Crosshead bearing Crosshead with Connecting Rod Purpose The purpose of the crosshead is to translate reciprocating motion of the piston into the semi rotary motion of the con rod and therefore bearings are required for this purpose. It is also necessary to provide guides in crosshead piston engine to ensure that the side thrust due to angularity of the connecting rod is not transmitted to the piston but to the Crosshead guides. Crosshead also ensures that the piston remains central in the cylinder thus limiting wear in the liner. Effect of gas load from piston on cross head bearing In large slow-speed engines the gas pressure acting on the piston and through the piston rod puts a load of approximately 750 tons on the crosshead pins with maximum combustion pressure. RB 18/10/04 1/14 Trunk piston engine Crosshead piston engine Side thrust
Transcript
Crosshead bearing
Crosshead with Connecting RodPurposeThe purpose of the crosshead is to translate reciprocating motion of the piston into the semi rotary motion of the con rod and therefore bearings are required for this purpose. It is also necessary to
provide guides in crosshead piston engine to ensure that the side thrust due to angularity of the connecting rod is not transmitted to the piston but to the Crosshead guides. Crosshead also ensures that the piston remains central in the cylinder thus limiting wear in the liner.
Effect of gas load from piston on cross head bearing In large slow-speed engines the gas pressure acting on the piston and through the piston rod puts a load of approximately 750 tons on the crosshead pins with maximum combustion pressure.
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Trunk piston engine Crosshead piston engine
Side thrust
Crosshead bearing
Crosshead pins are supported in bearings and the traditional way has been to mount the piston rod at the centre of the pin with a large nut and having two bearings alongside. This arrangement is like a simply supported beam and the pin will bend when under load. This gives rise to edge pressures which break through the oil film resulting in bearing failure. The Sulzer solution is to mount the bearings on flexible supports. When the pin bends the supports flex allowing normal bearing contact to be maintained.
In order to minimise the risk of bearing failure the actual force on the oil within the bearing should be kept within reasonable limits this can be achieved by having as large a bearing area as possible. Increasing the diameter of the pin and hence the bearing will minimise the problems as this not only allows for a large bearing area but it also avoids the problem of pin bending. Pin bending is further prevented by means of a continuous bearing. This also avoids the loss of oil which can take place with short bearings. Most modern engines tend to have single continuous bearings. Oil loss from the ends of bearings is prevented by means of restrictor plates. Some engine builders provide booster pumps which increase the oil pressure to the crosshead during the critical firing period. Cross heads do not have complete rotary motion and so a complete oil wedge does not form. The use of means for preventing oil loss are therefore useful in maintaining an oil film between pin and bearings.
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Crosshead bearing
DescriptionThe crosshead is provided with two guide shoes fitted on the crosshead ends.
The centre part of the crosshead is designed as a bearing journal which is housed in the crosshead bearing.
The crosshead bearing cap is provided with a cut-out enabling the piston rod to be assembled with the crosshead journal.
The crosshead bearing is equipped with bearing shells lined with white metal.Further, the lower shell has an overlay coating.
The piston rod foot rests on the crosshead, and is guided by a pipe in the crosshead.
A shim is inserted between the piston rod and the crosshead. The piston rod is fastened to the crosshead by means of studs and nuts.
The thickness of the shim is calculated for each engine in order to match the different engine layouts
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Crosshead bearing
A telescopic pipe, which supplies lubricating and cooling oil to the crosshead, crankpin and piston, is mounted on top of one of the guide shoes. The guide shoe is also fitted with a counterweight in order to balance the weight of the telescopic pipe.
The outlet pipe for piston cooling oil is mounted on top of the other guide shoe. The outlet pipe slides within a slotted pipe inside the engine frame, and from there the oil is led through a control device for each cylinder for the purpose of checking the temperature and flow before the oil is passed on to the lube oil tank.
The crosshead is provided with bores for distributing the oil supplied through the telescopic pipe, partly as cooling oil for the piston, partly as lubricating oil for the crosshead bearing and guide shoes and through a bore in the connecting rod> for lubricating the crankpin bearing.
The sliding faces of the guide shoes are lined with cast-on white metal.
The guide shoes are guided by crosshead guides in the engine frame box and properly secured against displacement by guide strips fastened to the guide shoes.
The crosshead bearing is tightened together by means of four studs and nuts. The nuts are tightened by means of hydraulic tools.
The crankpin bearing is fitted with bearing shells lined with white metal and assembledin the same manner as the crosshead bearing.
Both the crosshead bearing shells and the crankpin bearing shells are retained in position by means of screws fitted in the bearing housings.
Question:In Sulzer engines, there is a separate crosshead lubricating oil pump delivering oil at 12 to 16 bar for crosshead lubrication.
MAN+B&W uses the same pump to supply oil to main bearings as well as crosshead at a much reduced pressure (around 3 to 5 bar).
Both engines run satisfactorily without any crosshead problems and certainly MAN+B&W arrangement is more simple.
The question is what is the difference from design aspect of the crosshead lubrication arrangements in the two engines that make it necessary to have a separate high pressure pump in one make of engine, while the other one does not need such pump?Answer:"More than 90% of the circulated oil has the sole purpose of cooling the bearings. If you study antique machines with open crankcases, you will see that
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Crosshead bearing
the amount of oil for lubrication is a few drops per minute. This is enough for maintaining the oil film in the bearing and with an open crankcase the friction heat is removed by air-cooling. Modern engines have closed crankcaes and a much higher bearing load - hence the need for oil cooling.
In a main bearing, the oil is pumped into the upper shell and it will cool the upper part of the journal. Since the shaft is rotating, it is cooled on all sides and because the oil film thickness is very small in the loaded part, the shaft will cool the loaded bearing half as well.
A crosshead bearing is only oscillating and the lower shell is always loaded. The cooling oil must be injected between shaft (crosshead pin) and lower bearing.
In MAN B&W engines, we have machined a set of channels in the lower crosshead bearing, in which the cooling oil can pass. The geometry is designed in such a way that all the loaded square centimeters of the pin are flushed with cooling oil twice, every engine cycle.
In contrast, the Sulzer croshead has a plain lower bearing without channels. In order to inject oil between pin and bearing, they have to supply oil at a much higher pressure. The injection will take place at around 20 degrees crank angle before TDC, where the cylinder pressure is still low and upward inertia forces on piston is still high. There is a short interval, in which the bearing pressure is lower than the oil pressure"
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MAN B&W
Sulzer
Crosshead bearing
Cross head Thin Shell Bearing The type of bearing used in the crosshead assembly is a thin shell (insert) bearing The lower shell is a trimetal shell, i.e. the shell is composed of a steel back with cast-on white metal and an overlayer coating. The upper shell is a bimetal shell, as it does not have the overlayer coating; both the upper and lower shells are protected against corrosion with tin flash.
Bearing Function and ConfigurationBecause of the oscillating movement and low sliding speed of the crosshead bearing, the hydrodynamic oil film is generated through special oil wedges on either side of the axial oil supply grooves situated in the loaded area of the bearing.
The oil film generated in this manner can be rather thin. This makes the demands for pinsurface roughness and oil wedge geometry very important parameters for the assemblyto function. A further requirement is effective cooling which is ensured by the transverseoil grooves. The pin surface is superfinished.The lower shell is executed with a special surface geometry (embedded arc) which extends over a 120 degree arc, and ensures a uniform load distribution on the bearing surface in contact with the pin. The lower shell is coated with an overlayer, which enables the pin sliding geometry to conform with the bearing surface.
Guide Shoes & guide strips
The guide shoes, which are mounted on the fore and aft ends of the crosshead pins, slide between guides and transform the reciprocating movement of the piston/piston rod via the connecting rod into a rotational movement of the crankshaft.
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Crosshead bearing
The guide shoe is positioned relatively to the crosshead pin with a positioningpin screwed into the guide shoe, the end of the positioning pin protrudes intoa hole in the crosshead pin and restricts the rotational movement of the crossheadpin when the engine is turned with the piston rod disconnected.
The guide strips are bolted on to the inner side of the guide shoes and ensure the correct position of the piston rod in the fore-and-aft direction. This alignment and the clearance between the guide strips and guide is made with shims between the strips and the guide shoe.
The sliding surfaces of the guide shoes and guide strips are provided with cast-on white metal and furnished with transverse oil supply grooves and wedges
Guide Clearance
The usual way of checking guide clearance is by means of a feeler gauge with the piston forced hard against one face and the total clearance taken at the other face. This gives a reasonable estimation as wear should be approximately the same in the ahead and astern faces. A more accurate idea can be gained by chocking the piston centrally in its bore than measuring the clearance at each face. This will also give the athwartships alignment. The edges of the guide shoes are also white metal faced and these run against rubbing strips. Clearance at these faces can be checked with feelers and this gives the fore and aft alignment.
Guide clearances are usually adjusted by means of shims between the hardened steel guide bars and the mounting points. Bolts are slackened off allowing
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Guide bar
Guide Shoe
Fwd/Aft Clearance
Port/Stbd Clearance
Crosshead bearing
slotted shims to be inserted or removed. Note, care must be taken when handling these shims.
Crank pin bearingThis assembly is mounted with a thin shell, and has two or four tensioning studs, depending on the engine type. Crankpin bearing assemblies with four studs must be tensioned in parallel, for example first the two forward studs and then the two aft most studs; the tensioning may be executed in two or three steps. This procedure is recommended in order to avoid a twist (angular displacement) of the bearing cap to the mating face on the connecting rod. The oil supply groove transition to the bearing sliding surface is similar to that of the main bearing geometry.
Check without Opening upa) Stop the engine and block the main starting valve and the starting air distributor.b) Engage the turning gear.c) Just after stopping the engine, while the oil is still circulating, check that uniform
oil jets appear from all the oil outlet grooves in the crosshead bearing lower shell and the guide shoes. The oil flow from the main and crankpin bearings must be compared from unit to unit; there should be a similarity in the flow patterns.
d) Turn the crank throw for the relevant cylinder unit to BDC position and stop the lube oil circulating pump
e) Check the top clearance with a feeler gauge. The change in clearances must be negligible when compared with the readings from the last inspection (overhaul).
f) Examine the sides of the bearing shell, guide shoes and guide strips, and check for squeezed-out or loosened metal; also look for white metal fragments in the oil pan.
g) In the following cases, the bearings must be dismantled for inspection, 1. Bearing running hot.2. Oil flow and oil jets uneven, reduced or missing.3. Increase of clearance since previous reading larger than 0.05 mm.4. White metal squeezed out, dislodged or missing at the bearing, guide shoe or guide strip ends.
Checks ObservationsOil Flow OK, similarity
Uneven
Oil JetsCrosshead, Guide strips
OK, similarityReduced
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Crosshead bearing
MissingTwisted
White metal OKSqueezedCracksLooseMissing
Crosshead guides OKScratchesCorrosionSilvery White
Oil pan OK, cleanWhite metal fragments
Oil condition OKDarkWater traces
Open up Inspection and OverhaulNote: Record the hydraulic pressure level when the nuts of the bearing cap go loose.Carefully wipe the running surfaces of the pin/journal and the bearing shell with a clean rag. Use a powerful lamp for inspection. Assessment of the metal condition and journal surface is made in accordance with the directions given below. The results should be entered in the engine log book.
Back of Shell • OK• Fretting• Trapped Hard Particles
Types of failureVarious types of failure occur in crosshead bearing linings. Some, if found early, can be rectified so that the bearing can satisfactorily continue in service, in other forms of failure the lining must be condemned and the bearing remetalled.
Failure of the white-metal in some less severe forms usually progresses so that eventually the bearing must be remetalled.The forms of failure are as follows.
1. Cracking of white metal due to fatigueFailure of the white-metal by cracking or fatigue due to increased dynamic stress levels in local areas of the bearing metal. It does not usually manifest itself under normal operation. If it is present it is usually found at the time the lower half of the cross head bearing is exposed for examination.
In the event of excessive local heat input, the fatigue strength of the white metal/tin aluminum will decrease, and thermal cracks are likely to develop at the normal dynamic stress level.
A small cluster of hairline cracks develops into a network of cracks. At an advanced stage, increased notch effect and the influence of the hydrodynamic oil pressure will tear the white metal from the steel back and produce loose and dislodged metal fragments.
Cause for Cracksa) Insufficient strength of the bonding between the white metal and the steel back (tinning or casting error).b) Crack development after a short working period may be due to a misalignment(e.g. a twist between the bearing cap and housing) or geometric irregularities (e.g. a step between the contact faces of the bearing shell, or incorrect oil wedge geometry).
c) High local loading: for example, if, during running-in, the load is concentrated on a few local high spots of the white metal.
Note: Bearings with cracks cannot be repaired.
2. Squeezing of white-metal so that oil grooves are partially blocked or obliterated; oil holes may be partially blocked, or wholly blocked in extreme cases.
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Crosshead bearing
Failure by squeezing or overheating may become apparent during engine operation; it is often noticed when checking bearing clearances or during crankcase examination
3. Failure by overheating, when the bearing surface of the white-metal becomes plastic or even melts.
4. Corrosion.Failure by corrosion is usually found when examination is carried out on the bearings, corrosion of the white-metal usually being accompanied by corrosion of the crosshead pins. The form of the corrosion will depend on the nature of the contamination of the lubricating oil.
Bearing damage can be prevented by correct maintenance and operation which address the following bearing failures:
As there are many and widely varying causes of failure, each case must be considered individually and the cause ascertained from the findings of careful examination. Bearings have been known to fail on new engines when the engine is being worked up to full power for the shop trials, or after many years of satisfactory service.
a) Tearing of the overlayer is due to sub-standard bonding. The damage is not confined to specific areas of the bearing surface. The white metal in the damaged area is seen clearly with a sharply defined overlayer border. This defect is regarded as a cosmetic defect, if it is confined to small areas of bearing surface.
b) Wiping of overlayer manifests itself by parts of the overlay being of the overlayer being smeared out. Wiping of overlayer can take place when running-in a new bearing; however, if the wiping is excessive, the cause must be found and rectified. One of the major causes of wiping is pin/journal surface roughness.
c) White metal wiping is due to metal contact between the sliding surfaces which causes increased frictional heat, resulting in plastic deformation (wiping).
Causes for Wipinga) Hard contact spots, e.g. originating from:
1. Defective pin/journal, bearing, or crosshead guide surfaces.2. Scraped bearing or guide shoe surfaces.3. Hard particles trapped between the shell.4. Fretting on the back of the shell and in the house boring
b) Increased pin/journal surface roughness:In most cases the increase in roughness will have occurred in service, and is attributed to:
1. Hard particle ingress:Hard particle ingress may be due to the malfunction of filters and/or centrifuges or loosened rust and scales from the piping. Therefore, always pay careful attention to oil cleanliness.
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Crosshead bearing
2. Corrosive attack:a) If the oil develops a week acidb) If the salt water content in the lube oil is higher than 1%. The water will attack the white metal, and result in formation of a very hard black tin-oxide encrustation (SnO) which may scratch and roughen the pin surface.
c) Inadequate lube oil supply
d) Impure lubricant or water contamination
e) Misalignment of running gear
f) Excessive firing pressure
Ships using heavy fuel are timed to inject fuel early to obtain good combustion. When during maneuvering such engine uses diesel oil , which is faster burning, the rate of pressure rise increases and the maximum combustion pressure may well exceed that for which the engine was designed. Therefore fuel quality lever or VIT needs to be adjusted. Excessive combustion pressure will show up as by cylinder relief valve lifting.
Repair of Oil Transitions(Wedges, tangential run out and bore relief)
Formation of sharp ridges or incorrect inclination of the transition to the bearing surface will seriously disrupt the flow of oil to the bearing surface, causing oil starvation at this location.
Oil transitions are reconditioned by carefully cleaning accumulated metal with a straight edge or another suitable tool. Oil wedges should be rebuilt to the required inclination (maximum 1/100) and length,
Bearing Wear RateThe reduction of shell thickness in the loaded area of the main, crankpin and crosshead bearing in a given time interval represents the wear rate of the bearing. Average bearing wear rate is 0.01 mm/10,000 hrs. As long as the wear rate is in the region of this value, the bearing function can be regarded as normal.
For crosshead bearings, the wear limit is confined to about 50% reduction of the oil wedge length
Journal Roughness
Limits to surface roughness The surface roughness should always be within the specified limits.
1. For main and crankpin journals:a) New journals 0.8 Rab) Roughness approaching 1.6 Ra (journal to be reconditioned).
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Crosshead bearing
2. For crosshead pins:a) New or repolished 0.05 Rab) Acceptable in service 0.05-0.1 Rac) Repolishing if over 0.1 Ra
Repairs of Journals/Pinsa) Crosshead pinsPin surface roughness should be less than 0.1 Ra . If the Ra value is higher than 0.1 µm, the pin can often be repolished on the spot,
If there are also deep scratches, these must be leveled out carefully with 3M polishing paper, or similar, before the polishing process is started.
Use a steel ruler, or similar, to support the polishing paper, as the fingertips are too flexible.The surface roughness after polishing should be 0.05 Ra.
The following methods are recommended for repolishing on the spot.1. Polishing with microfinishing film The polishing process is carried out with a ``microfinishing film'', e.g. 3Maluminium oxide (30 micron and 15micron), which can be recommendedas a fairly quick and easy method, although the best solution will oftenbe to send the crosshead ashore.
2. Braided hemp rope methodThis method is executed with a braided hemp rope and jeweller’s rouge.A mixture of polishing wax and gas oil (forming an abrasive paste of a suitably soft consistency) is to be applied to the rope at regular intervals.During the polishing operation, the rope must move slowly from one end of the pin to the other.
Measurement of clearance for SMC Engine
a) Turn the crank throw concerned to 90° before BDC on the exhaust side.
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Crosshead bearing
b) Measure the clearance in the crosshead bearing by inserting a feeler gauge between the bearing cap and the crosshead journal, exactly next to the landing surface for the piston rod foot.
c) The wear limit for a crosshead bearing shell is confined to 50% reduction of the oil wedge length (L).
