Will a lack of grease damage the electric motor shaft?

No. the bearings, bushings and end covers may damage. Not the shaft.

That would be a rare circumstance though it is possible. Lack of grease in a bearing will result in excessive heating of the bearing. If the motor continues to operate under such conditions the heat may tranfer to the shaft of the motor which may cause weakening of shaft. In some instances when the bearing inner race overheats it can expand more than the shaft thereby creating a small clearance on shaft. The inner race then rotates around the shaft thereby digging it. The dammage in this case would be extensive and needs to be rebuilt and machined. 

the bearings jammed but not stopped so relay will not sense ( like starting on load it has time setting ) 

check this one

and of course grease or lubrication must be checked regularly 

Regards

Not Instantly, but due course the motor shaft will get damaged, if the maintenance of motor is not carried out by checking the bearing noise level & apply sufficient grease. When grease gets dried in the bearing causing rubbing of steel balls & bearing heats up & chances of bearing getting snapped due to constanly running on load, hence damages the motor as well as shaft. 

The purpose of greasing (Lubrication) is to minimise the effects of friction by getting a layer of mulecules between two surfaces, this in effect causes a micro gap, thus minimising the effect of friction. It is well known that correct lubrication prolongs the life of any machine. Would you drive your car without oil? Certainly not. Lubrication is not just pumping grease or topping up with oil. It means doing sampling on a regular basis to test for lubrication purity and mechanincal wear. Lubrication function is a good tool to do proper predictive maintenance. So in essence, no lubrication or lack thereof will cause early deterioration of any machine. An electric motor shaft/bearing already is heated by eddy currents and would expand with heat, lack of lubrication would then cause bearing failure, and secondary to that cause shaft damage.

Automatically NO, but gradually yes because any motor machines has an exact amount grease or oil to be used. Maintenance of lubricants is very important in motor shaft.

In modern day industry like Combined cycle Gas Power Plant are very cleaned plant, where ash % is very less & less amount of fluids because of High calorific value. There motor requires less grease for PM. If PM schedule is followed strictly we are safe or the motor.

But in coal fired boiler or STG's, where ash % is more there the motor requires regular monthly greasing schedule. In that dusty atmosphere if greasing schedule disturbed then it may cause trouble to the running motors like in CHP we require Higher greasing / Higher cleaning schedule otherwise motor may burn. Or may damage motor shaft due to high vibration. 

Say Belt driven Impact crusher motor running continuously for a longer time in a dirty / in polluted area & with less grease. Micro-fine dust will enter inside the motor/bearing, Grease will become hard, now slowly slowly motor lubrication will be weaker. Results motor heavy vibration & noise. If PM is not done in time, vibration will transfer to the rotating side,Due to heavy load in the coupling belt it may damage the Motor stator, rotor shaft & it  as may damage the Crusher part also.

Lack of grease will result in developing heat & very rarely can damage the shaft.

Impotent to check periodical noise checkups & grease applying.

Need to be done periodic bearing change. 

Also need to follow up periodic records & on date bearing replacement.

It's impotent to do PPM on time.

dear all

first thank you for participants

 i had seen many motor with lack of grease or more grease i didn't see any motor the shaft is melted we are talking about shaft diameter 120mm (12cm) the temperature required to melted the Carbon steel is more 1800C

let us discussion the probability of unusual cases

1- i think what if there are current drain from the Squirrel cage through the shaft to the earth it will be working as welding machine is this possible 

the current in squirrel cage bars is very high with 0 voltage and low frequency

The lack of grease can lead to damage to bearings, seizure of shaft leading to damage to end sheilds etc.

It cannot lead to melting of shaft.

The rotor can get heated up in case of unbalance current or single phasing. Thsi can be detected by negative phase current measurement by relay and the protection settings for negative phase sequence in the relay.