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Lubrication types explained – hydrodynamic and elastohydrodynamic

Full-film lubrication conditions occur when a machine part has started to rotate and the load and speed rate result in a layer of oil forming between the shaft and the surfaces of bearings. This oil formation effectively lifts the shaft from the bearing surface, ensuring there is minimal chance of contact with asperities.

As the oil’s molecular structures slide past one another in operation, this is the only friction occurring inside the lubricant itself. These layers of oil film are usually somewhere between 0.00008 to 0.004 inches in thickness, but they can be much larger.

In the following sections, we’ll take an in-depth look at the two types of full-film lubrication: hydrodynamic and elastohydrodynamic.

Hydrodynamic lubrication

This type of lubrication occurs between a machine’s sliding surfaces in instances where a full film of lubricating oil supports and creates clearance between parts, like shafts and bearings, for example. For complete and successful hydrodynamic lubrication, however, machine components must conform geometrically, and when the surfaces are in motion, there must be low-contact pressure.

In order for hydrodynamic lubrication to work efficiently, an oil’s viscosity must allow the hydrodynamic condition to be successfully maintained under a wide range of different operating conditions. If operating conditions result in the working clearance being excessively reduced, there is a danger of metal-on-metal contact being made between asperities or metal high points.

If a lubricating oil’s viscosity is overly high, the internal resistance of its molecules can significantly decrease operating efficiency and can cause temperatures to rise.

Elastohydrodynamic lubrication

This lubrication condition occurs when there is a rolling motion happening between the moving machine parts, and the contact area has low-degree conformity levels. When machine components are exceptionally disparate, they will have far smaller contact zones. However, this means that any existing pressure is not distributed evenly, but is instead concentrated into a single area, which is sometimes calculated at hundreds of thousands of pounds per square inch (PSI).

Oil will roll into a contact area between a raceway and ball, and the lubricant’s pressure will spike swiftly. This high pressure has the knock-on effect of substantially increasing the lubricant’s load-holding capability and viscosity. This concentrated load will then slightly flatten the metal parts of the rolling components and race into the contact area. This deformation only happens within the contact area, and the metal is able to elastically return to its original form as the rotation process continues.

The nature of this process makes heat treatments of the metal involved exceptionally important. As the oil’s viscosity is impacted directly by temperature, any incorrect operating temperatures can easily interfere with the elastohydrodynamic lubricating film being successfully formed.

Oil film thicknesses for elastohydrodynamic lubrication are incredibly thin and are used in a wide range of mechanical applications, from gear teeth and rolling element bearing to cam contacts, where high-rolling contact loads take place. Providing that recommended temperatures, loads and speeds are never exceeded, this lubrication can ensure no asperities ever make contact.

Whether hydrodynamic or elastohydrodynamic, a full-film lubrication regime creates the most optimal operating conditions for preventing unnecessary wear and friction to machine components.

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