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This description of reduction gears is limited to the older type installation. Each reduction gear reduces the high main motor speed of approximately 1300 rpm to the propeller shaft speed of 280 rpm. The ratio of reduction is determined by the maximum efficiency obtainable from the propellers without loss of power at varying motor and propeller speeds.

The gears are single reduction, double helical type, a right- and left-hand helix being used to balance the fore and aft components of the tooth pressure. These helical gears produce a smoother action and avoid the tooth check of spur gears.

13A2. Description and operation. With the exception of minor differences in design, gear units produced by various manufacturers and installed on fleet type submarines today are similar. Specifications to which they are built will be found in the manufacturer's instruction book pertaining to the unit in question. The two units used on each ship are alike except that one is for port propulsion and the other for starboard propulsion. Facing aft, the port shaft rotates clockwise, and the starboard shaft rotates counterclockwise.

The main gear is pressed and keyed to the gear shaft. The aft end of the shaft is coupled to the propeller shaft. On the forward end of the main gear shaft is mounted the collar of the main thrust bearing which absorbs the propeller thrust. The gear and shaft are carried on two sleeve bearings.

The sleeve bearings consist of steel shells lined with babbitt. The bearing shells are split

Figure 13-1. Reduction gear, top case removed.

13A3. Flexible couplings between pinion and motor shafts. The couplings between the two main motor armature shafts and the pinion shafts of the reduction gear are of the enclosed flexible type. Each coupling consists essentially of two hubs with external spur gears, and two sleeves with internal spur gears. The hubs are pressed on and keyed to their respective shafts. The floating sleeves fit around the hubs so that the spur gear teeth are permanently meshed. The floating sleeves are bolted together.

This type of coupling provides longitudinal flexibility between the driving and driven shafts and thereby permits the pinion to trail the main gear. Movement of the main gear is in turn limited by the clearance in the thrust bearing. The coupling permits a small amount of misalignment of the hubs to occur without causing operational difficulties. However, it is not advisable to operate continuously with the hubs out of alignment because the coupling is not intended to function as a universal joint. Continuous operation with the hubs out of alignment will result in excessive friction and gear teeth wear, and eventually will cause a breakdown.

The couplings are lubricated by a continuous stream of oil supplied by the main motor and reduction gear lubricating oil pump. Oil enters through a nozzle and after passing between the gear teeth is discharged through holes in the sleeve.

13A4. Maintenance. a. Machinery history. It is of great importance that the machinery history contain a complete record of the installation from the time of commissioning. Complete installation data as furnished by the contractor should be entered in the machinery index by prospective engineer officers at the contractor's yard. This should include the original bearing crown thickness or bridge gage readings, bearing clearances, thrust settings and clearances, and tooth clearances (backlash and root) of the gear wheel and pinion teeth. It is essential that these data be on hand when the alignment is subsequently checked.

b. Unusual sounds. A properly operating reduction gear has a certain definite sound which the trained operator can easily recognize. The cause of any unusual noises should be investigated, and the gears should be operated with caution until the source is located and remedied.

c. Tooth contact. It is essential, for proper operation of the gears, that the total tooth pressure be uniformly distributed over the total area of the tooth faces. This is accomplished by accurate alignment, and adherence to the designed clearance limits. Alignment should be checked at the time the gear is installed, during each major overhaul, and after any casualty severe enough to threaten the alignment. Operating gears with faulty alignment are detrimental to the life and performance of the teeth. Continued quiet operation and good tooth contact are the best indications of proper tooth alignment.

d. Backlash. Backlash is measured by locking the main gear in its forward position and then moving each pinion just far enough forward and aft to make firm contact each way. The total lengthwise movement measured when doing this is the axial backlash. The backlash will increase with wear, and it can increase considerably without causing trouble. The actual longitudinal movement, as measured at the time the unit was built at the factory, should be found stamped on all pinion shafts except spares, and should be recorded in the machinery history. This measurement is the minimum allowable backlash.

e. Flexible couplings. The coupling backlash should be checked at regular intervals to see that it has not increased excessively. A dial indicator is used to measure the total backlash without dismantling the coupling. The one shaft is held stationary, and the dial indicator is mounted on the opposite or moving shaft with the indicator needle on some Dart of the coupling housing. By twisting the movable shaft back and forth without allowing the stationary shaft to move, the total backlash will be indicated on the dial indicator.

Since the condition of the bearing surfaces depends upon the axial alignment of the shafts, regular inspection should include a check to see that proper alignment is maintained. To check the alignment, the flexible coupling must first be dismantled. To accomplish this, the manufacturer's instruction book should be consulted.

f. Bearings. All of the bearing caps may be removed for bearing inspection or replacement without disturbing the gear case. The pinions are light enough so that no trouble should be experienced when rolling out the lower halves of the pinion bearings once the shaft has been raised.

When assembling, all bearing shells should be replaced in their original positions. Old cement should be cleaned off the mating surfaces of the bearing caps, end caps, and case, and a new coat of oilproof cement applied to these surfaces before reassembling. Do not permit the cement to contact the surface of the bearing. The dowel bolts should be tapped back into position before the bearing cap bolts are tightened.

Before starting the gear unit, sufficient oil should be pumped through the system by the standby pump to indicate pressures not less than 15 pounds on the two gages and to show steady flow through the thrust bearing sight flow indicator.

After starting the unit and securing the standby pump, the oil inlet temperature should not exceed 130 degrees F. Bearing temperatures should not exceed 180 degrees F, and the temperature rise should not exceed 50 degrees F. At full speed, lubricating oil pressure at the reduction gears should be at least 15 pounds. At any value above 25 percent of full speed, the pressure should not fall below 4 pounds. For continuous operation below 25 percent of full speed, the low limit pressure is 2 pounds.

Pressures and temperatures, as well as the flow through the thrust bearing flow indicator, should be observed at regular intervals during operation.

13A5. Special precautions. a. In case of churning or emulsification of the oil in the gear case, the gear must be slowed or stopped until the defect is remedied.

b. If for any reason, the supply of lubricating oil to the gears fails, the gears should be immediately stopped until the cause can be located and remedied.

c. When bearings are known to have been overheated, gears should not be operated, except in cases of extreme emergency, until bearings have been examined and the defects remedied.

d. If excessive flaking of metal from gear teeth occurs, the gears should not be adjusted, except in case of emergency, until the cause has been determined. Care should be taken, however, to prevent the entry of the metal flakes into the general lubricating system.

e. Unusual noises should be investigated at once, and the gears should be operated with caution until the cause is discovered and remedied.

f. No inspection plate, connection, fitting, or cover that permits access to the gear casing should be removed without specific authority of the engineer officer.

g. The immediate vicinity of an inspection plate joint should be kept free from paint.

h. When gear cases are open, precaution should be taken to prevent the entry of foreign matter. The openings should never be left unattended unless satisfactory temporary closures have been installed. Before replacing an inspection plate, connection, fitting, or cover, a careful inspection should be made by a responsible officer to insure that no foreign matter has entered or remains in the casing or oil lines.

i. Lifting devices should be inspected carefully before being used and should not be overloaded.

j. Naked lights should be kept away from vents while gears are in operation, as the oil vapor may be explosive.

The standby pump is placed in operation in the event of failure of one of the attached pumps, and when the propeller shaft speed is below 34 rpm. The standby system is also used to prime the main motor and reduction gear bearings after a shutdown period.

The piping on the gear unit is arranged so that the oil flow divides, part of it going to the after bearings and inboard pinion spray box, and the remainder flowing to the forward bearings, outboard pinion mesh, and the flexible couplings.

All of the gear lubricating oil drains into the lower casing and is returned to the sump through a fitting connected to the bottom of the casing. A sounding rod may be inserted into the sump tanks for checking the oil level.

A hand pump is provided for sampling the contents of the sump tanks. Before starting the machinery, samples should be taken from the tanks and examined for presence of water and dirt. When the hand pump brings up water, the pump should be operated until the water is removed. The engine should not be started until all of the water is removed. The hand pump is fitted with one suction line which takes a suction from either of the two sump tanks.

When filling the sump tanks from the filling line, the oil enters the sump tanks through the filling and transfer line. New oil may be transferred from the normal lubricating oil tank to the sump tank by means of the standby pump.

Low-pressure alarms are installed in the supply lines from the reduction gear to the main motors. The contact maker is set to close an alarm circuit when the lubricating oil pressure

13B2. Maintenance. Efficient lubrication of reduction gears is of the utmost importance. It is essential that oil at the designated working pressures and temperature be supplied to the gears at all times while they are in operation.

The proper grade of lubricating oil must be used. The oil must be so thin that the film will be squeezed from between the teeth, with resultant damage that may be beyond repair, nor so heavy that it will not flow through the restricted oil passages.

The lubricating system must be kept clean at all times. Particles of lint or dirt in the system are likely to clog the oil spray nozzles. The lubricating oil must be free from all impurities such as water, dirt, grit, and any particles of metal that may enter the system. Particular care must be taken to clean out metal flakes and fine chippings when new gears are wearing into a working fit. Magnets are fitted in lubricating oil strainers for this purpose.

The importance of taking immediate corrective measures when salt water is found in the reduction gear lubricating oil cannot be emphasized too strongly. The immediate location and sealing of the leak or removal of its source are not enough. Steps must also be taken to remove the contaminated oil from all steel parts. Several instances have occurred where, due to deferring this treatment, gears, journals, and couplings were so badly rusted and pitted that the gears had to be taken out by naval shipyard forces for reconditioning of teeth and journals. This condition can be reached in a week or less and may, result in burned-out bearings.

Frequent tests should be made to determine whether salt water is present in the oil, and the reduction gears should be inspected through the inspection plates for signs of salt water pitting. The oil level in the bottom of the gear case must not rise above the proper height predetermined for the particular installation. If the oil level is too high, the rotation of the gears will churn and aerate the oil, causing a sudden

Spray nozzles to gears should be kept open at all times. No oil spray apparatus fitted for the

The shoes are the bearing members in this type of bearing. They are supported in a manner that permits them to tilt and form a wedge shaped oil film between the shoe surface and the collar. The total end play permitted by the bearing is determined by the thickness of a spacer which rests against the end cover. This end play is fixed by the manufacturer at 0.015 to 0.030 inch.

The reduction gear oil pump supplies oil under pressure at a rate of approximately 3 gallons per minute. This quantity should be sufficient to limit the normal temperature rise between the oil inlet and outlet to about 15 degrees F. The oil pressure required is comparatively low, because the passages within the bearing are large. There are two oil inlets, one at each end of the bearing, and a single outlet as shown in Figure 13-4.

The line admitting oil to the bearing contains a needle valve that may be operated to obtain the desired flow. With the valve closed, sufficient oil will be delivered through a drilled hole in the valve seat for ordinary running conditions.

13C2. Maintenance. During normal operation, the thrust bearing will require no attention

Figure 13-4. Cross section of reduction gear thrust bearing.

except to see that the necessary circulation of clean, cool oil is maintained.

Since the bearing surfaces, when running, are completely separated by oil, there is practically no wear, and therefore, no take-up is provided except by shimming.

During the general overhaul period, the thrust bearings should be disassembled and thoroughly cleaned. Cleaning cloths that deposit lint should not be used. A coarse stone, a scraper, or a file should not be used on the collar surfaces.

13D2. Nomenclature. Terms used in describing a propeller and relative to propeller operation are as follows:

The pressure face is the after face of the propeller blade. It is customary to design the blade section by using this face for datum line. This is the driving side of the blade which pushes the water astern when the propeller is in action.

The suction face is the forward face of the blade. As this face is under a relatively low pressure, small irregularities in the surface will cause cavitation. It is therefore important that this surface be maintained fair and smooth.

Diameter of a propeller is twice the distance from the shaft center to the extreme blade trip.

Pitch is defined as the distance the blade element would move in one revolution of operating in a solid medium. Unless otherwise defined, it is the designed pitch and equals the pressure face pitch of the blade section at the .7 radius. When the leading or trailing edge of the pressure face is not a true helix, the design pitch is considered to be the pitch of that part of the section which is a true helix.

Projected area is the area of the projection of the propeller blades upon a plane normal to the shaft axis.

PA/DA represents the ratio of the projected area to the disk area.

Developed area. The helicoidal (curved) surface of a propeller blade can be represented only approximately by a plane area. The developed area therefore approximates the sum of the actual areas of the pressure faces of all of the blades. Note: For convenience all areas are measured from the maximum hub diameter. This introduces a slight error due to the fact that the hub is not cylindrical.

Mean width ratio (MWR) is the ratio of the average width of the developed blade to the diameter of the propeller.

Pitch ratio is equal to the pitch divided by the diameter of the propeller.

Cavitation. When a propeller turns at high speed, the resulting high velocity between the propeller surface and the water, augmented by surface irregularities, tends to form a vacuum adjacent to the propeller. When the absolute pressure is reduced below the vapor pressure of the fluid, vapor pockets are formed, which break the continuity of flow and reduce the efficiency of the propeller. This phenomenon is called cavitation. When the cavitation bubbles collapse on the blade surfaces due to condensation, erosion of these surfaces results.

True slip, or slip ratio, is equal to unity minus the ratio-of the speed of the water relative to the propeller in feet per minute divided by the pitch in feet times the rpm. When the speed of the ship through the water is used instead of the speed of the water relative to the propeller, the resulting slip ratio is known as apparent slip. The difference between these two velocities is due to the wake created by the ship's hull.

S = I- V_a / (rpm X pitch) where
V_a = Velocity of water relative to the propeller in feet per minute.

Propeller numbering. Every propeller has been assigned a serial number for identification and to enable the Bureau of Ships to maintain a complete history. When referring to propellers,

13D3. Propeller inspection and maintenance. a. Inspection. Whenever the vessel is in drydock, the propeller should be inspected for possible damage. If there is reason to suspect that the propeller blades have been sprung or bent, and the fact is not obvious from a visual inspection, the pitch should be checked with a pitchometer. Whenever the propeller is removed, the tail shaft and hub bore should be inspected for corrosion and fractures.

Propellers are dynamically balanced to prevent vibrations. If an inspection and test show the need of removal of metal to obtain a balance, the metal must be taken from the pressure (after) face of the blades.

If an inspection shows small pieces broken off the blades or slight cracks, repair may be possible by the hot melt process. If the breaks or cracks appear to be so serious that either blade or hub strength is affected, expert consultants should be called in to survey the damage before any attempt is made to repair. If there is any question as to the suitability of the damaged propeller, it should be replaced. Pitting or erosion found during inspection should be considered from the viewpoint of cause and the elimination of the cause if possible. Fast runs or steady runs under high power in prolonged heavy weather will sometimes erode the backs of the blades. This erosion is due to cavitation and usually appears at the tips of the blades. Erosion under these conditions cannot be prevented, but pitting or erosion at any other point on the blades is usually the result of a fault that can be eliminated.

b. Propeller blade maintenance. The casting and machining of the propellers require extreme care to maintain the relationship between the engineering calculations of proper pitch, diameter, and area and the actual physical dimensions of the propeller.

Navy propellers are invariably made of cast solid manganese bronze. Usually small propellers and frequently large propellers (up to destroyer size) are machined to their true pitch. The tolerance allowed is from 1/2 to 2 percent, the amount depending on the application.

Propeller blade surfaces must be fair and free from humps and hollows. Many different blade thicknesses are used at numerous places on the blade and each must be accurate to less than 2 percent. Inaccuracies will set up forces that cause vibrations resulting in excess noise which is not acceptable on submarine installations.

Bent blades cause hydrodynamic irregularities which will cause vibrations and sometimes severe damage to struts and bearings.

Blade fillets located near the hub should be fair and should change uniformly, decreasing near the ends. There should never be any knuckles or sharp corners for the water flow to break over. Irregular contours always result in erosion.

Blade edges and tips must be maintained as sharp and clean as called for on the propeller drawing. A propeller can vibrate in many different ways, and each vibration is associated with a definite frequency. Forces that cause vibrations of a definite frequency are sometimes the result of blunt edges on the blades near the tip, and these vibrations result in a noisy or singing propeller. Propellers are dynamically balanced to prevent vibrations when in service.

Hub taper. The hub of the propeller is accurately bored out to receive the propeller shaft. One or more keys are used to insure a tight fit and to prevent movement between the shaft and the hub. These keys must fit uniformly and snugly in both shaft and hub, and no movement should be permitted. Loose keys work back and forth and may eventually result in the loss of a propeller.

Fairwater cap. The after end of the propeller tail shaft is sealed against water by a fairwater cap which is filled with hot tallow. Some ships have a separate nut behind the fairwater cap for holding the propeller hub on the tail shaft, and some fairwater caps have the nut integral with the cap. In either case, the cap and nut must be fitted so that there is no play. The nut is kept from working loose by a locking key. The faces of the cap must be smooth and free of sharp corners or irregularities.

13D5. Routine tests and reports. a. Whenever a ship is-docked the engineer officer (of the ship) should examine the propellers, and the result of the examination entered in the engineering log and in the ship's log.

c. The stenciled hub and blade data should be verified and recorded at each drydocking. Wear of bearings, adjustments made, general conditions found, and work performed should be recorded in the machinery history.

d. At each interim and regular naval shipyard overhaul docking, the hub cap should be removed from each propeller and the propeller nut examined.

e. When repairs are made to propellers, the activity performing the work should make a detailed report, including repairs effected, condition of propeller, location and extent of defects, data stamped on hubs and blades, and pitch measurements, if taken. Applicable forms should be submitted if major repairs affecting pitch and blade dimensions have been accomplished or if the propeller was balanced dynamically.