Talk about the causes and countermeasures of crankshaft bearing damage in marine diesel engines

Diesel engine installed on board, long-term work in the harsh environment of the sea, inevitably there will be a variety of failures, the light cause parts damage, increase the cost of ship operations; heavy machine damage endanger the ship and life safety, bringing huge economic losses. In a diesel engine, the value of the diesel engine crankshaft accounts for about one-third of the whole machine, the crankshaft in the bearing protection work, if not well managed, may cause failure, repair and replacement difficulties and long repair cycle. The author cited some cases of crankshaft bearing damage in the ship diesel engine, the diesel engine crankshaft bearing damage causes and how to reduce its damage to do a shallow analysis of the problem for the reference of engine managers.

A ship diesel engine crankshaft bearing damage case

in 2008, a ship (diesel engine model B&W 8TLH), the incident found some changes in the slip oil pressure, sound some abnormal, cleaning the slip oil filter, found that there are alloy shedding material, after a thorough inspection found that all the eight connecting rod shaft tiles cracked, crank pin hair pulling. 2009, a ship (diesel engine model: 8PSHTB -26D) No. 2 In 2010, a wheel (diesel engine model 6270C) was found to have abnormal diesel engine sound and stopped at a reduced speed when the incident occurred. After investigation, a total of four main bearings burned, holding the crankshaft, resulting in the whole crankshaft scrap.

Second, the form of bearing damage and cause analysis

through the above case is not difficult to see, crankshaft bearing damage is mainly on the wear-resistant alloy damage, its main damage form: excessive wear, cracking and spalling, corrosion and burning melt.

1, excessive wear of the shaft tile and its cause analysis of the diesel engine after a period of operation so that the main bearing under the tile, crosshead bearing under the tile and crank pin bearing on the tile to produce excessive wear. Excessive wear of the shaft tile will make the bearing clearance increase, oil film damage, oil pressure drop increases the impact, intensifying wear. Causes excessive wear of the shaft tile is mainly:

(1) poor purification of lubricating oil, oil contains more mechanical impurities.

(2) journal surface accuracy level is too low, geometry error is too large and crankshaft deformation, etc.

(3) Frequent starting and stopping of diesel engines and long time over-speed and over-load operation.

(4) Other daily maintenance is not good, and even illegal operation, etc. All the above reasons, either make the bearing lubricating oil film can not be established or established bad; or due to abrasive particles, journal surface finish or excessive bearing load, damage has formed the oil film, resulting in excessive wear of the shaft tile.

2, shaft tile cracking and peeling and its cause analysis cracking and peeling mainly occurs in the thick-walled alloy shaft tile. Initially due to a variety of reasons in the shaft tile work surface produced small fatigue cracks. As the diesel engine continues to run, the cracks on the shaft tile expand and extend to cause the wear-resistant alloy on the shaft tile to flake off. The reasons for cracking and flaking are:

(1) The fatigue strength of the white alloy material is low, and fatigue cracks are easily produced under alternating loads.

(2) The geometric shape of the journal error is too large and excessive wear of the shaft tile will make the shaft tile subject to excessive impact load, so that the shaft tile cracks.

(3) Diesel engine overload makes the bearing load too large to cause cracking of the shaft tile.

(4) poor casting quality of the shaft tile, such as alloy layer and tile shell fit poorly or between the two embedded foreign objects, etc., in the alternating load to make the shaft tile cracking and alloy layer flaking.

3, shaft tile corrosion and burning melt and its cause analysis of shaft tile corrosion, including chemical corrosion and electrical corrosion. As the lubricating oil contains water or slip oil oxidation, gas or fuel oil mixing into the slip oil deterioration will make the shaft tile working surface macro and micro chemical corrosion pockmark; ship’s stray current is electrical leakage caused by, it will also make the shaft tile inside and outside the surface of the local pockmark electrostatic corrosion.

Shaft tile alloy burning melt is a common bearing failure, mainly due to bearing clearance is too large, lubricating oil oil pressure is insufficient or loss of pressure and foreign body into, in the bearing internal continuous destruction of alloy surface so that the oil film can not be established, journal surface finish is too low or geometric shape error is too large, etc. so that the oil film destruction. Oil film can not be established or destroyed will make the shaft and tile metal direct contact, dry friction generated by high temperature so that the alloy melted.

Third, to prevent bearing damage management countermeasures

Through the analysis of bearing damage form and damage causes, to change the diesel engine crankshaft bearings frequently damaged condition, can take the following countermeasures:

1, the requirements of the crankshaft diesel engine long-term operation so that the crankshaft main journal and crank pin neck uneven wear. Usually in the inspection, its diameter is reduced, geometric accuracy is reduced, the production of non-roundness and non-columnar, beyond the specification should be repaired, otherwise it will affect the bearing life.

2, the requirements of the new tile check whether the new tile deformation and other defects, such as alloy layer and tile shell whether bonding, alloy layer whether cracks, oil groove and rubbish groove is normal, measurement and record shaft tile thickness, width and expansion amount and so on is in line with the requirements.

3、For bearing installation requirements
(1) Shaft tile and bearing seat hole with good surface fit: shaft tile installation should be with the bearing seat hole inside the round surface fit closely and uniform contact, check the use of 0.05mm ruler, such as inserted into, that the two fit closely. Thick-walled shaft tile under the tile installed in the bearing seat fit the situation available tile, seat surface coloring to check, if the back of the shaft tile dip in 25mmx25mm area to not less than 3 points when good. Thin-walled shaft tile of the lower tile and bearing seat close fit can be achieved through the shaft tile and bearing seat hole interference fit. Shaft tile in the free state expansion amount is usually recommended for no flange tile 0.3 ~ 1.0mm, flange tile 0.1 ~ 0.4mm.

(2) journal and bearing under the tile should be in a certain angle of uniform contact installation requirements contact area should be greater than 75%, diesel engine main journal and main bearing under the tile contact angle should be in the body centerline on both sides of 40 ~ 60 ° range; crank pin neck and big end bearing. The contact surface of the upper tile should be in the range of 60~90° on both sides of the centerline of the connecting rod.

(3) bearing clearance should be in line with the provisions of the appropriate bearing clearance is the formation of lubricating oil film to achieve the important conditions of liquid dynamic pressure lubrication, bearing clearance is too large, too small oil film can not be established; clearance exceeds the limit, the loss of lubricating oil and produce impact, so that the shaft tile alloy cracks, broken, so the requirements of the shaft and the bearing clearance between the shaft tile to be in the range of clearance specified in the manual.

4、Requirement in daily management
(1)To the lubricating oil must do: regular laboratory analysis, master the oil used viscosity, flash point, freezing point, total alkali value, moisture, impurity situation; seriously do a good job of separating impurities, moisture and oil purifier maintenance work, so that the oil used to keep in the best condition; regularly clean the lubricating oil filter, check whether there is metal powder; pay attention to the influence of ambient temperature on the viscosity of lubricating oil.
(2) The inflection gear difference, bearing sinkage and bearing clearance must be measured regularly to judge the bearing wear according to the figures measured. The bearings should be dismantled and replaced according to the running time specified in the manual, and the diesel engine should be strictly controlled to start and stop frequently, and never allowed to work at excessive speed and load for a long time and operate against regulations.
(3) Strictly implement the duty system, always pay attention to the changes of oil lubrication pressure, temperature and cooling water temperature, and judge whether the oil pipeline leaks, breaks and changes in the viscosity of lubricating oil, so as to ensure that the bearings are well lubricated.

5、The requirements for engine management personnel and engine supervisors
Accidents must have precursors. Take crankshaft bearing damage as an example, its main manifestation is the oil pressure, oil temperature and oil cooling water temperature abnormal. As long as the engine personnel have a strong sense of responsibility, carry out the duty system and duty tour inspection system carefully, and check, touch and see diligently, all these problems can be found in time. Once the bearing is found to have signs of damage, stopping measures should be taken in time and checked and eliminated, avoid sailing with disease, causing crankshaft damage and expanding the accident. The mechanic should often go into the front line of the ship to solve the problems for the ship. According to the monthly report, the ship’s machinery and equipment not maintained according to the requirements of supervision and inspection or shore-based support should be carried out to prevent the ship’s maintenance personnel not in place or human factors caused by machine damage accidents.
IV. Conclusion
Damage to the crankshaft bearings of marine diesel engines is the first in the annual statistics of machine damage accidents, and its great harm is self-evident. Diesel engine crankshaft bearing damage for complex reasons, in the analysis of the machine because of the time, combined with the specific circumstances of comprehensive consideration, to avoid one-sided thinking. Such as a single parameter by virtue of a (normal oil pressure) that there is no problem with the bearing idea is wrong. After the correct analysis of the causes, repair to follow, and repair to be in place, the most avoid perfunctory, leaving the accident hidden trouble.
In short, the most effective way to control crankshaft bearing damage is to rely on the correct operation and maintenance of engine management personnel, and diligent observation and diligent thinking, to eliminate accidents in the bud, to truly achieve the goal of reducing marine diesel engine crankshaft bearing damage.

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