Liquid lubricants (lubricating oils). Lubricating oils are mostly purified oils with special additives, which allow to increase the service life of oils by 2-4 times. Oils without additives are used to lubricate light-loaded, high-speed units in machines and mechanisms of industrial equipment. The performance properties of such oils are provided by their natural oil base.
The main characteristics common to all liquid lubricants are:
Viscosity is one of the most important characteristics of a lubricant, which largely determines the frictional force between the moving surfaces on which the lubricant is applied.
Since the viscosity is inversely proportional to the temperature (in the temperature range from -30 … + 150 ° C it changes by a factor of thousands), special viscous additives are added to their viscosity-temperature properties to stabilize the viscous-temperature properties of the oils, relatively low viscosity of the base oil at low temperature, but significantly increasing the viscosity with increasing temperature. The viscosity value of the lubricant is always indicated at a specific temperature, usually at 40 ° C.
The pour point (point of leakage) is the lowest temperature at which the oil spreads under the influence of gravity. The concept of pour point is used to determine the pumpability of oil through pipelines and the possibility of lubricating friction units operating at a reduced temperature. The temperature of solidification of the oil means the temperature at which the oil placed in a test tube and inclined at an angle of 45 ° does not change its level for one minute. The pour point should be 5 … 7 ° C below the temperature at which the oil should to be pumped.
The flash point is the lowest temperature at which the oil ignites when exposed to a flame. The flash point of oil vapors should be known when the oil is supplied to friction units operating at elevated temperature. The flash point is determined in an open or closed crucible. Usually, the directories indicate the flash point of oil vapors in an open crucible.
The acid number is a measure of the content of free organic acids in the oil. The acid number is determined by the number of milligrams of potassium hydroxide (KOH) required to neutralize all the acidic components contained in 1 g of oil. As the oil ages, the acid number rises. In many cases, this number is the main indicator for oil changes in circulating lubrication systems.
When choosing liquid lubricants for specific operating conditions, the following characteristics are guided:
viscosity index – an estimate of the change in viscosity of a lubricant as a function of the temperature change;
Oxidability – an assessment of the ability of the oil to react with oxygen. Resistance to oxidation is an indicator of the stability of an oil;
extreme pressure (EP) – a measure of the quality of the strength of an oil film, is used to characterize lubricants of heavily loaded friction surfaces;
sticking (stick-slip) – evaluation of the ability of the lubricant to prevent jumps or unstable movement of the power table or the machine carriage even at extremely low speeds.
The service life of the lubricating oil depends on the rate of accumulation of harmful impurities in it and its aging. The essence of aging lies in the fact that during the process of oxidation of the oil with air oxygen with the formation of soluble acids and sludge. The oil must be replaced if an increase in its kinematic viscosity is detected by more than 30%; increase in the value of the acid number to 3 mg KOH per 1 g of oil; water content more than 0,2%; the content of non-abrasive mechanical impurities (sludge, admixture of grease) is more than 0.1%.