The topic of lubrication can be easily divided into two specific areas, 1) the science and chemistry of lubricating materials, and 2) the practical application of lubrication knowledge and materials to effectively reduce the friction, wear and energy loss of moving parts. LMT’s mandate is to inform its readers, from those who are highly experienced to those who are less so, not just about what’s new in the world of lubrication, but about the basic concepts and principles that guide GLP (Good Lubrication Practices). These fundamentals—or “Elements”—are what make up the International Council of Machinery Lubrication (ICML) Domain of Knowledge. They’re also the focus of a new 12-part series that kicks off in this issue.
Keep in mind that the articles in this series aren’t meant to replace requisite formal preparatory training. The goal is to introduce (or refresh readers on) the fundamental knowledge requirements for working in a best-practice lubrication environment, spur them to pursue a certification path (if they haven’t already) and give the lubrication community a stronger voice in the world of asset reliability.
Friction is why we lubricate
The word “tribology,” coined by Sir H. Peter Jost (see Sidebar below), comes from the Greek word tribos, meaning “to rub,” and is used to describe what happens when two hard surfaces move over one another. The resistive force causing this “rubbing” action is known as friction, and was first recognized by Sir Isaac Newton in his laws of motion as an external force to motion.
Webster’s describes friction as “the force, which opposes the movement of one surface sliding or rolling over another with which it is in contact.” Simply put, friction is the resistive force that retards motion. And it’s not necessarily a bad force: We employ frictional forces when we want to intentionally slow a body in motion (i.e., retarding the movement of a rotating machine or automobile by applying a rough and soft consumable braking material with a high coefficient of friction against a smooth, hard [less-consumable] surface).
Friction becomes an undesirable force when it robs energy from an applied force used to intentionally move an object. Frictional forces have, in fact, been estimated to consume over one-third of the world’s energy. When ignored in such cases, friction causes heat, wear and, sometimes, catastrophic failure of the moving body. To understand friction we must recognize that there are two unchanging fundamental laws that govern it:
Fig. 1 Forces on bodies at rest (Source: Engtech Industries, Inc.)
Figure 1 depicts the forces at play on two bodies at rest. To begin to move Body “A” across Body “B,” we must first overcome its resistive frictional force. This resistive force is a result of the load N representing the weight of the body multiplied by the coefficient of friction. For example, if the upper body represented a full steamer trunk resting on a concrete floor, using the formula F = uN we can calculate the initial (static) resistive force we need to overcome to start the trunk moving across the floor. Thus, if we assume the loaded trunk weighs 100 lbs., and the coefficient of friction of wood on dry concrete is 0.65, the applied force required to start the trunk moving would be 0.65 x 100 = 65 lbs. Once the trunk has begun to move, the static friction barrier has been broken and the force required keeping the trunk moving reduces somewhat as long as the body remains moving. The frictional force has changed from a static frictional load to a kinetic frictional load
The Coefficient Of Friction (COF) (which is different for every material and fluid) is represented by the Greek letter mu (u). COF values range from almost 0 to well over 1, and the lower the value, the lower the resistance and retardation effect. Therefore whenever we want to produce work from moving parts, lower COF values are preferred because they require less energy expenditure to achieve movement—or work (i.e the motor requires less amperage draw, or the engine requires less fuel to achieve the desired work performance). Obviously, we would expend enormous amounts of energy to move things around if we were only to allow surface-to-surface contact on all moving parts. To reduce these forces and overcome the large static and kinetic forces we must introduce a fluid film to separate the two moving parts.
Fig. 2. Magnified cross-section of two bodies in motion separated by a fluid film (Source: Lubrication for Industry, by Ken Bannister, Industrial Press)
The fluid film is referred to as the lubricant. The principle of reducing friction while supporting a moving sliding load is referred to as lubrication. Lubricants or fluid films are not themselves “frictionless,” as they rely on an action known as “shearing,” depicted in Fig. 2, whereby fluid friction occurs between the molecular shear planes of the lubricant as they move across one another when the load moves. The following exercise demonstrates this point: First sweep your hand back and forth quickly on the surface of a table. Then place your hand atop a deck of cards on the table and move it back and forth. Compared to your hand's movement on the table, sweeping it back and forth over the deck of cards will be relatively effortless. That’s because the movement generates little or no heat as the cards “shear under the load” (slip over one another). Fluid friction is a similar phenomenon, in that it increases as viscosity becomes thicker or as a lubricant becomes dirty. Although a small amount of energy is required to overcome fluid friction, it’s negligible compared with having no fluid film present.
When mechanical moving parts are present, the amount of lubrication required depends on the state of friction that manifests itself in three specific ways:
Sliding friction is common where any plain surfaces move over one another (like in plain bearings where a journal moves within a sleeve). Sliding friction arrangements require the most lubricant, as the friction is evident over a larger surface contact area.
Rolling friction is found in all rolling element bearings that at one time were described as “friction-less” bearings. The contact surface is considerably smaller than in sliding friction bearings, and thereby requires much smaller amounts of lubrication to achieve a protective full-fluid film.
Combination friction, on the other hand, is unique to meshing gears. This is due to the changing gear-tooth profile that requires the teeth to slide on one another until the opposing pitch surfaces meet and rolling friction takes over as they disengage. Certain types of gears, such as hypoid gears and worm gears, are capable of producing much higher degrees of sliding friction.
Whenever moving parts are present, friction will be present—ever present, that is. Understanding friction helps us develop effective lubrication practices that, in turn, help us tame the harmful effects of friction and increase the life cycles of our equipment components.
The next issue will cover ICML's Body (Domain) of Knowledge Element #2: “The Functions of a Lubricant.” LMT
In the mid-1960s, a groundbreaking study by the British government (under the charge of Sir H. Peter Jost) quantified the tangible effects of poor lubrication practices on the nation's gross national product. That study, now referred to as the Jost Report, introduced us to the word “tribology” (the science of lubrication, friction and wear). For the first time, lubrication was recognized for its role as a bone fide science in the area of asset reliability, and for its fiscal impact on industry when practiced poorly.
Once awakened, sleeping giants take time to stir and get moving. Unfortunately, it wasn’t until the turn of the new millennium that a heightened awareness surrounding the field of lubrication began to emerge on a global scale. This has been emphasized through industry’s rapidly growing recognition of—and demand for—certified lubrication specialists in the practical application and lubricant diagnostic/analysis fields.
Although there are no specific apprenticed trade designations for lubrication specialists, over the past two decades many of the world’s leading lubrication experts and proponents (including scientists, engineers, consultants, suppliers and practitioners) have worked to develop certification programs backed by a body and domain of knowledge. Today, there are three lubrication certifying bodies: STLE (Society of Tribologists and Lubrication Engineers); ICML (International Council of Machinery Lubrication); and ISO (International Organization for Standardization).
Originally designed for engineers, the STLE Certified Lubrication Specialist (CLS) program has been in place since 1993. ICML has developed two certifications for “hands-on” lubrication practitioners: the MLT (Machine Lubrication Technician) and MLA (Machine Lubrication Analyst) designations.
A relative newcomer, ISO’s lubrication certification program has chosen to adopt the ICML model and, in fact, has collaborated with the ICML to use its body of knowledge. Participants who attend the requisite preparatory formal training associated with ICML certification are also eligible to take corresponding ISO exams (upon payment of the appropriate examination fees).
Of these three programs, ICML’s (currently offered in nine languages) has issued the most certifications around the world. For information on the ICML program, please visit: www.lubecouncil.org.