Rolling Bearings

Add caption

The advantages and disadvantages of rolling bearings

Advantage
1. Low starting torque
2. compact and ready to install machine members
3. It is generally easier to lubricate rolling bearings than plain bearings
4. Combined radial and thrust loads can be carried readily in a rolling bearing
5. High overloads can be carried for short periods without undue or adverse effects
6. Rolling bearings become noisy when nearing the end of their natural life.
7. Running clearances is less than plain bearings, preloading can reduce deflection and increase accuracy.
8. Standardisation by international agreement makes replacement and servicing easier
9. Better electrical insulation is possible than with plain bearings
10. Can be multifunctional e.g. combined water pump and fan on the same shaft

Disadvantage
1. More expensive than plain bearings with more parts to go wrong
2. They are noisier than plain bearings
3. Their use at high speeds and loads are limited by centrifugal, fatigue and brinelling effects. Plain bearings can run faster and carry heavier loads.
4. Brinelling, fretting can occur during static conditions when the bearing is subjected to vibration
5. All rolling bearings have a finite life. If a large number of a particular bearing are produced for a particular application a percentage of premature failures can be anticipated
6. Repair of rolling bearings is almost impossible in an average workshop. The entire unit has to be replaced when worn or failed

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. Anouther 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.

Premature failure

Bearing failure not associated with fatigue can occur due to several reasons but most commonly misalignment, over greasing and contamination

Over greasing

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

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

Misalignment

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

Lubrication failure

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

Rusting

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.

Brinelling

This comes in two forms;

True Brinelling

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. Hematite rust may be evident 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.

Overloading

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

Indentations

Typically uneven craters and pits. Associated with contaminated or harderned grease but also occurs with oil lubricated bearings containing wear debris

Cracking

Associated with incorrect fit, uneven race support or severe overloading

Poor Fit

Loose fit
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

Tight fit

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

Cage Wear

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.

Smearing

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.

Electrical discharge

This causes either deep cratering or thin pits containing dark product ( refered to fluting).

Installation

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.

Case Study- Running in rolling element bearings

Despite good alignment one bearing out of four on a gas freeing fan shaft was found to overheat within 10 minutes of first start up. The bearing was allowed to cool, from 100 to 65'C then restarted. On the second instance the fan run for 30 minutes before a temperature of 100'c was reached. After a cool down and restart this was extended to over one hour. After repeating this procedure several more times the bearing temperature stabilized and the fan was found to operate reliably. A second instance was on a thruster motor where zero pitch amps was found to be 500 amps against 340 amps normal after motor overhaul. The bearing temperature and vibration where monitored and the unit run off load. The amps were found to reduced steadily over a period f 24hours until normal currents where experience. Again the unit entered into service successfully.

Case Study - Electrical discharge through bearing

There are benefits to inspecting all ball bearings removed from a machine to ascertain that no abnormalities exist within the installation A vessel had experienced a history of failure over a 25 year period in its horizontal twin screw Cargo pumps. Inspection after one such failure of the intact bearing set indicated an unusual form of damage.

This took the form of axial deep grooves ( sometimes referred to as Cratering) of a form not normally associated with false brinelling. A very similar form of failure is seen in electric motors caused by electrical discharge through the bearing. It should be noted that the pump was driven by a diesel engine which was also coupled to a generator. It was possible although not proven due to the vessel sale soon after that this was a partial cause.
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.