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In hydrodynamic and hydrostatic sliding bearings, what do the terms ‘hydrodynamic’ and ‘hydrostatic’ refer to? What kind of force is involved here? Below are the concepts of hydrodynamic and hydrostatic sliding bearings, which help us understand what type of bearings they are; however, I still don’t understand the difference between ‘hydrostatic’ and ‘hydrodynamic’. I hope someone experienced can clarify this for me. Thank you. Hydrodynamic sliding bearings are the most widely used type of sliding bearing, and they include two categories: lubrication using liquids (both oil-based and non-oil-based lubricants) and gas for hydrodynamic lubrication. Oil-lubricated hydrodynamic bearings come in various types, including single oil wedge (monolithic), double oil wedge, multiple oil wedges (monolithic or tilting-pad type), and stepped surfaces, each with distinct lubrication characteristics. Generally, it is required that a dynamic pressure effect be generated during rotation, with a small distance between the spindle and the bearings (high-precision machine tools require this distance to be 1–3 μm), high stiffness, and low temperature rise. A hydrostatic bearing is a sliding bearing that is lubricated using the principle of hydrostatic lubrication. The main shaft is supported by external pressure oil; at any rotational speed (including start-up and shutdown), an oil film separates the friction surfaces of the journal and bearings. Regardless of the shaft’s speed or the viscosity of the oil, the friction pairs are in a state of fluid lubrication, with no metal-to-metal contact occurring. Therefore, the friction is extremely low, with a friction coefficient of 0.0003 to 0.001. It can withstand changes in load even when using low-viscosity liquids, water, hydraulic fluids, etc.