Here is an example of a flat grinding head spindle adopting a spindle bearing structure with one end sliding and one end rolling. Among them, there are three kinds of friction states between the sliding bearing and the main shaft: boundary friction, liquid friction, and mixed friction.
Boundary friction means that the two sliding surfaces are separated by a boundary adsorption film formed by the direct combination of polar molecules (usually present in surface active additives such as oleic acid and fatty acid) in the lubricating oil and the metal surface, or by the boundary adsorption film containing sulfur and chlorine. Active substances such as, phosphorus and the metal surface are separated by the inorganic salt film formed by the chemical action.
Liquid friction means that two sliding surfaces are completely separated by a layer of lubricating oil (carrying oil film).
Mixed friction refers to between the above two states. Or there are more cases of boundary friction; or more cases of liquid friction.
For convenience, boundary friction and mixed friction are collectively referred to as non-liquid friction hereinafter. The sliding bearing is in the most ideal state of liquid friction. Because the friction resistance at this time is small, the friction factor is close to the equivalent friction factor of the rolling bearing. In addition, the bearing oil film can completely eliminate wear and tear, and at the same time has good vibration absorption, cushioning and other advantages. But when the sliding bearing is in the boundary friction state, if the boundary oil film is destroyed, the friction and wear between the metals (the first is the abrasive wear) will increase sharply. At the same time, the local temperature caused by metal-to-metal friction increases sharply. The increase in temperature leads to rapid expansion of the metal surface, which in turn intensifies the friction between metals. When the local high temperature generated by friction reaches the melting point of the material, adhesion will occur between the main shaft and the bearing, that is, shaft holding.
The material of the plain grinding sliding bearing is tin bronze. When the shaft is held slightly, there are black burn marks on the surface of the main shaft. The sliding bearing is a copper-based alloy material, which is relatively soft. In addition to black burn marks, pits will also be formed on the burned surface. In this case, you should first analyze the cause of the shaft holding, eliminate the fault, repair the surface of the main shaft and the sliding bearing, and re-scrape and assemble. The main shaft and sliding bearings can still be used. Severe holding of the shaft will cause the main shaft to crack, in which case the main shaft should be replaced.
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Analysis of the Causes of Sliding Bearings Holding Shaft
(1) The lubricating oil and sliding bearing oil pool have impurities, dirt, and poor oil cleanliness. When the oil ring rotates with the main shaft, the oil supply system starts to work. Impurities and dirt enter between the main shaft and the sliding bearing along with the lubricating oil. The gap between the main shaft and the sliding bearing is generally 0.03-0.04mm. When impurities and dirt pull the main shaft or sliding bearing, non-liquid friction is prone to occur, resulting in shaft holding.
(2) The grinding head lubrication system is malfunctioning. Plain grinding sliding bearings are lubricated by oil rings. This lubrication method has the advantages of simplicity, automaticity, and low oil consumption. 16 small holes of 5mm are evenly distributed on the circumference of the oil ring. When the oil ring rotates with the main shaft, the small hole on the oil ring splashes the lubricating oil in the oil pool, and the lubricating oil splashes on the main shaft. The spiral oil groove on the main shaft inputs the lubricating oil between the main shaft and the sliding bearing. When the lubrication system of the grinding head does not supply oil or the oil supply is unstable, the phenomenon of shaft holding will occur.