Correlation between the Flow Point Method and Traditional Evaluation Methods for Characterizing Grease Flow Performance
With the development of modern machinery and equipment of large-scale,heavy-duty,automation trends,high-end equipment manufacturing industry has encountered a bottleneck in the development of the research in the field of aerospace,precision machine tools,automotive industry,rail transportation and energy industry.Bearing has been clearly placed as the development focus in the high-end manufacturing industry in Made in China 2025,and high reliability and long life of bearings have become its core requirements.Grease is the most commonly used type of lubricant in rolling bearings,so grease becomes an important factor affecting the reliability of bearings.Whether the grease can be effectively delivered to the friction contact area is critical to the reliability of grease lubrication.Difficulties in grease delivery in centralized lubrication systems can lead to equipment damage due to lubrication failure,so research on the flow-ability of grease has received extensive attention.Grease is a stable semi-solid product formed by dispersing a thickening agent in a liquid lubricant,it combines the advantages of a liquid and a solid:the grease does not flow like a solid at room temperature and standstill,and grease precipitates base oil to lubricate the friction pair.As a non-Newtonian fluid,the flow condition of grease is that external force must be applied to make the flow possible,therefore the core content of the study for the flow performance of grease is the relationship between temperature and shear stress.Some traditional test methods for evaluating grease flow properties were established based on applied research,including similar viscosity,flow pressure,low-temperature torque,taper entry and other methods.Flow pressure is the yield shear stress of the grease,and the physical meaning of yield shear stress of the grease is that the maximum shear stress formed by the grease under the external force from standstill to produce flow,and a more visual representation of the energy output that overcomes the grease from standstill to start flowing in the outside world.The flow point is sometimes called the flow stress and refers to the value of shear stress at the intersection of the energy storage modulus and the loss modulus are equal.At the high shear,the viscous part dominates and the sample is in a flowing state.In order to improve the efficiency of grease flow performance testing,establish a more complete evaluation system of grease flow characteristics,and solve the practical problem that the core lubrication point sample collection in the lubrication monitoring process is too small to be analyzed by conventional test methods,this paper used rotary rheometer,grease and petroleum grease cone penetration tester,grease similar viscosity tester,FFK low temperature flow pressure tester to test the cone penetration,similar viscosity,flow pressure and flow point of three different types of thickeners:lithium complex grease,urea base grease and calcium sulfonate base grease.The change law of flow point and one penetration,flow point and similar viscosity,flow point and flow pressure with the temperature of the three types of grease were researched.Meanwhile,the person correlation coefficient of flow point/cone penetration,flow point/similar viscosity and flow point/flow pressure of three different types of thickeners were calculated.The results indicated that the flow point of grease had a linear relationship with cone penetration,similar viscosity and flow pressure.Where the flow point was moderately correlated with the cone penetration,and the flow point was highly correlated with the similar viscosity,and the flow point was also highly correlated with the flow pressure.Therefore,the flow properties of grease could be characterized by flow point which tested by rotary rheometer.
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