Overview of Tribology
Tribology is about Rubbing Surfaces
When two surfaces, round or flat, are in contact and one is moving, we recognize that friction is preventing smooth and easy movement, that we may wish to squirt a bit of lubricant onto one surface to reduce the friction, and that one or both surfaces may show wear tracks after some time has passed. When engineers change the design of a shaft and bearing, the friction may be reduced and the bearing may show less wear. For existing machines, such as our cars, we make decisions about lubricants that will change friction and wear, hopefully in the direction of a longer useful life.
In our everyday life, we need to be concerned with lubrication of equipment such as cars, outboard motors, sewing machines, snowmobiles, and smaller equipment and tools. Then there are the workplaces such as textile mills, mines, steel mills, power plants, and other industries, where lubrication is essential. Industries such as steel mills would grind to a screeching halt without proper attention to lubrication.
Friction creates heat, promotes wear, and wastes power, so the reduction of friction, by any means, is vital. It is estimated, that from 1/3 to 1/2 of the total energy produced in the world is consumed by friction. It is also estimated that the cost of wear in the U .S. is equivalent to 2/3 the cost of energy. If we add these two together, we see that the cost of friction and wear in the U .S. is equivalent to the cost of energy.
Take your own car for an example. The engine delivers useful work only after overcoming the friction of the moving parts such as bearings, valves, pistons, cams, etc. The useful work is then consumed in gear friction, the rolling friction of tires, brake friction, and wind friction. Wear is the cause for most maintenance and repair of brake, clutches and gear boxes, as well as replacement of the entire automobile. By the time that you are tired of your car because it is dripping oil onto the driveway and the brake pads and clutch plates need replacement, it has lost, in wear debris, less than 3/1000 of its original weight. Of even greater concern are the millions of dollars lost due to friction and wear in various industrial machines. Reduction of friction and wear leads to increases in service life, less downtime and lower operating costs, which can add up to tremendous savings.
The Good, the Bad, and the Friction
Friction is not entirely bad.
Sometimes frictional force must be increased to improve safety conditions, such as in automobile brakes, friction clutches, and tires on icy roadways.
Friction is the resistance to relative motion of two adjacent bodies, whether they are solids, liquid or gas molecules. When the sliding velocity is zero, the friction force required to start sliding is generally called the "static friction". Static friction holds nails in wood, keeps your parked car from sliding down a hill, and holds your body from sliding off from a wooden chair.
The friction force between two bodies that are moving relative to each other is “kinetic friction”. Kinetic friction is present as gear teeth in your car slide over each other, as ice skates slide over ice and as force holding the cereal box back as you slide it off the shelf.
For a given system, the static friction is usually greater than kinetic friction. It takes more force to start moving a chair across a wooden floor and less force to keep it moving across the floor.
Historically, the study of friction began approximately 500 years ago with Leonardo da Vinci (1452-1519), whose notebooks show that he found that:
From his experiments and calculations he estimated that the frictional resistance of a body is about 1/4 of its weight.
Amontons, a 17th century physicist, was unaware of da Vinci's work and published his own work on friction in 1699. Attempting to explain friction, he theorized that friction is caused by surface roughness. The peaks of one surface lay in the valleys of the adjoining surface and Amontons beleived that friction is the force required to pull the peaks up the other surface until they clear.
Coulomb published a paper in 1785 on his investigations of both static and kinetic friction, with five conclusions:
As to the cause of friction, Coulomb theorized that at least part of the frictional force might result from cohesion of molecules of the two sliding surfaces.
Friction is the force that resists motion between two bodies. It acts at the common boundary between the two bodies. Frictional force is proportional to the load.
F = µp, with
F = Force
µ = Coefficient of friction
P = Load
Example Problem 1 A 1000 lb landscaping rock is on a concrete driveway. It has to be dragged 50 feet to be placed in a small garden. The coefficient of friction between the rock and the concrete is 0.3. If a rope is wrapped around the rock to drag it, what will be the force in the rope? F = (0.3) (1000)
F = 300 lb.
Example Problem 2 We need to know the coefficient of friction for aluminum oxide sliding on steel. The experiment can be done in a number of ways. One method uses a spring balance to pull the aluminum oxide along the steel surface. If the aluminum oxide piece weighs 150 gm and the force required to pull it along the steel surface is 30 grams, then
Many older reference books contain tables of friction coefficients. These should be used with extreme caution. The coefficient of friction for a given material pair, such as aluminum oxide and steel, depends on surface cleanliness. You will note in most tables that the values listed for lubricated material pairs are significantly lower than the nominal values. The same will be true for your system. If you have oil present, the force required to move the object will be lower than if it is a clean, dry surface. Measure the friction coefficient for the materials that you are using in the condition that they will be used in service, paying particular attention to surface cleanliness and lubrication.
Exceptions to Coulomb's laws:
First Law: The frictional properties of some very hard materials such as diamonds and certain very soft materials such as Dupont's Teflon do not obey the first law. For these special materials, friction is not proportional to the load; instead it is proportional to some reduced value of the load.
Friction is independent of the contact area between two surfaces. If we have to drag a sheet of plywood from the wood storage rack at the lumber yard up to the cashier, because all of the carts are being used by other customers, the dragging force is the same if we slide on the 1 inch by 96 inch edge or on the flat side which is 48 inches by 96 inches. It is not less by a factor of 48 as might be expected. The reason that force is the same in either case is due to the differences between the apparent area of contact and the actual area of contact of the plywood sheet and the floor.
Apparent Area of Contact (AA) vs. Real Area of Contact (AR) Surfaces are generally wavy and bumpy, even at the microscopic level. Therefore, two adjacent surfaces, as shown in the sketch, are never in total contact. The upper body is supported by the lower body surface at the top of roughness irregularities, called asperities. Under load, these asperities bend and deform until the load is fully supported. At that point, less than 1 part in 10,000 of the apparent area is usually in contact.
Friction is indirectly proportional to the real area of contact. Since this is almost always a small portion of the total area, friction is effectively independent of the apparent area of contact.
Exceptions to Coulomb's Law Second Law: Friction is not independent of the apparent area of contact with extremely clean and highly polished surfaces. Here, the 'real area of contact' may become a significant portion of the apparent area of contact.
Kinetic friction, the resisting force that occurs between moving bodies, is independent of velocity. Again, if we have to drag a sheet of plywood from the wood storage rack up to the cashier, the force is the same if we slide it slowly or quickly.
It is important to distinguish non-lubricated surfaces from lubricated surfaces. In the case where a layer of lubricant separates two surfaces, the resisting friction force can change.
Exceptions to Coulomb’s Laws
Third Law: Friction is not always independent of velocity. If we exclude very low speeds and very high speeds, the friction coefficient is constant and independent of sliding velocity. But at very high speeds, the friction coefficient generally has a slightly negative slope; that is, the friction coefficient decreases gradually as the speed increases.
At very low speeds, the friction coefficient generally increases gradually with a decrease in sliding velocity.
Friction is basic to certain musical instruments and to squeaky doors. Ask your favorite violinist about putting rosin on a bow before drawing it across violin strings to make music. The bow has to be treated daily. That person would be horrified if you used oil instead of rosin, because the bow would be ruined. The bow would slide more easily, but no music would be produced. On the other hand, we get rid of squeaks by spraying oil on hinges. A thin film of oil eliminates the noise and lasts a long time. Violins and squeaky doors are examples of stick slip behavior.
Experiments have shown that the static friction of a system is not constant but depends on the duration of static contact. It increases rapidly during the first 1/10 of a second that two surfaces are in contact and then increases more gradually with longer contact. In sliding systems where the static friction is greater than the kinetic friction, a type of intermittent sliding, called "stick-slip," may occur if the system has sufficient elasticity.
An example of this is the squeal of automobile brakes just as the car is about to stop, or that of the hinges of an old door. This is a particularly serious problem in slow moving mechanisms such as the feed mechanism of metal cutting machines.
Stick-slip occurs because static friction is usually greater than kinetic friction. When a force is applied to the part that is to be moved, this force must overcome the static friction. Then when motion begins, the friction of the moving system (kinetic friction) is lower than the static friction. The stationary part of the system bas been deflected due to its elasticity by this frictional drag, and the sudden reduction in frictional drag causes it to rebound and stick to the moving part. This results in an intermittent relative motion. This intermittent motion can be as mellow as a violin sonata or as jarring as a squeaky door.
The commonly accepted theory regarding the cause of friction is the adhesion theory. When two surfaces are brought into contact and a load is applied forcing them together, junctions are formed where asperities come into contact. The total area of contact of these junctions is a function of load and penetration hardness of the softer material.
This relationship is approximated by the equation,
When sliding is initiated, the junctions must be sheared. Therefore, friction force must be equal to the shear stress S of the softer material times the real area of contact A.
F = S x A
Since F = µ x P, than µ is equal to shear stress/penetration hardness, or, µ = S/H The value of the ratio S/H is roughly the same for most metals and is about 0.17 for dry surfaces. For clean metals, actual values for the friction coefficient are about 0.3 to 0.4. Normally, clean new surfaces that are exposed to air have oxide layers and usually have absorbed layers of water vapor and oxygen. These layers generally reduce the actual friction coefficient.
For very clean and outgassed surfaces high friction coefficients of around 1.0 are observed. In this case, the real contact area is further increased and surface energy contributes a significant component to the friction.
Other factors contributing to fiction force are surface roughness and electrostatic effects. Some early investigators thought all of the friction was caused by roughness, but generally it contributes only about 1/20 to the overall friction coefficient. The friction coefficient remains fairly constant for the normal range of surface roughness. It decreases for smooth surfaces because of the increase in real contact area, and it increases for rough surfaces because of asperity interlocking. The effect of electrostatic attraction occurs only for dissimilar materials, and is usually extremely small.
In practice, therefore, friction is a sum of adhesive force, a force caused by surface roughness and possibly an electrostatic force. The most effective ways to optimize friction are the selection of the mating materials, application of lubricants, and optimization of surface roughness.
You have completed the materials for Fundamentals of Lubrication and Tribology I - Friction.
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