Axle Alternator

The Bach-Simpson Axle Alternator is designed to translate the rotational speed of a locomotive or transit vehicle wheel and axle into an electrical signal which represents the linear ground speed of the conveyance. The Axle Alternator is mounted on the locomotive or car truck journal box by means of an adapter and is driven by the axle through a flexible connector or pegbar. The signal produced can be used by speed-actuated devices such as locomotive speed indicators, speed recorders and transition controls.

Operation
The components visible in Figure 1 which produce the electrical speed signal are a fixed magnetic pickup and a rotating spur gear. The teeth of the gear pass very close to the permanent magnet pole piece of the pickup. As each tooth 'cuts' through the flux field of the pickup, one cycle of sine-wave voltage is generated by the pickup. Since the spur gear is driven through the axle alternator shaft and flexible connector by the locomotive wheel and axle, the frequency of the electrical signal is directly related to locomotive speed. The actual frequency produced is dependent on the size of the locomotive wheel and its rate of rotation. The unit requires no external electrical power source.

Physical Design
The Bach-Simpson Axle Alternator was designed specifically for railroad and transit service. Its rugged, cast iron housing is designed to withstand the extreme vibration and shock loads present at the axle journal. The Alternator's low-profile reduces the possibility of damage from flying objects. The fact that thousands of these units have been in use for many years attests to the reliability, longevity and acceptance by the railroad industry.

Maintenance
The Axle Alternator's lightly-loaded main shaft is supported by permanently lubricated and sealed bearings. Because of the limited number of moving parts, maintenance requirements are minimal. A simple resistance check Is usually all that is necessary to test the Unit. This can be done from the cab or equipment cabinet of the locomotive. Any Axle Alternator which has been accidentally knocked off can quickly and economically be replaced.

Performance
The following operating and non-operating requirements for the Axle Alternator are established by Bach-Simpson Engineering Standards:

Signal Characteristics
The magnitude of the electrical speed signal's wave form is related to the distance between the outer diameter of the Axle Alternator spur gear and the pole piece of the magnetic pickup. It is also dependent upon the rotating speed of the gear. The graph in Figure 2 demonstrates the sine wave signal's strength (Eo ) for a series of air gaps over a frequency range of 0-1500 Hz. A similar graph in Figure 3 shows the signal magnitudes for the high output magnetic pickup used in the dual range axle alternator which is employed in those applications which require a higher output at extremely low speeds.

FIGURE 2	FIGURE 3

Signal Frequencies Tables
Table 1 lists the speed signal frequencies for Axle Alternators with 60 and 20 tooth spur gears for use on a wide range of locomotive wheel sizes.

Styles Available
Table 2 shows that a number of different axle attachment styles are available In single and dual range models for basic alternator frequencies of 60 and 20 cycles per revolution. Most locomotives are equipped with splined axles to which the Axle Alternator shaft can be attached by a flexible connector. Other locomotives have axle end covers attached by special bolts which can be used to turn the alternator shaft by means of a peg bar in contact with the bolts. One style of Axle Alternator is provided with a Plexiglass* cover, (see Table 2) which will allow an observer to check the rotation of the alternator spur gear as the locomotive is slowly moving. Plexiglass covers can be applied to other alternator styles. Consult factory. Figure 4 shows the dimensions of Axle Alternators by drive type. A. for splined axles, B. and C. for covered-end axles.
*Plexiglass is a Rohm & Hass Trademark.

Axle Drive Types

Type "A"	Type "B"	Type "C"