When the crank throw is loaded by the gas pressure through the connecting rod mechanism, the arms of the crank throw deflect in the axial direction of the
crankshaft, generating axial vibrations.
These vibrations may be transferred to the ship’s hull through the thrust bearing.
The thrust bearing is incorporated in the aft end of the bedplate as differential expansion of the shaft and hull is
minimum at the aft due to fuel heating in tanks.
The aft-most cross girder is therefore designed with ample stiffness to transmit the variable thrust from the thrust collar to the engine seating.
It is advised to align the thrust bearing positions along the length of the pin. The
measurement is taken at Port-Starboard and Top-Bottom positions. Handle the micrometer carefully to avoid scratching the pin while taking measurement.
Top
Bottom
Stbd
Port
Crankpin
Crankpin Bearing Clearance
Crankpin Measurement
www.marineinsight.com 85
when main bearing alignment is carried out to achieve accuracy.
Material
Michell type pads bearing arrangement consists of a steel forged thrust shaft, a bearing support, and segments of cast iron with white metal.
The thrust shaft is connected to the crankshaft and the intermediate shaft with fitted bolts.
The thrust shaft has a collar for transfer of the ‘thrust’ through the segments to the bedplate.
Lubrication of the thrust bearing takes place from the system oil of the engine.
At the bottom of the bearing there is an oil sump with an outlet to the oil pan.
Clearance
The clearance in the thrust bearing is measured during test bed trials of the engine.
For a new engine the clearance is 0.5-1.0 mm, and for an engine in service it must not exceed 2.0 mm.
Dismount the foremost segment stopper On top of the thrust segment, a wear groove of 1mm is provided (a segment with thermometer). To measure the wear, push the thrust pad with crowbar against thrust cam to eliminate any gap at the back
While Inserting feeler gauge in the
groove, if 0.1 mm is not able to enter, it indicates wear is more then 0.9 mm and the bearings need to be overhauled.
A Guide to Two Stroke Marine Engine Components If the white metal is found scored, fine
scrapping can be done to wipe off the scoring marks. The liner shims can be inserted at the back of the thrust shoes to make the clearance of all thrust pads
equal. This avoids uneven loading of pads
Shaft collar
shaft
Pads back plate
When removing thrust pad from the housing, ensure to insert a dummy or
wooden pads of same dimension to avoid shifting of shaft.
Pad with thermometer
Clearance
Pad
www.marineinsight.com 87
10
Camshaft
The engine camshaft is used to drive the fuel pump and to operate the exhaust valve mechanism. It is located in the housing for the roller guides. The camshaft bearings consist of two
mutually interchangeable bearing shells, which are mounted in hydraulically
tightened casings.
A Guide to Two Stroke Marine Engine Components
A number of segments are joined
together to form a complete camshaft unit for the engine. These segments are connected to each other by SKF sleeve coupling.
In SULZER engine, the reversing of the engine is done by changing the direction of rotation of the camshaft. For this, a separate hydraulic reversing servomotor is fitted within the pump housing in every segment of the crankshaft, to assist in engine reversing.
In MAN engine, the camshaft rotates only in one direction and does not integrate any hydraulic means for reversing the camshaft.
The Camshaft is driven by crankshaft either by chain drive or by gear drive.
www.marineinsight.com 89
MAN engine uses chain drive and SULZER uses gear drive.
These driving mechanisms (gear or chain drive) are located either at the end or at the center of the engine, depending upon the number of units. System oil is used as lube oil in the camshaft for lubricating the bearings.
Cam Profiles and Types:
The most important part of the camshaft is the CAM, which controls or operates the exhaust valve and the fuel pump. The cam consists of two parts, a profile and a base circle.
The curvature shaped cam profile drives the follower and regulates the injection time of the oil. Base circle, which acts as the base, is the smallest circle of the cam
profile. When the follower is over the
base circle, no operation of exhaust valve or fuel pump takes place.
Following are the different types of CAMs used in Marine engines:
• Single lobe cam
• External cam
• Regular Cam
A Guide to Two Stroke Marine Engine Components Material:
The Cams are made from heat-treated hardened steel. The camshaft bearings are shell type bearings, which are white metal lined bearings.
Inspection:
• Check the condition of cam surface for scoring marks. As the inspection must include the entire surface of the cam, the cam should be turned the whole way round during the inspection
• Check the condition of camshaft bearing clearance
• Check correctness of cam timing with the help of pin gauge
91
• Multi lobe Cam
• Internal Cam
• Inverse Cam
www.marineinsight.com
Checking Camshaft Timing (MAN engine):
• Ensure that the main lube oil pump is isolated well in advance
• Bring the No.1 unit to Top Dead
Centre (TDC). Ensure this position by checking the flywheel marking
• There are markings provided in the camshaft housing and at the center of each cam.
• With the help of pin gauge, check the deviation from the marking. If the values are outside maximum
permissible lead angle corresponding to the housing markings, adjusting of camshaft is required
• Clear the cams by lifting the exhaust
valve and fuel cam roller
• Remove the shield covering the coupling flange from the camshaft, which is nearest to the chain drive
• Remove the oil tray from the bottom
• Dismount the plug in the coupling flange and mount a ‘three snap-on connector”
• Connect a hydraulic jack and pump to the connectors
• Apply light oil pressure to vent the system. Once venting is done, tight the coupling
• A special spanner is provided to rotate the cam, mount the same
• Apply high pressure oil until oil seeps shaft, below the coupling flange
• Rotate the camshaft by means of a tool till the mark matches both the sides of the pin gauge