The starting motor for a diesel or a gasoline engine operates on the same principle as a direct current electric motor. The motor is designed to turn extremely heavy loads but tends to overheat quickly because it draws a high current (300 to 665 amperes). To avoid overheating, NEVER allow the motor to run for more than the specified amount of time. Then allow it to cool for 2 or 3 minutes before using it again. Refer to the NA VSEA technical manual for your engine for the recommended cranking and cooling periods.

The starting motor is located near the flywheel. (See fig. 10-1) The drive gear on the starter is arranged so that it can mesh with the teeth on the flywheel (or the ring gear) when the starting switch is closed. The drive mechanism has two functions: (1) to transmit the turning force to the engine when the starting motor runs and to disconnect the starting motor from the engine immediately after the engine has started and (2) to provide a gear reduction ratio between the starting motor and the engine. (The gear ratio between the driven pinion and the flywheel is usually about 15 to 1. This means that the starting motor rotates 15 times as fast as the engine, or at 1500 rpm to turn the engine at a speed of 100 rpm.)

The drive mechanism must disengage the pinion from the flywheel immediately after the engine starts. After the engine starts, the engine speed may increase rapidly to approximately 1500 rpm. If the drive pinion were to remain meshed with the flywheel and locked with the shaft of the starting motor, at a normal engine speed (1500 rpm), the shaft would spin at a rapid rate of speed (between 22,500 and 30,000 rpm). At such a rate of speed, the starting motor would be badly damaged.

Bendix Drive Mechanisms

Figure 10-2 illustrates a starting motor equipped with a Bendix drive friction-clutch mechanism. The drive mechanism moves the drive pinion so that it meshes with the ring gear on the flywheel.

The pinion of the Bendix drive is mounted on a spiral-threaded sleeve so that when the shaft of the motor turns, the threaded sleeve rotates within the pinion, moving the pinion outward, causing it to mesh with the flywheel ring gear and crank the engine. A friction clutch absorbs the sudden shock when the gear meshes with the flywheel.

As soon as the engine runs under its own power, the flywheel drives the Bendix gear at a higher speed than that at which the shaft of the starting motor is rotating. This action causes the drive pinion to rotate in the opposite direction on the shaft spiral and automatically disengages the drive pinion from the flywheel as soon as the engine starts.

Special switches are needed to carry the heavy current drawn by starting motors. Starting motors that have a Bendix drive use a heavy-duty solenoid switch (relay switch) to open and close the motor-to-battery circuit and a hand-operated starting switch to operate the solenoid switch. The starting switch is on the instrument panel and may be a push-button or a lever type. The solenoid switch (fig. 10-3) is mounted on and grounded to the starting motor housing so that the wires that must carry the heavy current required by the motor may be as short as possible to prevent voltage loss and overheating resulting from current draw. When the solenoid is energized by the starting switch, the plunger is drawn into the core and completes the circuit between the battery and the starting motor.

Operating precautions on the Bendix drive must be strictly followed. There are times when the engine may start, throw the drive pinion out of mesh, and then stop. When the engine is coming to rest, it may often rock back part of a revolution. If at that moment the pinion is engaged, the drive mechanism may be seriously damaged. There-fore, you must wait several seconds to be sure that the engine is completely stopped before you use the starting switch again. Sometimes the pinion will fail to engage immediately after the starting motor has been

Figure 10-3.-A typical solenoid switch assembly.

energized. When this happens, you will not hear the engine turning over and the starting motor will develop a high-pitched whine. You should immediately de-energize the starting motor to prevent overspeeding. An electric starting motor operating under no-load conditions can quickly overspeed and can be seriously damaged.

If the pinion is to engage and disengage freely, the sleeve and the pinion threads should be free from grease and dirt. The Bendix drive should be lubricated according to instructions in the NAVSEA technical manual.

Figure 10-2.-Cross section of a starting motor with a Bendix drive.