PRINCIPLES OF NAVAL ENGINEERING 



vibration should be maintained. The turbine is 

 not ready for normal operation until vibration 

 has disappeared at all possible speeds. 



NOISES AND VIBRATION. -On steam- 

 turbine driven ships, noises may occur at low 

 speeds or when maneuvering, or when passing 

 through shallow water. Generally, these noises 

 do not result from any defect in the propulsion 

 machinery and will not occur during normal 

 operation. A rumbling sound which occurs at 

 low shaft rpm is generally due to the low pres- 

 sure turbine gearing floating through its back- 

 lash. This condition has also been experienced 

 with cruising reduction gears. The rumbling and 

 thumping noises which may occur during maneu- 

 vering or during operation in shallow water, are 

 caused by vibrations initiated by the propeller. 

 These noises referred to are characteristic only 

 of some ships and should be regarded as normal 

 sounds for these units. These sounds will dis- 

 appear with a change of propeller rpm or when 

 the other causes mentioned are no longer 

 present. These noises can usually be noticed in 

 destroyers when the ship is backing, especially 

 in choppy seas or in ground swells. 



A properly operating reduction gear has a 

 definite sound which an experienced watch- 

 stander can easily learn to recognize. At dif- 

 ferent speeds and under various operating 

 conditions, the operator should be familiar with 

 the normal operating sound of the reduction 

 gears on his ship. 



If any abnormal sounds occur, an investiga- 

 tion should be made immediately. In making an 

 investigation, much will depend on how the oper- 

 ator interprets the sound or noise. 



The lube oil temperature and pressure may 

 or may not help an operator determine the 

 reasons for the abnormal sounds. A badly wiped 

 bearing may be indicated by a rapid rise in oil 

 temperature for the individual bearing. A certain 

 sound or noise may indicate misalignment or 

 improper meshing of the gears. K unusual 

 sounds are caused by misalignment of gears or 

 foreign matter passing through the gear teeth, 

 the shaft should be stopped and a thorough in- 

 vestigation should be made before the gears are 

 operated again. 



For a wiped bearing, or any other bearing 

 casualty that has caused a very high tempera- 

 ture, this procedure should be followed: If the 

 temperature of the lube oil leaving any bearing 

 has exceeded the permissible limits, slow or 

 stop the unit and inspect the bearing for wear. 



The bearing may be wiped only a small amount 

 and the shaft may be operated at a reduced speed 

 until the tactical situation allows sufficient time 

 to inspect the bearing. 



The most common causes of vibration in a 

 main reduction gear installation are: faulty 

 alignment, bent shafting, damaged propellers, 

 and improper balance. 



A gradual increase in the vibration in a main 

 reduction gear that has been operating satisfac- 

 torily for a long period of time can usually be 

 traced to a cause outside of the reduction gears. 

 The turbine rotors, rather than the gears, are 

 more likely to be out of balance. 



When reduction gears are built, the gears 

 are carefully balanced (both statically and dy- 

 namically). A small amount of unbalance in the 

 gears will cause unusual noise, vibration, and 

 abnormal wear of bearings. 



When the ship has been damaged, vibration of 

 the main reduction gear installation may result 

 from misalignment of the turbine, the main 

 shafting, the main shaft bearings, or the main 

 reduction gear foundation. When vibration occurs 

 within the main reduction gears, damage to the 

 propeller should be one of the first things to be 

 considered. The vulnerable position of the pro- 

 pellers makes them more liable to damage than 

 other parts of the plant. Bent or broken pro- 

 peller blades will transmit vibration to the main 

 reduction gears. Propellers can also become 

 fouled with line or cable which will cause the 

 gears to vibrate. No reduction gear vibration 

 is too trivial to overlook. A complete investiga- 

 tion should be made, preferably by a shipyard. 



MAINTENANCE AND INSPECTION.-Under 

 normal conditions, major repairs and major 

 items of maintenance on main reduction gears 

 should be accomplished by a shipyard. When 

 a ship is deployed overseas and at other times 

 when shipyard facilities are not available, emer- 

 gency repairs should be accomplished, if pos- 

 sible, by a repair ship or an advanced base. 

 Inspections, checks, and minor repairs should 

 be accomplished by ship's force. 



Under normal conditions, the main reduction 

 gear bearings and gears will operate for an in- 

 definite period. If abnormal conditions occur, 

 the shipyard will normally perform the repairs. 

 Spares are carried aboard sufficient to replace 

 50 percent of the number of bearings installed 

 in the main reduction gear. Usually each bearing 

 is interchangeable for the starboard or port in- 

 stallation. The manufacturer's technical manual 



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