Labels

Bearings



Properties of bearing materials 

 
Bearings provide support to journal, prevent metal to metal contact, transmit the load via lubricants and reduce the rotational friction. 
Properties of material required in a bearing are:- 
1. Antifriction resistance 
 2. Running In & Grinding Inability 
 3. Non-corrosive by lubricants. 
 4. Should not scratch and score the journal 
 5. Build up adhesive oil film under boundary lubrication 
 6. Allow abrasive particles to embed in it without major functional disability 
 7. Tensile and compressive strength 
 8. Fatigue resistance 
 9. Thermal conductivity 
 10. High melting point. 
 11. Ductility 
 12. Conformability, and 
 13. Load carrying capacity.
 Conformability is the ability of the bearing material to accommodate geometry misalignments of the bearing, its housing or journal. 
 Embeddability is the ability to bearing material to accommodate (or embed) small particles of dust, grit, etc. without scoring the material of the journal. 

 

Thick and Thin shell white metal Bearings 

 
Bearings support the journal, prevent metal to metal contact and reduce rotational friction. White metal lined bearings are applicable to the main, bottom end and crosshead duties. In all cases, the steel bearing shell is lined with a layer of white metal. 
 White metal:- less cost, easy casting and tolerance of lubricant impurities. 
 
Thick shell bearing:-They were used for main bearing duties in old engines. The edges of the steel backing shell are provided with lips that allow for correct location within the bearing keeps. They have stiff steel backing for counter distortion of the contour geometry of the bearing, which is essential for their effective sliding mechanism. If higher loads to withstood Top clearance is adjusted by inserting shims. 
 Thick shell bearing white metal:- 
 Tin : - 80-88% 
 Antimony : - 8-10% 
 Copper : - 4-5% 

Thin shell Bearing:- Also called Tri-metal bearing. Wall thickness is 2-2.5% of journal diameter. They have no edge lips and the layer of white metal is much thinner in comparison with the thick shell type. The quality of the running surface of the thin shell bearing is much higher than the thick shell type as cooling is easier to control during casting, allowing a much finer-grained structure, so can carry an increased load. crosshead and bottom end bearings are thin shell types. 
Thin shell bearing white metal: - 
Tin : - 88% 
Antimony : - 8% 
Copper: - 4% 
Cadmium : - Traces 
Strongback does not have an adequate stiffener. Manufactured with Nip/Crush. 
Layers of Thin shell bearing 
1. Flash layer- the 1micro-meter layer of lead/tin, it avoids corrosion during storage. 
2. Overlay - 20micrometer layer of white metal, 
3. Interlay - 5microns nickel dam helps to reduce corrosion of white metal. 
4. Lining - 1mm thick lead/bronze. 
5. backing layer - steel backing for shape and support. 


Load on a bearing 

 
Load a bearing can withstand is given by:- $\displaystyle \mathrm{\frac{\mu .N.(2L).r^{3}}{C}}$  
 µ = Oil viscosity 
 N = Bearing speed 
 r = Bearing radius 
 L = bearing length 
 C = Clearance 

Q. How to check the load on each bearing of the shafting system? 
Ans:- To check the load on each bearing -hydraulic jacks and dial gauges used. 
Load on each bearing = total weight of the shaft/No of bearings. 
Permitted deviation +/- 50% designed load given in the handbook. 
 Hydraulic jacks are placed on each side of the bearings, to lift the shaft just clear off. A dial gauge fixed to the bearing to indicate the lift. Hydraulic jack pressure registered the load on the bearing. Fixed one dial gauge on adjacent bearing to ensure the lift is limited to bearing being checked. A plot of lift and load made, moving up and down. 
 

Main bearing 


 The main bearings at present are of the 'thin shell' type. They are white metal lined, as white metal provides greater safety and tolerance against misalignment, as compared to the conventional tin-aluminium bearings. They have a superior running-in characteristic as compared to tin-aluminium, which have a running-in layer of lead indium. 
     Main bearings provide support to the Crankshaft, and take the weight of the working parts, besides the loads due to combustion. The Main bearing clearances must be regularly checked, to ensure that lubrication is not reduced and alignment is not affected. The steel backing shells are held in place by the bore of the housing and are designed to provide 'nip', i.e. a good interference fit, to provide adequate grip on the bearing shell, preventing it from turning in the housing. The nip provided is not great enough to cause distortion, which could adversely affect the running clearances. Main bearings need to be inspected at regular intervals, as per the requirements of the CSM, and also in case, the Crankshaft deflections show this to be necessary. Main bearing clearance should be zero at the bottom, since the engine is stationary, and the journal is resting on the bottom shell. If this is not so, it means that the crankshaft is out-of-alignment. The clearance of the bearing can be checked by means of special feeler gauges, provided by the manufacturer for this purpose. 

Main bearing dismantling 

Upper half: 
a. Open all Crankcase doors and ventilate. 
b. Disconnect the lubricating oil inlet pipes. 
c. Loosen lockings on the nuts and slacken the nuts using the hydraulic pump 
d. Remove the nuts and lockings. Lift the cover by means of chain blocks, adequately supported on the A-Frame. 
e. Fit the eye-bolt on the top half-shell and lit it by the chain block. 
 
Bottom half 
a. After dismantling and removing the Upper main bearing shell, connect the oil pump and hoses to the bore below the Main bearing shell. 
b. Insert the supplied tool pin in the oil hale on the journal and tighten it. 
c. Pump oil to lubricate the underside or lower half. 
d. Turn the engine by means of the turning gear, so that the pin drives the lower half-shell around and out, to the upper side of the journal. 
e. Lift the lower half by the chain block, connected to the eye bolt. 

Alternative method : 
a. Jack up the webs, by means of the jacks, mounted the cross-piece, by approximately 0.2 mm, so that the lower half is free of the load. 
b. Attach the nylon wire rope, passing it around the ends, as shown. 
c. Pull the lower shell around, by means of the chain block and a wire passing over the small fairlead suspended over the bearing. 
d. The lower half shell will be rolled over the main bearing journal when it turns through 180°. e. The lower shell can now be lifted out by the chain-block arrangement. 
 

Bearing Clearances in Two-Stroke Engine.


ITEM
CLEARANCE (MM)
Main bearing                      
0.25 - 0.40
Crankpin bearing                
0.40 - 0.65
Crosshead bearing pin        
0.40 - 0.60
Crosshead Guide shoe
(A +B)
0.20 - 0.55
Crosshead Guide piece
(C1+C2+D1+D2)
1.40 - 2.00
Crosshead Pin thrust
 (E+ F)
0.10 - 0.70
Thrust bearing
0.50 - 1.00
Camshaft bearing                   
0.10 - 0.30



Thrust Bearing 


 The Thrust bearing clearance is checked with a Feeler gauge, as well as any special gauge if supplied. 
For a new engine, this is 0.5 mm to 1.0 mm. For an engine in service, it must not exceed 2 mm. 
Checking the clearances on the Thrust bearing is done as follows: 
Turn the engine so that the aftermost crank throw is at BDC and the thrust bearing collar bears on the foremost thrust bearing segments. (B = 0 mm). 
The gauges are to be inserted between the side of the after-most lower main bearing shell and the side of the after-most crank throw. If a feeler gauge of less than 2 mm minus (B+C) is able to enter, the Thrust bearing clearance is correct, else the bearing is to be overhauled. 



Bottom end and main bearing bolts. 


Bottom end bolts of 4 stroke engines:
  Bottom end bolts of 4 stroke engines are subjected to fluctuating cyclic stresses and therefore exposed to potential fatigue failure. They experience large fluctuations of stress during cycles. This is due to inertia forces experienced in reversing the direction of the piston over the TDC, on the exhaust stroke. The forces experienced in this situation are very high. 
 As, Stress = $\displaystyle \mathrm{\frac{Load}{Area}}$  
 It implies that for a given load, the stress can be reduced by increasing the area and therefore increasing the size and weight of the bolts in four-stroke engines. Piston scuffing stretches the Crankpin or bottom end bolts as the crank drags the piston along. The flexing of the Connecting rod foot causes the bottom end bolts to bend. The only way to safeguard against failure of the bottom end bolts is to renew them every 30,000 hours, or earlier (check the engine manual). 
 Every time the bottom end bolts are replaced, an entry is to be made in the Log; so that the running hours can be recorded. It is important not to exceed the Elongation limits of the bottom end bolt at any time. This value can be found in the engine manual. Before tightening the bottom end bolts, use a 'stretch' gauge.
 The problem occurs when any bolt comes loose during running. The bolts can be checked during the Crankcase inspection, either by checking the split pins or by hammer' test, to confirm that they have not worked loose. Never over-tighten the bolts, as thread friction varies from nut to nut, and can cause deformation of the bolt. Tighten both sides till the stretch gauge indicates exactly the same reading on each of them, Record the stretch of the bottom end bolts in the history register for the engine, so that comparative figures are available at the next check of the bolts. 
 
Main bearing bolts 
  These are subjected to heavy loads, fluctuating in magnitude and direction. They transmit the downwards force from the engine frame. 
 
Comparison between Bottom end and Main bearing Bolts:
  Bottom end bolts, as compared to Main bearing bolts, have: 
 a. Higher fatigue limit. 
 b. Higher tensile ductility, as conducive to higher fatigue strength. 
 c. Fatigue limit is increased by case carburising, nitriding or carbon nitriding. 
 d. They are manufactured to higher standards of finish. 
 e. They have increased diameter at mid-shank, to reduce vibration. 


 

Crosshead bearings failure 


Failure of the Crosshead bearing can occur due to various reasons: 
a. Misalignment of the running gear. 
b. Poor quality of the material. 
c. Poor surface finish, causing early deterioration. 
d. Starvation of lubricating oil. 
e. Overload of the engine. 
 
Signs of failure are: 
 Squeezing of the white metal leads to lubrication problems. Cracking of the white metal shell. Fatigue failure. Melting of the white metal, due to overload (overheating). Wear of the bearing due to abrasion or corrosion. 

Inspection of crosshead bearings 
 The low sliding speed and uni-directional loading cause severe conditions in the operation of the Crosshead bearing. They require to be carefully inspected, during fitting as well as during service. 
Checks without dismantling : 
 a. Feel the bearing, while still warm, for the abnormal rise in temperature. 
 b. While the oil is still in circulation, after just stopping the engine, check that uniform jets appear from all the outlet grooves in the lower shells. 
 c. With oil circulation stopped, and the crank in BDC position, check the top clearance with a feeler gauge. 
 d. Check for signs of squeezed out metal, wiped out or loosened overlay. Also, check for fragments of white metal in the oil. 
 If the following are noticed, the Crosshead bearing needs to be dismantled: 
a. Bearing running hot. 
b. Oil jets reduced/absent or twisted. 
c. Excessive clearance. 
d. Signs of damaged white metal. 
 
Dismantling of the Crosshead bearing 
a. Turn the engine till you have access to the nuts on the piston rod, the telescopic pipe and the Crosshead bearing cap. 
b. Loosen and remove locking from piston rod studs and piston rod foot. 
c. Loosen and remove the lock nuts from the Crosshead bearing cap. 
d. Mount the spacer rings and hydraulic jacks for loosening the nuts on the piston rod and the Crosshead bearing cap. 
e. After removing the nuts, and taking out the hydraulic jacks. 
f. Remove the two studs for the piston rod from the Crosshead, by means of the stud setter tool. 
g. Loosen the screws of the Telescopic pipe and mount the lifting tool for suspending the Telescopic pipe. (Never turn the engine without doing this.) 
h. Turn the engine to TDC to dismount and suspend the Telescopic pipe. 
i. Turn the engine to take the Crosshead downwards, to give access to mount the chains and eye-bolts for suspending the piston from the cylinder frame. 
j. Suspend the piston rod, by turning the Crosshead downwards. 
k. Tum the Crosshead to BDC. Hook on the Bearing cap and raise to inspect the upper bearing shell. 
l. By means of tackles, raise the. Crosshead from the Connecting rod, to inspect the lower half of the Crosshead bearing and Crosshead pin. 
m. If all is in order, the Crosshead may be re-assembled. 

Damage to the Crosshead bearing 
The Crosshead Pin or Journal may become oval or roughened if the loaded part of the Journal has worn heavily, or there are scratches on more than one-third of the contact area, or the white metal has been 'wiped out' over a wide area. The Crosshead Pin then needs to be re-conditioned ashore. 
The Bearing shells may need to be replaced, if there are crack formations or the white metal is smeared out (wiped-out) due to heat softening and plastic deformation. 
 


Connecting rod and Bottom end Bearing 


 The Connecting rod transmits the gas forces from the Piston to the Crankshaft. The bearings of the Connecting rod are the Top end (Crosshead) and Bottom end (Crankpin). They are machined from a steel forging shaped at each end to accommodate the bearings. Usually, the oil hole is bored through the centre of the rod, for the flow of the lubricating cum cooling oil. This flow is downwards in the Crosshead engines, while it is upwards in the Trunk piston engines. For Trunk piston engines, the connecting rod usually has an obliquely split bottom end or small palm type, with shims for adjustment of the compression pressure. 

Inspection procedure: 
 The Connecting rod and the Bottom end bearing are inspected for fretting and cracks. Any defects noticed should be made good. In the case of trunk piston engines, the Connecting rod alignment is to be checked after piston seizure. In order to do this, the Connecting rod is to be placed on the surface plate, with suitable mandrels closely fitting in the top and bottom end bearings. By traversing a dial gauge on both ends of both mandrels, any twist in the Connecting rod can be detected. 
 
Dismantling of the Crankpin bearing 
a. Remove the Crankcase doors and turn the crank of the unit to TDC. 
b. Lockings to be removed, and nuts slackened by hydraulic jacks. 
c. Suspend the chain blocks, on each side. With wire slings, shackles and eye-bolts, lower the Bottom half into the Crankcase. 
d. Lift out the bottom end bearing outside the crankcase, by appropriate shackles and chain blocks. 
e. Fit the Crosshead holding pin device to the Guide. Turn engine till the Crosshead rests on the holding pin. 
f. Support the Connecting rod adequately and continue turning the engine, till the Crankpin is clear of the upper half, in order to have sufficient space for the inspection.


Q. A. Describe the procedure for opening a bottom end bearing for inspection making reference to the positioning of the crank and the safety precautions to be observed. 
 B. State how the bearing clearance may be checked and adjusted when necessary 
 C. State TWO defects, which may be encountered during inspection of the bottom end bearing and crankpin giving possible causes of EACH. 
 D. State TWO checks, which should be made before returning the engine to service following an overhaul of the bottom end bearing. 


Labels:

Comments

Post a Comment