Couplings of the induction type are used in some ships in the Navy. (See fig. 12-13.) These couplings use the force of magnetism to couple the outer shaft to the input shaft. The inner cage is very similar in construction to the rotor of an electric motor. When the outer member is energized through the collector rings, a magnetic field is produced which will lock in the inner rotor. An instant disconnect of the driver from the driven shaft is accomplished by de-energizing the coupling excitation circuit. The induction coupling limits maximum torque by pulling out of step when excessive torque is applied. The induction coupling also allows for a small amount of misalignment.

Figure 12-13.-Induction coupling.

In operating equipment that has induction couplings, observe the following directions and precautions:

1. Do NOT attempt to alter plant performance by changing control settings to settings other than those recommended in the NAVSEA technical manual for the equipment.

2. Ensure the coupling does not overheat because of insufficient ventilation when the fields rotate at slow speeds.

3. Ensure proper alignment is maintained (although most clutches of this type are capable of operating satisfactorily with a limited amount of misalignment).

4. Be thoroughly familiar with the means to permit the rotating members to be mechanically coupled in the event of total failure of the coupling excitation system.

5. Be thoroughly familiar with any interlocks that serve to PREVENT the following:

a. Operation at reduced excitation EX-CEPT when the prime mover is operating at specified reduced speeds.

b. Excitation of the field windings UN-LESS the throttle control is in the proper position.

c. Excitation of the field windings UN-LESS the shaft turning gear and shaft-locking devices are disengaged.

d. Excitation of the field windings at a time when the clutch would turn the driven gear counter to the direction in which it is already be-ing driven by another coupling or clutch.

BEARINGS

Bearings in main propulsion units vary in size, composition, and lubrication requirements, but their purposes are the same. Bearings guide and support rotating elements, prevent free radial movement, and limit the axial movement of these elements. Radial or journal bearings carry loads applied in a plane that is at a right angle to the axis of the shaft. Thrust bearings carry loads applied in the same direction as the axis of the shaft and restrict axial movement.

Lubrication of bearings differs according to type. Babbitt-lined bearings are lubricated by a constant flow of lubricating oil. Stern tube and strut bearings, which are lined with hardwood, phenolic, or a rubber composition, are lubricated by a constant flow of seawater. Bearings operate with a small lubricant clearance. (The lubricant clearance is the difference between the outside diameter of the journal and the inside diameter of the bearing.) This clearance must always be maintained within specified limits. With proper clearances and proper lubrication, bearings will last for many years.

The information in this training manual is of a general nature. You can find additional information in chapters 244, 9420, and 9430 of the Naval Ships Technical Manual. For details concerning a particular unit, consult the applicable NAVSEA technical manual and current MRCs.