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Materials Used in Bearings

One of the major problems when dealing with dynamic machines is friction. Increased friction leads to a slower process and inefficient operation. It may also cause damage to the machine as a result of frequent metal to metal contact between surfaces. 

Friction can be addressed by the use of lubricants, but even lubricants are not a very efficient solution. A suitable lubricant must be used to fit a certain application, or it simply won't be effective. In addition to that, lubricants require frequent replacement, and disposal must be done properly to avoid any environmental contamination. These downsides are overcome by a another alternative - bearings

Bearings are mechanical assemblies that allow machinery to carry heavy loads with efficiency and ease and move at extremely high speed in linear or rotational motion while reducing friction. Practically all applications involving moving parts utilise one or more bearings. They play a key role in almost all systems, from ceiling fans to automobiles to industrial machinery [1]. 

Various materials have been used by the bearing industry for the manufacture of different bearing components. These bearing materials undergo different heat treatment and processes in order to attain the desired properties to maximise bearing life and performance. 

Here, you will learn about: 

  • The different types of bearings
  • The most common materials used in bearings and bearing components 
  • The typical application areas of bearing materials and the factors considered in selecting the right material 

Bearings 3.jpg

Types of bearings

Listed below are some of the most commonly used bearings, each type utilised for their own application [2].

Type of bearing

Application

Example

Ball bearings

Light thrust and radial load

Computer fans

Roller bearings

Heavy radial load

Conveyor belt rollers

Ball thrust bearings

Light radial load and low-speed applications

Barstools

Roller thrust bearings

Large thrust load

Gear sets

Tapered roller bearings

Large thrust and large radial load

Car hubs

Metallic materials used in bearings

Chrome steel SAE 52100

Roller, ball and tapered roller bearings are usually made of chrome SAE 52100 bearing steel. This material has excellent wear resistant capacity and a Rockwell hardness of about 64 HRC. Bearings made of chrome steel can function at temperatures up to 120°C, and upon heat treatment, they can even go as high as 220°C. However, one disadvantage of chrome steel bearings is the low chromium content that makes them prone to corrosion [3].

Stainless steel AISI 440C

Stainless steel bearings have a higher chromium content (up to 18%) that makes them superior to SAE 52100 in terms of corrosion resistance. They can function at temperatures as high as 250°C but are low in hardness and overall load-carrying capacity, in addition to having a higher production cost [3]. 

AISI 440C is a high-carbon stainless steel used in rolling-contact bearings (ball or roller bearings). It is broadly utilised in corrosive environments and in applications where corrosion resistance is more important than load capacity, such as in instrument ball bearings [4].

 

Bearing steel chemical composition [3]


ELEMENT


% WEIGHT

SAE 52100 (USA)

AISI 440C (USA)

Carbon

0.95 - 1.1%

0.95 - 1.2%

Chromium

1.3 - 1.6%

16 - 18%

Silicon

0.15 - 0.35%

1% max

Manganese

0.5% max

1% max

Molybdenum

0.08% max

0.75% max

Phosphorous

0.012% max

0.04% max

Nickel

0.25% max

0.25% max

Sulphur

0.25% max

0.30% max

 

AISI 440C can be heat-treated and hardened up to 60 HRC. Its major disadvantage is its shorter fatigue life compared to SAE 52100. However, some applications integrate the properties of high fatigue and corrosion resistance by using chrome-coated bearings made of the standard SAE 52100 steel [4].

Other metallic materials used in bearings include [1], [4]:

Non-metallic materials used in bearings

Non-metallic bearing materials provide a variety of properties that are suitable for a large number of applications. They are generally used in low PV (pressure-velocity)-value applications due to their relatively lower heat conductivity as compared to metals, in addition to satisfying self-lubrication, low cost, chemical resistance, and high temperature stability requirements. Non-metallic bearing materials can be classified into plastics, ceramics, rubber, carbon graphite, and other materials. Here are some of the common materials used.

Nylon

Nylon is a polyamide, a crystalline material widely used in bearing materials as a sintered layer inside a metal sleeve or an injection-moulded sleeve. It has excellent toughness, wear characteristics, and fatigue and chemical resistance. Nylon bearings are commonly used in lightly loaded applications such as in household appliances [4].

Phenolic materials 

Phenolic materials are usually used for cages of high-speed ball bearings such as in machine tool equipment spindles. They are good resistors to water, acidic, and alkali solutions. However, they have a low thermal conductivity and they tend to expand, which render them less desirable in high load-speed applications [4]. They are used as a replacement to metal bearings in electrical switch gears, water turbine bearings, and ship propeller shaft bearings. Although phenolic cages have high strength and are lightweight, they are expensive to produce, thus, plastics are often utilised in their place [5]. 

Nitrile rubber 

Buna or nitrile rubber is one of the most common bearing seal materials. It is resistant to many chemicals and can be used in a wide temperature range. It also has good mechanical properties and is cheap to produce. Bearing seals can also be made of silicone and viton. However, they are only used when a specific application requires their unique properties due to their high cost [5]. 

Teflon

Polytetrafluoroethylene (PTFE), commonly known as Teflon, is a thermoplastic polymer material whose high chemical inertness, self-lubricating property, and low friction coefficient makes it a suitable material for rolling-element bearing cages, journal, and sliding bearings, among others. Teflon is a soft material with a low resistance to wear and creep but can be improved by a thousand times through the incorporation of additives, such as fibres or particulates of harder materials [4].

 

Bearing design properties of non-metallic materials [4]

Material

Maximum Pressure(MPa)

Maximum Velocity (m/s)

Maximum Temperature (°C)

PV (Pa.m/s)

PTFE (Teflon)

3.4

0.51

260

35,000

Phenolics

41.4

12.7

93

525,000

Acetal

6.9

5.1

82

105,000

Polycarbonate (Lexan)

6.9

5.1

104

105,000

Nylon

6.9

5.1

93

105,000

Selection of bearing materials

There are no ideal bearing materials – they can be made of plastics, metals or composite materials depending on the type of bearing, application, speed, load and operating conditions. The method of lubrication is also a key factor. It is critical to consider that bearing material is merely one detail in the process of bearing selection, and even the costliest bearings cannot assure a successful performance if other design principles are ignored [4].

In terms of application requirements, bearing material should satisfy a balance between two opposing requirements. One is that the metal used should be sufficiently hard and strong to prevent issues like creep, and should have a suitable level of fatigue and impact resistance. On the other hand, the matrix should be soft and plastic enough to conform to machining. Of the characteristics to be considered during material selection are [4]:

Bearings 2 compressed.jpg

Sources

[1] N.K. Jha, “Bearings and Green Engineering”, Green Design and Manufacturing for Sustainability, Boca Raton: CRC Press, 2015.

[2] K. Nice, “How Bearings Work”, n.d., Available [Online]: https://science.howstuffworks.com/transport/engines-equipment/bearing3.htm [Accessed on 14.11.2019]

[3] R.K. Upadhyay and L.A. Kumaraswamidhas, “Bearing failure issues and corrective measures through surface engineering”, In A.S.H. Makhlouf and M. Aliofkhazraei (Ed.s), Handbook of Materials Failure Analysis With Case Studies from the Construction Industries, Oxford: Butterworth-Heinemann, 2018.

[4] A. Harnoy, “Selection and Design of Rolling Bearings”, Bearing Design in Machinery: Engineering Tribology and Lubrication, NY: Marcel Dekker, 2002.

[5] “Bearing Materials - Ceramics, Chrome Steels, Stainless Steels, and Plastics”, n.d., Available [Online]: https://www.astbearings.com/bearing-materials.html [Accessed on 14.11.2019]

Fun Facts

  • Bearings are used even in space, where machines such as the Hubble telescope, weather satellites, and the Mars Space Rover rely on bearings during their movement to reduce friction.
  • The great pyramids of Egypt were built with the help of ancient ball and roller bearings that allowed for the transport of massive bricks.
  • Bearings are also used in devices we experience everyday, such as fans, washing machines, microwave ovens, mechanical toothbrushes, garage door openers, and computer hard drives.