/16" from mechanical center and preferably right on mechanical center.

We have an 1800 rpm 2500 hp horizontal sleeve bearing motor driving double-suection single stage centrifigual pump via a shim pack coupling. When at low flow, the motor shaft moves axially almost 1/8". When at normal flow the condition goes away. The pump thrust bearing clearance is less than 0.020".... very small compared to the movement. I picture that the broadband thrust excitation created at low flow conditions (for our pump) excites the resonant frequency of a simple SDOF system where the mass is the motor rotor and the spring is the coupling. The motor shaft oscillates at 1 - 2hz which is a reasonable resonant frequency given the large mass of the rotor and low stiffness of the coupling. The only concern we would have is contact of motor against shoulder or fatigue damage of the coupling. We haven't seen any problems yet during inspections of motor bearings or couplings (fatigue cracking should show long before failure on these couplings).

ep,
Couldn't have said it better. So much concern is placed on mag center. I always as the customer how he sets mag center on a ball bearing motor! Can't! So, how do you know the rotor and stator iron are really lined up? Can't! Must not make that much of a differnce then.
As far as hunting, I have seen it caused by a motor not being level, but you nailed the oscillation cause.