Propulsion Train & Shaft Line Components Introduction • Reduction Gears - fast to slow • Lubrication System - overcome friction • Shaft components - turbines to the working medium (ocean) • Propeller - transform rotational energy into thrust Reduction Gears • Purposes – Allow turbine and propeller to operate at most efficient speeds – Combine two turbines to common shaft Reduction Gears • Gear Types – Straight • excessive vibration • low power-transfer ability – Helical • Reduces vibration, quieter • Higher power transfer ability • Excessive axial thrust – Double Helical • Two sets of teeth cut at opposite angles • Eliminates axial thrust Reduction Gears • Reduction Process – Pinion (small) gear drives reduction (large) gear – Reduction ratio = turns of pinion : turns of reduction gear – Double-reduction: reduction in 2 steps (more compact design) – For naval reduction gears, normally 30:1 Reduction Gears • Locked Train – Two sets of gears and shafts – Torque transmitted equally – Increases ability to transmit torque using smaller components • Turbine shafts connected to reduction gears by flexible couplings to allow for thermal expansion Shaft Turning/Jacking Gear • Electric motor that rotates reduction gears, turbines, and shaft w/o using steam – Cool down turbines after operation – Prior to startup for even heating – Position for maintenance • Can be used to lock shaft in place – In event of casualty (i.e., loss of lube oil) Shaft Bearings • Designed to support the moving parts of: – Shaft – Turbines • Thrust bearings – Absorb axial forces – Ex: Kingsbury Thrust bearing • Radial (Journal) bearings – Absorb radial forces Lube Oil System • Provide lubrication and remove heat generated by bearings in overcoming friction • Major components: – Sump – Pump Cooler SW – Strainer Moving Parts – Cooler – Bearings Strainer Pump Sump Lube Oil System • Lube oil can be kept in service for a long time if kept pure (two methods) • Batch Purification – In-port only – Uses heated settling tank • Continuous Purification – At-sea method – Centrifugal purifier separates oil & contaminants Propulsion Shaft • Shaft is hollow: reduces weight & increases resiliency • Consists of four sections – Thrust shaft - from thrust bearing in reduction gears to end of engineroom – Line shaft - located in shaft alley (supported by line shaft bearings) – Stern shaft - part of shaft which penetrates hull (supported by Stern Tube bearings) – Propeller shaft - shaft connected to propeller (supported by Strut Bearings) Propulsion Shaft • Different sections needed for easy installation, removal, & maintenance Propeller • Made of hub and blades & creates the thrust necessary to propel the ship through the water • Terms: –Pitch: axial distance advanced during one complete revolution of screw –Face: the pressure side –Back: the suction side Propeller Types • Constant vs. Variable Pitch – Variable has the twisted look – Adv: more efficient over wide range of speeds • Fixed vs. Controllable Pitch – In controllable, blades can rotate on hub to change pitch (change direction) • Right vs. Left Hand Screw – Viewed from aft of ship – Twin-screw ships have one of each CRP System Propeller • Cavitation – Formation and subsequent collapse of bubbles as propeller turns – Occurs at critical speed • Effects – Excessive noise – Erosion of blades – Decreased efficiency Propeller Power vs. Shaft RPM • Flow a RPM; Thrust (head) a RPM2; Power a RPM3 • So, if 10% power yields 100 RPM, how much power will produce 200 RPM? 10% x% x = 10 * (200/100)3 1003 2003 = 80% power Sample Problems • Shaft hP • shp=2πNT/33,000 • Effective hP • Propulsive efficiency • Slip ratio Questions?
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