<<

FEATURE ARTICLE Jeanna Van Rensselar / Contributing Editor Beneath the Why and are a natural match

KEY CONCEPTCONCEPTSS

• CoContacntact mechanics iiss a sometimes oveoverlookedrlooked bbutut ccriticalritical asaspectpect of ttribology.ribology. • FriFrictionction rereductionduction iiss maximizmaximizeded by lleveragingeveraging the dydynaminamic bbetweenetween ssurfaceurface mmaterialaterial gengineeringg aandnd lublubricationrication genengineering. gineering. g. ••A Atetexturedxtured bbearinbearingg susurfacrface acts ass a lublubricantricant ,reservoirreservoir,, sstoringtoring daandnd ddispens-ispens- ing ththee llubricantubricant directly taatt ththee ppotinintt of ccontactontact wherwheree iitt makes mmostost ssense.ense.

52 • JULY 2015 TRIBOLOGY & TECHNOLOGY WWW.STLE.ORG surface:

Leveraging both disciplines together can results neither can achieve alone.

WHEN THE THEORY OF CONTACT MECHANICS WAS FIRST DEVELOPED IN 1882, were not factored into the equation at all. Experts say that may be due to the lack of powerful ana- lytical and numerical tools that necessitated simplicity.1 The omission of lubrication from contact mechanics theory was resolved four years later when Osborne Reynolds developed what is now known as the Reynolds Equation. It describes the distribution in nearly any type of fluid film .

WWW.STLE.ORG TRIBOLOGY & LUBRICATION TECHNOLOGY JULY 2015 • 53 Even with that breakthrough, how- WHY IS IT THE STRIBECK CURVE?4 ever, it was decades later before contact mechanics was actually factored into Fundamental to both contact mechanics and tribology, the Stribeck curve is named after tribology. Richard Stribeck. Developed in the first half of the 20th Century, it defines the form of the Today tribology is an com- curve for -lubricated surfaces and is a foundational tribology element. ponent of contact mechanics, but con- Stribeck performed his research at Berlin’s Royal Prussian Technical Testing Institute tact mechanics still doesn’t have the (now the BAM Federal Institute for Materials Research and Testing) around 1900. Similar same status in tribology. Experts say work was previously performed at the same institute 15 years earlier by Adolf Martens it’s time for a change. and 15 years before that by Robert H. Thurston at the Stevens Institute of Technology in Currently, machine elements are Hoboken, N.J. required to work under increasingly So why is the curve attributed to Stribeck, though both Thurston and Martens severe conditions such as thinner lu- achieved similar results considerably earlier? Possibly because Stribeck published in one brication, minimum lubrication and of the most important journals for engineers at that time, Germany’s Zeitschrift des Ver- higher temperatures. Because of this, eins Deutscher Ingenieure (VDI). Martens published his results in the lesser known Journal the effect of surface topography on lu- of the Royal Prussian Technical Testing Institute. bricated contacts is more relevant than ever. This is especially true for heavily loaded lubricated contacts, which are very dependent on the correct descrip- tion of and its associ- Principles of contact mechanics numerical data. If a surface ated behavior. Today, with the help of are applied in all tribology-related has inherently very low friction, faster computers, better software and disciplines that involve the contact of it may be difficult to distinguish significantly more efficient numerical one or more solid surfaces; this would between friction from solid contact methods, it’s possible to start modeling include just about every discipline. versus shearing. It is also surface failure modes in real time.2 In fact, contact mechanics is one very difficult to generate theoretical of the fundamental tools for designing Stribeck curves that are quantitatively bearings. It lays the foundation for race- correct.” way profile design and surface rough- ness specifications. For example, the HISTORY Today tribology is an profile of the rough surface needs to be In 1882 solved the integral component of considered in order to predict the mini- contact problem of two elastic bodies mum effective film thickness. For bear- with curved surfaces. He published contact mechanics, but ings operating in a mixed or boundary his theory in the seminal work On the lubrication environment, contact me- Contact of Elastic and is consid- contact mechanics still chanics is even more important. ered the father of contact mechanics. doesn’t have the same Researchers are able to create Hertz’s solution became the foundation coupled models that consider both for contact mechanics and is still rel- status in tribology; the lubricant behavior and the solid evant today. experts say it’s time contact between the rough surfaces Hertzian contact usually re- (the relationship between lubricants fers to the stress close to the area of for a change. and asperities); this is essential for contact between two different-sized generating a Stribeck curve (see Stribeck . In tribology, Hertzian contact Curve sidebar). This also has opened up stress is the stress within pairs of parts the possibility of designing surfaces that operate in the same system (the matched to lubricants (i.e., surface ball and socket of a hip replacement CONTACT MECHANICS texturing and controlled machining). or elements and raceways of a Contact mechanics is the study of how But STLE-member Rob Jackson, bearing in a gearbox). Hertzian contact solids that touch each other at one or professor of mechanical stress assumes the following: more points change shape. Central to at Auburn University, the only school 5 contact mechanics are the offering a tribology minor, has this • There is very little pressure on ei- and to the caveat, “The Stribeck curve is a ther and the surfaces have contacting surfaces (the normal direc- popular method often employed to very little yield. tion).3 Contact mechanics is a some- characterize the lubrication regime • The surfaces are continuous and times overlooked but critical aspect of by the friction coefficient. It can non-conforming; the area of contact tribology. be created from experimental or is a point or line.

54 • JULY 2015 TRIBOLOGY & LUBRICATION TECHNOLOGY WWW.STLE.ORG NA-LUBE® KR Series Broadest Range of Alkylated Naphthalenethalene

® SPECIALTY CHEMICALS

NEW BROADEST RANGE NA-LUBE NA-LUBEA-LUB NA-LUBEA-LUB Property KR-008KR-00 KR-009R-00 Supported with Custom Solutions KR-007A NA-LUBE® KR synthetic base oils are available in a Visc. @ 40˚C 22 cSt 36 cSt 39 cSt broad range from 22-193 cSt at 40°C and Visc. @ 100˚C 3.8 cSt 5.6 cSt 5.7 cSt 3.8-20 cSt at 100°C. King has the expertise and capabilities to customize products to meet your Noack Volatility 39.0% 12.0% 7.7% specific performance requirements. Pour Point < -48˚C -33˚C -33˚C

EXCELLENT SOLUBILITY Extremely Versatile Solubility NA-LUBE® KR Series synthetic base oils provide excellent additive solubilizing properties in a wide selection of natural and synthetic base oils and are highly effective as base oil modifiers.

LOW VISCOSITY PLUS Low Volatility NA-LUBE® KR-009 alkylated naphthalene combines low viscosity with low volatility for enhanced fuel economy.

EXCELLENT STABILITY Highest Thermal-Oxidative Stability NA-LUBE® KR Series synthetic base oils have exceptional thermal-oxidative stability to provide excellent varnish control and to help extend drain intervals.

www.kingindustries.comg www.kingindustries.comg www.kingindustries.comg www.kingindustries.comg ndustries.com www.kingindustries.comg ndustries.com for Specialty www.kingindustries.comCallgCallndustt ries.com Alkylated Naphthalene www.kingindustries.com...and..andandg aska ndust our chemists chemtries which co Alkylated Naphthalene product is best for you! www.kingindustries.com 1-800-431-7900 Toll Free www.kingindustries.com • Each sphere can be considered an Shortly after the JKR model was Hertz’s theory significantly more relevant. elastic half-space. developed, Derjaguin, Muller and Timken Co. scientist and ASME • The surfaces are frictionless. Toporov developed the DMT model— Fellow Xiaolan Ai explains, “Numerical which also accounts for . Both simulations have come to a stage where So basically all this assumes that the JKR and DMT models still are used they can take any digitized surface the surfaces are smooth and friction- today, and both are still valid. The profile into consideration and study the less, that there is elastic difference is that each model is more effect of roughness under more realistic and half of each surface represents the applicable to certain materials. The conditions. However, experimental entire sphere. So while no one would trick is determining which model to studies are not quite up to modern argue that Hertz’s theory is valid, its ap- use—another field of study in itself. standards.” plicability was and is limited to contact In the 1950s Bowden and Tabor between surfaces that are smooth and were the first to take into account the CONTACT MECHANICS where the contact is lightly loaded and importance of surface roughness for AND BEARINGS frictionless. bodies in contact—this is the point Part of the reason that Hertz failed to In 1970 Johnson, Kendall and of intersection for contact mechanics take asperities into account was the Roberts discovered a similar solution and tribology. Bowden and Tabor pos- crudeness of microscopes in the late for contact (meaning that tulated that, because of positive and 1800s. If a surface looked smooth un- the contact itself creates an adhesive negative surface asperities (protrusions der the most powerful microscope, boundary between the two surfaces). and cavities), the actual contact area is one could assume that it actually was This theory, known as the JKR less than the apparent contact area; the smooth. In actuality, most bearing sur- model, takes into account the contact degree of contact reduction is propor- faces are rough at today’s microscopic area created by the elastic material tional to the degree of roughness. Also levels and nearly all are rough at the properties plus Hertz’s interfacial in the 1950s, Archard added the di- nanoscopic level. interaction strength: Pressing a ping mension of to the equation. And Jackson says, “Surfaces are now pong ball into a foam ball would in 1966, Greenwood and Williamson known to possess features over many increase the contact area and also produced the first practical, engineer- different scales. This means that there create an adhesion.6 While Hertz’s ing-oriented asperity model for contact are small features or asperities on the theory only takes the point of contact mechanics. larger ones. This continues all the way into account, the JKR model goes on to So while Hertz laid the foundation down to the atomic scale. They are address what happens after the contact for contact mechanics, Johnson, Kendall then often characterized using fractals is made. Like the Hertzian theory, the and Roberts; Derjaguin, Muller and or spectral . It is very dif- JKR model only applies to sphere-to- Toporov; Bowden and Tabor; Archard; ficult to determine the effect of these sphere contact. and Greenwood and Williamson made multiple scales of roughness features on lubrication.” When two bodies come into con- tact, it’s the positive asperities that STAGES OF touch. Thus, the real area of contact is significantly lower than the geometrical Per basic , the wear rate of contacting surfaces normally goes through contact area (JKR/DMT models). But three stages: as the asperities become bigger and greater in number, the contact stress in- 1. Run-in. Surfaces are adapting to each other—so the wear rate is difficult to creases. So asperities equal less contact predict. This stage can last longer than either of the two stages that follow—it area but more stress. Because of this, depends on the materials and the degree of deformity. it’s important to determine the spatial 2. Steady state (also called secondary or mid-age). At this point the two surfaces pattern of peaks and valleys in addi- have adapted to each other and are exhibiting a steady rate of wear; the wear rate tion to the height variation. All of this is fairly predictable. This stage will continue until just before one or both of the makes the accurate measurement of contacting surfaces fail. Most of the components’ operational lives occur in this bearing asperities extremely important stage. The duration of this stage is affected by severe environmental and operating to contact mechanics and by extension, conditions such as higher and lower temperatures, load, stress and speed. lubrication. The lubricant also is a major determinant of the duration of this stage. Ball bearings are common subjects 3. Catastrophic (also called tertiary or old-age). The components rapidly fail due of contact mechanics for a number of to either a single catastrophic event or rapid aging. reasons, not the least of which is that the contact area and stresses are rela- tively simple to determine.

56 68% of plants are in danger of going extinct. Scientists believe the extinction rate is 5,000 times FRICTIONAL MECHANICS7

Contact mechanics includes compressive, adhesive and frictional , but tribology is mostly concerned with frictional mechanics, which is a subdiscipline of contact mechanics. It’s the study of the deformation of objects in the presence of frictional forces. • At the macroscopic scale, it applies to the observation of contact- ing objects in (a baseball bat contacting a ball). • At the intermediate scale, it applies to the local stresses, strains and deformations of the contacting objects in and near the contact area in order to validate macroscopic contact events, or investigate wear and damage of the contacting objects’ surfaces (examining the surfaces of the bat and ball post-contact). • At the microscopic and nano scales, frictional mechanics increases the understanding of tribological systems and adds to the friction knowledge base.

Ball bearings are typically loaded • Loss of tolerance. Usually the result ENGINEERING BEARING SURFACES against the conforming grooves in the of excessive or repeated deformation TO ENHANCE LUBRICATION inner and outer raceways. For radially of the components. Surface engineering, as a method for loaded ball bearings (where the ball is improving the friction and lubrication loaded against the conforming grooves One of the primary functions of performance of mechanical compo- in the inner and outer raceways), the lubrication is to reduce or eliminate nents, is a relatively new direction for contact between the ball and either the each of these friction-induced tribology. It improves the performance inner or outer raceway will be ellipti- situations. But bearing friction also of load-bearing surfaces in hydrody- cal. Determining the radii (ball radius, can be reduced by the shape, material namic lubrication by mathematically groove radius and radius of the con- and surface of the bearing. Friction formulating an optimum texture, tact track) is straightforward. Assum- reduction is maximized by leveraging which is then etched, stamped or sim- ing that the load carried by a roller the dynamic between surface material ply machined onto the bearing surface bearing is distributed among the indi- engineering and lubrication engineering (or the mold). Engineered surface tex- vidual balls. The load and the contact (see Frictional Mechanics sidebar). tures are usually in the form of positive radii can then be used to determine the Jackson says, “Surface roughness and negative asperities of a specific size contact area and stresses. is extremely important in determining and geometric pattern. the required thickness of a lubricant Considering that it’s generally the THE RELATIONSHIP BETWEEN film. One of the main requirements properties and characteristics of a sur- CONTACT MECHANICS AND in bearing design is to make sure face—not the bearing as a whole—that LUBRICATION that the film thickness is large determine system performance, surface Excessive contact stress or deformation enough so that the surface peaks engineering has proven to be a cost-ef- can lead to bearing failure through: are not in contact. If there is contact fective performance improvement. wear, micropitting can occur. The Guillermo E. Morales-Espejel, • Overload. Components either break general rule is to have a thickness in principal scientist for SKF’s Engineering apart or fracture from the contact order of magnitude larger than the and Research Centre in the Netherlands, load. roughness of the surface. However, explains, “In the lubrication of lightly • Wear. Material is removed from the there is some debate as to what is the and heavily loaded contacts like sliders contacting surfaces, usually through actual limit. Depending on the rough and rolling bearings, roughness and 9 abrasion (see Stages of Wear sidebar). surface contact model, this minimum micro-geometry in general are very thickness could be between one to six important to the understanding of real • Rolling contact . Cyclical con- times the roughness. The roughness surface behavior. Apart from locally tact stress may cause fatigue cracks. is also a fairly simplistic surface modifying the film thickness and • Seizure. The surfaces weld due to parameter, and other quantities such surface separation, micro-geometry high contact stress (usually high as spectral amplitude, skewness and plays an important role in surface heat). kurtosis could also be useful.”8 fatigue and surface life.”

more than it should be. Unlike animals, plants can’t easily migrate to more hospitable environs. 57 He adds, “Phenomena such as • Lower operating temperatures surface distress (micropitting), scuffing • Improved performance MIXED EHL and wear are substantially influenced • Lower running costs by the surface micro-geometry. In the • Longer service intervals. EHL (elastohydrodynamic lubrication) case of heavily loaded contacts (gears, refers to the full film formation that is roller bearings), the research on The design as well as the direction enhanced by surface elastic deformation mechanical effects of micro-geometry of the surface texture will affect the and the resulting increase of lubricant is very advanced (e.g., elastic-plastic performance of tribological compo- viscosity due to high pressure. There are deformation, temperature calculation, nents (both bearings and lubricants). situations where both boundary lubrica- contact area fraction, hydrodynamic The texture is a well-defined (usu- tion and full film lubrication are key fac- pressures, material in-homogenates and ally micro-dimpled) pattern on com- tors in the lubrication of the contacting even surface fatigue). This also includes ponents like bearings. Other shapes objects. The lubrication regime between rheological effects from the lubricant include squares, triangles, circles, boundary and elastohydrodynamic and sliding. In the case of mechanical crosshatches, rectangles, hexagons, a lubrication is termed mixed EHL (or partial effects, very fast models are needed to fan of straight lines and curved lines. EHL). Mixed EHL deals with the simultane- simulate the evolution in time and the Ai explains, “Surface texturing ously occurring solid-to-solid contact and fatigue damage accumulation, since could also lead to noticeable contact elastohydrodynamic lubricated contact. millions of cycles must be modeled.” pressure fluctuations, which may The prevailing theory is that if the have adverse effects on rolling contact average film thickness is less than three Coatings and surface treatments. Surface fatigue. Thus selecting a right surface times the composite surface roughness, engineering technologies span a range texture for a right application is by no the surface asperities will force direct of coatings and surface treatments that means an easy task.” solid-to-solid contact, resulting in mixed- combat friction, prevent corrosion and The most favorable pattern is the EHL.11 reduce wear. result of careful research and complex Rob Jackson, professor of mechanical A straightforward method for re- modeling. The main challenge is to engineering at Auburn University, ex- ducing wear on bearings is to increase determine the optimum value of key plains, “A great deal of progress has been the hardness of the surface. This can dimensions between various geometri- made toward including roughness in the be achieved through processes such as cal parameters of the surface for a par- prediction of hydrodynamic lift. For the hard-anodizing. Hard coatings also can ticular bearing operating in a particular elastohydrodynamic regime encountered be used to prevent (degradation set of conditions. These geometrical pa- in rolling element bearings and gears, caused by cyclic rubbing). rameters are length, width and depth of this has received more attention lately Diffusion processes (like case- the texture; the distance between tex- due to the problem of micro-pitting. hardening) broadcast elements such as ture such as pitch, array and angle; as Micro-pitting has been a major issue in nitrogen and carbon into the surface. well as density and orientation of sur- wind energy applications.” High-energy processes, beyond the lim- face textures. The various techniques its of traditional diffusion techniques, that can be employed for surface tex- incorporate ions to increase the relative turing include: atomic amount of nitrogen/carbon in the surface layer. • Photolithography • Laser surface texturing Surface texturing. Surface engineering • Etching (wet and dry) techniques and lubrication can be syn- • Mechanical indentation (micro-pits) chronized to further improve bearing • Abrasive jet machining properties. A textured bearing surface • Vibro-mechanical texturing.10 acts as a lubricant reservoir, storing and dispensing the lubricant directly at the For roller bearings, which are getting point of contact where it makes most progressively smaller and are operating Timken Co. scientist and ASME Fellow sense. under conditions of increasing speed Xiaolan Ai adds, “I believe the work Benefits of surface texturing in- and load, the role of surface engineer- on molecular dynamic simulation on clude: ing is becoming increasingly important. lubricated contacts will help us to better Morales-Espejel says, “The main understand mix mode lubrication contact. • Reducing the coefficient of friction deficiencies in theoretical predictions In a broader sense, it may help us to • Reducing seal leaks are related to physical/chemical redefine the very fundamental concept of • Improving load-carrying capacity aspects, the real behavior of the contact.” • Reducing metal-to-metal contact lubricant at the nano-metric scale, the

58 • JULY 2015 TRIBOLOGY & LUBRICATION TECHNOLOGY WWW.STLE.ORG Simple… Better… Simply Better!

The Most Versatile Tribology System Ever Designed Bruker’s Universal Mechanical Tester (UMT) platform has set the industry standard for flexibility in tribology testing. Now, the next-generation UMT TriboLab™ offers higher speeds, more torque, and better force measurements, all on a single system. Within minutes the platform can be transformed from rotary to reciprocating motion, from sub-newton to kilonewton force measurement, or from room temperature to 1000°C for environmental testing. No other mechanical testing system on the market can match the range of capabilities of the UMT TriboLab:

„ Exceptional Modularity – „ Environmental Testing – Interchangeable drives, and force Tribology testing at temperature and sensors, all with TriboID automation humidity from -25°C to 1000°C TriboScript™ user interface „ Broadest Capabilities – „ Unmatched Ease of Use – Rotary, reciprocating and block-on-ring Tool-less, blind-mate connects, and testing intuitive TriboScript software

Be part of the latest thing in tribology! Visit www.bruker.com/tribology, email [email protected] or call +1.408.376.4040/866.262.4040 for more information today. Innovation with Integrity Tribology & Mechanical Testers current and future state of contact Ai adds, “In the area of lubricated mechanics and tribology: contact, theoretical predictions of film Surface engineering, as thickness and contact pressure are very a method for improving 1. Micro-EHL (elastohydrodynamic accurate and match well with experi- lubrication) concepts and models mental results. However, when it comes the friction and lubrica- are now being used not only to to friction or predictions, there tion performance of understand the simple interaction is still a ways to go before high fidelity of fluid and surface, but also as a predictions can be obtained.” mechanical components, tool to model the service life of Jackson believes that multiple mechanical components (see Mixed surface feature scales and how the is a relatively new EHL sidebar on page 58). properties change with scale will direction for tribology. receive more attention in the near 2. The inclusion of local material future. He says, “Several researchers behavior in micro-EHL, when it are already looking into this using comes to pressure and clearance molecular , but it is extremely calculations and assessment of difficult to measure experimentally. The effect of tribolayers (including micro- damage, is something that is begin- mechanical properties of the surfaces slip) on the surface walls, wet-ability ning to be addressed by the scien- can also effectively change with scale; effects and the wet-ability history. The tific community. a small asperity on the surface can be micro-geometry is constantly evolving much harder than its bulk material 3. Despite what is written in the litera- during running conditions. With properties.” ture, the mixed lubrication problem the proper understanding of surface needs to be properly solved, bring- micro-geometry effects in lubrication ing together an understanding of and surface life, better tools can be and engineering. developed for life predictions of machine elements by separating out 4. Promising future developments in the surface from the subsurface; this is molecular dynamics and computa- what we are doing in rolling bearings.” tional may help to clarify the mechanisms behind lubricant Jeanna Van Rensselar heads her own CONCLUSIONS film rupture, wall slip and friction, communication/public relations firm, Smart PR Morales-Espejel has come to the nano-wear, tribolayer build-up and Communications, in Naperville, Ill. You can reach following conclusions about the destruction, etc.12 her at [email protected].

REFERENCES 1. From Elastohydrodynamic Lubrication: A Gateway to Interfa- 7. From Wikipedia entry Frictional Contact Mechanics. Available at cial Mechanics—Review and Prospect, by Dong Zhu and Q. Jane http://en.wikipedia.org/wiki/Frictional_contact_mechanics. Wang, Transaction of the ASME, Vol. 133, Oct. 2011. Avail- 8. Kurtosis refers to the sharpness of the peaks. able at http://asmedigitalcollection.asme.org/solr/searchresults. aspx?q=Elastohydrodynamic%20Lubrication%3A%20A%20Gate- 9. Waviness, roughness, indentations. way%20to%20Interfacial%20Mechanics%E2%80%94Review%20 10. From Study on Effect of Surface Texture on the Performance of Hydro- and%20Prospect&f_ContentType=Journals&f_JournalDisplayName dynamic Journal Bearing (sic), by Krishan Kumar Gupta, Raj Kumar, =Journal%20of%20Tribology&ResourceTypeID=2&SearchSourceType=3. Harendra Kumar and Medha Sharma. Available at www.ijeat.org/ 2. Information in part from “Surface roughness effects in elastohydro- attachments/File/v3i1/A2145103113.pdf. dynamic lubrication: A review with contributions,” by Guillermo 11. From Mixed EHL Lubrication, Purdue University. Available at E. Morales-Espejel, Proceedings of the Institution of Mechanical https://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd Engineers, Part J: Journal of Engineering Tribology, 2014, 228: 1217. =1&ved=0CB8QFjAA&url=https%3A%2F%2Fengineering.purdue.edu Originally published online Dec. 9, 2013. %2FME556%2FDocuments%2FMixed%2520EHL%2520Lubrication. 3. Contact mechanics also includes the tangential frictional stresses pdf&ei=iYQXVYfnFJbpoAS8_YK4Aw&usg=AFQjCNGnmQFRhfRxrRq_U3KtIp between the surfaces. 5zJyzvJw&sig2=cviCmrbtqkneW_7DOi_YGg. 4. From Wikipedia entry Tribology. Available at http://en.wikipedia.org/ 12. From “Surface roughness effects in elastohydrodynamic lubrication: wiki/Tribology. A review with contributions,” by Guillermo E. Morales-Espejel, Proceedings of the Institution of Mechanical Engineers, Part J: Journal 5. The pressure is lower than the pressure required for the surface to of Engineering Tribology, 2014, 228: 1217. Originally published yield. online Dec. 9, 2013. 6. Adhesion is roughly defined as the force required to separate two bodies in contact with each other.

60 • JULY 2015 TRIBOLOGY & LUBRICATION TECHNOLOGY WWW.STLE.ORG