Principles of naval engineering - Historic Naval Ships Association

PRINCIPLES OF NAVAL ENGINEERINGvibration should be maintained. The turbine isnot ready for normal operation until vibrationhas disappeared at all possible speeds.NOISES AND VIBRATION. -On steamturbinedriven ships, noises may occur at lowspeeds or when maneuvering, or when passingthrough shallow water. Generally, these noisesdo not result from any defect in the propulsionmachinery and will not occur during normaloperation. A rumbling sound which occurs atlow shaft rpm is generally due to the low pressureturbine gearing floating through its backlash.This condition has also been experiencedwith cruising reduction gears. The rumbling andthumping noises which may occur during maneuveringor during operation in shallow water, arecaused by vibrations initiated by the propeller.These noises referred to are characteristic onlyof some ships and should be regarded as normalsounds for these units. These sounds will disappearwith a change of propeller rpm or whenthe other causes mentioned are no longerpresent. These noises can usually be noticed indestroyers when the ship is backing, especiallyin choppy seas or in ground swells.A properly operating reduction gear has adefinite sound which an experienced watchstandercan easily learn to recognize. At differentspeeds and under various operatingconditions, the operator should be familiar withthe normal operating sound of the reductiongears on his ship.If any abnormal sounds occur, an investigationshould be made immediately. In making aninvestigation, much will depend on how the operatorinterprets the sound or noise.The lube oil temperature and pressure mayor may not help an operator determine thereasons for the abnormal sounds. A badly wipedbearing may be indicated by a rapid rise in oiltemperature for the individual bearing. A certainsound or noise may indicate misalignment orimproper meshing of the gears. K unusualsounds are caused by misalignment of gears orforeign matter passing through the gear teeth,the shaft should be stopped and a thorough investigationshould be made before the gears areoperated again.For a wiped bearing, or any other bearingcasualty that has caused a very high temperature,this procedure should be followed: If thetemperature of the lube oil leaving any bearinghas exceeded the permissible limits, slow orstop the unit and inspect the bearing for wear.The bearing may be wiped only a small amountand the shaft may be operated at a reduced speeduntil the tactical situation allows sufficient timeto inspect the bearing.The most common causes of vibration in amain reduction gear installation are: faultyalignment, bent shafting, damaged propellers,and improper balance.A gradual increase in the vibration in a mainreduction gear that has been operating satisfactorilyfor a long period of time can usually betraced to a cause outside of the reduction gears.The turbine rotors, rather than the gears, aremore likely to be out of balance.When reduction gears are built, the gearsare carefully balanced (both statically and dynamically).A small amount of unbalance in thegears will cause unusual noise, vibration, andabnormal wear of bearings.When the ship has been damaged, vibration ofthe main reduction gear installation may resultfrom misalignment of the turbine, the mainshafting, the main shaft bearings, or the mainreduction gear foundation. When vibration occurswithin the main reduction gears, damage to thepropeller should be one of the first things to beconsidered. The vulnerable position of the propellersmakes them more liable to damage thanother parts of the plant. Bent or broken propellerblades will transmit vibration to the mainreduction gears. Propellers can also becomefouled with line or cable which will cause thegears to vibrate. No reduction gear vibrationis too trivial to overlook. A complete investigationshould be made, preferably by a shipyard.MAINTENANCE AND INSPECTION.-Undernormal conditions, major repairs and majoritems of maintenance on main reduction gearsshould be accomplished by a shipyard. Whena ship is deployed overseas and at other timeswhen shipyard facilities are not available, emergencyrepairs should be accomplished, if possible,by a repair ship or an advanced base.Inspections, checks, and minor repairs shouldbe accomplished by ship's force.Under normal conditions, the main reductiongear bearings and gears will operate for an indefiniteperiod. If abnormal conditions occur,the shipyard will normally perform the repairs.Spares are carried aboard sufficient to replace50 percent of the number of bearings installedin the main reduction gear. Usually each bearingis interchangeable for the starboard or port installation.The manufacturer's technical manual106