False brinelling

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False brinelling of a bearing FB-geschaedigtes Lager.jpg
False brinelling of a bearing

False brinelling is a bearing damage caused by fretting, with or without corrosion, [1] that causes imprints that look similar to brinelling, but are caused by a different mechanism. False brinelling may occur in bearings which act under small oscillations [2] or vibrations. [3]

Contents

The basic cause of false brinelling is that the design of the bearing does not have a method for redistribution of lubricant without large rotational movement of all bearing surfaces in the raceway. [4] Lubricant is pushed out of a loaded region during small oscillatory movements and vibration where the bearings surfaces repeatedly do not move very far. [5] Without lubricant, wear is increased when the small oscillatory movements occur again. It is possible for the resulting wear debris to oxidize and form an abrasive compound which further accelerates wear.

Mechanism of action

In normal operation, a rolling-element bearing has the rollers and races separated by a thin layer of lubricant such as grease or oil. [6] Although these lubricants normally appear liquid (not solid), under high pressure they act as solids and keep the bearing and race from touching. [7] [8]

If the lubricant is removed, the bearings and races can touch directly. While bearings and races appear smooth to the eye, they are microscopically rough. Thus, high points of each surface can touch, but "valleys" do not. The bearing load is thus spread over much less area increasing the contact stress, [9] causing pieces of each surface to break off or to become pressure-welded then break off when the bearing rolls on.

The broken-off pieces are also called wear debris. Wear debris is bad because it is relatively large compared to the surrounding surface finish and thus creates more regions of high contact stress. Worse, the steel in ordinary bearings can oxidize (rust), [10] producing a more abrasive compound which accelerates wear.

Simulation of false brinelling

The simulation of false brinelling is possible with the help of the finite element method. For the simulation, the relative displacements (slip) between rolling element and raceway as well as the pressure in the rolling contact are determined. For comparison between simulation and experiments, the friction work density is used, which is the product of friction coefficient, slip and local pressure. The simulation results can be used to determine critical application parameters or to explain the damage mechanisms. [11]

Comparison between simulated frictional work density and wear Simulation der Reibarbeitsdichte.png
Comparison between simulated frictional work density and wear

Physical simulation of the false brinelling mechanism has been standardized since the 1980's in the Fafnir Bearing test instrument, where two sets of thrust ball bearings are compressed with a fixed load, and the bearings are oscillated by an excentric arm under standardised conditions. This culminated in the ASTM D4170 standard. [12] Although an old method, this is still the leading quality control method for greases that need to avoid the false brinelling damage.

Test bearings for ASTM D4170 False Brinelling fretting wear test Test bearings Fafnir False Brinelling test.jpg
Test bearings for ASTM D4170 False Brinelling fretting wear test


False Brinelling Fretting wear tester FafnirFrettingWearTester.jpg
False Brinelling Fretting wear tester




Examples

False brinelling was first mentioned by Almen in 1937. [13] Almen found that wheel bearings were damaged before they were used by customers. Furthermore, he found that the bearings were more damaged for long-distance shipping of the cars and that the season of shipping also had an influence. The reason for the damaged bearings were micro-oscillations [14] which occurred due to the shipping. The small oscillations result in fatigue cracking, followed by release of particles that subsequently start to abrasively damage the contact area between a ball and the bearing race, resulting in a typical wear damage. Because the damage has a similar look to brinelling, it was called false brinelling. [15]

Example of an application in which false brinelling may occur Bike-Steering.png
Example of an application in which false brinelling may occur

Although the car-delivery problem has been solved, there are many modern examples. A major maintenance problem are the pitch bearings in wind turbines, for which specialty greases had to be developed that result in almost no false brinelling damage. Similar damage may also occur in electric and electronic contacts that are subjected to vibrations during use, think of aerospace and automotive connectors and even remote control battery compartments. Although the damage in these areas may not be as severe as the false brinelling in bearings, the damage mechanisms are similar and result in the creation of particles in the contact that can severely influence the electrical connection.

Also, generators or pumps may fail or need service because of this damage, so it is common to have a nearby spare unit which is left off most of the time but brought into service when needed. Surprisingly, however, vibration from the operating unit can cause bearing failure in the unit which is switched off. When that unit is turned on, the bearings may be noisy due to damage, and may fail completely within a few days or weeks [16] [17] even though the unit and its bearings are otherwise new. Common solutions include: keeping the spare unit at a distance from the one which is on and vibrating; manually rotating shafts of the spare units on a regular (for example, weekly) basis; or regularly switching between the units so that both are in regular (for example, weekly) operation.

Until recently, bicycle headsets tended to suffer from false brinelling in the "straight ahead" steering position, due to small movements caused by flexing of the fork. Good modern headsets incorporate a plain bearing to accommodate this flexing, leaving the ball race to provide pure rotational movement. [ citation needed ]

Bearings of modern wind turbines are often affected by false brinelling. Especially the pitch bearing, [18] which is used under oscillation, shows often false brinelling damages. [19]

Related Research Articles

<span class="mw-page-title-main">Friction</span> Force resisting sliding motion

Friction is the force resisting the relative motion of solid surfaces, fluid layers, and material elements sliding against each other. There are several types of friction:

A lubricant is a substance that helps to reduce friction between surfaces in mutual contact, which ultimately reduces the heat generated when the surfaces move. It may also have the function of transmitting forces, transporting foreign particles, or heating or cooling the surfaces. The property of reducing friction is known as lubricity.

<span class="mw-page-title-main">Ball bearing</span> Type of rolling-element bearing

A ball bearing is a type of rolling-element bearing that uses balls to maintain the separation between the bearing races.

Fluid bearings are bearings in which the load is supported by a thin layer of rapidly moving pressurized liquid or gas between the bearing surfaces. Since there is no contact between the moving parts, there is no sliding friction, allowing fluid bearings to have lower friction, wear and vibration than many other types of bearings. Thus, it is possible for some fluid bearings to have near-zero wear if operated correctly.

<span class="mw-page-title-main">Bearing (mechanical)</span> Mechanism to constrain relative movement to the desired motion and reduce friction

A bearing is a machine element that constrains relative motion to only the desired motion, and reduces friction between moving parts. The design of the bearing may, for example, provide for free linear movement of the moving part or for free rotation around a fixed axis; or, it may prevent a motion by controlling the vectors of normal forces that bear on the moving parts. Most bearings facilitate the desired motion by minimizing friction. Bearings are classified broadly according to the type of operation, the motions allowed, or to the directions of the loads (forces) applied to the parts.

<span class="mw-page-title-main">Lubrication</span> The presence of a material to reduce friction between two surfaces.

Lubrication is the process or technique of using a lubricant to reduce friction and wear and tear in a contact between two surfaces. The study of lubrication is a discipline in the field of tribology.

<span class="mw-page-title-main">Wear</span> Damaging, gradual removal or deformation of material at solid surfaces

Wear is the damaging, gradual removal or deformation of material at solid surfaces. Causes of wear can be mechanical or chemical. The study of wear and related processes is referred to as tribology.

Tribology is the science and engineering of interacting surfaces in relative motion. It includes the study and application of the principles of friction, lubrication and wear. Tribology is highly interdisciplinary, drawing on many academic fields, including physics, chemistry, materials science, mathematics, biology and engineering. People who work in the field of tribology are referred to as tribologists.

<span class="mw-page-title-main">Plain bearing</span> Simplest type of bearing, comprising just a bearing surface and no rolling elements

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<span class="mw-page-title-main">Rolling-element bearing</span> Bearing which carries a load with rolling elements placed between two grooved rings

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Fretting refers to wear and sometimes corrosion damage of loaded surfaces in contact while they encounter small oscillatory movements tangential to the surface. Fretting is caused by adhesion of contact surface asperities, which are subsequently broken again by the small movement. This breaking causes wear debris to be formed.

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Brinelling is the permanent indentation of a hard surface. It is named after the Brinell scale of hardness, in which a small ball is pushed against a hard surface at a preset level of force, and the depth and diameter of the mark indicates the Brinell hardness of the surface. Brinelling is permanent plastic deformation of a surface, and usually occurs while two surfaces in contact are stationary and the material yield strength has been exceeded.

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Pitch bearing Component connecting a turbine blade to the hub allowing pitch variation

The pitch bearing, also named blade bearing, is a component of modern wind turbines which connect the rotor hub and the rotor blade. The bearing allows the required oscillation to control the loads and power of the wind turbine. The pitch system brings the blade to the desired position by adapting the aerodynamic angle of attack. The pitch system is also used for emergency breaks of the turbine system.

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References

  1. ↑ Schwack, Fabian (25 May 2017). "Time-dependent analyses of wear in oscillating bearing applications (PDF Download Available)". ResearchGate. Retrieved 27 June 2017.
  2. ↑ Schwack, Fabian; Poll, Gerhard. "Service Life of Blade Bearings - Problems Faced in Service Life Estimation of Blade Bearings". ResearchGate. Retrieved 27 June 2017.
  3. ↑ Pittroff, Hans (1 September 1965). "Fretting Corrosion Caused by Vibration With Rolling Bearings Stationary". Journal of Basic Engineering. 87 (3): 713–723. doi:10.1115/1.3650657. ISSN   0098-2202.
  4. ↑ Schwack, Fabian; Bader, Norbert; Leckner, Johan; Demaille, Claire; Poll, Gerhard (15 August 2020). "A study of grease lubricants under wind turbine pitch bearing conditions". Wear. 454–455: 203335. doi: 10.1016/j.wear.2020.203335 . ISSN   0043-1648.
  5. ↑ Feng, Chen; Maruyama, Taisuke; Saito, Tsuyoshi (2009). "Oil Film Behavior under Minute Vibrating Conditions in EHL Point Contacts". Advanced Tribology. Springer, Berlin, Heidelberg. pp. 42–43. doi:10.1007/978-3-642-03653-8_16. ISBN   978-3-642-03652-1.
  6. ↑ Maruyama, Taisuke; Saitoh, Tsuyoshi; Yokouchi, Atsushi (4 May 2017). "Differences in Mechanisms for Fretting Wear Reduction between Oil and Grease Lubrication". Tribology Transactions. 60 (3): 497–505. doi:10.1080/10402004.2016.1180469. ISSN   1040-2004. S2CID   138588351.
  7. ↑ Godfrey, Douglas. "Fretting Corrosion or False Brinelling | Wear | Surface Science". Scribd. Retrieved 27 June 2017.
  8. ↑ Errichello, Robert (April 2004). "Another perspective: False brinelling and fretting corrosion (PDF Download Available)". Lubrication Engineering. 60: 34–36. Retrieved 27 June 2017.
  9. ↑ Tonazzi, D.; Komba, E. Houara; Massi, F.; Le Jeune, G.; Coudert, J. B.; Maheo, Y.; Berthier, Y. (15 April 2017). "Numerical analysis of contact stress and strain distributions for greased and ungreased high loaded oscillating bearings". Wear. 21st International Conference on Wear of Materials. 376–377, Part B: 1164–1175. doi:10.1016/j.wear.2016.11.037.
  10. ↑ Tomlinson, G. A. (1 July 1927). "The Rusting of Steel Surfaces in Contact". Proceedings of the Royal Society of London A: Mathematical, Physical and Engineering Sciences. 115 (771): 472–483. Bibcode:1927RSPSA.115..472T. doi: 10.1098/rspa.1927.0104 . ISSN   1364-5021.
  11. ↑ Schwack, F.; Prigge, F.; Poll, G. (October 2018). "Finite element simulation and experimental analysis of false brinelling and fretting corrosion". Tribology International. 126: 352–362. doi:10.1016/j.triboint.2018.05.013. ISSN   0301-679X. S2CID   139773784.
  12. ↑ ASTM D4170, Standard Test Method for Fretting Wear Protection by Lubricating Greases
  13. ↑ Almen, J.O. (1937). "Lubricants and False Brinelling of Ball and Roller Bearings". Mechanical Engineering. 59 (6): 415–422.
  14. ↑ Pittroff, Hans (1965). "Fretting Corrosion Caused by Vibration With Rolling Bearings Stationary". Journal of Basic Engineering. 87 (3): 713–723. doi:10.1115/1.3650657.
  15. ↑ Schwack, Fabian; Poll, Gerhard. "Service Life of Blade Bearings - Problems Faced in Service Life Estimation of Blade Bearings". ResearchGate. Retrieved 27 June 2017.
  16. ↑ Schwack, Fabian (2016). "Comparison of Life Calculations for Oscillating Bearings Considering Individual Pitch Control in Wind Turbines". Journal of Physics: Conference Series. 753 (753): 11. Bibcode:2016JPhCS.753k2013S. doi: 10.1088/1742-6596/753/11/112013 .
  17. ↑ False brinelling standstill marks on roller bearings. www.tib.eu. Technische Informationsbibliothek (TIB). 2011. ISBN   9783901657382 . Retrieved 27 June 2017.
  18. ↑ Stammler, Matthias (March 2015). "Blade bearings: Damage mechanisms and test strategies". CWD 2015: 371–379.
  19. ↑ Schwack, Fabian (2017). "Time-depending analyses of wear in oscillating bearings". STLE (72nd).
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