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Selection principle, injection method and injection amount of bearing grease for repair and maintenance

Selection principle, injection method and injection amount of bearing grease for repair and maintenance

2022-02-08

First, the selection principle of bearing grease


When selecting grease, it is mainly based on the working environment, working temperature and load conditions of the bearing.


1. Working environment


It mainly refers to the minimum and maximum temperature of the working environment, air humidity, whether it contains corrosive gas or excessive dust, etc. For example, when used outdoors in severe cold areas, lithium-based low-temperature grease should be used; in places with high humidity and more moisture, calcium-based grease should be used; base grease.


2. Working temperature


One is the lowest temperature of the use environment, and the other is the highest temperature that may occur during operation. When the operating temperature is high, a high-temperature grease with a high dropping point should be selected. The actual maximum operating temperature should be 10-20C lower than the dropping point of the grease (20-30C for synthetic grease).


3. Load condition


For heavy loads, grease with a smaller taper should be used. When working under high pressure, in addition to requiring a small taper, it also requires high oil film strength and extrusion performance.


Second, the injection method of bearing grease


There are two types of grease injection tools for rolling bearings, manual grease injection and pressure grease injection, commonly known as oil guns; larger production, use and repair units may use special grease injection machines (tanks or barrels) with metering devices.


3. Grease injection amount of bearing grease


1. The effect of grease injection on the use of bearings


For sealed bearings, appropriate grease has been filled during production; for open bearings, although a sealing device is provided to prevent the leakage of grease, it will gradually decrease due to a small amount of leakage during use. Continually replenish.


Practice has proved that the amount of grease injected into the bearing will have a certain impact on its operation. Too little grease can easily lead to insufficient lubrication. In severe cases, it will lead to dry friction, high heat generation, and the bearing will be scrapped in a short period of time due to wear, which may have a serious impact on the entire equipment. Excessive amount of grease injection will lead to high rolling resistance of the bearing rolling elements, and the temperature of the grease in the bearing will be high. When it reaches its dropping point, it will be converted into liquid and lost, and finally the serious consequences of the above-mentioned too small amount of grease injection will occur. Leaked grease is both wasteful and polluting the environment.


2. The principle of grease injection


The principle of the amount of grease injection: the less the better on the premise that the bearing can be fully lubricated.


Through practical experience, the following principles are more appropriate.


For open-type bearings, the appropriate amount of grease injection should be based on the bearing chamber cavity volume (after the two bearing caps and the bearing are installed, the air occupied part of the inner space contained by them, see Figure 2 except the bearing balls). The blank part) and the bearing speed used (for AC motors, the number of poles can also be used instead of the speed) to briefly calculate the amount of grease,


For the bearing chamber structure with the oil baffle (also known as the oil slinger), the amount of grease injected for the first time should be appropriately increased, and no grease should be injected into the bearing outer cover cavity (here is to accept the "waste grease thrown out"). ”, the grease in it will not enter the bearing for lubrication, so the newly injected grease will be wasted). With this structure, the grease in the bearing chamber will become less and less. If the grease is not added regularly as required, the lubricating effect will be reduced due to too little grease. Eventually, the grease will dry out due to overheating, and the entire bearing will be damaged.


3. Matters needing attention


When injecting grease, the site should be clean and the tools used should be cleaned with gasoline. After the grease is injected, other parts should be assembled as soon as possible to prevent the grease entering the bearing from entraining dust and debris, especially sand and iron filings.


4. Grease injection of rolling bearings during operation


For machines using open bearings, after running for a certain period of time, grease should be added to the bearing chamber. For those with oil injection system, it can be carried out without disassembling the machine, as shown in Figure 4a, otherwise, the bearing cover should be disassembled and oil should be directly injected into the bearing chamber. Attention should be paid to using the same grade of grease as the original grease to avoid Avoid harmful reactions of different components of grease to reduce or even lose lubricating effect, resulting in bearing overheating damage.


Note: If the original grease has deteriorated, it should be completely removed with a solvent such as gasoline, and then refilled with new grease.


5. Time interval for replenishing grease during operation


The life expectancy of a grease is affected by a number of factors. For example, the type of grease, the rotational speed and temperature of the bearing, the amount of dust and corrosive gases in the working environment, the design of the sealing device and the actual function of the situation.


For sealed or smaller bearings, both the bearing itself and the grease in it determine the life of a set of bearings. There is no need or possibility to add or change grease midway.


As a rule of thumb, the effect of temperature on the time interval for replenishing grease is: when the temperature (measured at the outer ring of the bearing) reaches above 40C, the time interval for replenishing grease will be shortened by half for every 15C increase.


For open-type bearings, refer to Figure 5 for the interval between regreasing. Figure 5 shows the replenishment time interval of the grease in the bearing of the horizontal axis in the fixed machine under normal working conditions, based on the lithium base grease containing oxidant (the Y axis of the ordinate is the replenishment time interval tq, the unit is Working hours, the abscissa X axis is the running speed n, the unit is r/min, and d is the inner diameter of the bearing, the unit is mm). Among them, a coordinate is radial bearing; b coordinate is cylindrical roller and needle roller bearing; c coordinate is spherical roller, tapered roller and thrust ball bearing. If it is a full complement cylindrical roller bearing, the interval is 1/5 of the corresponding value of the b coordinate; if it is a cylindrical roller thrust bearing, a needle roller thrust bearing, and a spherical roller thrust bearing, the interval is the corresponding value of the c coordinate 1/2 of the value.


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