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Understanding Lubrication Fundamentals

Basic Fluid Lubrication and Protection Fundamentals

Different methods of lubrication protect machines from wear.

Lubrication of worm gear

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

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.

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.

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Ultra-low-sulfur diesel doesn’t provide sufficient lubricity

Ultra-low-sulfur diesel lacks Needed Lubricity

Mark Nyholm | TECHNICAL MANAGER, HEAVY DUTY AND MECHANICAL R&D

Fortunately, we have a simple solution.

It feels like forever ago, but it’s only been 13 years since the U.S. Environmental Protection Agency (EPA) mandated reduced sulfur content in diesel fuel, in 2006. Boy, were people angry. They knew sulfur helped protect their fuel pump and injectors from failure. Change is scary, and the thought of replacing expensive components even more so.

Fast-forward to 2019, and I rarely hear anyone talking about this. But the problem is even more prevalent now than it was then. Modern diesels demand even more from the fuel pump and injectors than before, increasing the potential for failure. So, why aren’t people still up in arms? My hunch is they have accepted the new reality. Or, they just don’t know what they don’t know when they buy a new truck today.

Today’s ultra-low-sulfur diesel fuel (ULSD) provides significantly reduced lubricity – a critical property in controlling fuel-pump and injector wear. While diesel fuel has traditionally had high lubrication properties, the desulfurization process used to strip the diesel fuel of the sulfur content to meet ULSD requirements also strips the fuel of some of its organic compounds responsible for lubrication. The ASTM D975 diesel-fuel standard mandates a minimum lubricity level. However, the Engine Manufacturers Association (EMA) wants the standard to provide for increased lubricity, but lost out on the control of the specification. While the EMA claims there’s a problem, it doesn’t carry enough clout to change the specification.

Since 2006, ULSD has accounted for nearly all diesel available in North America because the EPA mandated reduced sulfur to curb harmful emissions. ULSD now contains a maximum of just 15 ppm sulfur, compared to fuel that had up to 5,000 ppm sulfur prior to EPA regulations.

Waxes in diesel fuel lubricate the fuel pump and injectors, helping fight wear. Without them, the highly engineered components in modern diesels, particularly high-pressure common rail (HPCR) engines, can wear out and cost thousands in repairs. They can also develop deposits that interfere with an optimum spray pattern, reducing power and fuel economy. The editors of Diesel Power Magazine covered the problem of ULSD in the April and May 2019 issues. As reported, the Bosch* CP4.2 fuel pump that comes stock on 2011-2016 Duramax* engines has led to thousands of catastrophic failures. It’s culminated in class-action lawsuits in Texas and California against Bosch, GM*, Ford* and other vehicle manufacturers on behalf of individual diesel owners whose vehicles use that pump. When the CP4.2 fuel pump fails, it instantly contaminates the entire fuel system with metal particulates, costing $8,000 to $12,000 in repairs. The magazine reiterates what AMSOIL has been saying for years: “The way to be proactive in protecting a CP4.2 equipped diesel from an early demise is being diligent about using fuel additives that add lubricity with every fill-up.”

 

The CP4.2 pump is said to fail because of two reasons: 1) It’s designed with about 20 percent reduced flow volume than the previous generation pump, requiring it to work even harder. 2) ULSD isn’t providing enough lubricity.

Our testing of base fuels across the U.S. confirms the second point. ASTM D975 requires diesel fuel to limit the wear scar in lubricity testing to 520 microns. The EMA, meanwhile, sets its own, stricter requirement of 460 microns. As the chart shows, many of the fuels (blue bars) failed to limit wear to 520 microns. And none of them met the EMA’s 460-micron limit.

Fuel treated with AMSOIL Diesel All In-One (ADB) performed far better (red bars). It not only met the ASTM D975 standard, it also met the stricter EMA lubricity requirement. You can find the same technology in Diesel Injector Clean (ADF) and Diesel Injector Clean + Cetane Boost (ADS). Our diesel additives deliver a healthy boost in lubricity to help lubricate diesel fuel pumps and injectors. The extra lubrication helps prevent wear in fuel pumps and injectors. I strongly recommend that all diesel owners use AMSOIL diesel fuel additives with every tank of fuel.

We keep this in large supply in Sioux Falls – Both our Stan Houston’s location and the Tea Exit location (exit 73). Buy in the half gallons to save money.