Ships’ Controllable Pitch Propellers: Maintenance, MSO Control, and Servo System

foreign [Music] [Music] foreign this Minesweeper is driven by controllable pitch propellers the controllable pitch propeller system is made up of the controls the servo system and the propeller assembly this film will describe two types of controls the electro hydraulic the electromechanical and the associated propeller Servo system particular emphasis will be given to maintenance troubleshooting Electro hydraulic controls are manufactured by the Norfolk Naval Shipyard and the feral Birmingham company the controls of both these manufacturers are so similar that they can be described by use of this schematic the electro-hydraulic control system consists of a double throw toggle switch a pitch indicator solenoid valves the control unit and connecting wiring and piping the signal for propeller pitch change is initiated at the Pilot House console by moving this toggle switch in the desired direction of pitch change forward for a head pitch and back for a Stern pitch the control unit is a hydraulic cylinder with a differential area piston this area being greater than this area for a head pitch this solenoid valve is electrically opened allowing oil to drain out of the forward end of the control unit cylinder high pressure oil continuously supplied to the AFT end forces the piston forward for a Stern pitch this solenoid valve is opened emitting high pressure oil to the forward end with equal oil pressure on both sides the Piston will move aft due to the higher total pressure acting on the larger area of the forward side of the piston movement of the control unit piston and the valve stem positions the control valve of the main Servo motor the servo motor in turn positions the blades in the control unit the Piston follow-up Rod by means of a rack and gear positions the Synchro generator the Synchro generator transmits blade pitch to all indicators these two valves have been shown here for Simplicity of illustration in reality the solenoid valves are normally located at some distance from the control unit and have a manual operation feature associated with the solenoid valves is the emergency control panel equipped with manually operated valves for use when necessary in place of the solenoid valves the most common symptom that indicates a need for a maintenance troubleshooting is lack of propeller pitch change in response to a signal the electrical system the trouble may be a fuse or circuit breaker in the hydraulic system the trouble may be a closed oil supply valve a loose connection or defective toggle switch might be the answer a burned out solenoid valve can be detected by first listening for the normal sound of the valve in operation if you cannot hear the valve work then check the lead wires for power with a test light if power has not been interrupted but the valve cannot be heard in operation the solenoid should be replaced the second symptom that requires maintenance troubleshooting is drifting propeller pitch an oil leak of some kind is generally the cause if a pitch is drifting ahead first examine the forward end of the pitch control unit for obvious oil leaks oil leaking from either the follow-up Rod o-ring as is the case here or from the servo cylinder end plate should be readily visible if the follow-up Rod o-ring seal is the leaked Source examine the follow-up rod for signs of scoring and replace if necessary if no leaks can be found in the pitch control unit examine all joints and fittings in the control circuit piping finally check the needle valves and solenoid valves first make sure that the ahead manual needle valve is tightly secured next open and completely close this valve this action should flush any foreign material from the valve seat if this does not eliminate drifting pitch close the ahead solenoid shutoff valve if this eliminates drifting pitch the trouble is in the solenoid valve if you examine the solenoid valve be sure to examine the valve slide and Shear seals for scoring and check the condition of all o-ring seals in the valve if the pitch still drifts with the solenoid shutoff valves closed remove and examine the head manual needle valve or wear on the valve seat and check the needle if the pitch is drifting a Stern the most probable cause is high pressure oil leaking past the Piston o-ring in the pitch control cylinder increasing pressure here on the forward side of the piston forcing it out before disaster and solenoid and needle valves by using the procedure previously described for the ahead solenoid and needle valve if the trouble is not in the stern solenoid or needle valve then disassemble the pitch control unit and replace the O-ring seal when doing this be sure to examine the cylinder liner for scoring or unusual wear replace if necessary if the propellers go to full a stirring pitch without a pitch change order and without any indication on the pitch indicator look for failure of the bearings in the pitch control unit piston when installing new bearings be sure that the correct bearings are properly installed erratic or lack of pitch indication on the remote electrical indicators is generally caused by defective synchros there are two general causes for Synchro failure slippage in the mechanical drive to the Synchro and an electrical failure within the Synchro itself electromechanical controls are manufactured by the American engineering company and the control engineering company the controls of both these manufacturers are so similar that they can be described by use of this schematic the signal for propeller pitch change is made at the Pilot House console by depressing the toggle switch in the desired direction of pitch change the proper motor contacts located in the relay box are activated as these contacts are activated the magnetic brake is simultaneously released allowing the reversible shifting motor to turn in the desired Direction the shifting motor through a gear train and Lead screw positions the yoke valve stem and valve of the main Servo motor the servo motor piston by means of the control rod and the Hub mechanism positions the propeller blades a set of Gears working off the end of the lead screw positions the Synchro generator giving an accurate and instantaneous indication of Blade pitch on all indicators a manual control is provided at the engine room control station the most common symptom that indicates a need for maintenance troubleshooting in the electromechanical control system is a lack of response to a propeller pitch change order this is often due to an electrical failure within the system checked fuses circuit breakers toggle switch electrical connections a defective motor starter may be the cause check to see that electrical power is being supplied the motor starter if power is being supplied and the starter cannot be heard in operation a tear down and inspection is required inspect the shifting motor in the same manner as the motor stocker particular attention should be given to the magnetic brake if the electrical system is not defective check the pitch control unit for binding as a last resort the engine room control linkages should be examined for binding if without an order the blades go to full a Stern pitch without the change registering on the indicators the pitch changes probably due to failure in the mechanical pitch changing mechanism of the servo control unit erratic or lack of pitch indication on the remote electrical indicators is probably due to defective synchros or connecting wiring check for an electrical failure or slippage in the mechanical drive to the Synchro propellers going to full a Stern pitch without an order and oil leaking from the pitch control unit usually indicate that the oil supply tubes have become detached due to improperly installed or faulty lock washers both the electro hydraulic and the electromechanical control systems are used with one type of Servo system the servo motor and Associated components make up the servo system these Associated components are the oil injector filters relief valve pumps strainer hydraulic tank and the various connecting Pipelines in the servo motor itself the servo motor piston positions the control rod which ultimately changes blade pitch the servo motor is always fully charged with oil high pressure oil from the pump is introduced here to drive the piston forward and here to drive it aft the high pressure hydraulic oil flows in these channels of the servo motor piston the hollow stem of the spool valve provides entry for the high pressure oil when the spool valve is moved for or aft the high pressure oil supply is open to one end of the servo motor and the low pressure return is open to the other end when the spool valve is stopped the Piston closes the ports shutting off oil flow as the propeller revolves in the water there are forces acting on the blades which tend to rotate them toward reverse pitch these forces are transmitted to the power piston and tend to force it out of position the slightest uncovering of this port however allows the high pressure oil to automatically correct and hold the Piston position and blade pitch the servo system incorporates one of two types of injectors one type for the electro hydraulic and the other for the electromechanical the injector used with the electro hydraulic system consists of two housings one is for the high pressure Inlet and the other is for the low pressure return oil the forward end of the high pressure housing is formed by the pitch control unit cylinder and piston high pressure oil is piped into the housing here the high pressure oil flows into the valve stem through three holes and back to the servo motor a gland and packing prevents the high pressure oil from leaking into the low pressure housing the return oil is collected here in the low pressure housing at the end of the shaft and piped back to the sump the oil injector used with the electromechanical system consists of a rotary seal and a low pressure return housing the rotary seal attached to the end of the valve stem is connected to the servo system piping by a flexible hose the high pressure oil flows into the valve stem through the rotary seal and back to the servo motor the return oil is collected here in the low pressure housing and piped back to some a vicar is constant pressure variable delivery pump is used to supply high pressure oil to the injector this pump maintains a constant pressure by means of a compensator that is instantaneously sensitive to pressure changes any change in pressure is immediately compensated for by changing the pump stroll which controls the volume output of the pump the pump is now operating at short stroll before an ordered pitch change a pitch change order is given and the pump stroke automatically increases to Long stroke once the pitch change has been completed the pump returns to Short Stroke a sluggish response to an ordered propeller pitch change indicates that the oil supply to the propeller Servo motor is either too small or that the pressure is too low for normal operation check the oil temperature at the pump drain line if the temperature is warmer than at the pump discharge line the pump has excessive internal leakage and should be repaired or replaced if a high temperature does not exist listen for the sound of cavitation within the pump if cavitation is heard the suction strainers in the hydraulic sump tank are probably clogged and starving the pump another probable cause of sluggishness is the oil supply pump itself check the stroke of the pump a stroke limit stop is Incorporated in the pump and should be set for a 30 degree stroke micronic full flow filters are provided in the discharge line of the hydraulic pumps the elements in these filters should be changed every 250 hours of operation internal leakage in the servo motor can cause a sluggish pitch response this leakage is usually caused by either a stored Servo motor cylinder or worn piston rings or by damaged o-ring seals between the valve body and Servo motor piston only a visual inspection of parts will determine the leak Source within the servo motor high pressure oil leaking past the V packings from the high pressure housing into the low pressure return housing can cause a sluggish pitch response this leakage is usually caused by worn V packings or scoring of the valve stem inspect these parts to determine whether they are defective and should be replaced if during a sluggish pitch change there is a drop in pump discharge pressure followed by a return to normal it is probably due to a dirty or sticking pump compensator control if this is the case remove and thoroughly clean the compensator control if a sluggish pitch response is accompanied by a general overheating of the whole hydraulic system check to see that all cross-connecting valves are closed if extensive heat is being generated by a relief valve raise its setting and readjust the pump compensator control if necessary if the cause for sluggish pitch change cannot be found in the servo system the trouble can probably be traced to the propeller hub in order to temporarily smooth out propeller pitch response adjust the pump compensator control to give the desired response be sure that steps are taken to inspect the Hub during the next overhaul the procedure for correct Hub maintenance is contained in another film in this series let us briefly re-examine the four basic symptoms that indicate maintenance is required on the control and Servo system on the MSO the first basic symptom is propeller pitch moving to fully Stern without an order complete loss of control the second major symptom is a lack of response to pitch change order the third symptom is drifting pitch this will occur on the electro hydraulic system only the fourth major symptom is a slow or sluggish pitch response troubleshooting the control and Servo system on your ship will be minimized if you recognize these basic symptoms and follow the procedures outlined in your technical manuals thank you