Mechanical Engineering Design, Stage 2 - BEng: DSGN 215, DSGN 223 and BSc: DSGN 221 - Bearings

Introduction

Bearings minimise the wear / friction between 2 surfaces in relative motion. This can be done in a number of ways:

This 'interface material' might be:

  • A grease film

  • A soft metal or plastic layer on one of the surfaces.

Rolling bearings

Link to INA Bearing Company Ltd site which contains extensive information about their products, applications and technical papers.

Features:

  • Friction is very low, typically the coefficient of friction is between 0.0015 and 0.002.

  • Have to be manufactured to very close tolerances - can be expensive.

  • Need to be fitted (carefully) to accurately manufactured housings / shafts.

  • Because of very high stresses in small contact zones between the rolling elements and the tracks, bearings are usually made from hardened 52100 chromium steel, which limits the maximum operating temperature to about 180o C. For operation at higher temperatures, more exotic materials must be used, which is likely to be very expensive.

  • Some lubrication is essential, if the operating (or storage) environment is corrosive, appropriate seals must be fitted.

  • Recently one rolling bearing manufacturer, SKF, have introduced 'NoWear' bearings which feature a highly wear resistant diamond-like carbon (DLC) coating, which extends the life of the bearings by a factor of 5 to 10 times compared to conventional components. The DLC is applied to the components by a chemical vapour deposition process and is thermally stable up to 200oC.These coatings are ideal for bearings with inadequate lubrication or applications involving only small repetitive movements which can result in fretting corrosion in normal bearings.

  • Usually fail by fatigue - hence some short lives can occur. However the use of highly refined high purity steel virtually eliminates early fatigue failure.

Bearing lives may be determined by the ISO equation for the basic rating life used in conjunction with manufacturers charts and data for basic load ratings, static or dynamic.

or

where L10 = basic rating life, millions of revolutions, the life that 90% of a large sample of bearings is expected to equal or exceed.

C = basic dynamic load rating, N

P = equivalent dynamic load, N

p = life equation exponent, 3 for ball bearings, 10/3 for roller bearings.

Other Materials

While the vast majority of rolling bearings use hardened steel for the rolling elements and tracks, some applications require other materials, such as plastics or ceramics. Link to Euro Bearings which contains information about plastic and other specialist rolling element bearings.
One manufacturer has improved the performance of rolling bearings made from steel by the use of very clean steel. The reduced impurity content reduces the sites available for crack initiation and improves fatigue performance.

Silicon nitride (Si3N4) bearings are in regular use in Formula 1 racing cars. Wheel bearings (angular contact ball bearing type) are usually 'hybrid', silicon nitride rolling elements (balls) and steel races. To give a higher temperature capability than is available from standard 52100 steel, so called high nitrogen steels have been developed. This type of bearing typically gives an increase in life by a factor of 10. However when used in a Formula 1 car the life is about 2000 km.
Hybrid bearings are also used in formula one gear boxes. Costs presently limit wider application - a set of hybrid bearings for a Formula 1 gearbox costs about 5000 Euros - over 3000.
Link to Cerobear GmbH (in German) who make ceramic bearings.

Fluid Film Bearings

The coefficient of friction in a lubricated bearing varies depending upon the absolute viscosity, the speed and the pressure per unit projected area, but has the same general form shown in this 'Stribeck' plot. To the right of the dividing line the surfaces are totally separated whereas in the zone to the left, some surface contact is occuring.

A fluid film dragged (due to its viscosity) into converging gap, can generate extremely high pressures (high enough in some circumstances to cause plastic deformation of a metal). The pressure generated depends upon the fluid viscosity, the taper of the gap and the relative speed of the moving surfaces. A second order effect is the viscosity - temperature characteristic of the fluid. This mechanism is very reliable in continuous (rotational) motion, but for high speed - high load applications an oil cooler may be needed.

Notes on hydrodynamic fluid film librication.

Until fairly recently almost all lubricating oils were based on refined mineral oil, with appropriate additives. Now synthetic oils which are not directly derived from refined mineral oils are increasingly being used for arduous applications.

At low speeds the lubricating film is likely to break down and the surfaces come into contact. This must be allowed for in reciprocating motion (piston - cylinder liner) and also when normally continuous motion is starting or stopping. To accommodate intermittent motion, a thin soft layer (plastic or metal alloy depending upon the load) may be fixed or plated to one of the surfaces. When the two surfaces come into contact, the softer material tends to smear rather than wear. Any hard particles at the interface should be pushed into the softer material before causing too much damage.

For applications involving high local contact stresses and high sliding speeds, where the geometry does not favour the maintenance of full fluid film lubrication (hypoid gears) extreme pressure additives (sulphur and chlorine compounds) are added to the oil, then when metal to metal contact occurs, high local temperatures are generated, giving a local chemical reaction forming solid film lubricant in situ on the surface where required.

Friction speed characteristic is given by the Stribeck curve.

Greases

For applications involving slow, occasional or intermittent motion where the loads are not very high, greases may be used. A grease is a solid or semi - fluid lubricant consisting of a dispersion of a thickening agent in a lubricating fluid. The thickening agent may consist of a clay or a soap, the lubricating fluid is usually a mineral oil, a diester or a silicone. A mineral oil will run out of a bearing over a short period of time, a grease should remain in the bearing for a very long time.

Plain Bearings

Where slow speed - low load components rub together, possibly lubricated by grease, one of the surfaces will often be a non - metal, frequently a plastic.

Porous bushes, frequently made from sintered brass and impregnated with oil, are often used in domestic appliances if the loads are not excessive and where no maintenance is expected.

Return to Module Introduction.

David J Grieve, revised 26th November 2009; revised 20th July 2004; revised 5th June 2003; original: 7th September 2000.