The Mechanical Requirements for Bushings
November 20, 2022When a lubrication film is present between two comparably moving surfaces, the friction conditions of the two contact surfaces can be categorised as hydrodynamic lubrication, boundary lubrication, or solid lubrication, depending on the nature of the lubrication film.
Continue reading to discover more about the technical conditions that must be met to keep Bushings in excellent operating condition and how Global Bearings may be a tremendous assistance in reaching this overarching objective!
Lubrication of the Boundaries
If there is no viscous hydrodynamic oil film between the two contact regions and the lubricating oil film between the two contact surfaces is exceedingly thin, then the only film between the two contact areas is the film of absorbed oil molecules. The organised oil molecules stuck to the solid surfaces make up what is known as a porous oil film. The shear resistance of a porous oil film is higher than that of a hydrodynamic oil film. In this region, the friction force is larger than the lubrication provided by hydrodynamics. At the places of contact where there is the most friction, the oil film usually breaks. “Boundary lubrication” is the name given to the lubrication situation that results in a frictional condition such as the one described here. A self-lubricating bushing might be the best option to use in this situation to reduce the amount of friction experienced.
Lubrication Based on Hydrodynamic Forces
When the lubrication film between two contact surfaces is sufficiently thick, a phenomenon known as hydrodynamic lubrication can take place. This happens when a film of viscous oil entirely separates the two contact surfaces. At this point, the viscous resistance of the lubricant determines the frictional force between two surfaces that are in touch with one another, which results in a very low number (coefficients of friction can be 0.0001-0.01). When this occurs because of the viscosity of the lubrication oil, the oil that surrounds the shaft likewise spins in response to the rotation of the shaft, which results in oil pressure being generated at the load region. The name given to this phenomenon is the wedge effect.
Changes in the temperature and viscosity of the lubricating oil and variations in surface roughness, clearance, and rotational speed of the shaft all affect the oil pressure P created inside the lubricating oil film.
Solidification of Lubrication (Dry friction)
Under these working conditions, the two solid surfaces will be close to one another, and there will be no external lubrication, such as a hydrodynamic or porous oil film. The only determining how proportionate the frictional force is is how much vertical load is delivered to the solid body’s contact surface. The sliding velocity has no effect on the coefficient of friction, which is always positive; therefore, static friction is always larger than kinetic friction. Choosing a material that can act as its lubricant in a bushing is essential to performance in such an application.
Please remember that Global Bearings offers design engineering and application assistance as part of our value-added services to make your work life easier. These services are available to you if you require assistance in selecting the products that will work best for the specific application you have in mind.
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