วันพฤหัสบดีที่ 11 พฤศจิกายน พ.ศ. 2553

Friction Reducing Technologies – Fact or “Friction”

Friction Reducing Technologies – Fact or “Friction”

Abstract

By John Schlobohm, Senior Global Technical Advisor – bp/Castrol

Do anti-friction additives actually enhance lubricant performance? Or is that just a myth circulated by the oil companies? This paper will explore the efforts of producers to enhance lubricants through the use of general additive systems and advanced technologies. It will examine additive technologies that reduce generated heat, increase equipment life, save energy, and reduce overall maintenance costs.

In a discussion of both non-solid and solid lubricant additives, this paper will examine how specifically engineered additives work to not only provide anti-friction protection, but also “repair” damaged gears and bearings.

Introduction

Tribology, the study of friction and sliding surfaces, is a relatively new scientific and engineering discipline. Yet the goals of Tribology – to reduce friction, to improve the design of bearings and other mechanisms, to extend the life of parts, and to develop superior lubricants – have challenged industry since making tools and using machines began.

Lubrication has three goals:
                        Reduce friction
                        Carry away heat
                        Reduce wear



The friction between working surfaces, a rotating shaft and journal for example, can be significantly reduced by separating those surfaces with a film of oil.

Asperities

Even the finest machining leaves a surface of peaks and valleys on bearings and gear teeth. Examined under a microscope, the working surface of a bearing takes on the rugged appearance of a lunar landscape (figure 1). Metallurgists and other specialists call these peaks and valleys asperities.


Figure 1  Bearing rolling surface as seen under a microscope

When a film of conventional lubricant is ruptured by heavy shock loads, contacting surfaces cannot be separated and opposing asperities tend to interlock or “cold weld.” As a result, the peaks fracture repeatedly under the forces of equipment operation, and an irreversible sequence of destructive wear begins.

When asperities interlock, energy is required to overcome the resistance. Besides squandering energy, uncontrolled friction generates excessive heat and wear. For the maintenance department, the bottom line is often premature parts failure, costly unscheduled downtime, frequent or expensive repairs and needlessly high energy bills. In fact, according to many authorities, over 1/3 of the world’s energy production is consumed in overcoming friction.

Despite its polished appearance, a microscope reveals pits, valleys and jagged peaks on even a finely machined bearing. Actual contact between working surfaces occurs only where opposing peaks touch. The contact area may be as little as 1/1,000 the apparent surface area (Figure 2).


Figure 2  Typical bearing or gear work surface

There are some advanced technology lubricants that offer a different dimension of friction-fighting capabilities for improved equipment performance.

Fluid Film

Although less than a few thousandths-of-an-inch thick, a layer of oil is sometimes called a thick film because its depth is several times greater than the height of the asperities on opposing surfaces. The combination of journal speed and lubricant viscosity tends to form a “wedge” of oil under the load. At moderate speeds and loads, the journal lifts up on this wedge and literally “floats.” Lubrication engineers call this “fluid film” or “hydrodynamic lubrication.”

Mixed Film

As loads and temperatures increase or as steady loads become shock loads or as operating speeds diminish, the lubricating oil film becomes thinner and thinner. Eventually, some asperities penetrate the oil film and come into contact with each other some of the time. This is known as “mixed film lubrication” (Figure 3).

Elastohydrodynamic Lubrication

Elastohydrodynamic lubrication occurs when concentrated loads produce high local pressure in the contact region. This will elastically deform the surfaces in an amount that can approach, and even exceed, the hydrodynamically generated lubrication film thickness. At the same time, due to the high local pressures, the lubricant viscosity increases many times over its normal value. The combustion of these two events result in the formation of a fluid film with greater load capacity and a shape that prevents asperity interaction between the surfaces. Several oil companies have propriety anti-wear and extreme-pressure additive technologies that synergistically work with the elastohydrodynamic process to impart increased surface smoothness, durability and protection against wear (Figure 3).

Boundary Lubrication

Conditions of sustained metal-to-metal contact are known as boundary conditions or “boundary lubrication.” Under conditions of ongoing and destructive metal-to-metal contact, friction is excessive. The abrasive wear particles that are formed can contaminate the oil and can rapidly deteriorate working surfaces, leading to parts failure and unscheduled down time for repairs.

However, the cost of parts, repair labor, and production losses can be controlled by specifying antiwear, EP (extreme pressure) or solid film high performance lubricants for machines operating under boundary conditions (Figure 3).




วันพุธที่ 27 ตุลาคม พ.ศ. 2553

A Basic Analysis of Grease Thickeners

A Basic Analysis of Grease Thickeners

John Schlobohm
Castrol Specialized Industrial Business Unit

1001 West 31st Street
Downers Grove, Illinois 60515
USA
(630) 743-3571
john.Schlobohmsr@castrol.com

Key words: Thickener, Grease, Soap, Base Oil

ABSTRACT
Greases are complex compounds constituted of base oils, thickeners and additives designed to effectively meet specific lubrication requirements. There are several ways greases can be classified: synthetic or mineral based, biodegradable or non-biodegradable, high temperature or low temperature, or according to the product’s thickener system. The most widely used classification is the one based on the thickening system. This paper will discuss the various thickeners used in making greases today, along with their advantages and disadvantages.

Grease development

In simplest terms, lubricants are divided into oils and greases. In general, oil tends to flow and grease tends to stay put. However, advances in the formulation of lubricants have made those simple definitions incomplete. The first greases were simple animal fats and lards smeared on wooden axels and shafts to reduce friction. Greases were also used to maneuver stone blocks used in construction. The early settlers used hand soap and animal fat to lubricate machines that developed friction. As mechanical systems became more complex, base oils were refined and thickener systems developed to meet changing needs. Today, greases are expected to work without failure under adverse conditions, including extreme heat, massive water wash, tremendous shock loads and frigid cold.

The carefully engineered properties of premium and high-performance greases enable them to change viscosity to meet changing operating conditions – blurring the once clear distinction between greases and oils. Modern greases have three elements: base oils, additive packages and thickeners.

Base oils are of three general types: mineral, animal/vegetable and synthetic. Most industrial oils are mineral based. However, synthetic oils have moved rapidly into the marketplace. Although called synthetic, these fluids are often manufactured from petroleum feedstock.

Prior to around the 1950s, base oils were the sole lubrication property within greases. Today, however, advanced thickener and additive packages are available that assist in the lubricating operation. Additive packages aid in failure prevention. They are used to reduce wear, inhibit corrosion and rust, and reduce the impact of shock loading on bearing elements. Other additives contain anti-oxidants, anti-wear (AW), extreme pressure (EP) and solid packages.

Greases can be classified by a variety of terms: synthetic or mineral based, biodegradable or non-biodegradable, high temperature or low temperature, or according to the product’s thickener system. Thickener classification is the most frequently used and most helpful method of determining which grease is appropriate for a given situation. Therefore, this paper will discuss greases based on their thickening systems. Advantages and disadvantages of various types of thickeners will be discussed as well as some applications for the different grease types.

Thickening systems

The thickener provides the gel-like consistency to hold the liquid lubricant (base oils and additives) in place. The gel-like characteristic make greases preferred over oil lubrications in applications where leakage can occur, or where the sealing action of the extra film thickness is needed.

Soap thickened greases



Many thickeners are the result of a chemical reaction between a long-chain fatty material (animal or vegetable origin) and an alkali in the base oil. Thickeners fall into several categories as shown in the following table.



NLGI Grade

In general, lubricating greases are composed of 85 to 95% base fluid and 5 to 15% thickener and other additives. The National Lubricating Grease Institute (NLGI) Grade is a standard used to classify the consistency (thickness) of grease. The NLGI grade is determined using the American Society for Testing Materials ASTM D-217 cone penetration test. In the test, a standard cone-shaped weight is placed on the surface of a cup of grease. The weight is allowed to penetrate the sample for five seconds. The NLGI grade assigned to the grease is based on a micrometer measurement of the depth of penetration. NLGI grades range from 000 (softest or thinnest and pourable at room temperature) to 6 (hardest or thickest and resembling a solid block of grease at room temperature).

Final statement


Other advanced thickeners are currently undergoing testing. If history is an indicator, tomorrow’s greases will some day relegate today’s products to the same antique status as unrefined animal fats and hand soap. Possibly the first “perfect grease” will emerge -- one that will eliminate friction and never break down under high temperatures or become unpumpable in the cold. But until that product is developed, an expanded knowledge base regarding today’s products will make selecting the proper grease for specific applications easier.

No matter what the incentive is when purchasing greases -- cost, performance or special needs -- the oil industry has a product that will work in most any application. As noted at the outset, thickeners are only one-third of the overall lubrication puzzle. Additive packages and base oils make up the other two-thirds. Greases are engineered so that these three parts work in harmony to produce the best possible results at a particular cost level. Four elements demand consideration when making a grease choice: operating environment, equipment cost, lead-time on replacement parts and delay cost if the equipment is down. If any one of these items is a factor, making an informed choice of the best product available will deliver overall cost savings.