Friction(frictional interaction) - the process of interaction of bodies during their relative motion (displacement) or when a body moves in a gaseous or liquid medium.

Friction processes are studied in a branch of physics called tribology(mechanics of frictional interaction).

Friction is usually divided into:

• dry when interacting solids are not separated by any additional layers / lubricants (including solid lubricants) - a very rare case in practice; a characteristic feature of dry friction is the presence of a significant static friction force;
• boundary when the contact area can contain layers and areas of various nature (oxide films, liquid, and so on) - the most common case of sliding friction;
• liquid(viscous), arising from the interaction of bodies separated by a layer of a solid (graphite powder), liquid or gas (lubricant) of various thicknesses - as a rule, it occurs during rolling friction, when solids are immersed in a liquid, the amount of viscous friction is characterized by the viscosity of the medium;
• mixed when the contact area contains areas of dry and liquid friction;
• elastohydrodynamic(viscoelastic) when internal friction in the lubricant is critical. It occurs with an increase in the relative speeds of movement.

Friction force Is a force that arises at the point of contact of bodies and prevents their relative movement.

Causes of friction force:

• roughness of the contacting surfaces;
• mutual attraction of molecules of these surfaces.

Sliding friction- the force arising from the translational displacement of one of the contacting / interacting bodies relative to the other and acting on this body in the direction opposite to the sliding direction.

Rolling friction- the moment of forces arising from the rolling of one of the two contacting / interacting bodies relative to the other.

Rest friction- the force that arises between two contacting bodies and prevents the occurrence of relative motion. This force must be overcome in order to set two contacting bodies in motion relative to each other.

The friction force is directly proportional to the force of the normal reaction, that is, it depends on how strongly the bodies are pressed against each other and on their material, therefore the main characteristic of friction is coefficient of friction, which is determined by the materials from which the surfaces of interacting bodies are made.

Wear- change in the size, shape, weight or condition of the surface of the product due to the destruction (wear) of the surface layer during friction.

The operation of any machine is inevitably accompanied by friction during the relative movement of its parts, therefore, it is impossible to completely eliminate wear. The amount of wear in direct contact of surfaces is directly proportional to the work of friction forces.

Abrasive wear is partly caused by dust and dirt, so it is very important to keep the equipment clean, especially the rubbing parts.

To combat wear and friction, some metals are replaced with others, more stable ones, thermal and chemical treatment of rubbing surfaces, precise machining are used, and metals are replaced with various substitutes, the design is changed, lubrication is improved (change the appearance, add additives), etc.

In machines, they tend to prevent direct sliding friction of hard surfaces, for which they either separate them with a layer of lubricant (liquid friction), or add additional rolling elements between them (ball and roller bearings).

The basic rule for the design of rubbing machine parts is that the more expensive and difficult to replace element of the rubbing pair (shaft) is made of a harder and more wear-resistant material (solid steel), and simpler, cheaper and easily replaceable parts (bearing shells) are made of a relatively soft material with a low coefficient of friction (bronze, babbitt).

Most of the machine parts fail precisely due to wear, so reducing friction and wear by even 5-10% gives huge savings, which is of exceptional importance.

List of links

1. Friction // Wikipedia. - http://ru.wikipedia.org/wiki/Friction.
2. Wear (technique) // Wikipedia. - http://ru.wikipedia.org/wiki/Iznos_(technique).
3. Friction in machines, friction and wear in mechanical engineering // Project-Technar. Progressive auto technologies. - http://www.studiplom.ru/Technology/Trenie.html.

Questions for control

1. What is friction?
2. What are the different types of friction?
3. What causes frictional forces?
4. How is friction classified depending on the acting forces?
5. What is wear and how is it dealt with?

Types of dry friction. Friction at rest is a force between bodies at rest relative to each other. Sliding friction is the force that occurs when one body slides over the surface of another. Rolling friction - occurs when one body rolls on the surface of another body. When solids move in liquids, a viscous friction force arises.

Physics grade 7

"Gas pressure grade 7" - Completed the work: student of grade 7 "a" Magomedova Bazhi. From temperature From concentration (number of particles per unit volume) From mass of particles. 7th grade. Gas pressure. Gas pressure. Content. Gas pressure depends:

"Ships sailing physics" - Waterline. Displacement. Content: Sailing ships. Carrying capacity. Now let's look at a physical phenomenon -. Draft. The principle of navigation of ships. The principle of navigation of ships. 2. Characteristics of the vessel: draft; waterline; displacement; carrying capacity. Displacement of the vessel? is determined by summing the weight of the empty vessel and the deadweight.

"Grade 7 lesson Friction force" - Lesson topic: "Friction force". Task 1. Determine the dependence of the friction force on body weight. Practical work “Measuring force. During the classes. Ftr. What to do? 7th grade. Friction force of rest arises during friction at rest. Friction force.

"Pressure problems" - M = 60 kg. 2. Solution: Strength. 5. (Pa). 10.S = 400 cm2. Units of pressure. Pressure =. F. N. Dano:

"Physics Grade 7 Pressure" - Barometer, aneroid. 11.Pumps 12.Pulling force. H 0.57 kN =? rH =? Pa. H 380 H =? kN II. Option: 1. Determine the atmospheric pressure in Pa, kPa, mm Hg with a barometer. 2. Express: 3 n / cm2 =? Aeronautics. IV. Why do gases exert pressure? Liquid pressure at the bottom of the vessel. Formula for calculating pressure? Lesson course: I. Organizational moment II.

"Pressure grade 7" - Test. Reducing pressure in technology. Power. 300000 kPa. Laying the foundation of the building. Attached to a support or hanger. Gravity. Increased pressure in nature. Repeat 10 times. How to increase pressure How to decrease pressure. Force is a measure of the interaction of bodies. P = F / S. Insect sting. Body weight. The force with which the Earth pulls the body towards itself. The unit of pressure is the newton per square meter, called Pascal.

DEFINITION

From the second equation:

Friction force:

Substituting the expression for the friction force in the first equation, we get:

When braking to a full stop, the bus speed drops from a value to zero, so the bus:

Equating the right-hand sides of the ratios for the acceleration of the bus during emergency braking, we get:

where is the time until the bus stops completely:

Free fall acceleration m / s

Substituting the numerical values ​​of physical quantities into the formula, we calculate:

EXAMPLE 2

Report

Dry friction law. Modeling of frictional self-oscillations. Burridge and Knopoff models

Dry frictionoccurs between the surfaces of solids in the absence of lubrication.

Dry friction, in turn, is subdivided into friction slipand friction rolling.

Dry friction lawswere formulated by Coulomb. The magnitude of the maximum force of friction at rest / Nr depends on the magnitude of the force of normal pressure between the surfaces. If in our experiment (Fig. 87) to increase the force of normal pressure, then approximately proportionally to this force, the value of the weight that must be placed on the cup will also increase in order for sliding to occur.

Dry friction lawssuitable for hard surfaces. Lubrication of skis is needed not so much to reduce friction as to eliminate possible snow adhesion to the skis.

Consider dry friction laws.

Consider first dry friction laws... Such friction arises not only when one body slides over the surface of another, but also during any attempt to cause such a slip. In the latter case, friction is called static friction or clutch friction. The presence of static friction is a characteristic feature of dry friction. In a more general sense, regardless of which bodies friction occurs between, it is called dry if the friction forces do not disappear when the relative velocities of the contacting bodies vanish. Otherwise, friction is called fluid friction. We then apply a horizontal force to the bar, lying in a vertical plane passing through its center of mass, as close as possible to the table surface, in order to prevent the bar from overturning when it starts to move. Experience shows that if the force does not exceed a certain certain value, then the bar does not move. This is the frictional force, namely the static friction. The same frictional force, but in the opposite direction, acts on the table surface from the side of the bar.

Vibration diagnostics of the parameters of dry non-cumulative friction during frictional self-oscillations

Dynamic processes in mechanical devices with contacting and rubbing elements in kinematic pairs, such as machine tool slide guides, robotic systems, friction clutches, clutches, shaft bearings, etc., can be accompanied by the emergence of complex and poorly controlled, and therefore such difficult to eliminate phenomena such as frictional self-oscillations. The result of frictional self-oscillations in machines is a decrease in the quality indicators of technological processes, positioning accuracy, fatigue failure and increased wear of parts.

The reason for the occurrence of frictional self-oscillations is the nonlinear "falling" characteristic of the dry friction force from the speed of the relative sliding of the contacting surfaces. In this regard, the definition of dynamic, that is, constantly changing in time and in function of other quantities, parameters of dry non-cumulative friction is important and relevant. Solving this problem will make it possible to more efficiently diagnose rubbing and contacting machine units, to make a reliable forecast of the dynamic behavior of their kinematic pairs, for example, to predict frictional self-oscillations, and will also provide the possibility of purposeful control of the friction process. The identified parameters of the acting nonlinear dry friction forces can be used as diagnostic indicators for assessing the technical state of this class of objects, including at the dichotomous level (“good” - “bad”).

However, direct measurement of the acting forces of dry friction is possible only by tribotechnical methods and approaches and is very difficult to implement in elastic oscillatory systems. Therefore, to identify the dynamic parameters of dry friction, one has to use indirect methods based on measuring the vibrational response in a dynamic system.

Let us consider one of the proposed methods for identifying the parameters of dry non-cumulative friction, which is implemented in the study of frictional self-oscillations.

The calculated dynamic diagram of the system under consideration is shown in Fig. one.

Rice. 1. Calculated dynamic scheme of the system for the study of frictional self-oscillations

The differential equation of the dynamics of this system has the form:

,

where C is the coefficient of stiffness of the elastic element; is the coefficient of viscous resistance; = const is the constant speed of movement of the right end of the elastic element;

the law of variation of the force of dry non-cumulative friction with a falling characteristic as a function of velocity (Fig. 2).

Thus, the proposed method, a test bench, and a hardware-computing complex for studying the dynamics of frictional self-oscillating processes allow one to identify the dynamic parameters of the force of dry non-cumulative friction and to implement, on this basis, the procedure for vibration diagnostics of various pairs of friction, predisposed to the occurrence of frictional self-oscillations.

Burridge and Knopoff model

The Burridge-Knopov (BK) model was created more than 40 years ago to explain the occurrence of repeated shocks in earthquakes.

friction pendant rolling sliding

The essence of the B-K model can be understood from the figure, which shows that a moving plate is connected to a stationary plate by means of N discrete elements (blocks) connected to each other and the plates by means of "springs". Let's consider one of the blocks. The idea of ​​this model is that as long as a force less than a given threshold is acting on this block, it is motionless. When the threshold is reached, the block "breaks down" abruptly. The mutual influence of the blocks, which is that the one that has fallen off pulls others along with it, can lead to the simultaneous breakdown of several neighboring elements of the system at once. This, according to BK, is the "main blow" of the earthquake, while the "jumps" of other blocks are repeated blows, or aftershocks. The BK model was investigated in the laboratory experimentally and on a computer - numerically. As a result, it was shown that the model exhibits properties inherent in the experimental law of recurrence of Gutenberg-Richter earthquakes. In the experiments, a similarity of the main shock, foreshocks and aftershocks was observed.

In an experimental study of the behavior of rock specimens under external pressure loading, it was found that the force acting on the specimen changes depending on the magnitude of the recorded change in the length of the specimen in the "saw shape". The BK model found a geological explanation for these results as a “stick-slip” of two plates against each other along a fault in the presence of friction.

Despite the fact that the B-K model was proposed in the second half of the last century, the interest in it among scientists has increased only in recent years. This is due to the fact that there have been some successes in the physics of nonlinear phenomena, in particular, in the field of self-organizing systems. The BK model was recognized as a very suitable basis for developing these ideas and modeling the corresponding systems. In addition, at present it is generally accepted that this model, of all the others, is the most adequate for describing the earthquake process.

All BK models obey the experimental Gutenberg-Richter law, according to which the number of earthquakes N with energy E:

Let us describe in detail a two-dimensional version of the BK model. All blocks of the system are located on the platform. There is friction between the platform and the blocks. Each block of the system is connected to four neighbors using springs. Also, each block is connected by another spring to the upper large moving platform. The movement of the blocks is caused by the relative displacement of the two plates. When the force acting on the block becomes greater than a certain threshold (F c ritical , the maximum value of friction at rest), the block "breaks down". The model assumes that after a breakdown, a zero force acts on the block (i.e., the resultant is zero), and the forces acting on the neighbors are recalculated. This can lead to a breakdown of one of the neighbors, which means to a chain reaction (earthquake). The total number of cells torn off in one such process determines the size of the corresponding earthquake. To begin with, we will present this two-dimensional block-spring model in the form of a cellular automaton. Let's set an array of blocks of size L 1xL 2, each block is assigned its coordinates (i, j). 1≤i≤L 1, 1≤j≤L 2.

Through x i, j the displacement of the block (i, j) from the equilibrium position is indicated. The total force applied to this block is given by:

Where K 1, TO 2, TO L are the stiffness coefficients of the corresponding springs, x i, j - displacement of the block (i, j) relative to the equilibrium position. When one of the plates moves relative to the other, the force acting on each block grows constantly until it reaches a critical value, after which the relaxation process begins.

Frictional forces can also arise in direct contact of solids. These forces are characterized by the fact that they act along the contact surface and are always directed in such a way that they prevent the movement of the contacting bodies relative to each other. These forces are often referred to as dry friction forces. We will consider only two types of dry friction forces: static friction and sliding friction.

Try to budge some heavy object on the floor (fig. 3.34). If you act with low force, the object will not budge. He will remain at rest because, simultaneously with the force, the rest friction force will begin to act on him from the floor. This force is equal in magnitude to the force but is directed in the opposite direction and prevents the occurrence of movement. Simultaneously with changes in the modulus and direction of the external force, the static friction force also changes its modulus and direction. This is the first important feature of static friction forces.

The forces of friction at rest can take any values: from zero to some maximum value. The modulus and direction of the static friction forces depend on the nature of external influences to which the contacting bodies are exposed. The greatest value of the static friction force depends on the material from which the bodies are made, on the quality of processing and the state of the contacting surfaces.

The maximum value of the static friction force can be determined using a simple experiment, the diagram of which is shown in Fig. 3.35. If you gradually increase the load, then at some load, the bar will slide over the table surface. In this case, the static friction force will take the greatest possible value and become equal to the gravity of the load

Using the same setup, one can notice the second important feature of the static friction forces: the maximum value of the static friction force grows in proportion to the normal pressure force of the body pressing against each other. Indeed, loading the bar with an additional load (Fig. 3.36), we will increase the force of normal pressure and observe an increase in the greatest friction force, proportional to the change.Therefore, we can write:

Here is the force of normal pressure; constant coefficient of friction.

Finally, with the help of the same installation, you can find the third feature of the static friction forces (Fig. 3.37): with a constant force of normal pressure, the largest value of the friction force does not depend on the size of the contact area of ​​the bodies.

The features of sliding friction forces can be determined in exactly the same way. To do this, you need to pick up the load so that after the start of sliding, the body moves evenly. In this case, the tension force of the thread will be equal in modulus to the sliding friction force.

A number of such simple experiments make it possible to establish all the basic properties of sliding friction forces. Experiments show that the sliding friction force is slightly less than the maximum static friction force.

The sliding friction force depends on the material of the bodies and on the quality of the contacting surfaces. It is also proportional to the force of normal pressure pressing the bodies against each other, and does not depend on the size of the contact area. The sliding friction force is always directed in the direction opposite to the direction of the speed of the relative motion of the bodies. The sliding friction force is slightly, but rather difficult, changes with increasing this speed.

When solving problems, a number of simplifications are usually introduced. For example, the difference between the greatest force of static friction and the force of sliding friction is neglected and they are considered equal to each other; or neglect changes in sliding friction force with changes in speed. It is believed that the force of sliding friction in its value remains constant at all speeds. Taking these simplifications, in the future, in the calculations, we will use the formula to determine the sliding friction force.

Static friction and sliding friction play a very important role in technology and in everyday life. Very often friction is seen only as a hindrance, which does not allow creating and maintaining unchanged body movements. But at the same time, without the existence of friction, the movement of bodies on the surface of the earth would be impossible. Using the friction of the wheels on the ground or on the rails, cars and trains are set in motion.

Therefore, in technology they solve the problem not only of how to reduce friction where it interferes with movement, but also how to increase it where it helps to create or convey movement. For example, diesel and electric locomotives are made as heavy as possible. Clutches in a car transfer movement from the engine to the wheels using frictional forces, which must be large. To achieve this, the clutch discs of the car are pressed against each other by strong springs (Fig. 3.38). This creates a large force of normal pressure and achieves a significant increase in forces

friction at rest, transferring movement from one part of the machine to another.

The same is done when frictional forces are used to connect parts in various mechanisms. For this, the parts are pressed into each other (Fig. 3.39). In this case, elastic forces arise, which create a large normal pressure on the surface of the pressed-in part. Due to this, the necessary large static friction forces are developed at the junction. The same frictional forces hold in place any tightly screwed nut (Figure 3.40).

In the future, when solving problems, we will use the equation as an additional one, expressing the special properties of sliding friction forces.