Gearing. In most cases, spur gearing is used to transmit power from the armature shaft to the car axle, although a few motors with armatures mounted directly on the car axle are in use. Various gearings other than the simple spur gear have been tried, such as worm gears, chain and bevel gears. Practically all have been abandoned in favor of the single-reduction spur gearing, which is the most satisfactory from the standpoint of wear and efficiency. This gearing is shown in Figs. [3] and [9]. The gearing is covered with a gear case ([Fig. 9]), which is usually of steel, though gear cases of thin sheet metal and wood are sometimes used. A solid gear is shown in [Fig. 11], and a split gear in [Fig. 12]. The gear ratios in common use vary from 5 to 1 to 2 to 1, the larger ratio being common on the smaller motors. A ratio often used on motors of 30 to 50 horsepower is 4.78 to 1, the gear having 67 teeth, the pinion 14 teeth.
Street car wheels are usually 33 inches in diameter. This makes necessary 612 revolutions per mile. With a gear ratio of 4.78 the armature revolves 2,925 times per mile. At 15 miles per hour, this gives 731 r.p.m.
Fig. 11. Solid Gear.
Fig. 12. Split Gear.
Lubrication. The lubrication of railway motors was for a number of years carried on almost exclusively with grease, which it was customary to place in the gear casing and in grease boxes over the armature and car-axle bearings. Grease becomes most efficient as a lubricant only when the bearing is heated sufficiently to make the grease run like oil. Oil is now being used to a considerable extent, especially for larger motors. It is fed to the bearings by various devices that allow a very slow feed, such as wicks and lubricators adjusted to pass a small amount of oil per hour.
Bearings. Railway motor bearings are usually of Babbitt metal, which metal is cast into a steel shell. This shell fits into receptacles in the motor casing, which can be seen in Figs. [3] and [9]. A steel shell is used so that the worn-out bearings can be easily renewed and the shells taken to a Babbitt melting furnace to have new Babbitt poured into them.
The motor has two sets of bearings, those for the armature and those for the axle upon which the motor is mounted. The axle bearings are always split diametrically to avoid removing a wheel when a bearing is replaced. On the later designs of motors these are of brass, no Babbitt metal being used. The armature bearings are distinguished by the terms “gear end” and “commutator end” bearings. The gear end bearing is usually of larger diameter and of greater length because of the thrust of the gears it must take in addition to the weight of the armature. This bearing is split so that it may be removed and replaced without the removal of the gear. The commutator end bearing is in one piece. Armature bearings are shown in [Fig. 13].
