The application range of grease is very wide, and the working conditions are also very different. Different mechanical equipment have different requirements for the performance of grease.
The performance of lubricating grease is the comprehensive embodiment of the composition and preparation process of lubricating grease. The evaluation of grease performance is not only of decisive significance in production and research, but also essential in the selection and inspection of grease by the user department.
According to the specific conditions of grease parts for automobiles and construction machinery, the basic requirements for grease are: appropriate consistency, good high and low temperature performance, good extreme pressure and wear resistance, good water resistance, anti-corrosion, anti rust and stability.
Under the specified shear force or shear speed, the deformation degree of the grease structure system is measured to express the structure of the system, that is, the concept of consistency.
It is an important performance index related to the holding capacity and sealing performance of grease on the lubricated parts, as well as the pumping and filling mode of grease. The reason why some lubricating points use grease is that it has a certain consistency, so that it has a certain ability to resist loss.
Different consistency greases are suitable for different working conditions such as mechanical speed, load and ambient temperature. Therefore, consistency is an important index of grease.
The consistency grade of grease can be expressed by cone penetration. The cone penetration of lubricating grease refers to the depth of the standard cone of the specified weight penetrating the lubricating grease sample under the specified time and temperature conditions,
expressed in (L / 10) mm. The cone penetration of lubricating grease can be measured according to the method specified in the determination of cone penetration of lubricating grease (GB / t269-91).
The cone penetration of lubricating grease usually includes non working, working, extended working and block cone penetration. The non working cone penetration generally does not effectively represent the consistency of lubricating grease in use as the working cone penetration.
The working cone penetration is usually used when inspecting lubricating grease. The extended working cone penetration is applicable to the cone penetration measured after working for more than 60 times.
Summary of measurement method for cone penetration of lubricating grease: release the cone assembly from the cone penetration meter at 25 ℃ and make the cone sink into the sample for 5S to measure the above four cone penetration of lubricating grease respectively.
The higher the cone penetration value, the softer the grease, that is, the smaller the consistency, the easier it is to deform and flow; The lower the cone penetration value is, the harder the grease is, that is, the greater the consistency is, the harder it is to deform and flow.
It can be seen that cone penetration can effectively represent the consistency of grease and is an important basis for the selection of grease. In China, the cone penetration range is used to divide the consistency grade of lubricating grease.
GB7631. 1-87 is consistent with the consistency number of American grease Association (nlgi), which is widely used internationally.
2. High temperature performance
Temperature has a great influence on the fluidity of grease. With the increase of temperature, the grease becomes soft, which reduces the adhesion of grease and is easy to be lost.
In addition, it is easy to increase the evaporation loss of lubricating grease at high temperature, and the phenomenon of oxidative deterioration and condensation oil separation is serious.
The main reasons for the failure of grease are mostly due to the shrinkage of the gel and the evaporation loss of the base oil, that is, the speed of the lubricating grease is related to the temperature.
The grease with good high temperature performance can maintain its adhesion at high service temperature, and its deterioration and failure process is slow. The high temperature performance of lubricating grease can be evaluated by dropping point, evaporation and bearing leakage.
The dropping point of lubricating grease refers to the low temperature when it reaches a certain fluidity under specified conditions, expressed in ℃. The drop point has no absolute physical meaning, and its value varies with the equipment and heating rate.
The dropping point of lubricating grease mainly depends on the type and content of thickener. The dropping point of lubricating grease can roughly reflect the upper limit of its service temperature. Obviously, when the grease reaches the drop point,
it has lost its adhesion to the metal surface. Generally speaking, the grease should be used at a temperature of 20 ℃ ~ 30 ℃ or lower below the drop point.
The dropping point of lubricating grease can be determined according to GB / t4929-85 determination of dropping point of lubricating grease. Method summary: put the grease into the grease cup of the drop point meter, and record the temperature
when the grease reaches the specified fluidity during the test under the specified standard conditions. This standard is equivalent to ISO / dp2176. GB / t3498-83 is a wide temperature range drop point method for lubricating grease.
The evaporation degree of lubricating grease refers to the mass percentage of the loss of lubricating grease after evaporation under specified conditions. The evaporation degree of lubricating grease mainly depends on the type, fraction composition and molecular weight of the base oil used.
The measurement of evaporation degree of lubricating grease used under high temperature and wide temperature is particularly important. Evaporation degree can qualitatively represent the upper limit service temperature of lubricating grease.
The evaporation loss of grease base oil will increase the content of soap based thickener in the grease, change the consistency of the grease, increase the internal friction and affect the service life of the grease. Therefore, the evaporation index can indicate the high-temperature performance of lubricating grease to a certain extent.
SH / t0337-92 is a dish method for measuring the evaporation of lubricating grease. GB / t7325-87 is a method for determining the evaporation loss of lubricating grease and lubricating oil. The method is summarized as follows:
Place the grease sample placed in the evaporator in a constant temperature bath at the specified temperature, and the hot air passes through the sample surface for 22h. Calculate the evaporation loss according to the weight loss of the sample.
In order to better evaluate the high-temperature performance of lubricating grease used in vehicles and construction machinery, the working characteristics of bearing and bearing leakage under high-temperature conditions should be measured through simulation test.
According to statistics, most rolling bearings are lubricated with grease. Therefore, the service life of grease bearing is an extremely important performance index. The evaluation of grease on high temperature bearing life testing machine can simulate the working performance of grease under certain conditions of high temperature,
load and speed. Therefore, the measured results have a certain reference value for practical use. Generally, it is observed on the testing machine that when the lubricating grease reaches the service life, the grease film is damaged, the peak value of the damage torque appears, the test stops automatically,
and the bearing temperature rise record indication value rises sharply and dry friction noise appears. If it still cannot rotate after repeated startup, it indicates that the grease film has been damaged and the test is over,
The time of the test is the high temperature bearing life of the grease. Generally speaking, the longer the bearing life of grease, the longer its service life.
SH / t0428-92 is a method for measuring the workability of lubricating grease in anti-wear bearing under high temperature.
Measuring the leakage of grease bearing is to simulate the working performance of grease in wheel bearing of automobile and construction machinery. SH / t0326-92 “grease leakage test” specifies the leakage measurement method. Method summary:
take the grease sample GDG, install the grease sample 85g into the wheel Yi, and install the grease sample 29 ± 0 LG, install grease sample in the other bearing 39 ± 0.5 lg。 The rotating speed is 660r / min ± 3R / min, the bearing temperature is 105 ℃± L ℃, the temperature in the box is 113 ℃± l 0.5 ℃,
and the operation time is 10h. The working characteristics of the grease are measured by the amount of grease thrown out on the bearing, and pay attention to the surface condition of the bearing at the end of the test. Obviously, the greater the leakage, the worse the high-temperature performance of the grease.
3. Low temperature performance
The starting temperature of automobile and engineering alum machinery is almost the same as the ambient temperature.
When used in cold areas, it is required that the grease can still maintain good lubrication performance under low temperature conditions, which depends on the similar viscosity and low temperature torque of the grease under low temperature conditions.
We know that the viscosity of lubricating oil decreases with the increase of temperature, so the viscosity of the same lubricating oil is also different due to different temperature. This characteristic is called zhongzao extra collapse.
The viscosity temperature characteristics of grease are more complex than that of lubricating oil, because the viscosity temperature characteristics of grease structure system also change with the change of shear force.
The viscosity of lubricating grease at a certain temperature is a variable that changes with the shear rate. This viscosity is called similar viscosity, and the unit is pa.s. The similar viscosity of grease decreases with the increase of shear rate, but when the shear rate continues to increase,
the similar viscosity of grease will not change after it is close to the viscosity of its base oil. The variation law of similar viscosity and shear rate of lubricating grease is called viscosity velocity characteristic. The more obvious the change of viscosity with shear rate, the greater the energy loss. Generally,
the low-temperature service limit of lubricating grease can be determined according to the allowable value of similar viscosity of lubricating grease under low-temperature conditions.
The similar viscosity of lubricating grease also decreases with the increase of temperature, but it is only a few hundred or even one thousandth of that of base oil. Therefore, the viscosity temperature characteristics of lubricating grease are better than that of lubricating oil.
SH / t0048-91 specifies the determination method of similar viscosity of lubricating grease, which adopts non constant flow capillary viscometer.
Low temperature torque refers to the degree to which the grease blocks the rotation of low-speed ball bearings when used under low temperature conditions. The low-temperature torque can represent the low-temperature service performance of grease,
which is 9.8N Cm torque measures the low temperature when the bearing rotates for one cycle within 1min, which is used as the low service temperature of lubricating grease.
The low temperature torque of lubricating grease is not only related to the low temperature viscosity of base oil, but also related to the strength limit of lubricating grease.
SH / t0338-92 measurement method of low temperature torque of ball bearing grease specifies the measurement method of starting and running torque. This method can measure the starting and running torque of ball bearing grease at – 20 ℃ as an evaluation index to evaluate the running resistance of grease at low temperature.
The grease film formed by the grease applied between the metal surfaces in contact with each other can withstand the axial and radial loads. The load bearing characteristics of the grease film are called the extreme pressure of the grease. Generally speaking, after adding soap based thickener to the base oil,
the extreme pressure of grease is enhanced. Extreme pressure agent is often added to grease used under harsh conditions to enhance its extreme pressure. At present, four ball testing machine is widely used to measure the lipid film strength of lubricating grease.
SH / t0202-92 determination of extreme pressure performance of lubricating grease (four ball machine method) specifies the determination method of extreme pressure performance of lubricating grease, which is expressed by comprehensive wear value and sintering point. Comprehensive wear value, also known as load wear index,
is the average value of several corrected loads obtained under specified conditions when the extreme pressure performance of lubricant is measured by four ball method. Sintering point, also known as sintering load, refers to the low load (n) that causes the steel ball to sinter under specified conditions.
SH / t0203-92 determination of extreme pressure performance of lubricating grease (Timken tester method) uses 0k value (i.e. large application value) to represent the extreme pressure performance of lubricating grease.
The wei0k value refers to the small value (n) of the load applied when there is scratch or bite in the process of measuring the pressure bearing capacity of the lubricant by Timken Method.
The ability of lubricating grease to prevent metal to metal contact and wear by maintaining the oil film between the surfaces of moving parts is called wear resistance. The thickener of lubricating grease itself is an oily agent with good wear resistance.
The grease used under harsh conditions is added with antifriction agents such as molybdenum disulfide and graphite and extreme pressure agents, so it has stronger wear resistance than ordinary grease. This kind of grease is called extreme pressure grease.
SH / t0204-92 determination of antiwear performance of lubricating grease (four ball machine method) specifies the determination method of antiwear performance of lubricating grease. SH / t0427-92 determination of gear wear of lubricating grease is a method for measuring the wear resistance of lubricating grease with gear wear tester.
5. Water resistance
The water resistance of lubricating grease indicates the water absorption performance of lubricating grease under the condition of atmospheric humidity. It is required that the lubricating grease does not have the ability to absorb water during storage and use.
After the grease absorbs water, it will dissolve the thickener and reduce the drop point, causing corrosion and reducing the protective effect. Some greases, such as compound calcium base grease, absorb moisture in the atmosphere and will also lead to hardening and gradual loss of lubricating ability.
The water resistance of lubricating grease mainly depends on the water resistance and emulsification of thickener. During the use of automobiles and construction machinery, the friction points of the chassis may be in contact with water, which requires that the grease has good water resistance.
Lubricating grease with poor water resistance often reduces its consistency or even emulsifies and loses after absorbing water in the atmosphere or meeting water. SH / to109-92 specifies the method for determining the water resistance of lubricating grease by measuring the water spray resistance.
Method summary: under the specified conditions, add a known amount of sample into the bearing of the testing machine and be sprayed with water during operation. The amount of grease lost due to water spraying is calculated according to the poor quality of the sample in the bearing before and after the test.
The water resistance of lubricating grease can also be measured by measuring its water solubility. Method summary: add a certain amount of water into the sample step by step,
measure its 100000 extended working cone penetration, and then compare it with the 60 working cone penetration before the test. The difference can evaluate the water solubility of the sample.
Corrosion resistance is the ability of a grease to prevent corrosion of the metal in contact with it. The thickener of lubricating grease and base oil itself will not corrode metals. There are many reasons for the corrosiveness of lubricating grease,
mainly due to the production of acidic substances by oxidation. Generally speaking, too much free organic acid and alkali will cause corrosion. Corrosion test is to detect whether the grease has corrosive effect on the metal. There are several methods and different test conditions,
but they all fail to judge the corrosivity of the grease by observing the discoloration or spots on the metal sheet at a certain temperature and test time. SH / t0331-92 lubricating grease corrosion test method adopts 100 ℃, 3h, copper sheet and steel sheet for determination.
GB / T 7326-87 copper strip corrosion test of lubricating grease specifies the method for determining the acid corrosivity of lubricating grease to copper parts. It is determined by copper strip at 100 ℃ for 24 hours, which is divided into method a and method B.
Method a is to compare the test pot with the copper corrosion standard color plate to determine the corrosion level; Method B is to check whether the test copper is discolored. GB / t5018-85 test method for corrosion resistance of lubricating grease specifies the test method for corrosion resistance of lubricating grease.
Method summary: run the new bearing coated with sample under light thrust load for 60s, so that the grease is distributed as it is used. The bearing shall be stored at 52 ℃± L ℃ and 100x relative humidity for 48h, and then the bearing outer race raceway shall be cleaned and inspected for corrosion signs.
Corrosion in this method refers to any surface damage (including pitting, etching, rust, etc.) or black stain on the raceway of the outer ring of the bearing. This method can evaluate the ability of lubricating grease to prevent corrosion and other forms of corrosion of metal in contact with it under wet conditions.
7. Colloidal stability
Colloidal stability refers to the ability of lubricating grease to avoid colloidal decomposition and prevent the precipitation of liquid lubricating oil during storage and use. If the tendency of soap oil separation of lubricating grease is large, it indicates that its colloidal stability is not good,
which will directly lead to the change of lubricating grease consistency. Oil separation test can be used to evaluate the colloidal stability of lubricating grease. GB / T 392-90 lubricating grease – Determination of oil pressure – colloidal stability of lubricating grease is evaluated by measuring the oil content of lubricating grease.
Method summary: press out the oil from the grease with a pressurized oil separator, and then measure the amount of oil pressed out. SH / t0321-92 determination of lubricating grease funnel oil separation method specifies the method for determining the oil separation amount of lubricating grease
by funnel oil separation method. SH / t0324-92 steel mesh oil separation method for lubricating grease (static method) specifies the method for measuring the oil separation amount of lubricating grease by steel mesh oil separation method,
which is applicable to measuring the oil separation tendency of lubricating grease under the condition of temperature rise.
8. Oxidation stability
The ability of lubricating grease to resist the action of atmosphere and maintain its properties without permanent change during storage and use is called oxidation stability.
The oxidation of grease is related to its components, i.e. thickener, additive and base oil. The thickener and base oil in lubricating grease are stored or stored for a long time