Elastohydrodynamic lubrication

Elastohydrodynamic (EHD) lubrication is another form of full-film lubrication that exists when the lubricant reacts to pressure or load and resists compression, functioning as if it were harder than the metal surface it supports. As viscosity increases under pressure, the film becomes more rigid, creating a temporary elastic deformation of the surfaces. EHD occurs in the area where the most pressure or load affects the component. In roller bearings, for example, the metal surface deforms from the extreme pressure of the lubricant

The lubricant’s viscosity and additives work together to protect surfaces in an EHD system. Anti-wear additives are often used to protect engine bearings in high-load conditions, while both anti-wear and extreme-pressure additives work to protect gears in high-load conditions.

Hydrostatic-film lubrication

Hydrostatic-film lubrication is a full-film lubrication method common in heavily loaded applications that require a supply of high-pressure oil film. The high pressure in hydrostatic-film lubrication ensures that the required film thickness will be maintained to support a heavy load during extreme operation. Hydrostatic-film lubrication maintains a fluid film under high-load and low-speed conditions, such as those experienced at equipment startup.

Squeeze-film lubrication

Squeeze-film lubrication is a form of full-film lubrication that results from pressure that causes the top load plate to move toward the bottom load plate. As these surfaces move closer together, the oil moves away from the heavily loaded area.

As load is applied, the viscosity of the lubricant increases, enabling the oil to resist the pressure to flow out from between the plates. Eventually, the lubricant will move to either side, resulting in metal-to-metal contact. A piston pin bushing is a good example of squeeze-film lubrication.

Boundary lubrication

Boundary lubrication is a form of thin-film lubrication and occurs when a lubricant’s film becomes too thin and contact between the surface’s asperities occurs. Excessive loading, high speeds or a change in the fluid’s characteristics can result in boundary lubrication.

No surface is truly smooth, even when polished to a mirror finish. The irregularities, or asperities, on every surface may be so small that they are only visible under a microscope. When two highly polished surfaces meet, only some of these asperities on the surfaces touch, but when force is applied at right angles to the surfaces (called a normal load), the number of contact points increases.

Boundary lubrication often occurs during the start-up and shutdown of equipment. In these cases, chemical compounds enhance the properties of the lubricating fluid to reduce friction and provide wear protection. For instance, anti-wear additives protect the cam lobes, cylinder walls and piston rings in engine high-load conditions, while anti-wear and extreme-pressure additives protect ring and pinion gears in rear axles.

Other lubrication

Mixed-film lubrication is considered a form of thin-film lubrication, although it is a combination of hydrodynamic and boundary lubrication. In mixed-film lubrication, only occasional asperity contact occurs.

Solid-film lubrication is used in applications that are difficult to lubricate with oils and greases. To manage these difficult applications, solid- or dry-film lubrication is applied where the solid or dry material attaches to the surface to reduce roughness. Solid-film lubricants fill the valleys and peaks of a rough surface to prevent metal-to-metal contact. Common solid-film lubricants include graphite, molybdenum disulfide (MoS2, aka moly) and polytetrafluoroethylene (PTFE), also known as Teflon.*