I. Working Principle of Plain Bearing Lubrication
The core principle of lubrication of sliding bearings is to use lubricant to form a continuous film between the friction surfaces of the shaft neck and the bearing shell. The film completely or partially separates the solid friction pair, thus altering the friction state, reducing frictional resistance and minimizing component wear. In addition, it also has the functions of heat dissipation, vibration absorption, shock absorption, sealing, rust prevention and so on.
When the equipment is stationary or starting at low speed, the shaft neck and bearing shell come into direct contact and work under dry or boundary friction conditions, causing severe wear and tear. When the shaft neck rotates continuously, the viscous lubricant is sucked into the wedge gap of the bearing, producing a supercharged lubricating film. The pressure of the oil film balances the external load and completely lifts the axle neck. At this stage, there is no direct solid-solid contact between the axle neck and bearing shell; friction turns to internal fluid friction. This greatly reduces the friction coefficient, effectively protects the bearing's mating surfaces, and improves the operation stability and service life of the equipment. Lubrication states is divided into four kinds: dry friction lubrication, boundary lubrication, hydrodynamic lubrication and hydrostatic lubrication, of which hydrodynamic lubrication is the most common stable lubrication method in industrial equipment.
II. Essential Conditions for Hydrodynamic Lubrication
Hydrodynamic lubrication is a method of producing supercharged oil film by the relative motion of friction pair itself. Three core conditions must be met simultaneously, none of which can be ignored:
Geometric Condition: there is a wedge-shaped converging clearance
The bearing friction pair must form a wedge-shaped converging clearance --wide inlet and narrow outlet --as the structural basis for producing oil film pressure. Parallel clearance cannot concentrate the lubricant or generate pressure; only a convergent wedge structure can squeeze and retain the inflowing lubricant, thus establishing a stable membrane pressure field.
Kinematic Condition: Axle neck has plenty of speed
The shaft neck must be maintained at a certain speed so that the adhesion effect on its surface constantly drags the lubricant into the wedge gap. Under the condition of low speed, the lubricating oil supply is not enough to form an effective pressurized oil film. Only by reaching the critical speed can the lubricating oil be continuously replenished to form a hydrodynamic oil film that can withstand the load.
Medium condition: Lubricating oil appropriate viscosity.
Lubricant oil must possess a a specific viscosity, as this property is essential to create internal friction and keep the lubricant in the clearance gap. If the viscosity is too low, oil will become overly fluid, easy to leak from the gap, making pressure difficult to increase; conversely, excessively high viscosity will increase flow resistance and power consumption. Viscosity according to operation condition ensures the steady accumulation of oil in wedge gap and the integrity and pressure stability of oil film.