Selection And Use Of Engineering Machinery Grease

1. Main performance indicators of grease ① Dropping point: refers to the temperature when a certain fluidity is reached when heated under specified conditions. It can generally determine the operating temperature of the lubricating finger (the dropping point is 15 to 30 degrees higher than the operating temperature) ② Cone penetration: refers to the depth to which the test cone can freely penetrate vertically into the grease within 5 seconds under the specified temperature and load (unit: 1/10mm). It is a measure of the consistency and hardness of lubrication. ③ Colloidal stability (oil separation): refers to the ability of lubrication to retain oil in the skeleton of its thickener under the action of external force, and is judged by the amount of oil separated. When the oil separation amount of the grease exceeds 5%-20%, this grease basically cannot be used. ④Oxidative stability: refers to the ability to resist oxidation during storage and use. ⑤Mechanical stability: refers to the ability to resist changes in consistency under mechanical working conditions. Poor mechanical stability can easily cause the consistency of the grease to decrease. ⑥Evaporation loss: refers to the percentage of the total loss under specified conditions. It is an important factor affecting the service life of grease. ⑦Water resistance: refers to the ability to not dissolve in water, not absorb moisture from the surrounding medium, and not be washed away by water. ⑧Similar viscosity: refers to the ratio of shear stress and shear speed when non-Newtonian fluid flows. When the rotation speed is high, the viscosity is low, otherwise the viscosity is high.
   2. Failure analysis of grease (1) Failure caused by physical factors During use, the grease will be subjected to mechanical shear and centrifugal force at the same time. The grease will be thrown out of the friction interface to separate the oil, resulting in the reduction of the oil content of the grease, the reduction of the cone penetration and hardening. After reaching a certain level, the grease will It will completely fail; under the action of mechanical shear, the grease structure will be damaged (such as soap fibers detaching or orienting), causing its softening, reduced consistency and increased oil precipitation, etc., ultimately leading to failure. Under normal circumstances, if the transmission speed of grease increases by 2000r/min, its service life will be reduced by about half. Under high shear stress, the rotation speed doubles and the service life is only equivalent to 1/10 of the original life. (2) Failure caused by chemical factors Grease reacts chemically with oxygen in the air to produce acidic substances. It first consumes the antioxidant additives in the grease, but after reaching a certain level, the organic acid generated will corrode metal components and destroy the structure of the grease, causing it to drop. Decrease, base oil viscosity increase and fluidity deterioration, etc. A large number of tests have shown that the higher the temperature, the more obvious the life span of the grease decreases. For example, when the temperature is between 90 and 100 degrees, the life of the grease will be reduced by about half for every 19 degrees of temperature increase. When the temperature is between 10 and 150 degrees, for every 15 degrees of temperature increase, the life of the grease will also be reduced by half. In addition, moisture, dust and harmful gases in the environment in which grease is used are also important factors in its deterioration. For example: wear particles such as copper, iron, lead and bronze mixed into the grease will catalyze the oxidation of the grease. In short, there are many reasons for the failure of grease. Sometimes it may be caused by a certain reason, but more often it is the result of the simultaneous action of multiple factors, or one reason is the breakthrough, and then other reasons work together.
   3. Reasonable selection of grease When selecting grease, you should mainly consider the working conditions of the friction pair (load, speed, temperature), working status (continuous operation, intermittent operation, presence of vibration and impact, etc.) and working environment (humidity, temperature, air pollution level, etc. ). (1) The service temperature of grease should be at least 20 to 30 degrees lower than its dropping point. When the service temperature is high, greases with good antioxidant properties, small evaporation losses and high dropping points should be selected; when the service temperatures are low, greases with low starting torque and similar viscosity should be selected, such as greases with synthetic oil as the base oil. . (2) The selected grease should be suitable for the use speed of the lubricated friction pair. At high speeds, a grease with a larger penetration made from a low-viscosity base oil should be selected; for low-speed grease, a grease with a high penetration made from a high-viscosity base oil should be selected. (3) The selected grease should be suitable for the load. For heavy loads, greases with high base oil viscosity and high thickener content should be selected. When the load is particularly large, attention should be paid to selecting a grease with extreme pressure additives or fillers (molybdenum disulfide, graphite); when the load is medium to low, grease with a short soap fiber structure of No. 2 consistency and a medium viscosity base oil is generally used. (4) The selected grease should be suitable for the environmental conditions used In an environment where the air is humid or in contact with water, grease with good water resistance such as calcium-based, lithium-based, lithium complex-based, etc. should be selected; when there is a lot of dust, a thicker and harder grease (that is, a higher grade) should be selected to ensure better sealing Better, it can prevent impurities from being mixed into the friction pair. In environments with strong chemical media, synthetic oil greases that are resistant to chemical media, such as fluorocarbon grease, should be used. (5) The selected grease should be compatible with the grease supply method of the friction pair

For centralized grease supply, grease No. 00 to No. 1 should be selected; for parts that are greased regularly with grease guns, grease cups, etc., grease No. 1 to No. 3 should be selected; for parts that are used for a long time without changing grease, grease should be selected. Use No. 2 or No. 3 grease.

 (6) The selected grease should be compatible with the working condition of the friction pair

 For example, when the vibration is large, grease with high viscosity, good adhesion and vibration damping properties should be used, such as complex soap-based grease thickened with high-viscosity naphthenic or mixed-base lubricating oil.

 (7) The selected grease should be suitable for its purpose of use

 The grease used for lubrication must be specifically selected according to the type of friction pair, working conditions, working conditions, environmental conditions and grease supply methods; for the grease used for protection, it should be able to effectively protect the metal from corrosion, such as protection Mechanical parts in contact with seawater should choose aluminum-based grease with strong adhesion and water resistance; general protective grease can be made of solid hydrocarbon-thickened high-viscosity base oil. For sealing grease, attention should be paid to its resistance to the solvent of the sealed medium.

 (8) The selected grease should try to reduce the variety of grease and improve economic efficiency.

 When the requirements are met, try to use multi-effect general-purpose greases such as lithium-based grease, complex soap-based grease, and polyurea grease. In this way, the types of grease are reduced, grease management is simplified, and the long service life of multi-effect grease can reduce grease costs and maintenance costs.

 4. Correct use of grease

 (1) The amount of lubrication added must be appropriate

 If the amount of grease added is too large, the friction torque will increase, the temperature will rise, and the amount of grease will increase. If the amount of grease added is too small, reliable lubrication will not be obtained and dry friction will occur. Generally speaking, the appropriate amount of grease is 1/3 to 1/2 of the total void volume in the bearing. However, depending on the circumstances, sometimes the bearing edge should be greased for cavity lubrication.

 (2) Pay attention to prevent the mixing of different types, brands and new and old greases

Avoid cross-use of grease containers and tools, otherwise, it will have adverse effects such as lowering the dropping point of the grease, increasing cone penetration, and decreasing mechanical stability.

 (3) Pay attention to the replacement of new grease

 Since the variety and quality of grease are constantly improving and changing, when old equipment is replaced with new grease, it should be tested first and tried before official use; when replacing new grease, waste grease should be removed first and the parts should be cleaned. clean. When adding grease, waste grease should be squeezed out until new grease is seen at the grease outlet.

 (4) Pay attention to the management of the grease filling process

 Before receiving and adding grease, strict attention should be paid to the cleanliness of containers and tools. The grease supply port on the equipment should be wiped clean in advance to prevent the mixing of mechanical impurities, dust and sand.

 (5) Pay attention to the timely replacement of seasonal grease

 If the temperature difference between winter and summer in the environment where the equipment is located changes greatly, if winter grease is used in summer or vice versa, the results will be counterproductive.

 (6) Pay attention to adding and changing grease regularly

 The time for adding and replacing grease should be determined according to the specific usage conditions, so as to ensure reliable lubrication without causing a waste of grease.

 (7) Do not pack grease in wooden or paper containers

 Prevent oil loss from becoming hard, mixed with moisture, or contaminated and deteriorated, and should be stored in a cool and dry place.