OPERATION AND CARE OF ELECTRIC MACHINERY 441 



too small or the speed of the commutator too high, it would 

 be impossible to obtain sparkless commutation. These 

 points are well understood by designers so that such a 

 cause of sparking is not likely to exist. If it does, the 

 operator can do nothing to remove it and the machine 

 must be turned back to the factory as being improperly 

 designed. 



12. Heating of the bearings is generally due either to 

 lack of oil, to the poor grade of oil used, or else to the dirt 

 in the bearing. The attendant should keep constant watch 

 of all bearings and if one begins to heat excessively the 

 cause should immediately be discovered and removed. If 

 this is not done, the bearing is likely to get hot enough to 

 melt the Babbitt metal lining and then the machine is very 

 likely to be injured seriously. If the Babbitt metal runs 

 out, the armature may drop enough to rub on the pole 

 faces and tear the binding wires loose etc., and the shaft 

 is likely to be scored badly where it goes through the 

 bearing. 



If the bearing begins to get too hot, it is quite probable 

 that its temperature will continue to increase until the 

 lining melts. This is due to the fact that the excessive 

 heating of the box thins the oil to such an extent that it 

 no longer affords the requisite lubrication. If a bearing is 

 discovered dangerously hot, oil (preferably heavy, such as 

 cylinder oil) should be poured liberally into the holes on 

 the top of the box, the shaft should be cooled by pouring 

 water on it close to the bearing when possible (using care 

 to keep the windings dry) and the machine should be kept 

 turning slowly. 



Water should never be poured over the box and the machine 

 should never be allowed to stop turning until the bearing has 

 cooled sufficiently to be at a safe temperature. If these 

 two precautions are not heeded the shaft is likely to " freeze " 

 in the bearing, i.e., the bearing metal solidifies tight to the 

 shaft just as solder does in a soldered joint. When this 



