O’zbekistоn respublikasi оliy va o’rta maxsus ta`lim vazirligi

Figure 12.1 Dependence of the probability of failure-free operation of a simple device on time
It is beneficial to increase the resource of machines only up to a certain limit. If the machine works for a very long time, then during this time new, more productive machines appear. The operation of the old machine becomes economically unprofitable, and it becomes obsolete. To avoid unnecessary costs, the resource of the machine should be assigned taking into account the time of its obsolescence. At present, the equipment becomes obsolete on average over a period of 3 to 8 years.
18.4 Design methods
A number of measures to improve the wear resistance of friction pairs can be taken at the design stage of the machine. The main ones are the following.
The choice of the principle diagram of friction units is made taking into account their influence on the wear resistance and reliability of the machine as a whole. Below are some recommendations for the design of friction pairs developed by tribologists.
Depending on the ratio of hardness (H ₁ and H ₂ ) of materials and areas (S ₁ and S ₂ ) of the friction surface of mating parts 1 and 2, direct and reverse friction pairs are distinguished:
- in a straight pair H ₁ H ₂ , S ₁ S ₂ , i.e. a more rigid body slides over a larger body;
- in the return pair H ₁ H ₂ ,S ₁ S ₂ , i.e. a softer body glides over a larger body.
From the point of view of tribology, the reverse pair is preferable to the direct one. Under overloads, the plastic deformation of the softer element of the reverse pair does not prevent friction, but in the straight line it leads to cutting or plastic displacement of the material on the friction surface of the counterbody.
Ensuring favorable friction conditions is an important criterion when choosing the design of the friction unit. This is achieved by:
- complete lubrication of the friction surface, avoiding that there are areas not covered with lubricant on it;
— protecting the lubrication system from abrasive particles using filters, seals, sealed containers, etc.;
- devices for heat removal from the friction zone in a natural way or forced cooling;
- reducing, if possible, the pressure on the friction surface.
Compensation of wear of elements of a friction pair is a common design technique that makes it possible to increase the wear resistance of machines. For example, the gap adjustment in a friction pair of the “shaft-hole” type is carried out automatically or during the next maintenance of the machine. In places of intense wear, replaceable structural elements are used: cutters on cutters, teeth on an excavator bucket, inserts on a trolley bus current collector, etc. If the use of replaceable structural elements is impossible, it is advisable to increase the size of the parts by the amount of linear wear in the areas of greatest wear.
The choice of friction pair materials is an important design stage. It is determined by the design and purpose of the pair, its manufacturing technology, operating conditions, etc. General recommendations for the choice of materials for friction pairs are listed below:

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  It is advisable to combine hard material with soft. In order for the optimal ratio of hardness to be maintained during the technical life of the pair, the soft material should not harden (recrystallize) at the operating temperature of the friction pair. It is advisable to apply it in the form of a coating on the friction surface of a part made of structural material.
  
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  If the use of a soft material is impossible for reasons of strength, the friction pair is made of hard materials. This makes it possible to reduce wear, but imposes increased requirements on the pair running-in modes.
  
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  It is not recommended to combine two soft materials in a friction pair. In this case, the plastic deformation of microroughnesses predominates, which leads to an increase in wear.
  
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  From the point of view of tribology, it is undesirable to make elements of a friction pair from the same materials. When the lubricant and oxide films on the friction surfaces are destroyed, juvenile surfaces of the same nature come into contact. This leads to the formation of strong adhesive bonds, increasing the propensity of the elements of the pair to seize.
  
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  In structures that are difficult to access for lubrication, it is advisable to use self-lubricating and oil-impregnated porous materials. They provide extreme pressure resistance to the pair.
  

The principle of compliance of a friction pair is to ensure that the friction surface follows the deformations of the counterbody and adapts to inaccuracies in its geometric shape that have arisen during processing or due to wear.
Replacing external friction with internal one is expedient for small linear or angular displacements of kinematic pairs. During the operation of the pair, the mutual displacement of the rigid structural elements occurs due to the elastic deformation of the elastic element attached to them. Thanks to this, the most dangerous abrasive wear of parts is eliminated, the need for lubrication is eliminated, and the noise during the operation of the pair is reduced.
Replacement of sliding friction rolling friction is expedient from the standpoint of increasing the reliability of friction pairs. Rolling bearings have the following advantages over plain bearings:
a) lower friction losses, because at the same speeds and loads, the coefficient of rolling friction is much lower than that of sliding, it changes little in a wide range of friction modes and during the start-stop of the pair;
b) a large amount of non-ferrous metals (Cu, Sn, Pb) spent on the manufacture of plain bearings is saved;
c) the consumption of lubricants is reduced;
d) operating costs are reduced;
e) ball and roller bearings are standardized, which simplifies the design, installation and operation of the bearing unit.
Disadvantages of rolling bearings:
a) low reliability under shock and vibration;
b) frequent failures at high speeds due to rupture of the oil film and destruction of friction surfaces;
c) large diameters of rolling bearings compared to plain bearings;
d) noise at work;
e) non-separable design.