Lubrication and storage of bearing. Selection of Oil. Oil Lubrication. Additional Symbols for Lubricants for ClosedType Bearings, and Their International Equivalents (Analogs)

LUBRICATION AND STORAGE OF BEARING

LUBRICATION AND STORAGE OF BEARINGS

Selection of Oil

Proper lubrication is a vital prerequisite ensuring reliable operation of a bearing. Insufficient quantity or improper selection of lubricating material inevitably cause rapid wear of the bearing, cut down its estimated service life. In rolling bearings lubrication performs the following functions:

–  forms an elastic hydrodynamic oil film between the working surfaceswhich is required in order to cushion impacts of rolling elements against the rings and the bearing cage, increasing, in so doing, the bearing service life expectancy and reducing noise in its operation;

–  reduces sliding friction arising between the contacting elements of abearing;

–  protects the bearing against corrosion;

–  prevents from penetration of alien particles from the surroundings into thbearing.

The factors determining the selection of lubricating oil are the working con ditions, i.e., the operating temperature and rotational speed. The environmen tal conditions should be also taken into account.

Two kinds of lubricating materials are generally used to lubricate rolling contact bearings: liquid and greases. Each of these kinds of lubricating mate rials has its advantages and disadvantages. Selection of an appropriate type of lubricant depends on the operating conditions of bearings and the mecha nism design as a whole.

Oil Lubrication

Oil lubricants are preferable as compared to grease. Therefore, whenever possible, oils should be used. They penetrate to friction surfaces more readi ly, thus, substantially reducing the danger of oil "starvation", or lack of oil. The use of a noncirculating flow or circulating lubrication permits the removal of heat arising as a result of bearing operation, and carrying of wear products away from the bearing. In practice, liquid lubricants are only used if this is required by exceptional operational conditions of the bearing unit, for instance, when the rotational speed is abnormally high, when heat should be removed from the bearing or when the parts (seals, gears, and the like) adja cent to rolling bearings must be lubricated with oil. Oil is also recommended for lubrication of assemblies in which bearings undergo highspeed reversible rotation, in units with needle rollers.

The most common liquid lubricating materials are purified mineral (petrole um) oils, with viscosity being the main technical parameter which determines their operational properties and suitability for a given unit. Therefore, selection of the grade of oil to be used for lubrication of a given unit is based on its vis cosity. The kinematic viscosity of oils is measured at a specified temperature, most frequently at 50°C or 100°C, and is expressed in mm²/s (cSt). The higher the oil viscosity, the greater breakdown load can the lubricant film withstand; however, thick oils, on the other hand, offer greater resistance to parts move ment increasing, in so doing, energy consumption, impair heat exchange between oil and the bearing, etc.

Taking into account the above, it follows that thick oils ought to be used to lubricate bearings operating at high loads with low rotational speeds. High speed bearings need to be lubricated with lowviscosity oils.

Viscosity is not a constant quantity for a given grade of oil, it changes with the variation of temperature, and this circumstance determines the viscositytem perature characteristic of oil – a major parameter which is of particular impor tance for bearings operating at low and variable temperatures. Taking this into consideration, lowviscosity oils should be used for lubrication of bearings operating at low temperatures, and highviscosity grades – for hightempera ture applications.

For highspeed bearings, oil viscosity also determines the amount of heat dissipation from a bearing. With all the other conditions being equal, the higher the growth of oil viscosity, the higher heat dissipation from the bearing.

For largesize and mediumsize bearings operating under normal conditions, it is recommended to use the grades of oils which have, in the working temper ature range, viscosity of 12 mmVs for all types of ball and roller bearings, except for doublerow spherical tapered roller and thrust roller bearings. Recommended are grades of oils having the following viscosity: 20 mm²/s for spherical roller bearings, 2030 mm²/s for tapered roller bearings, and 30 mm²/s for thrust roller bearings.

Smallsize highspeed bearings, especially with low required starting force, can be lubricated with oil, whose viscosity is below 12 mm²/s.

Nomographs are usually used to facilitate selection of the required oil viscos ity for bearings of various sizes operating at various rotational speeds and tem peratures.

The method of selecting kinematic viscosity of oil with