The main crank-shaft bearings are more easily lubricated. Among the systems commonly used with good results may be mentioned that shown in Fig. 71, in which the half section represents a small tube starting from the bearing and terminating in the interior of an oil recess or reservoir cast integrally with the bearing-cap. This reservoir is filled up to the level of the tube opening. A piece of cotton waste held on a small iron wire is inserted in the tube, part of the cotton being allowed to hang down in the reservoir. This cotton serves as a kind of siphon and feeds the bearing by capillary attraction with a constant quantity of oil, the supply being regulated by varying the thickness of the
cotton. When the motor is stopped, the cotton should be removed in order that oil-feeding may not uselessly continue. Glass, sight-feed lubricators with stop-cocks, are very often used on crank-shafts. They are cleaner and much more easily regulated. Of all shaft-bearing lubricators, those which are most to be recommended are of the revolving-ring type (Fig. 72). They presuppose, however, bearings of large size and a special arrangement of bushings which renders their application somewhat expensive. Furthermore, the revolving-ring system can hardly be used in connection with engines of less than 20 horse-power. Since the system is applied almost exclusively to dynamo-shafts, it need not here be described in detail. As its name indicates, it consists of a metal ring having a diameter larger than that part of the shaft from which it is suspended and by which it is rotated. The lower part of the ring is immersed in an oil bath so
that a certain quantity of lubricant is continually transferred to the shaft.
Fig. 71.—Cotton-waste lubricator.
The revolving ring bearing should be fitted with a drain-cock and a glass tube in order to control the level of the oil in the bearing.
Many manufacturers have adopted lubricating devices for valve-stems, and especially for exhaust-valves. The system adopted consists of a small tube curved in any convenient direction and discharging in the stem-guide. The free end is provided with a plug. A few drops of petroleum are introduced once or twice a day.
Fig. 72.—Ring type of bearing oiler.
The lubrication of an engine entails certain difficulties which are easily overcome. One of these is the