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Some possible things you could try to find are (depending on the topic):

- In what kind of devices there are these kind of wear issues? What are the main reasons causing them?

- Which materials are usually used / present in the applications and why (what are their good properties for thespecific applications)? What are the effective materials in this tribosystem (e.g., if there is abrasive wear, which is theabrasive)?

- Which wear mechanisms / modes are present in the system and which are the most common ones? How do theyaffect the device – e.g., the usability? What is the most likely cause to the problems? Where specifically do thesewear mechanisms occur (which part of device / component)?

- How is the wear observed?

- Which environmental conditions or other parameters are important in these contacts (e.g. temperature, humidity orother things)? Which factors promote and prevent certain wear mechanisms - environmental conditions etc.?

 A ball bearing is a type of  rolling-element bearing that uses balls to maintain theseparation between the bearing races. The purpose of a ball bearing is to reducerotational friction and support radial and axial loads. It achieves this by using at leasttwo races to contain the balls and transmit the loads through the balls. In mostapplications, one race is stationary and the other is attached to the rotating assembly(e.g., a hub or shaft). As one of the bearing races rotates it causes the balls to rotate aswell. Because the balls are rolling they have a much lower  coefficient of friction than iftwo flat surfaces were sliding against each other.

All machined metal surfaces appear rough under the microscope. The individual roughnesses are of

varying height and depth. When two bearing surfaces are brought into contact, they touch at an

extremely small number of points.

How can the wear rate be tested? Which test methods could be used?

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Bearing test rig (http://www.nesbearings.com/index.php?testing)

Through testing, suitable materials can be selected for certain applications. To benefit fromthe results, a proper testing method must be selected to determine the important properties.

 A material that has been performing successfully in abrasive wear test, may have poor wearresistance in a different wear form. To make things complicated, a component can besubjected to multiple types of wear. In addition to wear mechanism, size matters Wearphenomena depend on the scale too. Testing can save time as the phenomena areaccelerated. However it must be ensured that the increase in load/speed does not alter theeffective wear mechanisms. More than just one kind of test may be necessary to evaluatethe suitability of the material.

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To obtain steady-state data, time-dependent changes in wear behaviour must be minimizedby subjecting the samples to a running-in period before the actual test. Several tests mustbe conducted with each material in each condition to evaluate the test repeatability.Four requirements for testing:

1. Understanding the characteristics of the test method being applied2. Expecting differing degrees of repeatability from different material types3. Selecting the right testing tool for the job4. Coupling measurements with physical observations of contact surfaces to ascertain

the causes for the physical behaviour.

Many of the wear tests are customized, non-standard tests to fulfill the requirements ofsimulating certain operating conditions and a plain number will not reveal much about amaterial’s wear resistance. It is useful to compare the acquired data with results from areference material. The reference material should be a well-known material with which thereis only very little scatter in the results meaning that the results are well repeatable. Withreference materials, the condition of the wear testing equipment and the reliability of theresults can be determined.

Documenting the tests:Wear testing equipmentGeometry of contactMotion type (sliding, rolling, continuous, oscillating)

 Applied load perpendicular to contact areaVelocityEnvironmental conditions (temperature, humidity)Wearing mediums (abrasives)Materials (also the counterface)LubricantsIf a run-in/wear-in period is applied, description of it

Specimen preparation (surface finish etc)DurationDateObservations

Many types of frictional tests exist:With proper lubrication: the material pair is separated by a lubricant film; the frictioncoefficient is the friction coefficient of the lubricant.

If partial contact exists, the friction coefficient is a sum of many aspects. With fluid, thefriction coefficients can be serveral orders of magnitude lower compared to those of solid

contacts.

There are various standards of different wear tests to enable comparison of test results.Some equipment are commercially available, for example pin-on-disc or rubber-wheels.

If the equipment is self-constructed even according to standards, some deviation in resultsmay occur. Standard or close-to-standard equipment are normally ‘model’ or ‘semi-tribocouple’ tests (e.g. pin-on-disk). When close to reality tests are required, often tailor-made ‘semi-tribocouple’ or ‘tribocouple’ test equipment is needed. 

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 Testing for abrasive wearThe abrasive is varied in terms of size, shape, hardness.Using a tribometer test. The

In adhesion, many phenomena may be effective. Even minor alterations in the environmentcan lead to changes. Test results might be difficult to interpret.

Which factors should be considered in testing these parts? What kind of

parameters should be used?

How varying certain parameters will affect the rates of the different wear processes.

Total wear rate is affected by the forces required to (1) shear metallic junctions (2) plowthrough softer metal (3) shear the surface films and/or lubricant.

MaterialsMaterial properties of Bearing & Test surface: Strength, Ductility, Corrosion resistance.Typical bearing materials have been:Chrome Steel (for environments where corrosion resistance is not critical)SAE 52100 Steel is most comon (extreme noise-sensitive applications)DD400 Matersitic stainless steel combined with heat treatment (superior hardness, lownoise characteristics, decent corrosion resistance)

Synergetic effects of the two surfaces, namely the Bearing and Test surfaces

The properties of the abrasive affect the abrasive wear rate

Specimen preparation and storage according to same procedure

Test design LoadingThe rate of corrosive wear may be increased by increasing the load

Motion type

Control of the apparatus operating parameters

Unwanted effects like uncontrolled vibration Environmental conditionsFailure to include corrosion-preventive measures during installation can result in rusting ofbearings, thus producing abrasive wear and pitting of the races.

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Temperature, humidity, etc. may vary depending on the season

The effect of temperature on wear is complicated by opposing factors. The increasingductility of metals (with increasing temperature) aids junction growth and seizure, loweringadhesion wear rate.

With increasing air temperature, the rate of oxide formation also increases, resulting intransition in the coefficient of friction at a critical value of oxide thickness.

In the event of a high vacuum, for example in space, the effect of oxide formation isnegligible.

Contamination

Wear can occur due to contamination by foreign matter and corrosion. Fretting corrosioncan be a resulting phenomena; it occurs at the contact area between two materials under loadand subject to minute relative motion by vibration or some other force. A rust like powder isresulted.

Chlorine gas, ozone, and other chemicals will shorten bearing life, due to contaminationhttp://www.nmbtc.com/engineering/documents/NMBBallBearingsEngineeringSection.pdf

(Seals are necessary to provide optimum ball bearing life by retaining lubricants andpreventing contaminants from reaching central work surfaces) Lubrication

Presence of lubricant

  Amount of lubricant: Excessive grease can cause shearing, heat buildup. 

Lubricant properties

  Type of lubricant:

Oils: Used to cover all liquid lubricants. Previously most lubricating oil was refined frompetroleum. Today, synthetic oils have found accpetance due to improved properties. Basiclubricant for ball bearings.

Greases: Oils which contain a thickening agent to make them semi-solid, preventing oilmigration from the lubrication site. Used when frequent lubricant replenishment isundesirable. Use of grease rather than oil results in higher starting and running torque andcan limit the bearing to lower speeds.

Dry lubricants: Include any lubricants in solid form such as loose powders

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 Gases: Such as air

  Low temperature properties, volatility, temperature/viscosity characteristics

Diesters in general have better low temperature properties, lower volatility, and bettertemperature/viscosity characteristics. Silicons and fluorinated compounds possess evenlower volatility and wider temperature/viscosity properties.

Petroleum mineral lubricant are useable only at moderate temperature ranges (-30 to 120degrees celsius)

  Chemical properties

Petroleum and diester oils contain additives that limit chemical changes/increase oxidationresistance and protect the bearing metal from corrosion.

(Improperly lubricated bearings and incomplete or flawed wear analysis of bearings canlead to destructive wear and bearing failures. Using an oil which is too volatile can result insignificant viscosity increase through evaporation, leading to excessive power demands onthe drive.)

Pressure, absence of oxygen, temperature extremes, electromagnetic radiation, candrastically affect lubricants and the bearing surface.

Thin film lubrication

Thick film lubrication