BEARINGS SHELL AND CRANKSHAFT
Q. ) How will you analyse whether journal bearing (fitted on place) is in good condition or not ? How to make out only by looking that clearance between bearing and keep has increased?
Ans.) while doing a crankcase inspection of we found red powder appearance due to fretting then it can be suspected that journal bearing is weared down.
Q.) What is the composition of thin shell bearings?
Ans.)Thin shell bearings are broadly of
1.Tin based White Metal: Tin based white metal is an alloy with minimum 88% tin(Sn), the rest of the alloy composition is antimony (Sb), copper (Cu), cadmium (Cd) and small amounts of other elements that are added to improve the fineness of the grain structure and homogeneity during the solidification process.
2. TinAluminium(AlSn40): Tin aluminium is a composition of aluminium (Al) and tin (Sn) where the tin is trapped in a 3 dimensional mesh of aluminium. AlSn40 is a composition with 40% tin.
Q.) Which part of main bearing bolt undergoes extreme stresses?
Ans.) shank
Q.) Give properties and composition of main bearing?
Ans.)Bearing
Q.) How to check bonding of white metal is intact?
Ans.)Cu coin test is carried out on bearing surface, if sound comes, it means that the overlay layer of bearing is gone leading to the interlay layer containing Ni to get exposed and hence showing that bonding is not intact.
Q.) By visual inspection how to check bearing overlay damaged?
Ans.) A shiny surface will be visible which is Ni which proves that overlay is damaged.
Q.) What damages can be observed in bearing shell? Give reason for black spot in main bearing shell.
Ans.) Damages in bearing:
fatique
wipping
corrosion
cavitation erosion
abrasion wear
spark erosion
bacterial attack damage (honey coloured deposits)
electrical corrosion
misalignment
white metal hardening.
Q. What is bore relief in a bearing?
Ans.)The bearing sliding surface is machined at the mating faces of the upper and the lower shells to create bore reliefs. Their main objective is to compensate for the misalignments, which could result in a protruding edge (step) of the lower shell’s mating face to that of the upper shell. Such a protruding edge can act as an oil scraper and cause oil starvation
Q. Explain bedding of crank pin bearing.
Ans.)Bedding is the process of accurately fitting a bearing to its shaft by scraping off the bearing until contact occurs uniformly over the surface.
Procedure:
1.Coat journal with prussian blue
2.Assemble the bearing
3.Rotate the crankshaft 360 deg on the journal.
4.Dismantle the bearing
5.Scrape off the high spots on the bearing indicated by blue
The above procedure is repeated until an even coating of blue is visible over the full bearing surface.
Q. AE bottom end bearing bolt, which part takes maximum load and design aspects?
Ans.) shank takes the maximum load.
Q. Explain thrust block arrangements. Why thrust blocks are installed in pieces and how propeller thrust is transmitted to hull? How does thrust pad generate hydrodynamic film?
Ans.)The thrust of the collar is transmitted through the oil film and pads to the casing. The white metal surface would be more likely to yield than the oil film at pressures as high as 500 bar (compressive yield of white metal say 560 bar = 56 MN/m2).
The oil scraper bears on the outer periphery of the thrust collar and delivers oil to the reservoir stop from where it cascades on to the pads and bearings. The pads fit radially in the inverted horse shoe castings, pads being secured circumferentially by the stop. The castings back on to liners so ensuring location in fore and aft direction and fixing the clearance, which can be adjusted. The radial pivot line on the pad back varies from
half to two thirds of the pad width from the leading edge. The lower half casting acts as an oil reservoir sump, being provided with oil level gauge glass and a cooling
coil. The total oil clearance is approximately 1 mm for say a 500 mm diameter shaft. The wedges at base have a slow taper of about 20 mm/m and act to relieve the holding down bolts of shear. The floors in the double bottom tank below the thrust stool are closely
pitched. Clearances are measured using wedges or hydaulic ram movement.
The pads are prevented from moving circumferentially but are free to tilt and incline to the direction of motion. Such tilt allows a self adjusting oil film wedge giving full film lubrication. This film fully carries the load and allows pressures of 30 bar and shows a coefficient of friction value of 0.003.By having pads in pieces, we get increased area, and also we get fulcrum action.
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Q.) How is thrust bearing lubricated, thrust bearing position and reason?
Ans.)The thrust collar of the main thrust bearing revolves in an oil bath. The revolving collar picks up lubricating oil from the bath which is scraped off the top of the collar by a diamond or rhombus shaped scraper or spreader. The oil is then diverted into two channels which keep the thrust faces of the collar adequately supplied with oil. The lubricating oil is supplied to the thrust bearing oil bath from the lubricating oil manifold or rail within the crankcase. Usually the thrust bearing is integral with the gear casing and the same oil which is fed to each journal bearing is also supplied to thrust block.
Q. )Explain crankcase inspection in detail, what all clearances to check?
1. Firstly check the oil condition for any smell,discolouration or degradation.
2. Turn the Engine to BDC & start checking from under stuffing box area for any signs of black oil, if so indication of stuffing box leaking.
3. Check piston rod surface for scoring marks & roughness.
4. Check Piston palm bolts & locking device for slackness & fretting.
5. Check guide & guide shoe bearing general condition & area around frame where guide is attached for any visible cracks.
6. Check guide shoe end cover bolts in place & not slack.
7. Check cross head general bearing condition.
8. Top & Bottom end of the con rod bolt, nut & locking devices for slackness, sign of fretting etc.
9. Check sliding of bottom end bearing (floating of con rod).
10. Check for slip of web & journal by checking the reference mark.
11. Check the web in the area of stress concentration & check tie bolts (bottom side).
12. Check cross girder, area around main bearing & bearing keep for signs of cracks & check the main bearing.
13. All bearings to be checked for silvery colour, (indicates bearing wiping)
14. Check all the surrounding oil pan area of all units for any sludge deposits, bearing metal pieces etc.
15. Check crankcase relief door (wire mesh should be wet, spring tension sealing condition etc.)
16. Check the teeth of transmission gears for signs of wear.
17. Check chain drive for tightness.
18. Oil mist detector sampling pipe to be checked for clear passage.
19. Clear all foreign materials from the C.C. & tools accounted for.
20. Start L.O.p/p & XHd p/p & check oil flow & distribution.
21 Check C.C.door sealing condition & close the door.
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