COC ORAL EXAM PREPARATION (PART- 11): PROPULSION MACHINERY

Marine-propulsion-systemImage Credit: Wikipedia.org

FUEL INJECTION TIMING

Checking of fuel injection pump timing:

  1. Turn 1 piston to TDC at the beginning of firing stroke.
  2. Turn backward to a point, a little earlier than fuel injection point.
  3. Shut fuel supply to engine, remove No. 1 fuel pump delivery valve assembly and put a bent pipe.
  4. Open fuel supply and when fuel lever is put to running position, oil will flow out at bent pipe.
  5. Turn engine towards TDC in its running direction slowly until fuel cease to flow.
  6. Check the marks on flywheel whether timing position is correct or not.

 Slight difference can be adjusted by:

For large engine:

    •  Timing can be altered by shifting the camshaft to the position relative to crankshaft, after removing the idler gear between crankshaft and camshaft.
    • Timing can be altered by individual fuel pump cam for adjustable cam type engine.

 

For small engine:

  • Adding or reducing shims on pump base.
  • Turning the plunger up & down adjustment screw on pump roller guide.
  • Shifting the coupling flanges between pump and drive side of the engine.

Crankshaft:

  1. Device for converting reciprocating motion of piston, driven by expansion of gases, to rotating motion.
  2. Power produced by engine is taken off the crankshaft by transmission.

Stresses in Crankshaft:

  1. Bending of crank pin, causes tensile, compressive and shear stresses. (Due to gas load)
  2. Twisting moment of journal, causes shear stress.
  3. Compressive stresses set up in journals and pins. ( Due to shrink-fit)
  4. Tensile stresses set up in webs. ( Due to shrink-fit)
  5. Torsional stresses due to power transmission fluctuate widely. ( In heavy sea)
  6. Shock loading on crank pin. ( Sudden fluctuation of engine speed )

Types of crankshaft:

  • Solid forged
  • Semi-built
  • Fully-built
  • Welded crankshaft.

In large marine engine which type is used and why?

Usually, Semi-built is used because:

  • Only one shrink-fit between web and journal as less chance of slippage.
  • Can get grain flow in way of web and pin.
  • Webs are smaller as there is no shrink-fit.
  • Can be repaired section by section when damage occurred.

Welded Crankshaft:

  • Die-forged crankthrow, consisting of thin webs and crankpin in one piece, having half a main journal on each side.
  • Welds are placed and welded at the middle of all main journals, to make complete crankshaft.
  • High technology Narrow gap welding process applied.

Fully-built Crankshaft manufacturing:

  1. Raw material melted in Cupola Furnace.
  2. Refined to remove impurities, by decarburising, controlling carbon amount and soaking time.
  3. Degassed in Vacuum Furnace, to remove H₂ and N₂.
  4. Molten metal is then poured into prepared mould.
  5. Removed from mould, after slow cooling, and casting is rough-machined.
  6. Normalised to improve grain structure, and tempered to remove stresses.
  7. Rough-machined to final dimensions.
  8. Cold roll the crank pin fillets, to increase bending and corrosion fatigue
  9. Finish machining.
  10. Shrink-fitting process [Shrinkage allowance: 1/570 to 1/660 of journal diameter.]
  11. Set upon a large lathe, and journals checked for throw, and throw errors machined out.

When to take Crankshaft Deflection:

  1. At initial installation and after 1000 running hours.
  2. At subsequent annual intervals if normal, ( 6000-8000 hrs. )
  3. At the time of main bearing overhaul or removal for survey.
  4. At foundation chock repair or renewal.
  5. Damage on bearing bracket, holding down bolt, chock.
  6. When major structure has been disturbed, such as: after fire breakout, propeller bending or impounding with something, ship grounding, before and after docking.

Causes of misalignment:

  1. Wear of main bearing lower shell.
  2. Wear and ovality of main journal pin.
  3. Main bearing damage.
  4. Main bearing pocket cracked.
  5. Bedplate deformed, transverse girder damaged.
  6. Foundation bolts loose or fractured.
  7. Foundation chocks broken, cracked or fretted.
  8. Slacked or broken tie bolts.
  9. Distortion of supporting ship’s structure.
  10. Defective structure due to corrosion.
  11. Defective propeller shaft bearing
  12. Lifting of flywheel side.
  13. Hull deformation due to: Improper loaded condition of vessel, grounding and fire.

Results of misalignment:

  1. Bending of crankshaft.
  2. Fatigue failure owing to cyclic stresses.
  3. Undue vibration within the engine.
  4. Damage to main bearing.

Why you measure crankshaft deflection?

  1. To ascertain whether or not, the axis of crankshaft journals deviates from theoretical shaft axis.
  2. Measuring is by a dial gauge, inserted between crank webs, and altered distances can be read, when turning the crankshaft.

How to know the amount?

  1. Difference between the values at TDC and BDC indicates the amount of crankshaft deflection, during one revolution.
  2. Interpretation of crankshaft deflections gives an indication of high and low bearings.

What will happen if a bearing is high or low?

  1. When a bearing between 2 cranks is higher than those on either side of it, both sets of crankwebs will tends to open out, when the cranks are on BDC, and close in when on TDC.
  2. Vice versa, if there is a low bearing between 2 cranks.

Requirements when taking crankshaft deflection:

  1. Hull deflection not excessive.
  2. Bed plate not distorted or bearing pockets not worn.

 Foundation Chock:

Purpose:

  1. To avoid misalignment on tank top surface.
  2. To carry out adjustments on individual chock.
  3. To correct any distortion.
  4. To absorb collision load by end chocks.
  5. To absorb side load, due to unbalanced reciprocating forces, by side chocks.

Advantages of Chockfast System: (Eposy Chock)

  1. Reliable and permanent alignment of machinery foundation.
  2. Resist degradation by fuel, LO and eliminate chock area corrosion.
  3. Give uniform and precise mounting.
  4. Non-fretting permanently.
  5. Reduce noise level.
  6. Can be used on all sizes and types of engines.
  7. Maintain C/S deflection, machinery alignment and even Hull fouling.
  8. Installation time is measured in hours, not in days.
  9. Withstand temperature up to 80°
  10. Give chock thickness up to 44 mm.

Chain Drive System:   

  1. Used for camshaft driving, on any length between shaft centres with very small friction loss.
  2. Fuel Pumps and Exhaust Valves are operated by Camshaft, driven from Crankshaft, by a roller chain [main] running over each sprocket wheel, being bolted to both shafts.
  3. Chain should wrap around at least 120°on both sprockets.
  4. Upward-running-side chain passes over an intermediate wheel, on which Tensioning Device is fitted.
  5. On another intermediate wheel’s shaft, there is another chain wheel and chain [smaller], to drive Start Air Distributor, Governor and Lubricators.
  6. Chain is lubricated by oil sprayer jets, with continuous stream of oil onto the chain.
  7. A roller chain consisting of side plates, bushing and rollers, and pin joints, which mesh with tooted sprockets.
  8. Shock-absorbing rubber clad guide bars, are provided to support the long chain, and to prevent transverse vibration.
  9. Renew cam chain after 15 years life.
  10. Factor of safety of chain: Never less than 25.

Slack chain: 

Symptoms:

  1. Excessive chain vibration and noise.
  2. Power loss in all units, indicated [by Power Card].
  3. Late injection, low Pmax, [by Draw Card].
  4. Late closing of Exhaust Valve, [by Light Spring Diagram].
  5. High exhaust temperature and smoke.

Effects:

  1. Impose heavy mechanical load, resulting fatigue failure.
  2. Damage to chain system and engine frame.
  3. Retardation of Fuel Pump and Exhaust Valve timings, resulting:
  4. Reduced Scavenge Efficiency, due to late closing of Exht: V/v.
  5. High exhaust temperature and smoke, due to after burning.
  6. Low Pmax, due to late injection.
  7. Reduced engine power.

Chain Casing Inspection: 

  1. Before 4000 running hrs and after lengthy voyage, chain tension is checked at mid span of slack side, in transverse direction.
  •    Limited transverse movement is ½ to one link pitch on slack side.
  •    Excessive tension may cause chain breakage.
  •    Excessive slackness may cause vibration and eventual failure.
  1. Elongation [chain wear] is checked between 3000–5000 running hours.
  •  Total length of 10 links drawn tight and measured, and chain-stretch calculated          in % by comparing with original length of 10 links.
  • Maximum elongation: ≯2%.  Over 2%, the whole chain must be renewed.
  • Due attention given when elongation reaches 1.5%
  • Stretching is the results of pin and bushing surface wearing out.
  • Chain length is measured in terms of number of links.
  1. Nozzle sprayers, LO pipes and oil flow and direction, checked.
  2. Loose bolts and pipe connections, checked.
  3. Every link checked for blemish, and bright marks due to misalignment of wheels.
  4. Sprocket teeth and wheel bearings checked for wear.
  5. Rubber clad guide bars, rollers and side plates, checked for crack or damage.

How to adjust chain tension: 

  1. Tensioning device [chain tightener] is used, and adjusting is limited to removing a maximum 2 chain links.
  2. Limited transverse movement is ½ to one link pitch on slack side.
  3. When tightening, engine is to be turned ahead, that the slackness of chain is on tightening side.

Advantages of Chain Drive over Gear Train: 

  1. Unaffected by foreign particles as gear trains.
  2. Class requires only a few links [6 links] for spares. The whole set required for gear train.
  3. Even if the chain breaks, engine can still be operated after repair.
  4. Accuracy of camshaft drive is very high, because chain tightener can adjust and compensate for inevitable mechanical wear. Gear train is non-adjustable.
  5. Enable camshaft position to be placed higher, thus shorten the hydraulic connections of fuel pumps and exhaust valves, and minimise timing error. 

Camshaft Timing Adjustment (By Pin Gauge):

  • As the chain stretches and re-tensioned camshaft is gradually retarded.
  • Thus camshaft must be repositioned relative to crankshaft to correct the timings of Fuel Pumps and Exhaust Valves.
  1. Engine must be in Ahead position.
  2. Bring cylinder no: 1 to TDC, and ‘0’ on flywheel.
  3. Check that cylinder no: 1 crank throw is in TDC; (with D-1 pin gauge)
  4. Check that camshaft position deviates from original marking; (with D-2 pin gauge) 

      If camshaft deviates:

  1. Connect high-pressure hydraulic pump oil connections to flanges next to chain drive, and pressurised until oil seep along the camshaft.
  2. Turn the whole camshaft using tackle until D-2 pin gauge mark is in line with original marking. Fixed markings are on roller guide housing [after removing cover] and on camshaft.
  3. D-3 pin gauge is for Lubricator Unit.

Methods of reversing:

  1. Direct reversal of engine: Propeller turns in opposite direction.
  2. CPP: Blade angle changes, as engine rotates in same direction.
  3. Diesel electric system: Engine and electric generator run in constant direction, supplying power to reversible electric motor.
  4. Reverse gears and clutches: Propeller turns in opposite direction.

Reversing Interlocks:

  1. Safety cut-out devices for JCW, Piston CW, and
  2. Reversing direction interlock.
  3. Hydraulic blocking device and mechanical blocking device for start air handle.
  4. Reversing Servomotor.
  5. Telegraph
  6. Turning Gear.
  7. Overspeed Trip.

Advantages of single cam and double cam:

  1. Single cam on camshaft is suitable for reversing of 2/S, large bore engine. But not suitable for 4/S engine, because reversing of 4/S engine requires turning of Inlet Valve cam, Exhaust Valve cam, Fuel cam and arrangement for Starting air Distributor, with their correct timings.
  2. Double cam on camshaft, is moved axially by means of servo system or manual system, so that all cams get their correct timings, in 4/S engine. (Used also for 2/S).

Lost motion:

Angular period between TDC points for Ahead and Astern running will be the “lost motion” required for Astern running.

  1. When reversing 2/S, exhaust ported engine, both Fuel Injection timing and Air  Starting timing must be changed.
  2. Retiming is carried out by altering camshaft position radially, relative to crankshaft. This is called “lost motion” of camshaft.

Why ‘lost motion’ necessary on some engine?

  1. Some 2/S, large bore, exhaust ported engines are Direct Reversing.
  2. Both Fuel Injection timing and Air Starting timing must be changed.
  3. Camshaft has single cam
  4. Retiming is carried out by altering camshaft position radially, (not axially), relative to crankshaft, by means of servo system.

Why ‘lost motion’ not necessary on some engine?

  1. Some 2/S and 4/S engines are Direct Reversing.
  2. Inlet Valve cam, Exhaust Valve cam, Fuel cam and arrangement for Starting air Distributor, with their correct timings, must be changed.
  3. Camshaft has double cam
  4. Retiming is carried out by altering camshaft position axially, from Ahead cams to Astern cams, by means of servo and manual systems.

Lost Motion Camshaft:

  1. When reversing 2/S Exhaust Ported Engine, both Fuel Injection and Air Starting timings must be changed.
  2. Lost Motion Clutch cam design can be used to alter reversing direction.
  3. Camshaft position is altered radially relative to crankshaft.
  4. Same cam is used for ahead and astern running.
  5. Reversing Servomotor, operated by Engine Reversing Controls, is fitted to camshaft drive mechanism to do this.
  6. Camshaft will lose motion or be retarded, through required angle (about 98°) by oil operated Lost Motion Clutch, causing the Reversing Servomotor to rotate the camshaft.
  7. Fuel Pump cam and Air Start cam will now operate the Engine in reversed direction.
  8. Lost motion is carried out while the Engine is at rest.
  9. For Uniflow Scavenge Engine, the second Servomotor is fitted to Exhaust Valve drive.

4 Stroke Engine Reversing Systems: 

  1. By means of camshaft, shifting axially. (Direct Reversing)
  2. By CPP.
  3. By gearing and clutch.

Tacho Generator:

  • AC or DC generator that provides an output voltage proportional to rotational speed, to remote rpm counter (tachometer).
  • It may be used to measure speed, or as part of automatic control system, to regulate speed.
  • In Sulzer RTA, also used for overspeed trip, using output current.
  • Fitted on ME intermediate shaft, for remote rpm counter.
  • Fitted on housing of Reversing Servomotor, and driven by gear wheel on Cam Shaft for overspeed cut-out.

V-Type Connecting Rod:

1)  Side by side   2)  Articulated    3)  Fork and Blade.

 Cross-head bearing is prone to failure, because of:

  1. High sudden load: Full effect of combustion, directly to the bearing.
  2. High bearing pressure: Bearing is placed high and the whole assembly reciprocates full length of stroke. So, limited bearing area results in high specific load.
  3. Distortion: Bending moment and deflection are maximum at centre, where pin is often bored to carry piston rod.
  4. Poor lubrication: Due to following factors:
  • Slow oscillating movement: Connecting rod swings through 25~30°, hence it is difficult to build up full fluid film.
  • Reciprocating movement: Vertical movement of pin and bearing disturbs oil supply. It is difficult to get smooth, uninterrupted oil flow.

5.  Two-stroke engine: No load reversal takes place, which does not help the oil flow into loaded part of bearing.

Different approaches adopted to overcome cross-head bearing problems:

  1. Conjugate deflection: Bearing deflection follows that of crosshead pin. Natural deflections of pin and bearing remain in line, resulting in lower specific load.
  2. Crosshead mounted LO pump: Attached high-pressure pump, operated by connecting rod movement, press oil into bearing gap when bearing load is lowest.
  3. Large diameter stiff crosshead pin: Reduced Length/Diameter ratio, but pin deflection is minimum for uniform distribution of oil films over the whole bearing
  4. Continuous full length bearing face under pin:
  • Low specific load on bearing.
  • Load is transmitted directly downwards.
  1. Large diameter pin and smaller “Connecting rod : Crank throw” ratio: Obtained higher sliding velocity of the bearing, with better LO oil film, to carry high loads.
  2. Hardened crosshead pin with high degree of surface finish: Surface finish is preferably better than  0.1 µm.
  3. Eccentric bored bearing: One of the finest designs for crosshead, which gives the same effect of load reversal. (GMT engine)
  4. Thin shell bearing: Bearing is renewable and pin is detachable. Produces high load carrying capacity, and better resistance against fatigue failure. Thin shell gives true circular shape, which improves lubrication characteristics.

Thrust block:

  • To prevent axial movement of crankshaft, resulting from propeller thrust.

Measurement of Axial clearance of thrust pads:

  1. Thrust block is cleaned by draining oil. And lift the top cover
  2. Place screw jack between casing and the back of the coupling, and push the thrust shaft aft until the collar is hard up on the
  3. Check alignment of shaft and take feeler gauge reading of open pads by using long feeler. It is inserted at one corner and ease diagonally across to the other.
  4. Repeat this operation, moving the shaft
  5. Difference between two readings is total axial clearance.
  6. Axial clearance is 1 ~ 2 mm. (0.5 ~ 1.0 mm for new engine and for engine in service, it must not exceed 2.0 mm.) 

Alternative method:

  • Bear the thrust collar on foremost thrust bearing segment, by pressing the crankshaft
  • Set dial gauge (zero position) to flywheel.
  • Bear the thrust collar on aftmost thrust bearing segment, by pressing the crankshaft
  • Check clearance by reading the indication of dial gauge.

Radial clearance of journal bearing:

  1. Remove end cover with oil seal.
  2. Radial clearance measured, by taking lead reading, or roughly by means of feeler gauge.
  3. Radial Clearance is 0.5 ~ 0.8 mm for 440 mm dia.shaft.

Advantages of Tilting Pad Bearing:

  1. Have ability to absorb, change in direction of load, more readily.
  2. Have greater flexibility to absorb shaft deflection or misalignment.
  3. Tilting of pads, allow oil to form wedge shaped film, between faces of collar and pads.
  4. Wedge shaped oil film prevents metallic friction and enables the thrust pads to carry loads.

Disadvantages:

  • Each pad in a set must be exactly the same thickness, and even a ‘thou’ difference might result in a single pad carrying the entire load, thus increasing the risks of failure.

Plumber block renewal:  (during heavy weather)

Practically it should be done in calm weather, but following ways can reduce overheating of plumber block bearing.

  1. By applying maximum lubrication.
  2. By applying maximum cooling after opening out the cooling coil out into bilge.
  3. By reducing to suitable speed.
  4. Then the ship proceeds to the sheltered sea and renew the plumber block bearing.

Removal of plumber block bearing.

  1. Take immobilisation permit.
  2. Mark the relative positions between each bearing halves and between the lower bearing halve and the
  3. Remove the upper bearing halve.
  4. Lift the shaft at the place close to bearing by jackscrew.
  5. Remove the lower bearing halve with chocks from the stool.
  6. Sent both bearing halves for repair.

Refitting Procedure:

  1. After repairing, place back the lower halve with chocks on the stool. But foundation bolts should not be placed.
  2. Remove the shaft-lifting device.
  3. Boxed back upper half.
  4. Remove all coupling bolts of intermediate shaft flange close to the bearing.
  5. Alignment checked by gap and sag method.
  6. After ensuring that the alignment is satisfactory, tightened foundation bolts.
  7. Refit and tighten the coupling flange bolts.

Allowances:

Gap method:   Equal to or less than  0.10 mm per meter for  1 to 2 pieces of shafts.

  • ≤  0.15 mm / m  for  3 to 4 pieces of shafts.
  • ≤  0.2 mm/m  for > 5 pieces.

Checked with a feeler gauge between the two coupling flange faces, at least at four places to check whether the bearing is in line with shaft or not.

 Sag method:

  • ≤  0.10 mm for 1 to 4 pieces of shafts.
  • ≤  0.15 mm for more than  4 pieces shafts.

Place a straight edge over the two flanges, at least four places around, to check whether the bearing is in line with shaft or not , or out of the shafting vertically and horizontally.

CPP:

Two main types:  Hub Servo and External Servo.

Hub Servo Type:

  1. Pitch altering mechanism, enclosed in propeller hub is most popular type, and used for higher power above 1000-Bhp.
  2. Propeller mechanism consists of 4 main parts:
  • Propeller hub incorporating servomotor, crank pin ring for turning blades, and necessary seals.
  • Oil distribution box (transfer box), mounted at forward end of tailshaft.
  • Control system; either pneumatic or
  • Hydraulic system; motor or shaft driven pumps, cooler, filter, and tank. etc.

Functioning Principle:

Movements of piston effect blade pitch:

  1. Servomotor in propeller hub consists of a piston rod with piston, which moves axially fore and aft when pressure oil is led to either side of piston.
  2. Piston rod is equipped with 4 or 5 “ears”, depending on number of propeller blades.
  3. Each ‘ear’ has a transverse slot in which a shoe slides.
  4. Eccentric crank pin fits into the hole of sliding shoe.
  5. Crank pin ring is supported on a bearing, which is built-in into hub body.
  6. When piston rod moves axially by pressure oil, crank pin ring rotates in circular motion, transmitted via piston, piston rod, slot, sliding shoe, and crank pin.
  7. Propeller blades, which are bolted to crank pin rings, turn.

Failure Arrangements:

  1. Hydraulic system failure:  
  • Safety springs, fitted in main servo, push the servo piston forward, to allow propeller pitch to full ahead position, in the event of hydraulic system failure. The springs are powerful enough to overcome friction, but RPM of 70% maximum should not be exceeded.
  1. Telemotor system failure:
  • Hand-operated control valve is used, in the event of telemotor failure.
  1. Main hub servo failure: 
  • If main servo fails, the system has either Emergency Servo or Mechanical Link.

CPP Bridge Control:

  1. CPP in large vessels are usually fitted with Combinator Control on the Bridge.
  2. A single lever controlling both propeller pitch and engine speed, either through pneumatic or electronic means.
  3. In either case, closed loop circuits are employed, so that feedback of propeller position and engine speed, balance off the control signal.
  4. In electronic control system, ME load is kept at desired value, by automatically changing the propeller pitch, irrespective of variation in external conditions; e.g. change in resistance in propulsion caused by wind and sea.
  5. Main panel receives, converts and transmits signals, and a potentiometer for adjusting ME load, and an instrument showing fuel pump setting, is provided.
  6. Control panel on Bridge contains instrumentation corresponding to that of Main panel.

Pilgrim Nut: 

  1. Pilgrim nut is a threaded hydraulic jack, screwed onto tailshaft, provided with hydraulic oil connection, steel jacking ring and nitrile rubber tyre.
  2. It gives predetermined frictional grip between tailshaft and propeller boss.
  3. Spherical graphite cast iron tapered sleeve is bedded onto shaft cone, before mounting the boss, to achieve better fit.
  4. When combined with Pilgrim Nut pushing up, it ensures a good frictional grip.
  5. No key is required; friction is sufficient to prevent slip.

Propeller mounting procedure:

  1. Tapered sleeve is bedded onto shaft cone, propeller boss is mounted, and pilgrim nut is run-down the shaft threads.
  2. Steel jacking ring on landing face of the nut, is loaded with hydraulic pump to predetermined pressure, and this forces the propeller hard on its cone.
  3. Pressure is released on jacking ring and air release plug
  4. Nut is hardened-up with spanner, and locked in normal way.

Propeller removing procedure:

  1. Pilgrim nut is taken-off the end of the shaft, reversed so that jacking ring is facing outward, and screw back the nut onto shaft, leaving some clearance between it and propeller boss.
  2. Studs are screwed into aft face of the boss and a “strong back” plate is fitted over the studs.
  3. Stud nuts are fitted so that the plate contacts with jacking ring.
  4. When hydraulic pressure is applied to jacking ring, propeller is pulled-off the conical end of the shaft.

Shaft Generator:

Shaft Generators are fitted on diesel engine propulsion ships, especially those sailing for long period at a constant ship speed.

Lloyd’s Requirements:

  1. Lloyd’s register would regard a shaft generator as a service main generator, if ME is intended to operate at constant speed. (CPP).
  2. If ME does not operate at constant speed, shaft generator would be disregarded as a service main generator, and at least 2 other independent generators would be required.

Running condition:

  1. Full generator capacity is available at within 60~100% of normal speed.
  2. More suitable for shaft with CPP, [constant shaft speed and variable blade pitch].

Advantages of shaft generator:

  1. Saving in fuel cost is main advantage.
  2. Saving in LO consumption, repair and maintenance cost due to reduced main generators’ running hours.
  3. Reduction in noise, space and weight, capital saving by reduction of numbers and ratings of main generators.

Disadvantages:

  1. Reduction in ship speed.
  2. Problems can arise to maintain electrical supply, during emergency manoeuvring astern.
  3. Increase in capital cost.

ME driven Generator:

  1. Fuel consumption is saved.
  2. Lower running and maintenance cost.
  3. Lower noise level in ER.
  4. Simple and most compact installation.

Varying speed of ME, driving a fixed pitch propeller, can be converted by variable gear ratio, to provide constant Generator speed.

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