Thin shell bearings are used for most bearing applications in the main engine. They consist of a steel backing strip coated with a layer of white metal. Bearings may be bi-metal or tri-metal. typical materials are steel-babbitt, steel-bronze or steel-tin/aluminium ( tin-aluminium has slightly greater load bearing capacity than white metal and maintains its fatigue strength over a greater range of temperatures. The bearing metal thickness is 0.5 to 3mm. An Overlay of 20 -40 micrometersmay be applied to improve conformity . This is generally a ductile coating of lead and tin. In addition new bearings may have a flashlayer of a few microns of tin to prvent oxidation
An intermediate layer may be used between the overlay and main bearing metal to avoid diffusion. This is particularly found where bearing loads are very such as in the lower half of the cross head bearing. The layer is galvanically applied.
Compared with the traditional cast bearing they have a number of advantages.
Shell bearings sometimes have a layer of copper or bronze between the steel and the white metal in order to improve adhesion of the white metal. This layer will also provide safe guard in the event that the white metal being worm away.
For camshafts shell bearings are still used in preference to ball or roller race, the action of the cam followers provides impact loading which can must be supported by the bearing. Ball or roller race would tend to suffer fatigue or brinelling damage. In addition to this replacement is simpler with the plain bearing.
Bearing wear must be checked as this can allow the camshaft to drop thus altering the timing.
WHITE METAL BEARING CORROSION
White metals are tin based, that is they contain a higher proportion of tin then other compounds. A typical composition might be 86% tin, 8.5% Antimony, 5.5% lead.
In the presence of an electrolyte corrosion of the tin can occur forming extremely hard, brittle, stannous and stannic oxides (mainly stannic oxide Sn20) normally in the presence of moisture. These oxides are usually of a grey to grey black coloured surface layer on the white metal, either in local patches or completely covering the bearing. The hardness of this brittle oxide layer could be as high as twice that of steel and if it became detached, possible due to fatigue failure, serious damage to bearing and journal surfaces could occur. The formation of the oxide layer is accompanied by an upward growth from the white metal, which can considerably reduce clearances and could lead to overheating and seizure etc.
Factors which appear to contribute towards the formation of tin oxides are
Additives in the lubricating oil can add some degree of protection as can efficient centrifuging.
Stannic Oxide being much harder than the white metal causes two problems:-
Both these result in scoring of the journal, these are normally considered as a low temperature type of failure. In addition the presence of water in the lub oil can cause the oxidation of the metals in the bearing causing the metal to grow. This reduces clearance and can lead to bearing failure.
Rolling Element Bearings
The advantages and disadvantages of rolling bearings
Rolling element bearing use Elasto-hydrodynamic lubrication. That is, the surface material deforms to assist with the formation and shape of the oil wedge.
As the roller moves over the surface the latter deforms in a pressure wave. Hertzian stress build up parallel to the surface of the material in which defects can occur. Defects may also form due to the pressure wave. This defects may open and close freely with the passing of the element. Should liquid enter the defect then hydraulic lock occurs and the defect grows to relieve the stress. When the surface defects join the subsurface hydraulic locking again occurs and these also grow. Eventually that portion of the material become weakened to an extent a portion is displaced. This is a generalisation of the mechanism associated to pitting. Another significant mechanism for the failure of rolling element bearing is spalling where subsurface defects lead to the detachment of sections of material
The most effective method for detection of bearing failure is by vibration monitoring. This may be by observing the vibration velocity history. Theoretically the vibration characteristic should have well defined nodes at such frequencies as outer race element pass. This is of use in determining that high vibration with an assembly is specifically caused by the rolling element bearings
Rolling element bearings traditionally generate vibrations over a wide frequency range. Trending of this vibration over a period of time will allow estimation of the current condition of the bearing. A general increase in the high frequency vibration is generally associated with the creation of microscopic cracks and spalling too small for human eye to detect. Although this is not considered destructive in itself it is a preliminary stage that leads to further degradation
Alternately the vibration acceleration level may be measured. In both cases deteriorating bearing conditions is indicated by increased vibration levels. Other methods include bearing temperature measurements and analysis of lubricating oil
Early detection of bearing failure prevents damage such as housing fretting and shaft distortion due to overheating remedy of which far exceeds that of bearing replacement only.
Normal Life Failure
Shown is a bearing demonstrating normal wear pattern for a vertically loaded shaft with rotating inner ring and fixed outer race. The pattern on the outer race is displaced depending on the line of action of the loading.
Although shown unifrom the actual wear can be of a more random pattern and careful interpretation is required.
Bearing failure not associated with fatigue can occur due to several reasons but most commonly misalignment, over greasing and contamination
Typically 1/3rd of the available space should be filed with grease . For some installations a weight of grease is given by the manufacturer.
Contamination is the most common cause of premature failure. This occurs usually during bearing installation and typically is caused by poor house keeping or dirt loaded grease. Excessive wear is indicated by a dulled appearance and indented running surface.
Bearings damaged due to this tend to have indentations, and scratches both on the race and the rolling elements. This effect is sometimes referred to as Scoring
Is sometimes identified by a wide contact track on the inner race and a thinner non perpendicular track on the outer race. Misalignment by as small amount as 1/1000 can lead to serious reductions in life expectancy
In the short term the removal of manufacturing asperities leads to a highly polished mirror like appearance. This may progress to a surface crystalline with dark lines showing a crystalline structure. This is caused by incorrect specification for lubrication or overheating reducing viscosity
Generally leads to overheating to a blue/black colour on the load surface reducing to a straw/gold color on the edges of the bearing
Caused by water ingress into the bearing leading to red of black patches on all surfaces. Generally associated with inefficient sealing and poor lubricant properties.
This comes in two forms;
this caused when the elastic limit of the race material is exceeded and a permanent deformation occurs. Associated with a sharp impact loading sometimes occurring during poor installation procedures. Impacts occur at rolling element pitch False Brinelling
Sometimes referred to as 'washboarding' and is caused by relative movement of the elements without formation of an oil film. Haematite deposits may be found in the pits
More associated with high background vibration. Damage appearance is distinctive and different to normal pitting. Shown is that seen with ball elements, roller elements are linear. Both are at element pitch or multiples of it. The use of quality EP additives can reduce progress of damage.
It should be noted that false brinelling can also lead to a similar fluting appearance but the pits are brighter and contain corrosion products, where due to the passage of electricity the pits are generally dark.
Roller bearings are generally more susceptible to this type of damage thus ball bearings are preferred in high vibration areas, part submersion in oil bath also reduces this effect.
Leads to overheating and spalling (surface material loss) of the running surfaces typically in the direction of overload. May be remedied by more appropriate selection of bearings
Typically uneven craters and pits. Associated with contaminated or harderned grease but also occurs with oil lubricated bearings containing wear debris
Associated with incorrect fit, uneven race support or severe overloading
This leads to fretting between the contact areas of the bearing/housing or bearing/shaft. Indication that this is occurring is by the presence of black or dark red coloration or deposits on the landing areas. As this worsens the inner or outer race may begin to creep or rotate relative to the adjacent landing area
The initiator for this is often the increased vibration from the bearing as it begins to reach the end of its life. This vibration is destructive before noise, heat or vibration level reach is detectable without the use of an instrument. Once the fit is lost in this manner the bearing no longer can offer reliable load bearing capacity. The use of 'Loctite Bearing Fit' can solve minor fit problems. The technique of raising the surface, for example using a centre punch has very limited benefit
Where the interference fit is greater than the radial fit very high bearing temperatures are generated due to overloading and premature failure results Discoloured wide contact areas, at the extremes leads to very heavy overloading, high temperatures and cracking. May lead to restriction in movement of the rolling elements and lead to scuffing of the running areas through welding
Associated with poor lubrication or incorrectly specified bearing type. Worsens to a point that the cage fails, the elements move allowing the relative positions of the outer and inner races to change.
This is caused by lack of lubrication and relative sliding motion between the element surfaces. It is commonly seen on the ends of roller bearings where they have been subjected to axial forces.
The material is generally heated and some transfer occurs between the parts. Surface rehardening can occur with tear fractures evident.
This effect may be seen where rollers are subject to high accelerations when entering the load zone ( this occurs as the elements are not driven out when out of the load zone and can be remedied by reducing bearing internal clearance), excessive preloading for taper bearings, or even in ball bearing where load is too light in relation to the speed of rotation. Careful selection of lubricant can avoid this damage.
Roller bearings may be subject to smearing on assembly due to poor assembly allowing the elements to 'scrape' the surface on the race
Smearing may occur on the non running outer edges of the races due to relative movements between housing/shaft and the bearing. This may be remedied by increasing the interference fit.
This causes either deep cratering or thin pits containing dark product ( refered to fluting).
Installation of rolling element bearings should be done in as clean an environment as possible. The leading cause of premature bearing failure is contamination.
Care should be taken not to cause impact lading to the bearing which may lead to brinelling. It is preferable that the bearing be preheated by oil bath or induction heater before fit. Where not possible then the bearing should be evenly pressed
The correct quantity and type of grease should be used.
Although not common it is not unusual for bearings to initially run with high temperatures or increased load. This may be associated to stresses left over from when the bearing was pressed onto the landing areas Care should be taken that the bearings are not allowed to overheat.
For smaller motors this may be successfully compensated by light tapping of the shaft I opposite directions until and 'tightness' is reduced. For larger assemblies a sequence of running to a limited temperature followed by a period of cooling generally leads to a stabilization and then normal running temperatures.
Improving Bearing Life
Improved bearing life may be gained by the use of better or more appropriate greases. Where possible the grease should be replaced to remove wear particles and restore levels of EP additives. For Oil lubricated bearings the use of very fine filtration is essential
It is understood that 'Hybrid' bearings consisting of a relatively standard steel rolling element bearing into which is inserted a single ceramic element. The ceramic element is believed to polish the running surface removing surface defects.
For bearing used in electrical installations the use of Teflon coatings to the outer race landing surface prevents electrical discharge through the bearings.