Ans. A heating of the bearings is liable to be occasioned through the attractive forces developed by the center of the armature core not being parallel with the centre of the armature chamber or bore, or through the core being nearer one pole piece than the other. This may result from unequal wearing of the bearings, and therefore the bearings should either be relined or the bolt holes of the bearings readjusted, or the bearings packed up until the armature is correctly centered. Ques. What happens in case of breaks in the armature coils? Ans. This fault results in local heating of the armature, for the reason that resistance is interposed in the path of the current at the fracture. It always results in sparking at the brushes, and the heating being confined to the neighborhood of the break. Ques. What are the effects of operation above the rated voltage and below normal speed? Ans. Voltage above normal is a possible cause of heating, and operation below normal speed calls for an increase of field strength and reduces the effective ventilation, thus tending to cause heating. [Illustration: Fig. 730.--Forced system of lubrication as applied to engine of the generating set shown in fig. 443. In engines employing the forced system of lubrication the crank pit, which is formed by the columns, is accessible through doors in the front and back of the engine. The base of the engine forms an oil tank to which is attached a small plunger pump driven by an eccentric on the shaft. The lubricant is carried under pressure to the various parts of the engine by the mechanism shown in the accompanying diagram. The oil is forced by a pump to a groove in the main bearing, and a drilled hole in the shaft connects this groove with the crank pin. From the crank pin box the oil is further forced to the wrist pin through the pipe running along the side of the connecting rod. The passage in the crosshead allows the oil to be forced from the wrist pin to the guides. As the oil is forced from one bearing to another, it is quite important that the bearing caps be set tight, otherwise the oil will escape before reaching the last bearing. After passing through the bearings, the oil is collected in the base, strained and used again. The oil should be free from foreign substances, and to guard against the introduction of any foreign matter, a strainer, which may be taken out for examination or cleaning, is attached to the suction valve of the pump. An oil pressure of from 10 to 20 lbs. should be maintained, and may be regulated by adjusting the set screw on the relief valve of the oiling system. The pressure gauge need not remain in the circuit continuously. Only mineral oils should be used for lubrication. A heavy oil gives better results and prevents knocking more effectively than thin oil. An oil which has been found to give good results, consists of two-thirds red engine oil and one-third heavy cylinder oil. As the oil passes through the bearings