FEATURE ARTICLE Jeanna Van Rensselar / Senior Feature Writer Rolling element bearing steels

KEY CONCEPTS

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50 • JULY 2017 TRIBOLOGY & LUBRICATION TECHNOLOGY WWW.STLE.ORG While excellent bearing steels exist for nearly every application, there is still room for improvement.

ROLLING ELEMENT BEARINGS OCCUPY THE on the basis of hardenability, fatigue Even with this new information, SWEET SPOT between cost, size, weight, strength, wear resistance, toughness several technical issues remain. accuracy, load capacity, friction, dura- and cost effectiveness. There may be a reason why bearing bility and accuracy. Modern bearing Bearing steels, when fatigue tested in steels produced post-1950s endure lon- steels are critically important ultra-high rolling contact, can typically withstand ger in rolling bearings, but it is difficult strength structural materials used in a billions of stress cycles. During opera- to pinpoint since several microstructur- multitude of industrial systems and sub- tion/testing, bearing steel is subjected al processes are involved. There is one ject to the highest loading conditions to a complex, multi-axial stress between view that the investigation of bearing for, in some cases, billions of cycles. the rolling element and bearing raceway. materials under torsion fatigue, which They are unique among structural ma- The bearing industry still refers to is used to observe shear-driven phe- terials because of the localized nature of empirical models of RCF (see What is nomenon, is pertinent to RCF of bear- rolling contact fatigue (RCF) loading, Rolling Contact Fatigue? on Page 52) ing steels. Recent investigations have leading to extreme sensitivity of fatigue data from the 1940s that are a very employed custom-built or commercial- life to microstructural attributes. conservative estimate of roller bear- ly available test rigs to characterize tor- Steel represents the material of ing life.1,2 Since then, there have been sion fatigue of bearing steel. Since bear- choice in the manufacture of bearings several advancements in the steel itself ing failures develop beyond 107 cycles, because of its ability to sustain severe and new insights from experimental some researchers have focused on rapid static as well as cyclic loads. Rolling el- and computational techniques (see A evaluation of shear fatigue properties ement bearing steels are always selected History of Bearing Steels). with ultrasonic torsional fatigue testers.

A HISTORY OF BEARING STEELS3

• 1734: Jacob Rowe applies for a British patent for a rolling element bearing. The rolling bearing materials used in Rowe’s era would have been wood, bronze and iron. • 1856: Modern rolling element bearing steel and metallurgy are made possible through the introduction of the Bessemer process whereby air is blown through molten pig iron to produce a relatively high grade of steel. • 1866: Open-hearth melting is invented, which further improves quality and makes steel far more accessible to the industry. However, because heat treatment of steel was still an art known only to a few, most rolling element bearings were probably made of unhardened steel. • 1879: A British patent is issued to J. Harrington and H. Brent for a hardened steel bushing, or inner shaft, fitted with a groove for balls. About the same time, W. Hillman of Coventry, England, constructs a machine for cutting balls from steel wire. • 1875-1900: The use of carbon and chromium steels for bearings gradually increases since the need for bearings capable of reliably supporting heavy loads increases. • 1900-1955: There is comparatively little progress in bearing steels. • 1955: The catalyst for quantum advances in bearing steels was the advent of the aircraft gas turbine engine. The engine creates unprecedented needs for better materials and designs for rolling element bearings. These needs included bearings for higher tem- peratures, higher speeds and greater loads. The ever increasing thrust-to-weight ratio for the aircraft jet engines requires the use of smaller and lighter bearings. • 1950-1970: Steel companies and research laboratories within the U.S. began to develop bearing steels with higher alloy content. • 1980s: AISI M-50 steel becomes the steel of choice for most high-temperature bearing applications over 149 C (300 F).

WWW.STLE.ORG TRIBOLOGY & LUBRICATION TECHNOLOGY JULY 2017 • 51 THE MOST COMMON BEARING STEELS4 WHAT IS ROLLING CONTACT FATIGUE? Dr. Xiaolan Ai, scientist for The Timken Co. in North Canton, Rolling contact fatigue (RCF) is fatigue spall of material Ohio, says, “The endurance life of a rolling element bearing originating from the bearing’s subsurface. It is caused is determined to a large degree by the strength of bearing by rolling contact and is the main failure mode of rolling material in relation to contact stresses the bearing experi- bearings. RCF results in metallic particles flaking from ences during operation. The quality of bearing steel, often the surface of the ball and rolling elements or raceways. measured by cleanliness among other factors, plays a critical RCF starts as a crack below the surface and propagates role in determining the strength of the steels (see Types of to the surface causing a pit or a spall in the bearing race- Rolling Elements).” way. While RCF is extremely variable, it is statistically In addition to steels, bearings also are made of ceramic, predictable depending on the steel type, processing, plastic and composites, depending on the performance char- manufacturing and operating conditions. Failures other acteristics for the application. The materials described here than those caused by RCF are avoidable if the compo- are the most commonly used steels. nent is designed, handled and installed properly and not overloaded. Chrome Steel – SAE 52100. This is a good general purpose steel with excellent hardness and wear but poor corrosion

TYPES OF ROLLING ELEMENTS

There are five basic types of rolling elements that are used in rolling element bearings: ball, cylindrical rollers, spherical rollers, tapered rollers and needle rollers.

a Ball bearing. Ball bearings have four main parts: a large outer ring, a small inner ring, steel balls between the rings and a cage to prevent the balls from contacting each other. Each race has a groove shaped so that the ball fits loosely (a). The ball deforms slightly where it contacts each race, and each race also yields slightly where each ball presses against it.

Cylindrical roller. Common roller bearings use cylinders that are slightly longer than the diameter. These b bearings usually have higher radial load capacity than ball bearings but a lower capacity and higher friction under axial loads. If the inner and outer races are misaligned, the bearing capacity can drop quickly (b).

Spherical roller. Spherical roller bearings have an outer ring with an internal spherical shape. The rollers are wider in the middle and narrower at the ends. Because of this, spherical roller bearings can accom- modate both static and dynamic misalignment. However, they are difficult to produce and, thus, expensive. c They also have higher friction than an ideal cylindrical or tapered roller bearing (c).

Tapered roller. Tapered roller bearings have conical rollers that run on conical races. Most roller bearings only accommodate radial or axial loads, but tapered roller bearings support both radial and axial loads and can carry higher loads than ball bearings. Tapered roller bearings are usually more expensive than ball d bearings and produce more friction (d).

Needle roller. Needle roller bearings have very long, thin cylinders. Often the ends of the rollers taper to points. Since the rollers are thin, the outside diameter of the bearing is only slightly larger than the hole in the middle. Since the small-diameter rollers bend sharply where they contact the races, the bearing fatigues relatively quickly (e). e Graphics courtesy of Wikipedia. of courtesy Graphics

52 Archimedes is credited with improving the power and accuracy of the catapult. Archimedes invented the odometer during scientist, computational bearings, and STLE-member Nick Weinzapfel, says, “There is a link in a very fundamental sense if we restrict bearing life to sub- surface initiated fatigue failures. Both involve alternating or cyclic application of a load to a component, which causes the atoms composing the material to rearrange and debond to form micro- scopic voids or cracks that grow with successive loading until the component fails.” Weinzapfel continues, “On the other hand, the particular stresses gen- erated by the applied loads can have Figure 1 | Subsurface cracks in rolling contact fatigue. (Figure courtesy of Sadeghi, F., et al. substantially different characteristics. (2009), “A review of rolling contact fatigue,” Journal of Tribology, 131, pp. 041403-1-041403-15.) Whereas the stresses in structural ele- ments generally rise and fall in propor- tion with the applied load, stresses gen- resistance. It also is the most widely for rings and balls in miniature erated in materials subjected to rolling used material. This steel has high car- bearings. This material has the contact follow a complex pattern with bon content and about 1.5% chromium same corrosion resistance as direct and shear components that are content. The finished components have 440C and is less noisy. out of phase with one another. The vol- a hard surface that resists subsurface • SV30. This steel, which is alloyed ume of heavily stressed material in roll- RCF (see Figure 1). The typical surface with nitrogen, has a lower car- ing contact is highly localized and also hardness for these components ranges bon content than other Martens- much smaller than in structural fatigue. from 60-64 on the Rockwell Hardness itic stainless steels. It is highly Still for both types of fatigue, the initial C Scale (Rc) (see The Rockwell Hardness resistant to corrosion and has formation of damage and micro-cracks Scale on Page 56). high strength and high hardness is believed to be primarily dependent Extra Clean 52100 Chrome Steel. This that can result in 100% greater on shearing stress. So despite their dif- steel is heat treated by thru hardening fatigue life. ferences, it is likely that a correlation it in a furnace with a controlled atmo- AISI316 Austenitic Stainless Steel. exists between them.” sphere. Because this steel can be highly Because these bearings have a low car- Nagaraj Arakere, professor of me- finished, bearings made from this ma- bon content, they are nonmagnetic and chanical and aerospace engineering at terial tend to be very quiet. They can have greater corrosion resistance. The the University of Florida in Gainesville, operate at continuous temperatures up downside is that this type of stainless Fla., notes that sensitivity of bearing fa- to 120 C (248 F). steel cannot be hardened—meaning tigue life to microstructural attributes Martensitic Stainless Steels bearings need to operate under low- arises from the localized nature of tri- • AISI 440C. The high chromium load, low-speed conditions. axial fatigue loading during RCF. This content and addition of nickel Medium Carbon Alloy Steel. Bearings leads to wide variation in subsurface makes stainless steel more re- made from this material, referred to as initiated fatigue life of bearings tested sistant to surface corrosion than commercial grade, tend to be less ex- under nominally identical conditions chrome steel. Because of reduced pensive than other bearings. The inner of geometry and loading. Because of hardness, the load-carrying ca- and outer rings are carburized. this, probabilistic models are widely pacity is 20% lower in bearings Low-Carbon Alloy Steel. Also referred used for characterizing bearing fatigue made from this material than to as commercial grade, low-carbon life. The localized cyclic loading also those made from 52100 chrome steel is used in bearing cages and metal induces plasticity at the scale of the mi- steel. Bearings made from this washers and shields. This type of steel crostructure, leading to microstructural material are both magnetic and is protected from corrosion through ei- evolution and phase transformation noisy. They can operate at tem- ther lubrication or plating. with cycles. The complex phenomena peratures up to 250 C (482 F). displayed by RCF from nanometer to • ACD34/KS440/X65Cr13. This BEARING LIFE PREDICTION millimeter length scales make reliable has a slightly lower carbon and Regarding the link between structural bearing life prediction in the gigacycle chromium content than AISI fatigue and bearing life, Sentient Sci- regime difficult. Current bearing life 440C and is a popular material ence’s (located in Buffalo, N.Y.) chief prediction models do not account for

the First Punic War. The device was a cart with a gear mechanism that dropped a ball into a container after each mile. 53 the microstructure-sensitive bearing tions and introduce modification fac- material properties that evolve with tors to account for other influences cycles. Bearing steels, when like contaminated lubricant or poor On the same subject, Ai says, “Qual- fatigue tested in rolling lubrication condition. However, they ifying the impact of the microstructure do not consider certain modes of of bearing steel to bearing fatigue life contact, can typically failure that involve wear, ring frac- would be considered as one of the most withstand billions of ture, etc. These unconsidered failure important improvements. It perhaps modes may render a bearing inoper- could be conquered in a number of stress cycles. able long before material fatigue sets steps by linking the material micro- in, or may act in concert with the structure to its mechanical properties fatigue process to accelerate the fail- and then linking the relevant material ure. Predicting the life of a bearing property to bearing fatigue life. of bearings in real-world operating governed by these effects presents a “Fatigue is a very complex and scenarios, the following three are es- significant challenge. challenging topic,” Ai continues. “Dif- pecially significant: ferent industries have different prac- 3. Conventional life theories have been tices and standards for fatigue failure 1. Operating conditions may deviate established on the assumption that prediction. The fundamental under- significantly from design cases due the bearing is operating at low to standing on the relationship between to random unpredictable environ- moderate speeds; high-speed effects material microstructure characteristics mental factors that push a bearing like centrifugal and gyroscopic forces and fatigue performance may help to beyond its intended limits. are considered insignificant. In cer- bridge the gap between various prac- tain applications these high-speed tices and standards.” 2. Life theories are generally formulated effects can play an important role in According to Weinzapfel, among for a particular mode of failure (i.e., the internal load distribution of the the challenges in predicting the lives material fatigue) in idealized condi- bearing and alter its life expectancy. ÎÎÎÎÎÎÎ

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54 • JULY 2017 TRIBOLOGY & LUBRICATION TECHNOLOGY WWW.STLE.ORG ÎÎÎÎÎÎÎ Weinzapfel explains that compu- 5 tational models enable investigation THE ROCKWELL HARDNESS SCALE of bearing life in a broad spectrum of scenarios that would not be timely or In 1914 Hugh M. Rockwell and Stanley P. Rockwell of Connecticut applied for a patent for cost effective through the sole use of their Rockwell Hardness Tester, with the goal of quickly determining the effects of heat experimental data. treatment on steel bearing races. The Rockwell Hardness Scale is based on indentation “Existing life theories require pa- hardness of a material. It determines hardness by measuring the depth of penetration rameters obtained from curve fits of of an indenter under a large load compared to the penetration made by a control. bearing endurance test data that take a There are different scales, each assigned a single letter, that use different loads or substantial amount of time to generate, indenters. The result is a number noted as HRA, HRB, HRC, etc., where the last letter making it practically impossible to tai- is the respective Rockwell Scale. When testing metals such as bearings, indentation lor the life prediction models for every hardness correlates linearly with tensile strength. This relationship permits economic new bearing material and processing nondestructive testing of bulk metal deliveries with lightweight, even portable equip- technique,” he says. ment such as handheld Rockwell Hardness Testers. Arakere points out that calibrat- ing empirical RCF models to material properties specific to ultra-clean bear- ing steels is important. “There is, as of namics calculations enable accurate institutes are best aligned to conduct yet, no clear link between fatigue prop- determination of the internal loading fundamental research work,” Ai says. erties (SN data using bulk specimens) distribution at any operating speed “They can certainly help the bearing of structural steels, which exhibit a and, thus, consider high-speed effects industry to advance our understand- fatigue limit, and RCF life of bearing of centrifugal and gyroscopic forces by ing on the micro-scale of the impact steels,” he says. “RCF manifests at the default. of material microstructure on bearing microstructural scale and bulk fatigue fatigue performance. This will enable data is perhaps not relevant. Measuring SUMMARY bearing manufacturers and steel sup- the micro-hardness evolution within Weinzapfel concludes that advance- pliers to explore cost-effective methods the RCF-affected zone is a useful mark- ments in simulation are needed to and practices to obtain desired bearing er for microstructural evolution with solve long-standing challenges, and steel quality that better serves their cycles. Bearing life, as estimated now he suggests areas to focus on. “There customers.” using virgin material properties, is not needs to be more ubiquitous, real-time a constant value and will continuously monitoring of the actual bearing oper- evolve depending on loading condi- ating conditions (e.g., loads, speeds, REFERENCES tions and cycles. Computational mod- lubrication condition, shaft-housing 1. See www.nsf.gov/awardsearch/ els that can characterize cyclic bearing misalignments, temperature, etc.) in showAward?AWD_ID=1434834&Histo stress fields, along with experimental order to improve the accuracy of inputs ricalAwards=false. approaches that can measure relevant to the models used to predict bearing 2. Dr. Xiaolan Ai, scientist for The material properties at the microstruc- life. Improvements in computational Timken Co., disputes this. tural scale, are required for reliable pre- speed and multi-scale modeling tech- 3. Rolling Bearing Steels—A Techni- diction of bearing fatigue life. niques will be needed to more rapidly cal and Historical Perspective. Available at https://ntrs.nasa. With computational models, a bear- consume increasing volumes of bear- gov/archive/nasa/casi.ntrs.nasa. ing can be decomposed into its basic ing operating data and evaluate the re- gov/20120008557.pdf. material building blocks that are vir- sponse of virtual bearings. Additionally, 4. From www.astbearings.com/bear- tually tested under a range of loading these improvements would enable fur- ing-materials.html. conditions. The relevant properties of ther consideration of microstructural 5. From https://en.wikipedia.org/wiki/ the material building blocks and their details not accounted for by computa- Rockwell_scale. statistical variation are determined em- tional models of today.” pirically at a fraction of the time and Arakere concludes that bridging the cost of full-scale bearing test data—an various length scales into a single com- advantage that scales rapidly with the putational framework that can be fed Jeanna Van Rensselar heads size of the bearing. Once the properties into finite element models to capture her own communication/public are determined, a bearing’s response to the 3D complexity of fatigue in bear- relations firm, Smart PR actual conditions can be composed ings is a long-term goal toward design- Communications, in Naperville, from the responses of the representa- ing bearings with increased life. Ill. You can reach her at tive building blocks. Computational And where will these advancements jeanna@smartprcommunica- models that involve multi-body dy- happen? “Universities and research tions.com.

56 He did not invent the lever but gave a full explanation of the lever and the fulcrum, including their principles and applications.