mechanical engineering

The relevant standard is BS 970 ‘Wrought Steels’. The standard is in six parts:
Part 1 Carbon and carbon manganese steels including free-cutting steels
Part 2 Direct hardening alloy steels
Part 3 Steels for Case Hardening
Part 4 Stainless, heat resisting and spring steels
Part 5 Carbon and alloy spring steels
Part 6 SI metric values (for use with Parts 1 to 5)

Each steel is designated by six symbols:
First three digits
000-199: Carbon and carbon-manganese steels. Digits represent 100 times the percentage of manganese.
200-240: Free cutting steels. Second and third digits represent 100 times the percentage of sulphur.
250: Silicon-manganese steel
300-449 : Heat-resistant, stainless and valve steels
500-999: Alloy steels
Letter
The letters A, M, H and S indicate if the steel is supplied to - chemical analysis, mechanical properties, hardenability requirements, or is stainless, respectively.
Last two digits
These give 100 times the percentage of carbon, except for stainless steels.
Example
070M20: A plain carbon steel with 0.2% carbon and 0.7% manganese. The mechanical properties, i.e. tensile strength, yield strength, elongation and hardness, are given in the standard.

Definition of EHD lubrication.
The lubrication principles applied to rolling bodies, such as ball or roller bearings, is known as elastohydrodynamic (EHD) lubrication.

Rolling body lubrication.
Although lubrication of rolling objects operates on a considerably different principle than sliding objects, the principles of hydrodynamic lubrication can be applied, within limits, to explain lubrication of rolling elements. An oil wedge, similar to that which occurs in hydrodynamic lubrication, exists at the lower leading edge of the bearing. Adhesion of oil to the sliding element and the supporting surface increases pressure and creates a film between the two bodies. Because the area of contact is extremely small in a roller and ball bearing, the force per unit area, or load pressure, is extremely high. Roller bearing load pressures may reach 34,450 kPa (5000 lb/sq in) and ball bearing load pressures may reach 689,000 kPa (1,000,000 lb/sq in). Under these pressures, it would appear that the oil would be entirely squeezed from between the wearing surfaces. However, viscosity increases that occur under extremely high pressure prevent the oil from being entirely squeezed out. Consequently, a thin film of oil is maintained.

 Effect of film thickness and roughness.

• The roughness of the wearing surfaces is an important consideration in EHD lubrication. Roughness is defined as the arithmetic average of the distance between the high and low points of a surface, and is sometimes called the centerline average (CLA).
• As film thickness increases in relation to roughness fewer asperities make contact. Engineers use the ratio of film thickness to surface roughness to estimate the life expectancy of a bearing system. The relation of bearing life to this ratio is very complex and not always predictable. In general, life expectancy is extended as the ratio increases. Full film thickness is considered to exist when the value of this ratio is between 2 and 4. When this condition prevails, fatigue failure is due entirely to subsurface stress. However, in most industrial applications, a ratio between 1 and 2 is achieved. At these values surface stresses occur, and asperities undergo stress and contribute to fatigue as a major source of failure in antifriction bearings