Friction overview

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Engineers Edge – Friction and Wear Knowledge Menu

Friction is a force that resists relative motion between two surfaces in contact. Depending on the application, friction may be desirable or undesirable. Certain applications, such as tire traction on pavement and braking, or when feet are firmly planted to move a heavy object, rely on the beneficial effects of friction for their effectiveness. In other applications, such as operation of engines or equipment with bearings and gears, friction is undesirable because it causes wear and generates heat, which frequently lead to premature failure.

From a perspective of engineering, the energy expended in overcoming friction is dispersed as heat and is considered to be wasted because useful work is not accomplished. This waste heat is a major cause of excessive wear and premature failure of equipment. Two general cases of friction occur: sliding friction and rolling friction.

Sliding Friction Overview

To visualize sliding friction, imagine a steel block lying on a steel table. Initially a force F (action) is applied horizontally in an attempt to move the block. If the applied force F is not high enough, the block will not move because the friction between the block and table resists movement. If the applied force is increased, eventually it will be sufficient to overcome the frictional resistance force f and the block will begin to move. At this precise instant, the applied force F is equal to the resisting friction force f and is referred to as the force of friction.

In mathematical terms, the relation between the normal load L (weight of the block) and the friction force f is given by the coefficient of friction denoted by the Greek symbol µ. Note that in the present context, “normal” has a different connotation than commonly used. When discussing friction problems, the normal load refers to a load that is perpendicular to the contacting surfaces. For the example used here, the normal load is equal to the weight of the block because the block is resting on a horizontal table. However, if the block were resting on an inclined plane or ramp, the normal load would not equal the weight of the block, but would depend on the angle of the ramp. Since the intent here is to provide a means of visualizing friction, the example has been simplified to avoid confusing readers not familiar with statics.

Laws of Sliding Friction Unlubricated Surfaces

Dry or unlubricated surfaces. Three laws govern the relationship between the frictional force f and the load or weight L of the sliding object for unlubricated or dry surfaces:

(a) “For low pressures (normal force per unit area) the friction force is directly proportional to the normal load between the two surfaces. As the pressure increases, the friction does not rise proportionally; but when the pressure become abnormally high, the friction increases at a rapid rate until seizing takes place.”

(b) The value of f/L is defined as the coefficient of friction µ. “The friction both in its total amount and its coefficient is independent of the area of contact, so long as the normal force remains the same. This is true for moderate pressures only. For high pressures, this law is modified in the same way as the first case.”

(c) “At very low velocities, the friction force is independent of the velocity of rubbing. As the velocities increase, the friction decreases.”

The third law (c) implies that the force required to set a body in motion is the same as the force required to keep it in motion, but this is not true. Once a body is in motion, the force required to maintain motion is less than the force required to initiate motion and there is some dependency on velocity. These facts reveal two categories of friction: static and kinetic. Static friction is the force required to initiate motion (F_s). Kinetic or dynamic friction is the force required to maintain motion (F_k).

Laws of Sliding Friction lubricated Surfaces

(a) “The frictional resistance is almost independent of the pressure (normal force per unit area) if the surfaces are flooded with oil.”

(b) “The friction varies directly as the speed, at low pressures; but for high pressures the friction is very great at low velocities, approaching a minimum at about 2 ft/sec linear velocity, and afterwards increasing approximately as the square root of the speed.”

(c) “For well lubricated surfaces the frictional resistance depends, to a very great extent, on the temperature, partly because of the change in viscosity of the oil and partly because, for journal bearings, the diameter of the bearing increases with the rise in temperature more rapidly than the diameter of the shaft, thus relieving the bearing of side pressure.”

(d) “If the bearing surfaces are flooded with oil, the friction is almost independent of the nature of the material of the surfaces in contact. As the lubrication becomes less ample, the coefficient of friction becomes more dependent upon the material of the surfaces.”

Coefficient of Friction Definition

The coefficient of friction depends on the type of material. Tables showing the coefficient of friction of various materials and combinations of materials are available. Coefficient of friction tables show the coefficient of friction for clean dry surfaces and lubricated surfaces. It is important to note that the coefficients shown in tables can vary.

Asperities / Friction

Asperities. Regardless of how smooth a surface may appear, it has many small irregularities called asperities. In cases where a surface is extremely rough, the contacting points are significant, but when the surface is fairly smooth, the contacting points have a very modest effect. The real or true surface area refers to the area of the points in direct contact. This area is considerably less than the apparent geometric area.

Shear Strength and Pressure

Shear strength and pressure, the primary objective of lubrication is to reduce friction and wear of sliding surfaces. This objective is achieved by introducing a material with a low shear strength or coefficient of friction between the wearing surfaces. Although nature provides such materials in the form of oxides and other contaminants, the reduction in friction due to their presence is insufficient for machinery operation. For these conditions, a second relationship is used to define the coefficient of friction: µ = S/P, where S is the shear strength of the material and P is pressure (or force) contributing to compression. This relationship shows that the coefficient of friction is a function of the force required to shear a material.

Stick-Slip / Friction

Stick-slip. To the unaided eye the motion of sliding objects appears steady. In reality this motion is jerky or intermittent because the objects slow during shear periods and accelerate following the shear. This process is continuously repeated while the objects are sliding.

During shear periods, the static friction force F controls the speed. Once shearing is completed, the kinetic friction force F_s controls the speed and F_k the object accelerates. This effect is known as stick-slip.

In well lubricated machinery operated at the proper speed, stick-slip is insignificant, but it is responsible for the squeaking or chatter sometimes heard in machine operation. Machines that operate over long sliding surfaces, such as the ways of a lathe, are subject to stick-slip. To prevent stick-slip, lubricants are provided with additives to make F_s less than F_k.

Rolling Friction

When a body rolls on a surface, the force resisting the motion is termed rolling friction or rolling resistance. Experience shows that much less force is required to roll an object than to slide or drag it. Because force is required to initiate and maintain rolling motion, there must be a definite but small amount of friction involved. Unlike the coefficient of sliding friction, the coefficient of rolling friction varies with conditions and has a dimension expressed in units of length.

Ideally, a rolling sphere or cylinder will make contact with a flat surface at a single point or along a line (in the case of a cylinder). In reality, the area of contact is slightly larger than a point or line due to elastic deformation of either the rolling object or the flat surface, or both. Much of the friction is attributed to elastic hysteresis. A perfectly elastic object will spring back immediately after relaxation of the deformation. In reality, a small but definite amount of time is required to restore the object to original shape. As a result, energy is not entirely returned to the object or surface but is retained and converted to heat. The source of this energy is, in part, the rolling frictional force.

A certain amount of slippage (which is the equivalent of sliding friction) occurs in rolling friction. If the friction of an unhoused rolling object is measured, slippage effects are minimal. However, in practical applications such as a housed ball or roller bearing, slippage occurs and contributes to rolling friction. Neglecting slippage, rolling friction is very small compared to sliding friction.

Laws of Rolling Friction

Laws of rolling friction. The laws for sliding friction cannot be applied to rolling bodies in equally quantitative terms, but the following generalities can be given:

(a) The rolling friction force F is proportional to the load L and inversely proportional to the radius of curvature r, or F = µ_r L/r, where µ_r is the coefficient of rolling resistance, in meters (inches). As the radius increases, the frictional force decreases.

(b) The rolling friction force F can be expressed as a fractional power of the load L times a constant k, or F = kLⁿ where the constant k and the power n must be determined experimentally. n

(c) The friction force F decreases as the smoothness of the rolling element improves.

Friction Wear

Friction wear is defined as the progressive damage resulting in material loss due to relative contact between adjacent working parts. Although some wear is to be expected during normal operation of equipment, excessive friction causes premature wear, and this creates significant economic costs due to equipment failure, cost for replacement parts, and downtime.

Friction and wear also generate heat, which represents wasted energy that is not recoverable. In other words, wear is also responsible for overall loss in system efficiency.

Effects and Classification of Friction Wear

The effects of wear are commonly detected by visual inspection of surfaces. Surface damage can be classified as follows:

(a) Surface damage without exchange of material:

• Structural changes: aging, tempering, phase transformations, and recrystallization.

• Plastic deformation: residual deformation of the surface layer.

• Surface cracking: fractures caused by excessive contact strains or cyclic variations of thermally or mechanically induced strains.

(b) Surface damage with loss of material (wear):

• Characterized by wear scars of various shapes and sizes.

• Can be shear fracture, extrusion, chip formation, tearing, brittle fracture, fatigue fracture, chemical dissolution, and diffusion.

(c) Surface damage with gain of material:

• Can include pickup of loose particles and transfer of material from the opposing surface.

• Corrosion: Material degradation by chemical reactions with ambient elements or elements from the opposing surface.

Wear may also be classified as mild or severe. The distinguishing characteristics between mild and severe wear are as follows (Williams 1994):

(a) Mild

• Produces extremely smooth surfaces – sometimes smoother than the original.

• Debris is extremely small, typically in the range of 100 nanometers (nm) (3.28 × 10 ft) -13 in diameter.

• High electrical contact resistance, but little true metallic contact.

(b) Severe

• Rough, deeply torn surfaces – much rougher than the original.

• Large metallic wear debris, typically up to 0.01 mm (3.28 × 10 ft) in diameter. -5

• Low contact resistance, but true metallic junctions are formed.

Abrasive Wear

Abrasive wear occurs when a hard surface slides against and cuts grooves from a softer surface. This condition is frequently referred to as two-body abrasion. Particles cut from the softer surface or dust and dirt introduced between wearing surfaces also contribute to abrasive wear. This condition is referred to as three-body abrasion.

Adhesive Wear

Adhesive wear frequently occurs because of shearing at points of contact or asperities that undergo adhesion or cold welding. Shearing occurs through the weakest section, which is not necessarily at the adhesion plane. In many cases, shearing occurs in the softer material, but such a comparison is based on shear tests of relatively large pure samples. The adhesion junctions, on the other hand, are very small spots of weakness or impurity that would be insignificant in a large specimen but in practice may be sufficient to permit shearing through the harder material. In some instances the wearing surfaces of materials with different hardness can contain traces of material from the other face. Theoretically, this type of wear does not remove material but merely transfers it between wearing surfaces. However, the transferred material is often loosely deposited and eventually flakes away in microscopic particles; these, in turn, cause wear.

Pitting Wear

Pitting wear is due to surface failure of a material as a result of stresses that exceed the endurance (fatigue) limit of the material. Metal fatigue is demonstrated by bending a piece of metal wire, such as a paper clip, back and forth until it breaks. Whenever a metal shape is deformed repeatedly, it eventually fails. A different type of deformation occurs when a ball bearing under a load rolls along its race. The bearing is flattened somewhat and the edges of contact are extended outward. This repeated flexing eventually results in microscopic flakes being removed from the bearing. Fatigue wear also occurs during sliding motion. Gear teeth frequently fail due to pitting.

While pitting is generally viewed as a mode of failure, some pitting wear is not detrimental. During the break-in period of new machinery, friction wears down working surface irregularities. This condition is considered to be nonprogressive and usually improves after the break-in period. However, parts that are continuously subjected to repeated stress will experience destructive pitting as the material’s endurance limit is reached.

Corrosive Wear

Corrosive wear occurs as a result of a chemical reaction on a wearing surface. The most common form of corrosion is due to a reaction between the metal and oxygen (oxidation); however, other chemicals may also contribute. Corrosion products, usually oxides, have shear strengths different from those of the wearing surface metals from which they were formed. The oxides tend to flake away, resulting in the pitting of’ wearing surfaces. Ball and roller bearings depend on extremely smooth surfaces to reduce frictional effects. Corrosive pitting is especially detrimental to these bearings.

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Other

In our textbooks, they say that friction originates due to electrostatic charges. But electrostatic charges themselves originate due to friction. Then what is the real reason behind friction? I have searched extensively on the internet, but nobody seems to know exactly how friction is caused. I know that friction is caused due to charges, but what is the actual process involved?

According to me, when atoms/molecules of one object come extremely close to atoms/molecules of another object, the atoms/molecules having greater number of valence electrons repel the valence electrons of the atoms/molecules on the surface of the other object (due to their larger value of negative charge), causing the surface atoms/molecules to polarize slightly. This develops a slight positive charge on the surface of the object (in other words, one surface causes slight polarization of the other). This attraction between the surfaces of the two objects results in formation of these weak bonds which we call “gluing up of surfaces” at the points of contact. The strength of the bonds, naturally, depends on the nature of atoms/molecules of the two objects.

Regarding emission of heat due to friction:
When bonds are formed, heat is released. When bonds break, heat is consumed. Now if we just place one object on top of the other, heat is produced due to formation of bonds, though it is quite small. When we slide one surface on top of another, there is a continuous formation and breaking of these bonds. The amount of heat produced is proportional to the weight of the body being moved: more the weight, stronger is the bond strength and more is the heat released.
But this is where I get stuck up. If one body is moving on top of another, then bonds are continuously forming and breaking. This means, emission and absorption of heat must be almost equal. Then why is so much heat evolved in movement?

This lesson will answer those questions. There is a mini-quiz near the end of the lesson.

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Answers to Readers’ Questions

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Resources

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Mini-quiz to check your understanding

If you got all three correct, you are on your way to becoming a Champion in Physical Science. If you had problems, you had better look over the material again.

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