CRANKSHAFT DEFLECTION :
In main and auxiliary reciprocating diesel engines, the variation of distance between the crankwebs of individual crank throws during a full engine turn are recorded and called crankshaft Reflection readings.
This deflection is indicative of true deflection of the crankshaft i.e. vertical hog or sog, and if excessive can cause dangerous bending stresses in web and fillet between crankpin and web.
Standard procedure is to fit a dial gauge between the webs, usually as close to the shaft circumference as possible (at opposite side to throw), and set to zero when crank throw is as close to 'bdc' as possible. Turning the crankshaft slowly and taking a readme at every 90 deg thereafter will provide top and bottom readings indicating the state of the shaft alignment in the vertical plane, and port and starboard readings indicating the state of shaft alignment in the horizontal plane.
When the dial gauge reading showing positive (web opening out) whiie throw at bdc and gradually reduces as the throw moves from bdc to tdc position and become negative (web closing in), it indicates 'low' journal bearings at two sides of that crank. Similarly if deflection gauge increases in reading from bdc to tdc, then it indicates 'high' bearings.
Readings from one unit alone do not enable the shaft alignment to be assessed, it is only from an overview of the interrelationship of all units that the lie of the shaft can be interpreted. The difference between 'tdc' and 'bdc' readings gives vertical plane misalignment whereas horizontal plane misalignment is the difference between port and starboard readings.
Crankshaft deflection can be assumed to be dependent on two main factors for a given mass per unit length (connecting rod, piston etc) ;-
a) Distance between supports of shaft (i.e. main bearing inner faces), as the further apart the supports the greater the sag effect. Vertical deflection for a simply supported uniformly distributed loaded beam is 5 mgl4 \348EI, where m is the mass per unit length, I is the length between supports and El is the flexural rigidity of the shaft. Deflection for a centrally concentrated load W is Wl3 \ 48EI
b) Distance out from shaft centre line the measurement is taken, further the distance greater is the gap. The web's, extreme distance and size of section is usually proportional to the engine stroke.
On the above assumption, total stroke versus deflection curve can be drawn for 1 m stroke, 1 m face distance, correct initial deflection is 70m and maximum 118m. Crankshaft needed to be realigned when deflection reaches 240m and dangerous condition reaches at 330m deflection.
Stresses caused by static deflection are difficult to assess but as a rough guide, each urn vertical crankweb deflection can be considered to be somewhere about 1m central vertical deflection, 2m change in bearing height and can cause a bending stress of about 33 KN/m2.
Not to keep any double bottom tanks below the engine bedplate overpressurised.
To
watch amperage as the engine is turning to get an idea of resistance
to turning.
Any sudden changes in reading should be investigated
immediately.
Crankshaft Stresses :-
(1) Torsional Stress :- Torsional stressing is being a result of the forces applied by the Connecting Rod to the bottom end, varies in magnitude with both the changes in Cyl. Pressure and the angle of thrust applied by the Connecting Rod. Cyclic variation in torsional stress causes fatigue and failure.
(2) Bending Stress :- (a) firing force causes bending stress.
(b) misaligned bearing support. fatigue failure or surface crack due to alternate compression and tension at the surface of the shaft.
(3) Shear force :- at 'tdc' positionwhere the piston rod, Connecting Rod and webs are in alignment and the turning moment is negligible.
Causes of slip in shrunk-fit Crankshaft :-
(1) Attempting to start the engine when the cylinder is partially filled with fluid.
(2) Propeller blade hitting a submerged object.
(3) Extreme or sudden overloading of an unit exceeding torsional stress limit may cause slip(as a result of a fault in fuel injection and timing system).
Effect of slip :-
(1) Change in fuel timing :- usually recoverable by adjustment of the pump or cam.
(2) "Breathing" of the engine :- Sequence of breathing from scavenge space will be changed by the displacement of the crank which controls the point at which the piston uncovers the ports. It can only be recovered by realignment of the crank system. In extreme cases, it can cause turbocharger surging and vibration of the scavenge space themselves.
(3) Exhaust :- The exhaust timing will be affected so that T/C may well surge and vibration of the uptake piping may occur. In mild cases the cam may be adjusted to return to an acceptable operating condition. For large twist, it is necessary to realign the slipped shrink fit.
(4)Balance :- Static and dynamic balancing will be affected. The critical speed will alter and unacceptable engine vibration may be set up, affecting chocking and other mountings throughout the engine.
Realignment of a slipped shrunk-fit crankshaft
(1)To chill as far as possible the crank pin (by dry ice)
(2)To warm up the web by applying a broad flame over a wide area.
(3)To lock the journal against rotation (removing shims and harden down the keep).
(4)To jack the web back into position by placing the jack against a wooden plank to distribute the load.
(5)To erect a stop at the web to prevent overshooting the desired position, before applying jacking force.
(6)Normally no dowel is fitted which reduces the factional grip.
(7)Witness mark is referred to adjust final alignment.
Checking of Crankshaft :-
(1) To check surface damage of journals
(a)scoring by impurities in lub.oil or particles embedded in white metal.
(b)Corrosion (usually apparent as discolouration ) possibly by weak organic acids caused by oxidation of lub.oil, bacteria in oil, products of combustion(trunk engines).
(2)Cracks at fillet radii, oil holes or other areas where stress concentrations occur caused by cyclic torsional stresses, crankshaft misalignment(worn main bearings or loose chocks), overloading of the engine.
(3)Slipped shrink fit( to check witness marks ) - (i) Caused by liquid in cylinder during starting,(ii) propeller collision with submerged object,(iii) extreme and sudden overloading of a unit or units.
(4)Ovality :- This occurs mainly in the bottom end journal, in extreme cases main and x-head pin may be affected. It is caused by (i) reduced effectiveness of lub.oil (ii) directional thrust of connecting rod, which is a maximum at 45°C after tdc, with a result that journal wears oval. Usually the maximum ovality that can be allowed is in the region of one-quarter of the working clearance of the bearing. Any more would affect hydrodynamic lubrication. Ovality can be recovered by in situ grindng .
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