Ceramic Bearings

7/27/2019 Ceramic Bearings

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Ceramic Bearings

D K SenGeneral Manager – Technical Training

SKF Bearings India Ltd

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Introduction :

Ceramic is a word that one would normally associate with things like Tiles and

Potteries and not with an antifriction bearing. But over the last couple of decades, it

has not only become associated with bearings, but has become one of the accepted

materials of bearings as well – so much so that SKF Ceramic Bearings is as much anentity within the SKF World as the SKF CARB Bearings.

This article is intended to throw some light on the reasons and the scope of use of

Ceramics in antifriction bearings.

This is the age of downsizing – as we all know, - and that applies to machines as

well. Gone are those days when the designers could afford the luxury of using large

safety factors while designing a machine. Now the users want machines to be smaller,

smarter, more productive as well as guaranteed to have zero breakdown – all with

minimal maintenance. Consequently, antifriction bearings – Ball and Roller Bearings,

to be more exact - are required to run at faster speeds under ever increasing loads andstringent working conditions with longer life and minimum power loss.

An antifriction bearing is required to carry static and dynamic loads. Scientists once

believed that an antifriction bearing can only fail due to fatigue. The life equation,

established by Dr. Arvid Palmgren and Goran Lundberg in the year 1947, took into

consideration only the load acting on a bearing.

Since then, our understanding of the the microworld of an antifriction bearing has

undergone a seachange. We now know that there are several other factors than load

alone that may affect the life of an antifriction bearing, such as the cleanliness of the

material that a bearing is made from, the correctness of lubrication, extent of

contamination present in the bearing and the lubricant, cleanliness of the operating

environment, proper mounting and dismounting of the bearings, protection against

passage of electric current through the bearing, thermal expansion of the bearing

components during operation, centrifugal forces within a bearing, etc.

Ceramics have some unique properties that the traditional bearing steel does not have.

And it is this consideration that has made it possible for Ceramics to enter the world

of Antifriction Bearings.

Material :

Normally, an Antifriction Bearing is made of a special category of steel called Ball

Bearing Steel in lay man’s language. It is a through hardened steel with 1% Carbon

and 1.5% Chromium. Some times, case haredening steel, containing 0.2% and high

Nickel is also used far manufacturing Antifriction Bearings. The choice mostly

depends on the nature of load that the bearing is required to carry during operation.

The Ceramic Material that is used as a material of construction of a bearing is Silicon

Nitride (Si3 N4). Bearing grade Silicon Nitride is a ceramic material belonging to agroup of materials called Engineering Ceramics. These are intended for demanding

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technical applications, as opposed to traditional ceramics used in potteries and

tableware etc. It is produced from powder in a compacting process acting at

temperature around 18000C and pressure of around 200 Mpa. This process, specified

by SKF, creates a solid Ceramic material, without any porosity and with good

toughness. It has higher hardness, lower density and better electrical insulation than

the traditional bearing steel. That is why it is considered to be a better material for certain applications that needs bearings which can handle:

1. Particle contamination

2. Choke residue

3. Poor lubrication

4. Passage of electric current

5. High speeds

6. Vibrations and oscillations

7. Corrosion

8. Vacuum

9. Process media lubrication

10. Magnetic fields

A comparison of material properties of Bearing Steel and Ceramics is given below:

SKF was the first bearing manufacturer that specified a classification of bearing grade

Silicon Nitride. There are three classes:

1. Class I for Aerospace applications.

2. Class II for General Bearing Applications

3. Class III for low duty applications

SKF always uses Class II or higher material for bearings.

Design :

Depending on the application criteria, a Ceramic Bearing may have one of thefollowing designs:

1. Ceralite : Bearing made of conventional material. Only

One ball is made from ceramic. Denoted by

Suffix VU 091

2. Hybrid Bearing : Rings are made from conventional material Only

the rolling elements are made from Ceramics.

Denoted by Suffix HC5

3. All-ceramic Bearing: Rings as well as the rolling elements are made

from Ceramics. Denoted by Suffix HC4.

Bearing Steel Silicon Nitride Porcelain

Compressive strength (MPa) 880 3000 950

Tensile strength (MPa) 800 800 150

Young’s modulus (GPa) 210 310 125

Hardness HV10 (Kg/mm2) 700 1600 800

Electr. Resistivity (Ω x mm2/m) 25x10-8 (Conductor) 1014 (Insulator) 1014 (Insulator)

Density (g/cm3) 7.9 3.2 2.9

Co-eff. Of thermal elongation(10-6 K) 12 3 5

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1. Ceralite :

In this bearing, the rings and all rolling elements except for one number is made of

Bearing Steel. Only one rolling element is made of Ceramic. The much harder

ceramic ball refreshes the surface of the raceway and keeps it free from foreign

particles. The Ceralite grinds the particles down until they are harmless. That is how it

renders a long service life.

Properties:

- Long service life

- Low noise

- Reduced wear

- Tolerant to contamination

2. Hybrid Bearing :

In this bearing, the rings are made of Bearing Steel while the rolling elements are

made of Silicon Nitride. Silicon Nitride balls weigh only 40% of its steel equivalent.

This leads to low centrifugal force, meaning lower load on the raceway at high

speeds, and lower friction. ( See Graph below).

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Since we use standard dimension steel rings, it is thus possible to switch from

Standard bearings to Hybrid bearings without making any design change.

Hybrid bearings function very well where sealing is impossible.

Properties:

- High tolerance to poor lubrication

- Electrical insulation

- High wear resistance

- High speed capabilities

- High precision

- Long service life

- High tolerance to contamination

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Grease life in Hybrid Bearings :

Ceramic rolling elements with SKF WT greases gives superior grease life at high

speeds and high temperstures. This has, for example, been demonstrated in SKF ROF

grease tests on bearings with TN9 cages, and shaft sizes of 20mm. Hybrid bearings

with SKF WT grease showed significantly extended grease life, compared to all steel

bearings and Hybrid bearings with ordinary grease. The test was performed at20000rpm and +1200C.

Hybrid Bearings under dry running conditions:

Tests have shown that Hybrid bearings tolerate dry running much better than all steel bearings. All steel bearings seize earlier and at higher vibration levels compared with

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Hybrid bearings. The good dry running properties of Hybrid bearings are due to the

thermal stability of Ceramic rolling elements. Ceramic will not weld to the steel rings.

Bearings of shaft size 25mm were tested without lubrication at a speed of 4500 rpm.

These tests showed that Hybrid bearings had smoother running with lower vibration

and did not seize.

Inner Ring Wear :

Longer Service Life :

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There are numerous test results to show that Hybrid bearings help extend the MTTF

in critical applications. The Bar chart above shows the results obtained from one such

case of Wire Roller Guide in a Wire Rod Mill in Peru.

Electrical Properties :

Ability to withstand vibration :

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3. All Ceramic Bearing :

As the name implies, in this type of bearings, the rings as well as the rolling elements

are made of Ceramics. The TNH cage is made of PEEK material, which can withstand

a continuous temperature of 1500C.

This bearing is of light weight, non-magnetic and non-corrosive, and can withstand

solvents. It may be process media lubricated. It is ideal for food processing and where

non-magnetic material is necessary. However, since the rings are not made of steel,

special care must be taken while mounting and dismounting.

Conclusion :

SKF is not the only company in the field of Ceramic Bearings. Most of the other

bearing companies are also active in the Hybrid bearing business. NSK, Koyo and

FAG have a considerable presence in the field of Hybrid bearings, while other

companies on scene are Torrington, SNFA, IBC and Cerobear.

For some applications and certain operating conditions, SKF offers alternative

solutions to Hybrid bearings. For example, the problem with electrical damage to

bearings may, in many cases, be solved with SKF INSOCOAT TM . For smearing, poor

lubrication and false brinelling problems, SKF offers NOWEAR TM as an alternative

solution, mainly for large size roller bearings. For temperatures upto 3000C, HTB

(High Temperature Bearing) is suitable.

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Ceramic Bearings