[Illustration: Fig. 727.--Ventilated commutator; sectional view showing air ducts. Air is frequently circulated through a commutator in order to maintain it at a sufficiently low temperature, suitable openings being provided for this purpose as shown.] Ques. What may be said with respect to excessive current? Ans. When a dynamo is overloaded, the temperature of the armature will rise to a dangerous extent, depending upon the degree to which the safe capacity of the machine is exceeded, and heavy sparking of the brushes will also result. If the overload be not removed, the insulation of the armature may be destroyed. Ques. State some causes of hot bearings. Ans. Lack of oil; presence of grit or other foreign matter in the bearings; belt too tight; armature not centred with respect to pole pieces; bearings too tight; bearings not in line; shaft rough or cut. [Illustration: Fig. 728.--Self-oiling and self-aligning bearing. The self-oiling feature consists of rings which revolve with the shaft, and feed the latter with oil continually, which they bring up from the reservoir below. The dirt settles to the bottom, and the upper portion of the oil remains sufficiently clean for a long time, after which it is drawn off, and a fresh supply poured in through holes provided in the top. These latter are often located directly over the slots in which the rings are placed, so that the bearings can be lubricated immediately by means of an oil cup if the rings fail to act or the reservoir become exhausted. The bearing is made self-aligning by providing the bearing proper with an enlarged central portion of spherical shape, held in a spherical seat formed in the pedestal by turning, milling, or by casting Babbitt or other fusible metal around it, thus allowing the bearing to adjust itself to the exact direction of the shaft. The upper half of the box can be taken off to facilitate renewal, etc., and to permit the armature to be removed.] Ques. What is the effect of hot bearings? Ans. Besides giving trouble themselves, the heat may be conducted along the armature shaft and core, thus giving rise to excessive heating of the armature. POINTS RELATING TO HOT BEARINGS 1. Use good oil; 2. See that oil cups or reservoirs are full and all oil passages clear; 3. In self-oiling and splash systems where the oil is used over again, it should be kept in clean condition by frequent straining; 4. Keep bearings clean and properly adjusted; 5. Maintain bearings in good alignment; 6. Avoid tight belts; 7. Examine the air gap or clearance between armature and pole faces and see that they are uniform. Ques. What troubles are encountered with short circuits in the armature or commutator? Ans. This results in sparking at the brushes, and in the heating of one or more of the armature coils, and even in the burning up of the latter if a bad case.