Rapid development of science and technology grease constantly update. The first generation of grease was used in the 1950s ~1960s: calcium-based grease, sodium-based grease; and the second generation of grease was used in the 1970s and 1980s: lithium-based grease, composite aluminum-based grease, composite lithium-based grease. In recent 20 years, countries have developed high-drop grease. Urea-based lipids, which are better than lithium-based lipids, can be used to measure the level of industrial development in a country in the 21st century.
Grease is composed of base oil, thickener and additive. The nature of the grease is determined by its composition and structure.
Base oil is divided into two categories: one is mineral oil, the other is synthetic oil. About 85%~93% mineral grease in the worldwide grease market.
thickener determines a series of properties of the grease. It is divided into four categories: soap group, hydrocarbon group, organic and inorganic thickener. Soap group: mainly fatty acid metal salt; hydrocarbon thickener is mainly paraffin and ground wax, commonly known as vaseline; inorganic thickener has bentonite, silica gel, boron nitride and carbon black; organic thickener is mainly: amide, polyurea, fluorocarbon and so on.
In addition to base oil and thickener, a third component, additive, is often added to the composition of the grease to improve certain properties of the grease. Such as: scaling improver, antioxidant additives, anti-rust additives, anti-wear and extreme pressure additives.
The main performance of grease and related terms explain:
First, taper - is a measure of the consistency of grease indicators. The greater the value, the softer the fat and the harder it is.
cone penetration is at 25℃, the cone of a certain mass is released from the cone penetration meter, and the cone lags behind the depth of the sample to be tested in 5 seconds. According to the size of taper, the grease is divided into 9 grades.
Work taper, is the first choice of grease index. usually according to the working conditions of friction parts and the way of adding grease. to select suitable grease consistency.
The speed factor DN value (inner diameter mm× rotational speed r/min) of the general rolling bearing is 15000-30000. If the DN value is greater than 30000 and less than 50000, no .3 is more suitable.
II. Drop points
The grease is heated under specified conditions, the grease becomes soft with the increase of temperature, and the temperature of the first drop dripping from the grease cup is the drop point. the drop point of the grease, which can roughly measure its maximum use temperature. generally, the maximum temperature limit for the use of grease should be less than its drop point 30~50℃. however, for low rpm usage, the maximum use temperature limit of grease can be lower than drop point 15~30℃.
it should be noted that the drop point is not the only parameter to determine the maximum temperature limit for grease use. To determine the maximum temperature limit of a grease, the consistency of the grease at high temperature, oxidation resistance of base oil and thickener, colloidal stability at high temperature, etc.
Grease in the storage and use of the process has the tendency to produce oil, the quality of better grease less. There are several methods to determine the amount of grease oil, commonly used is steel mesh oil.
Steel mesh oil separation is a 60 mesh stainless steel mesh filled with 10 g of grease sample, in 100℃ to maintain a certain time, weighing before and after the test of the beaker containing the separation oil, that is, the score oil mass fraction.
Grease in the rolling bearing due to heat extrusion, centrifugal force and penetration, will precipitate a small amount of oil, the precipitation part is thrown out, but also part of the oil used to lubricate the raceway. So the grease in the use of slight oil is beneficial, but not too large. Too much oil to burn the bearing will also pollute the equipment.
Evaporation is a measure of the tendency of grease to become dry due to evaporation of base oil during use and storage. Determination method: the steel dish filled with grease 1 mm after, placed in a special thermostat, at a specified temperature for a certain time. determine the mass of its evaporation loss, expressed by mass fraction, the smaller the evaporation is, the better.
V. Water leaching losses
The water resistance of grease refers to the ability not to dissolve in water, not to emulsify, not to absorb water from the surrounding medium, and not to be washed off by water.
The water resistance of grease mainly depends on thickener, followed by base oil. Silicone oil has better water resistance, followed by mineral oil, ester oil and polyether oil. In terms of thickener, urea-based lipids, hydrocarbon-based lipids have the best water resistance, aluminum-based, barium-based, calcium-based and composite aluminum, composite barium, composite calcium-based, again lithium-based lipids, the worst water resistance is sodium-based lipids and composite sodium-based lipids.
the amount of water leaching loss was tested in a water forest testing machine .4 g of samples were loaded into a 204 ball bearing, rotated at a speed of 600 r/min, the water temperature was controlled for 38℃, and sprayed on the protective plate of the bearing sleeve at a flow rate of 5 ml. the amount of grease washed out by water within 1 h was used to measure the resistance of grease to water leaching.
VI. Bearing Corrosiveness
For new bearings coated with grease, run for 60 seconds under light thrust loads so that the grease is distributed as it is used. bearing is stored at 52℃,100% relative humidity for 48 hours. Then clean and check the corrosion signs of the bearing outer ring raceway. Surface damage includes pockmarks, etching, rust or black stains, etc.
VII. Extreme pressure performance
Four-ball machine extreme pressure test and Timken test OK load method are the most commonly used methods to evaluate the anti-wear and extreme pressure performance of grease in laboratory.
Generally, the anti-wear and extreme pressure ability of grease is mostly achieved by adding anti-wear and extreme pressure additives. There are also composite titanium-based lipids, composite calcium sulfonate in the addition of anti-wear, extreme pressure additives themselves have good extreme pressure anti-wear.
Four ball machine extreme pressure performance test:
three steel balls with a diameter of 12.7 mm are clamped in the ball box and covered with grease by the test. another steel ball of the same diameter is placed on the top of the three balls. under the specified load, the upper steel ball rotates against the three steel balls at rest below, the rotation number is 1770 r/min, the grease temperature is 27℃, and then a series of 10 tests are carried out step by step to increase the load. after each test, the grinding diameter of any one or three steel balls in the ball box is measured until sintering occurs.
PB value is the sintering load, under the experimental conditions so that the rotating ball and three static balls of the minimum load of sintering, he said has exceeded the extreme pressure capacity of the grease.
PD value is the maximum load without bite, and the maximum load without bite occurs under test conditions.
the experimental grease is pressed down on the test ring at 24℃, and the test ring is driven by the spindle of the testing machine to slide on the static test block. the spindle speed is 800 r/min, the test time is 10 min.. By loading the lever loading mechanism, the pressure between the test ring and the test block is produced, and the load is increased step by step. By observing the grinding marks on the surface of the test block after each stage load test, the maximum load without scratch can be obtained, which is called the OK value.