Basic Fluid Lubrication and Protection Fundamentals

Different methods of lubrication protect machines from wear.

Gear oil on gear example

_by David Hilgendorf

The primary function of fluid lubrication is to provide a durable film that protects moving parts by reducing friction and wear between surfaces; however, the level of protection provided is enabled by different methods of lubrication:

The reduction of friction by using a fluid can be divided into two basic types: full-film and thin-film. Full-film lubrication consists of four sub-types and thin-film lubrication consists of two sub-types.

Full-film lubrication

Hydrodynamic
Elastohydrodynamic
Hydrostatic-film
Squeeze-film

Thin-film lubrication

Boundary
Mixed-film

We’ll discuss the differences in that order:

Hydrodynamic lubrication

Hydrodynamic, or full-film, lubrication exists when two surfaces are completely separated by an unbroken lubricant film so there is no metal-to-metal contact. The movement of the rolling or sliding action causes the film to become thicker and pressurized, which prevents the surfaces from touching.

When the two surfaces are moving in opposite directions, the fluid immediately next to each surface will travel at the same speed and direction as the surface. If two parts are moving in the same direction, a full hydrodynamic film can be formed by wedging a lubricant between the moving parts. Known as wedging film action, this principle allows large loads to be supported by the fluid. It works much like a car tire hydroplaning on a wet road surface. During reciprocating motion, where the speeds of the relative surfaces eventually reach zero as the direction changes, the wedging of the lubricant is necessary to maintain hydrodynamic lubrication.

The lubricant’s viscosity assumes responsibility for most of the wear protection and additives play a limited role. Although full-film lubrication prevents metal-to-metal contact, abrasive wear or scratching can still occur if dirt particles penetrate the lubricating film. Additional factors, such as load increases, can prevent hydrodynamic lubrication by decreasing the oil film thickness, allowing metal-to-metal contact to occur.

Engine components operating with full-film lubrication include the crankshaft, camshaft and connecting rod bearings, and piston pin bushings. Under normal loads, transmission and rear-axle bearings also operate under hydrodynamic lubrication.

Hydrodynamic lubrication diagram graphic

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.*

AMSOIL synthetic lubricants are carefully formulated with the optimum blend of the highest quality base stocks and additives, ensuring lubricated components receive outstanding protection from contact wear. AMSOIL synthetic lubricants are carefully formulated with the optimum blend of the highest quality base stocks and additives, ensuring lubricated components receive outstanding protection from contact wear.

Engine start-stop technology can increase bearing wear

Use only the best quality oil in these engines as the crankshaft needs to float. Even the “so called synthetics” don’t dampen the metal to metal issues mentioned below nearly as well as AMSOIL and you can tell due to the reduction in vibration or more consistent oil pressure as you rack up miles.

Yet another reason to upgrade to AMSOIL synthetic motor oil.

Matt Erickson | DIRECTOR, TECHNICAL PRODUCT MANAGEMENT

Nearly every technology shaping the auto industry can be traced to one goal: increased fuel economy. Engine start-stop technology is one more tool automakers have in their arsenals to ensure today’s vehicles meet tomorrow’s tightening fuel-economy regulations.

In principle, start-stop technology is simple: the engine automatically shuts off while you’re idling and restarts when you take your foot off the brake. This reduces fuel wasted while idling. Automakers introduced different startstop systems in the late ‘70s and early ‘80s; however, drivers found them awkward and unworthy of the higher vehicle price. Today’s start-stop systems are less obtrusive and are available on vehicle models from most automakers.

Should be called Metal to Metal Contact Engine

That’s not to say they’re without detractors. In fact, some automakers have installed off switches that allow motorists to disable the feature in response to negative driver feedback. But, despite their pitfalls, they’re likely not going anywhere. Consider these statistics:

According to bearing manufacturer MAHLE*, U.S. vehicles burned 3.9 billion gallons of gasoline while idling in 2017.
Buick* reports that engines with start-stop technology increase fuel economy 4-5 percent using the EPA test cycle.

Automakers leap for joy over minuscule fuel-economy gains, so you can bet they’re going to stick with anything that may provide a 4-5 percent boost.

So, what does that have to do with motor oil?

Maybe you’re aware that most engine wear occurs during cold starts. Well, engine wear occurs during warm starts, too, like every time an engine equipped with start-stop technology restarts.

We have to get technical to understand why.

The crankshaft spins thousands of times per minute in a running engine. As it spins, oil flows through tiny openings in the crankshaft journals and fills the spaces between the journals and main bearings. The crankshaft literally floats on an oil film and doesn’t contact the bearings. We call this scenario hydrodynamic lubrication. In this regime, the bearings suffer little wear and last a long time.

Run of the mill oils (95% on the shelf) are not going to provide protection with this condition

Stopping the engine, however, reduces oil film thickness. The crankshaft settles onto the bearing surfaces rather than floats over them. The oil film thickness shrinks to about the same thickness as the surface roughness of the crankshaft. This is called boundary lubrication. Starting the engine allows the microscopic peaks on the metal surfaces to contact and cause wear until the oil film has been reestablished and the crankshaft is once again floating over the bearings. This is where the oil’s additives play a huge role in protection.

Granted, only minimal wear may occur each time the engine is started. It’s not a big concern in a properly maintained traditional engine using a good oil. But what if you greatly increase engine startstop cycles?

Consider another statistic from MAHLE:

Start-stop cycles in equipped engines may triple over the engine’s lifetime compared to traditional engines.

That means three times more engine starts, three times more instances of boundary lubrication and three times more exposure to increased bearing wear.

Bearing wear can snowball out of control, too. Metal particles can break off and populate the oil. The bearing surface becomes rougher, encouraging adhesive wear in which peaks on metal surfaces grab and tear the mating surfaces. Eventually the crank journal and bearing can weld together, ruining the bearing.

This all points to a simple directive: make sure your customers with engines using start-stop technology are using AMSOIL synthetic motor oil to guard against bearing wear. Oil film thickness shrinks when engines start from a dead stop, placing even more importance on oil additives to maintain protection. Since engines equipped with start-stop technology spend so much more time under boundary lubrication, it’s vital to use an oil with superior film strength and additive quality. AMSOIL Signature Series Synthetic Motor Oil delivers. It provides 75% more engine protection against horsepower loss and wear** to help protect today’s advanced engines.

This is especially needed in vehicles calling for 0W-20, 5W-20 and 0W-16.

Tag: boundary lubrication

Understanding Lubrication Fundamentals