How to judge the shear stability of lubricant
author: Leila
2024-09-02
The shear stability of lubricating oil represents the property of lubricating oil to maintain stable viscosity when subjected to mechanical shearing and rolling, also known as mechanical stability. If the shear stability is good, the lubricating oil can maintain a relatively stable viscosity during mechanical shearing, otherwise the viscosity of the lubricating oil will decrease (the oil becomes thinner) due to shearing.
Temperature will affect the viscosity of the liquid. Lubricating oil with a high viscosity index has good viscosity-temperature performance and can maintain stable viscosity during temperature changes. In order to improve the viscosity index of lubricating oil, viscosity index improvers (also called viscosity-increase agents: viscosity-index improvers) are added to the lubricating oil. Viscosity index improvers are high molecular polymers that are sensitive to temperature. When the temperature rises, their molecular chains expand and stretch, and the friction between base oil molecules increases, which hinders the fluidity of the oil, that is, the viscosity increases, which to a certain extent alleviates the viscosity drop of the lubricating oil.
Viscosity index additives also have disadvantages. Under high shear rates, the molecular chains of viscosity index additives are easily cut off due to shearing, reducing or even completely losing the ability to improve viscosity. Shearing not only includes shearing during the movement of mechanical parts, but also high-temperature shearing. Under high temperature conditions, stretched molecules are more easily sheared by machinery. Therefore, attention should be paid to the thermal shear stability of viscosity index improvers.
Generally speaking, polymers with large relative molecular weights have better viscosity-increasing effects, but poor shear resistance. Polymers with small relative molecular weights are more resistant to shearing, but the viscosity-increasing effect is limited, so the amount used is relatively large. Some lubricating oils experience a sudden drop in viscosity and thinning of the oil during use because the viscosity index improver molecules are sheared and lose their viscosity-increasing effect. Viscosity index improvers are often used in multi-grade motor oils (automobile engine lubricants), gear oils, and high-viscosity hydraulic oils. Therefore, special attention should be paid to the shear stability of these oils.
Although all viscosity index improvers are subject to shearing, the viscosity stability of viscosity index improvers when subject to shearing varies. This requires us to judge the shear stability of oil products through professional testing:
The shear stability of lubricating oil is measured by shear stability test. Under certain conditions, the lubricating oil is sheared and then the viscosity change is tested. There are three main testing standards for shear stability:
DIN 51382: Bosch Injector Test - This is the lowest standard among the three testing methods. In this test, the oil is run 250 times under a pressure of 2550psi, and then the viscosity change rate of the lubricating oil is tested.
ASTM D5621: Sonic Shear Method is to place the oil in a sonic oscillator, and after being subjected to sonic shear, record the viscosity loss rate caused by shear. This test standard is generally used by American equipment manufacturers, but it is gradually being replaced by the following CEC-L45-A-99 KRL tapered roller bearing test.
CEC-L45-A-99: The KRL Tapered Roller Bearing Test is a popular test method in the world because it is generally believed that its requirements are the most stringent and are most closely related to the shear resistance of lubricants in actual use. The lubricant is used in the bearing, and the bearing runs continuously for 20 hours under load pressure. Then the viscosity of the lubricant before and after the test is recorded, and the viscosity loss rate is calculated.
In the shear test, the viscosity change rate of the lubricant is recorded as the shear stability index (SSI). The lower the value of the shear stability index, the smaller the viscosity change of the lubricant after shearing, the better the shear stability of the oil, and the better the shear resistance of the viscosity index improver. Multi-grade oils with good performance use high-quality viscosity index improvers. The SSI value of high-quality viscosity index improvers is small, and the molecular chain is not easy to be cut, such as polyolefin (OCP) polymers. Generally speaking, the shear index SSI of engine oil should be lower than 25.
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