Subject:“Fittings, characteristics of landings, graphical representation of landings in the hole system, in the shaft system.”

1.Mating and non-mating surfaces.

2. Characteristics of landings.

3. The gap and the conditions for its formation.

4. Preload and conditions for its formation.

5. Graphic representation of fits in the shaft system, in the hole system.

6. Determination of the landing group according to the drawings of the mating parts.

All various machines, machines, devices, mechanisms consist of parts that have mating and non-mating surfaces.

Mating surfaces– these are the surfaces along which parts are connected into assembly units (assemblies).

^ Non-mating (free) - These are structurally necessary surfaces that are not intended for connection with the surfaces of other parts.

The designs of parts connections and requirements for them may be different. Depending on the purpose of the connection, the structural elements of parts with mating surfaces having the same nominal size must, during operation of the mechanism or machine, either allow the parts to move relative to each other.

To ensure the mobility of the connection, it is necessary that the actual size of the female element of one part (hole) be greater than the actual size of the male element of the other part (shaft). The difference between the actual dimensions of the hole and the shaft, if the hole size is larger than the shaft size, is called gap.

To obtain a fixed connection, it is necessary that the actual size of the male element of one part (shaft) be greater than the actual size of the female element of the other part (hole). The difference between the actual dimensions of the shaft and the hole before assembly, if the size of the shaft is larger than the size of the hole, is called interference

The interface formed as a result of connecting holes and shafts (male and female elements of parts) with the same nominal dimensions is called landing.

Fit is the nature of the connection of parts, determined by the size of the resulting gaps or interference.

Since the actual dimensions of suitable holes and shafts in a batch of parts manufactured according to the same drawings can fluctuate between the specified maximum dimensions, then, consequently, the size of the gaps and interferences can fluctuate depending on the actual dimensions of the mating parts. Therefore, a distinction is made between the largest and smallest gaps and, accordingly, the largest and smallest interference.

Maximum gap S=D- d

Minimum clearance S =D- d

Where D, D are the largest and smallest maximum hole size

D, d - the largest and smallest maximum shaft size

Maximum interference N= d- D

Minimum interference N= d- D

Example: 1 The hole drawing shows size 50
, and on the shaft drawing – size 50
Let's carry out the necessary calculations.

Limit hole dimensions, mm: largest 50.0+0.02=50.02; smallest 50.00.

Limit shaft dimensions, mm: largest 50.00-0.03=49.97; smallest 50.00-0.06=49.94.

Gap, mm: largest 50.02-49.94=0.08; the smallest 50.0-49.97=0.03.

Example 2. The hole drawing shows size 50+ 0 - 02, and the shaft drawing shows size 50

Limit hole dimensions, mm: largest 50.00+0.02-=50.02; smallest 50.00.

Limit shaft dimensions, mm: largest 50.00+0.05=50.05; the smallest 50.00+0.03=50.03.

Preload, mm: maximum 50.05-50.00 = 0.05; the smallest 50.03-50.02=0.01.

The construction of a fit diagram begins with drawing a zero line corresponding to the nominal size of the connection (the nominal dimensions of the hole and shaft making up the connection, or, what is the same thing, forming the fit, are the same). From the zero line, common for the hole and shaft, they are laid off on a selected scale, taking into account the signs of the maximum deviations of the hole and shaft; Between the lines corresponding to the upper and lower deviations, we obtain the tolerance fields of the mating hole and shaft. And finally, in accordance with the above definitions, the largest and smallest gaps and interferences are identified in the diagrams

N smallest gap S mih

" On the largest is the gap

* tah

Largest clearanceS ma

The figure shows that when graphically depicting a fit with a gap, the tolerance field of the hole is located above the tolerance field of the shaft, i.e., the dimensions of a suitable hole are always larger than the dimensions of a suitable shaft, as was noted earlier when introducing the concept of “gap”.

Leastth interference/Ulnp

Largest interference N max/Ears

In the same way, the figure shows that when graphically depicting an interference fit, the tolerance field of the hole is located below the tolerance field of the shaft, i.e.

the dimensions of a suitable hole are always smaller than the dimensions of a suitable shaft, as was noted earlier when introducing the concept of “interference”.

The above numerical examples and the corresponding graphical constructions do not exhaust all possible groups of landings. Along with clearance fits and interference fits, when the clearance or, respectively, interference in the connection is guaranteed by mating any suitable holes and shafts, an option is also possible when the maximum dimensions of the mating parts do not guarantee only clearance or interference only. Such landings are called transitional. In this case, it is possible to obtain both a gap and an interference fit; the specific nature of the connection will depend on the actual dimensions of the mating suitable holes and shafts. Let's show this with an example.

Example 3. The hole drawing shows size 50 +0.02, and the shaft drawing shows -50
. Let's carry out the necessary calculations.

Limit hole dimensions, mm: largest 50.00+0.02=50.02; smallest 50.00.

Limit shaft dimensions, mm: largest 50.00+0.03=50.03; smallest 50.00+0.01=50.01.

If we imagine connecting a hole that has the largest maximum size with a shaft that has the smallest maximum size, then a fit with a gap is formed, since the hole is larger than the shaft, and the gap will be the largest and equal to 50.02-50.01 = 0.01 mm.

If we imagine connecting a hole that has the smallest maximum size with a shaft that has the largest maximum size, then an interference fit is formed, since the shaft is larger than the hole, and the interference will be the greatest and equal to 50.03-50.00 = 0.03 mm .

interference /Nmax

When graphically depicting a transitional fit, tolerance fields

holes and shaft overlap, i.e. the dimensions of a suitable hole may be larger and smaller size a suitable shaft, which does not allow us to say in advance, before manufacturing a pair of mating parts, what the fit will be - with clearance or interference.

A fit with a guaranteed clearance is used in cases where relative displacement of parts is allowed; fits with guaranteed interference - when it is necessary to transmit force or torque without additional fastening only due to elastic deformations that occur during the assembly of mating parts.

Transitional fits have small maximum clearances and interferences and therefore are used in cases where it is necessary to ensure the centering of parts, i.e., the coincidence of the axes of the hole and the shaft; this requires additional fastening of the parts to be connected.

Fitments of all three groups - with gaps, with interference, transitional with different values ​​of the largest and smallest gaps and interference - can be obtained with the same nominal size by changing the position of the tolerance fields of both mating parts - the hole and the shaft. But, obviously, there may be countless such combinations, which would lead to the impossibility of centralized production of measuring cutting tools (drills, countersinks, reamers) that shape the size of the hole.

It is much more convenient in technological (during production) and operational (during repair) terms to obtain a variety of different landings, changing the position of the tolerance field of only one part while the position of the tolerance field of the other remains unchanged.

For example, the different fits discussed in examples 1, 2, 3 are formed by changing only the tolerance fields of the shafts with a constant tolerance field of the holes. This method of forming various landings is called the hole system. A part in which the position of the tolerance field is basic and does not depend on the required nature of the connection is called the main part of the system (in the case considered, a hole). Similar fits can be obtained differently if the shaft is taken as the main part, and the tolerance fields of the holes are changed to form different fits. This method of formation is called the shaft system.

Thus, fits in a hole system are fits in which different clearances and interferences are obtained by connecting different shafts to the main hole; fits in a shaft system are fits in which different gaps and interferences are obtained by connecting different holes to the main shaft.

In mechanical engineering practice, preference is given to the hole system, since making a hole and measuring it is much more difficult and expensive than making and measuring a shaft of the same size with the same accuracy.

(drawing)

Thus, shafts of various (and high) precision can be processed and measured with universal tools - cutters, grinding wheels, micrometers, etc. And to process and measure precise holes, you will need special expensive tools (countersinks, reamers, broaches, plug gauges). The number of sets of such tools required to machine holes with the same nominal size depends on the variety of maximum deviations that can be assigned by the designer. Let's say you need to produce three sets of parts of the same nominal dimensions and the same accuracy to form fits with clearance, interference and transition. If we adopt the hole system, then the maximum hole sizes for all fits will be the same and only one set of special tools will be required to process and measure the holes.

To make it even more convenient to assign fits for the designer and process parts for the worker, it was agreed that the tolerance fields of the main parts of the fit systems must satisfy one mandatory condition: one of the maximum dimensions of the main part must coincide with the nominal size. Moreover, for the main hole, such a limiting size should be the smallest (or, what is the same, the lower limit deviation of the main hole should be equal to zero, and the main shaft should be the largest (or, what is the same, the upper limit deviation of the main shaft should be equal to zero ).

The tolerance of the main part of the landing system is always directed “to the body” of this part: in the case of the main hole - to increase the maximum size compared to the nominal one; in the case of the main shaft - to reduce the maximum size compared to the nominal one.

Control questions:

1.What is landing?

2.What is the landing characterized by?

3.What is a gap and what are the conditions for its formation?

4. What is interference and what are the conditions for its formation?

5.How are landings formed in the hole system?

6. How are landings formed in the shaft system?

7. How can you determine the nature of the connection based on the relative position of the tolerance fields of the hole and the shaft in a graphical representation of the fit?

Connections

Two or more fixedly or movably connected parts are called mating parts. The surfaces along which the parts are connected are called mating surfaces. The remaining surfaces are called non-mating (free).

In the connections of parts, a distinction is made between female and male surfaces.

The female surface is a part element with an internal mating surface (hole).

The male surface is an element of a part with an external mating surface (shaft).

The concepts male and female surfaces give a more general definition of the concepts “shaft” and “hole”.

Based on the shape of these surfaces, the following main types of joints are distinguished: smooth cylindrical; smooth conical; flat, in which the female and male surfaces are formed by planes (for example, the grooves of tables of metal-cutting machines); threaded of various shapes, profiles, purposes; splined; keyed; gears.

Fit is the nature of the connection of two parts, determined by the difference in their sizes before assembly.

There are three types of landings, which are called: landings with a gap; interference fits and transitional fits.

Landings with clearance

A clearance fit is a fit in which a gap is always formed in the connection, i.e., the smallest limit size of the hole is greater than or equal to the largest limit size of the shaft.

Gap 5 is the difference between the size of the hole (O) and the shaft (a1) before assembly, if the size of the hole is larger than the size of the shaft (Fig. 5.5), i.e.

From formula (5.9) it follows that for this type of fit the hole size is always greater than or equal to the shaft size. For clearance fits, it is typical that the tolerance field of the hole is located above the tolerance field of the shaft.

Rice. 5.5.

Since the dimensions of the shaft and bushing can vary within the tolerance range, the size of the gap is determined by the actual dimensions of the parts being connected.

The largest gap 5max is the difference between the largest maximum hole size and the smallest maximum shaft size (Fig. 5.6, a), i.e.

The smallest gap is the difference between the smallest maximum hole size and the largest maximum shaft size (Fig. 5.6, a), i.e.

In a particular case, the smallest gap can be equal to zero. Average gap 5" (arithmetic average of the smallest and largest gaps)

The actual gap Se is the gap determined by Kit to the difference between the actual dimensions of the hole and the shaft.

Clearance fit tolerance ITS is the sum of the tolerances of the hole and shaft that make up the connection. The fit tolerance can be determined in the same way as the difference between the largest and smallest gaps:

A graphical representation of the tolerance fields for clearance fits is shown in Fig. 5.7.

Rice. 5.6.

Rice. 5.7.

Preference fits

An interference fit is a fit in which an interference is always formed in the connection, i.e., the largest limit size of the hole is less than or equal to the smallest limit size of the shaft. Preference I is the difference between the dimensions of the shaft and the hole before assembly, if the size of the shaft is larger than the size of the hole (Fig. 5.5, b)

For interference fits, it is typical that the shaft tolerance field is located above the hole tolerance field.

The assembly of such parts is usually done using a press. The interference is usually designated by the letter N. The amount of interference is determined by the actual dimensions of the shaft and hole.

Rice. 5.8.

Maximum interference Ytzh - the difference between the largest maximum shaft size and the smallest maximum hole size before assembly (see Fig. 5.6, b and 5.8)

The smallest interference is the difference between the smallest maximum shaft size and the largest maximum hole size before assembly (Fig. 5.8)

Average tightness Yt - arithmetic mean of the largest and smallest tightness

The actual interference Ne is the interference defined as the difference between the actual dimensions of the shaft and the hole before assembly.

Interference fit tolerance ITN - the difference between the largest and smallest interference

i.e., the interference fit tolerance is equal to the sum of the tolerance fields of the hole and shaft that make up the connection.

Interference fits are used in cases where it is necessary to transmit torque and/or axial force mainly without additional fastening due to the friction forces created by the interference.

A graphical representation of the location of tolerance fields for interference fits is shown in Fig. 5.9.

Rice. 5.9.

Transitional landings

In this group of fits, it is possible to obtain both a gap and an interference, depending on the actual dimensions of the hole and shaft (Fig. 5.10). Characteristic feature Transitional fits are partial overlaps of the tolerance fields of the shaft and hole.

Transitional fits are characterized by the greatest interference and 5^. To determine the maximum interference and maximum clearance, you can use formulas (5.17); (5.18) and (5.10); (5.11).

Transitional fit tolerance /77^5 is determined by the formula

Rice. 5.10.

Let us rewrite formula (5.16) in this way: -(B - a). The expression in parentheses is the gap (5.9). Then we can write LG = -5, i.e. the interference is a negative gap. The minimum negative clearance is the maximum interference, and the minimum negative interference is the maximum clearance, i.e. the following relationships are valid:

Taking into account (5.24) and (5.25), formula (5.23) can be rewritten as follows:

i.e., the fit tolerance is equal to the sum of the tolerance fields of the shaft and hole that make up the connection.

A graphical representation of the tolerance fields in transitional fits is shown in Fig. 5.11.

Examples of determining the maximum dimensions, tolerances, gaps and interference in connections when various types landings

Clearance fit

The nominal shaft size is 100 mm, the lower shaft deviation is -160 microns (-0.106 mm), the upper shaft deviation e$ is -60 microns (-0.06 mm).

Nominal hole size 100 mm, lower hole deviation £7 = +72 µm (+0.072 mm), upper hole deviation £5_ +159 µm (+0.159 mm). A graphical representation of this landing is shown in Fig. 5.12.

Rice. 5.11.

Rice. 5.12.

Rice. 5.13.

Fit tolerance (clearance)

Interference fit

Example. The nominal shaft size is 100 mm, the lower shaft deviation is ~ 72 µm (0.072 mm), the upper shaft deviation is ~ 159 µm (0.159 mm).

Nominal hole size 100 mm, lower hole deviation

£7= -106 µm (-0.106 mm), upper hole deviation £5--60 µm (-0.060 mm).

A graphical representation of this landing is shown in Fig. 5.13.

Solution. Maximum maximum shaft size d^

dmax=d + es= 100+ (0.159) = 100.159 mm. Minimum maximum shaft size dm.n

4™= d + "= I* + (0.072) = 100.072 mm. Shaft tolerance range

Td = 4™, ~ 4*n = 0.159 - 100.072 = 0.087 mm

lTd = es-ei = 0.159 - 0.072 = 0.087 mm. Maximum hole size

Omw = D + ES = 100 + (-0.060) = 99.940 mm. Smallest hole size limit

Dmin= D+ E1= 100 + (-0.106) = 99.894 mm.

Let's determine the tolerance range of the hole

"™ = Ohm" " Rya1a = 99.940 - 99.894 = 0.046 mm

• 1TO = £5 - £/ = -0.060 - (-0.106) = 0.046 mm. Maximum tension in connection
• 4™- 4™ = 100.159-99.894 = 0.265 mm

N"1= E1= 0.159- (-0.106) = 0.265 mm. Minimum interference in the connection

4y"" A"* = 0.072 - 99.940 = 0.132 mm

^п"п = e" ~ £У= О"072 ~ (-0.060) = 0.132 mm. Fit tolerance (preference)

PI = - Yya.t = 0.265 - 0.132 = 0.133 mm

GYY = t + 1Ty = 0.087 + 0.046 = 0.133 mm.

Transitional fit

Example. The nominal shaft size is 100 mm, the lower shaft deviation a is +71 µm (+0.071 mm), the upper shaft deviation e$ ~ +93 µm (+0.093 mm).

Nominal hole size 100 mm, lower hole deviation £7= +72 µm (+0.072 mm), upper hole deviation £5_ +159 µm (+0.159 mm). A graphical representation of this landing is shown in Fig. 5.14.

Solution. The largest maximum shaft size dtzh

4™, = ^ + 00 + 0.093 = 100.093 mm. The smallest maximum shaft size is "

Shaft tolerance range

/Тс/ = с/^-с/^п = 100.093 - 100.071 = 0.022 mm

Rice. 5.14.

t = & - in! = 0.093 - 0.071 = 0.022 mm. Maximum hole size

Osh = O + £5 = 100 + 0.159 = 100.159 mm. Smallest hole size limit

Oyu.t = th + E1 = 100 + 0.072 = 100.072 mm. Hole tolerance range

/77) = Otaya - ya1a = 100.159 - 100.072 = 0.087 mm

/77) = £5- £7 = 0.159 - 0.072 = 0.087 mm. Maximum joint clearance

5"""= A™," 4-"= 100.159 - 100.071 =0.088 mm

= £5- e!= 0.159 - 0.071 = 0.088 mm. Maximum tension in connection

4Zh-/)m(n = 100.093 - 100.072 = 0.021 mm

M*,*, = ez-EG= 0.093 - 0.072 = 0.021 mm. Fit tolerance (clearance-tension)

/77У5 = 5^ + 0.088 + 0.021 = 0.109 mm

/7Zh = t + /77) - 0.022 + 0.087 - 0.109 mm.

Nature of the welding arc

Electric arc is one of the types of electrical discharges in gases, in which the passage of electric current through a gas gap under the influence of an electric field. The electric arc used to weld metals is called a welding arc. The arc is part of the electrical welding circuit and experiences a voltage drop across it. When welding with direct current, the electrode connected to the positive pole of the arc power source is called the anode, and to the negative pole is called the cathode. If welding is carried out on alternating current, each of the electrodes is alternately an anode and a cathode.

The space between the electrodes is called the arc area or arc gap. The length of the arc gap is called the arc length. IN normal conditions at low temperatures, gases consist of neutral atoms and molecules and do not have electrical conductivity. The passage of electric current through a gas is possible only if it contains charged particles - electrons and ions. The process of formation of charged gas particles is called ionization, and the gas itself is called ionized. The appearance of charged particles in the arc gap is caused by the emission (emission) of electrons from the surface of the negative electrode (cathode) and the ionization of gases and vapors located in the gap. The arc burning between the electrode and the welding object is a direct arc. Such an arc is usually called a free arc, in contrast to a compressed one, cross section which is forcibly reduced due to the burner nozzle, gas flow, and electromagnetic field. The arc is excited as follows. When there is a short circuit, the electrode and the parts where they touch the surfaces heat up. When the electrodes are opened from the heated surface of the cathode, electrons are emitted - electron emission. The yield of electrons is primarily associated with the thermal effect (thermionic emission) and the presence of a high-intensity electric field at the cathode (field emission). The presence of electron emission from the cathode surface is an indispensable condition for the existence of an arc discharge.

Along the length of the arc gap, the arc is divided into three regions (Fig. 1): cathode, anode and the arc column located between them. The cathode region includes the heated surface of the cathode, called the cathode spot, and the part of the arc gap adjacent to it.

The length of the cathode region is small, but it is characterized by increased tension and the processes of obtaining electrons occurring in it, which are a necessary condition for the existence of an arc discharge. The cathode spot temperature for steel electrodes reaches 2400 - 2700°C. It stands out up to 38% total heat arcs. The main physical process in this area is electron emission and acceleration of electrons. The voltage drop in the cathode region UK is about 12 - 17 V.

The anode region consists of an anode spot on the surface of the anode and part of the arc gap adjacent to it. The current in the anode region is determined by the flow of electrons coming from the arc column. The anode spot is the site of entry and neutralization of free electrons in the anode material. It has approximately the same temperature as the cathode spot, but as a result of electron bombardment, more heat is released on it than on the cathode. The anode region is also characterized by increased tension. The voltage drop in it Uk is about 2 - 11 V. The extent of this region is also small.

The arc column occupies the greatest extent of the arc gap, located between the cathode and anode regions. The main process of formation of charged particles here is gas ionization. This process occurs as a result of the collision of charged (primarily electrons) and neutral gas particles. With sufficient collision energy, electrons are knocked out of gas particles and positive ions are formed. This ionization is called collision ionization. A collision can occur without ionization, then the collision energy is released in the form of heat and goes to increase the temperature of the arc column. The charged particles formed in the arc column move to the electrodes: electrons to the anode, ions to the cathode. Some positive ions reach the cathode spot, while others do not and, adding negatively charged electrons to themselves, become neutral atoms. This process of particle neutralization is called recombination. In the arc column, under all combustion conditions, a stable equilibrium is observed between the processes of ionization and recombination. In general, the arc column has no charge. It is neutral, since in each section of it there are simultaneously equal numbers of oppositely charged particles. The temperature of the arc column reaches 6000 - 8000°C or more. The voltage drop in it Uc varies almost linearly along the length, increasing with increasing length of the column. The voltage drop depends on the composition of the gaseous medium and decreases with the introduction of easily ionized components into it. Such components are alkaline and alkaline earth elements (Ca, Na, K, etc.). The total voltage drop in the arc is Ud = Uk + Ua + Uс. Taking the voltage drop in the arc column in the form of a linear dependence, it can be represented by the formula Uc = Elc, where E is the tension along the length, lc is the length of the column. The values ​​of Uk, Ua, E practically depend only on the material of the electrodes and the composition of the arc gap medium and, if they remain unchanged, remain constant at different conditions welding Due to the small extent of the cathode and anode regions, one can practically consider lc = ld. Then the expression Ud = a + bld is obtained, showing that the arc voltage directly depends on its length, where a = Uk + Ua; b = E.

An indispensable condition for obtaining a high-quality welded joint is stable arc burning (its stability). By this we mean such a mode of its existence in which the arc long time burns at specified values ​​of current and voltage, without interruption or transition to other types of discharges. With a stable burning of the welding arc, its main parameters - strength current and voltage are in a certain interdependence. Therefore, one of the main characteristics of an arc discharge is the dependence of its voltage on the current strength at a constant arc length. A graphical representation of this dependence when operating in static mode (in a state of stable arc burning) is called the static current-voltage characteristic of the arc (Fig. 2).

With increasing arc length, its voltage increases and the static current-voltage characteristic curve rises higher, and with decreasing arc length it drops lower, while qualitatively maintaining its shape. The static characteristic curve can be divided into three regions: falling, hard and rising. In the first region, an increase in current leads to a sharp drop in arc voltage.

This is due to the fact that with increasing current strength, the cross-sectional area of ​​the arc column and its electrical conductivity increase. Arc burning in regimes in this region is characterized by low stability. In the second region, the increase in current strength is not associated with a change in arc voltage. This is explained by the fact that the cross-sectional area of ​​the arc column and active spots changes in proportion to the current strength, and therefore the current density and voltage drop in the arc remain constant.

Arc welding with a rigid static characteristic is widely used in welding technology, especially in manual welding. In the third region, as the current increases, the voltage increases. This is due to the fact that the diameter of the cathode spot becomes equal to the diameter of the electrode and cannot increase further, while the current density in the arc increases and the voltage drops. An arc with increasing static characteristics is widely used in automatic and mechanized submerged arc and shielding gas welding using thin welding wire. When mechanized welding with a consumable electrode, a static current-voltage characteristic of the arc is sometimes used, taken not at a constant length, but at a constant feed speed of the electrode wire (Fig. 3).

As can be seen from the figure, each electrode wire feed speed corresponds to a narrow range of currents with stable arc burning. Too little welding current can lead to short circuit electrode with the product, and too large - to a sharp increase in voltage and its breakage.

Features of an arc on alternating current

When welding with direct current in a steady state, all processes in the arc occur at a certain speed and the arc burning is highly stable.

When the arc is powered by alternating current, the polarity of the electrode and the product, as well as the conditions for the existence of the arc discharge, periodically change. Thus, an alternating current arc of industrial frequency 50 Hz is extinguished and re-excited 100 times per second, or twice for each period. Therefore, the question of the stability of alternating current arc combustion especially arises. First of all, the stability of the combustion of such an arc depends on how easily the arc is re-excited in each half-cycle. This is determined by the course of physical and electrical processes in the arc gap and on the electrodes in the periods of time between each extinction and new ignition of the arc. A decrease in current is accompanied by a corresponding decrease in the temperature in the arc column and the degree of ionization of the arc gap. When the current passes through zero and changes polarity at the beginning and end of each half-cycle, the arc goes out. At the same time, the temperature of the active spots on the anode and cathode also drops. The temperature drop is slightly behind the phase when the current passes through zero, which is due to the thermal inertia of the process. The temperature of the active spot located on the surface of the weld pool drops especially rapidly due to the intensive removal of heat into the mass of the part. At the moment following the extinction of the arc, the polarity of the voltage across the arc gap changes (Fig. 4).

At the same time, the direction of movement of charged particles in the arc gap also changes. In conditions low temperature active spots and the degree of ionization in the arc gap, re-ignition of the arc at the beginning of each half-cycle occurs only at an increased voltage between the electrodes, called the ignition peak or arc re-ignition voltage. The ignition peak is always higher than the arc voltage corresponding to a stable arc combustion mode. In this case, the magnitude of the ignition peak is slightly higher in cases where the cathode spot is located on the base metal. The magnitude of the ignition peak significantly affects the stability of the AC arc. Deionization and cooling of the arc gap increase with increasing arc length, which leads to the need for an additional increase in the ignition peak and leads to a decrease in arc stability. Therefore, attenuation and interruption of the alternating current arc, other things being equal, always occurs at a shorter length than for direct current. If there are vapors of easily ionized elements in the arc gap, the ignition peak decreases and the stability of the AC arc combustion increases.

As the current increases, the physical conditions for arc combustion improve, which also leads to a decrease in the ignition peak and an increase in the stability of the arc discharge. Thus, the magnitude of the ignition peak is an important characteristic of an alternating current arc and has a significant impact on its stability. How worse conditions to restart the arc, the greater the difference between the ignition peak and the arc voltage. The higher the ignition peak, the higher the open circuit voltage of the arc current supply should be. When welding on alternating current with a non-consumable electrode, when its material and the product differ sharply in their thermophysical properties, the straightening effect of the arc is manifested. This is characterized by the flow of a certain direct current component in the alternating current circuit, shifting the voltage and current curves from the horizontal axis in a certain direction (Fig. 5). The presence of a direct current component in the welding circuit negatively affects the quality of the welded joint and the process conditions: the depth of penetration decreases, the arc voltage increases, the temperature of the electrode increases significantly and its consumption increases. Therefore, it is necessary to apply special measures to suppress the action of the constant component.

When welding with a consumable electrode, similar in composition to the base metal, in modes that ensure stable arc burning, the rectifying effect of the arc is insignificant and the current and voltage curves are located almost symmetrically relative to the abscissa axis.

Technological properties of the arc

The technological properties of a welding arc are understood as the totality of its thermal, mechanical and physico-chemical effects on the electrodes, which determine the intensity of melting of the electrode, the nature of its transfer, penetration of the base metal, formation and quality of the weld. The technological properties of the arc also include its spatial stability and elasticity. The technological properties of the arc are interrelated and are determined by the parameters of the welding mode.

Important technological characteristics of the arc are ignition and arc stability. The conditions for ignition and burning of the arc depend on the type of current, polarity, chemical composition electrodes, interelectrode gap and its length. To reliably ensure the ignition process, blow? it is necessary to supply the electrodes with sufficient open circuit voltage from the arc power source, but at the same time safe for the worker. For welding sources, the open circuit voltage does not exceed 80 V on alternating current and 90 V on direct current. Typically, the arc ignition voltage is 1.2 - 2.5 times greater than the arc voltage on alternating current, and 1.2 - 1.4 times on direct current. The arc is ignited by heating the electrodes; arising when they come into contact. At the moment of separation of the electrode from the product, electron emission occurs from the heated cathode. The electron current ionizes the gases and metal vapors of the interelectrode gap, and from this moment electron and ion currents appear in the arc. The arc discharge establishment time is 10-5 – 10-4 s. Continuous burning of the arc will be maintained if the influx of energy into the arc compensates for its losses. Thus, the condition for ignition and stable burning of the arc is the presence of a special power source with electric current.

The second condition is the presence of ionization in the arc gap. The degree of occurrence of this process depends on the chemical composition of the electrodes and the gaseous environment in the arc gap. The degree of ionization is higher in the presence of easily ionized elements in the arc gap. The burning arc can be stretched to a certain length, after which it goes out. The higher the degree of ionization in the arc gap, the longer the arc can be. The maximum length of the arc burning without breaking characterizes its most important technological property - stability. The stability of the arc depends on a number of factors: the temperature of the cathode, its emissivity, the degree of ionization of the medium, arc length, etc.

The technological characteristics of the arc also include spatial stability and elasticity. This is understood as the ability of the arc to maintain a constant spatial position relative to the electrodes in a stable combustion mode and the ability to deflect and move without attenuation under the influence of external factors. Such factors may be magnetic fields and ferromagnetic masses with which the arc can interact. With this interaction, a deviation of the arc from its natural position in space is observed. The deflection of the arc column under the influence of a magnetic field, observed mainly during DC welding, is called magnetic blast (Fig. 6).

Its occurrence is explained by the fact that magnetic field strengths are created in places where the direction of current changes. The arc is a kind of gas insert between the electrodes and, like any conductor, interacts with magnetic fields. In this case, the welding arc column can be considered as a flexible conductor, which, under the influence of a magnetic field, can move like any conductor, deform and elongate. This leads to a deflection of the arc in the direction opposite to the greater tension. When welding with alternating current, due to the fact that the polarity changes with the frequency of the current, this phenomenon is much less pronounced. Arc deflection also occurs when welding near ferromagnetic masses (iron, steel). This is explained by the fact that magnetic field lines pass through ferromagnetic masses, which have good magnetic permeability, much more easily than through air. The arc in this case will deviate towards such masses.

The occurrence of magnetic blast causes lack of penetration and deterioration in the formation of seams. It can be eliminated by changing the location of the current supply to the product or the angle of inclination of the electrode, temporarily placing ballast ferromagnetic masses at the welded joint, which makes it possible to equalize the asymmetry of magnetic fields, as well as replacing direct current with alternating current.

The concept of welding and its essence

Complex structures, as a rule, are obtained as a result of combining individual elements (parts, assemblies, assemblies) with each other. Such associations can be performed using detachable or permanent connections.

In accordance with GOST 2601-74, welding is defined as the process of obtaining permanent connections by establishing interatomic bonds between the parts being welded during their local or general heating or plastic deformation or the combined action of both.

Permanent joints made by welding are called welded joints. Most often, metal parts are connected using welding. However, welded joints are also used for parts made of non-metals - plastics, ceramics or combinations thereof.

To obtain welded joints, the use of any special connecting elements (rivets, overlays, etc.) is not required. The formation of a permanent connection in them is ensured by the manifestation of the internal forces of the system. In this case, bonds are formed between the metal atoms of the parts being connected. Welded joints are characterized by the appearance of a metallic bond caused by the interaction of ions and shared electrons.

To obtain a welded joint, simple contact of the surfaces of the parts to be joined is completely insufficient. Interatomic bonds can be established only when the atoms being connected receive some additional energy necessary to overcome a certain energy barrier existing between them. In this case, the atoms reach a state of equilibrium. the action of tension and repulsion forces. This energy is called activation energy. When welding, it is introduced from the outside by heating (thermal activation) or plastic deformation (mechanical activation).

Bringing the parts to be welded together and applying activation energy - the necessary conditions for the formation of permanent welded joints.

Depending on the type of activation when making connections, two types of welding are distinguished: fusion and pressure. In fusion welding, parts along the edges being joined are melted under the influence of a heat source. The melted surfaces of the edges are covered with molten metal, which, merging into the total volume, forms a liquid weld pool. As the weld pool cools, the liquid metal solidifies and forms a weld. The seam can be formed either only due to the melting of the metal of the edges being welded, or due to them and the additional introduction of a molten additive into the weld pool.

The essence of pressure welding is continuous or intermittent joint plastic deformation of the material along the edges of the parts being welded. Due to plastic deformation and flow of the metal, the establishment of interatomic bonds between the parts being connected is facilitated. To speed up the process, pressure welding with heating is used. In some pressure welding methods, heating can be carried out until the metal of the welded surfaces melts.

Classification of welding types

Currently, there are more than 150 types of welding processes. GOST 19521-74 establishes a classification of welding processes according to basic physical, technical and technological characteristics.

The basis of the physical characteristics of the classification is the form of energy used to produce the welded joint. According to physical characteristics, all types of welding are classified into one of three classes: thermal, thermomechanical and mechanical.

To thermal class include all types of fusion welding carried out using thermal energy - gas, arc, electroslag, electron beam, laser, etc.

To thermomechanical class include all types of welding carried out using thermal energy and pressure - contact, diffusion, gas and arc press, forging, etc.

To the mechanical class include all types of pressure welding carried out using mechanical energy - cold, friction, ultrasonic, explosion, etc.

The technical characteristics of the classification of welding processes include methods of protecting the metal in the welding zone, continuity of the process and the degree of its mechanization (Fig. 7).

Technological classification characteristics are established for each type of welding separately. For example, the type of arc welding can be classified according to the following criteria: type of electrode, nature of protection, level of automation, etc.

Main types of arc welding

The heating source for arc welding methods is welding arc, which is a stable electrical discharge occurring in a gas environment between two electrodes or an electrode and a part. To maintain such a discharge of the required duration, it is necessary to use special arc power sources (APS). To power the arc with alternating current, welding transformers are used; for direct current, welding generators or welding generators are used. welding rectifiers. In Fig. Figure 8 shows a diagram of the electrical circuit of arc welding.

The development of arc welding was due to the discovery of the electric arc in 1802 by Russian physicist V.V. Petrov. For the first time, to connect metal parts using an electric arc burning between a non-consumable carbon electrode and the workpiece being welded, N.N. Benardos in 1882. If necessary, filler material was additionally supplied to the weld pool. In 1888, Russian engineer N.G. Slavyanov improved the process by replacing the non-consumable carbon electrode with a consumable metal electrode. Thus, the unification of the functions of the electrode for the existence of an arc discharge and the filler metal for the formation of the pool was achieved. Proposed by N.N. Benardos and N.G. Slavyanov methods of arc welding with non-consumable and consumable electrodes formed the basis for the development of the most common modern methods arc welding.

Further improvement of arc welding went in two directions: 1) finding means of protecting and processing the molten metal of the weld pool; 2) process automation. According to the nature of the protection of the metal being welded and the weld pool from environment arc welding methods with slag, gas-slag and gas protection can be distinguished. Based on the degree of automation of the process, methods are divided into manual, mechanized and automatic welding. Below are characteristics and descriptions of the main types of arc welding.

Arc welding with coated electrodes(Fig. 9). With this method, the process is performed manually. Welding electrodes can be consumable - steel, copper, aluminum, etc. - and non-consumable - carbon, graphite, tungsten.

The most widely used welding is with steel electrodes that have an electrode coating on the surface. The electrode coating is prepared from a powder mixture of various components and is applied to the surface of the steel rod in the form of a hardening paste. Its purpose is to increase the stability of the arc, carry out metallurgical processing of the weld pool, and improve the quality of welding. A weld is formed by melting the metal of the welded edges and melting the welding electrode rod. In this case, the welder manually carries out two main technological movements: feeding the coated electrode into the welding zone as it melts and moving the arc along the welded seam. Manual arc welding with coated electrodes is one of the most common methods used in the manufacture of welded structures. It is distinguished by its simplicity and versatility, the ability to make connections in various spatial positions and hard-to-reach places. Significant disadvantage it is the low productivity of the process and the dependence of the quality of welding on the qualifications of the welder.

Submerged Arc Welding(Fig. 10). An electric arc burns between a consumable electrode and a workpiece under a layer of welding flux, which completely covers the arc and weld pool from interaction with air. Welding electrode made in the form of wire, rolled into a cassette and automatically fed into the welding zone. The arc can be moved along the edges being welded either manually or using a special drive. In the first case, the process is carried out using semi-automatic welding machines, in the second - automatic welding machines. Submerged arc welding is characterized by high productivity and quality of the resulting joints. Disadvantages of the process include the difficulty of welding parts of small thicknesses, short seams and making seams in main positions other than the bottom ones. Detailed information Read about submerged arc welding in

Gas shielded arc welding(Fig. 11). The electric arc burns in an environment of protective gases specially supplied to the welding zone. In this case, you can use both non-consumable and consumable electrodes, and perform the process manually, mechanized or automatically. When welding with a non-consumable electrode, filler wire is used; when welding with a consumable electrode, no additive is required. Gas shielded welding is widely varied and is used for a wide range of metals and alloys.

Electroslag welding(Fig. 12). The welding process is arcless. Unlike arc welding, the heat generated when the welding current passes through the molten electrically conductive slag (flux) is used to melt the base and filler metals. After the melt solidifies, a weld is formed. Welding is most often performed with the parts being welded in a vertical position with a gap between them. To form the seam, copper slides-crystallizers, cooled by water, are installed on both sides of the gap. Electroslag welding is used to connect parts of large thickness (from 20 to 1000 mm or more).

Welded joints and seams

According to GOST 2601-84, a number of terms and definitions related to welded joints and seams are established.

Welded joint- This is a permanent connection of several parts made by welding. The structural type of the welded joint is determined by the relative position of the welded parts. When fusion welding, the following types of welded joints are distinguished: butt, corner, T, lap and end. An overlap connection with a spot weld, made by arc welding, is also used.

A metal structure made by welding from individual parts is called a welded structure. Part of such a structure is called a welded assembly.

Butt joint It is a welded connection of two parts located in the same plane and with end surfaces adjacent to each other (Fig. 13, a). It is most common in welded structures, as it has a number of advantages over other types of connections. Symbols for butt joints: C1 - C48.

Gusset It is a welded connection of two elements located at an angle to each other and welded at the point of application of their edges (Fig. 13, b). Symbols for corner joints: U1 - U10.

T-joint- this is a connection in which another element is adjacent to the side surface of one element at an angle and welded to its end. As a rule, the angle between the elements is straight (Fig. 13, c). Symbols for T-joints: T1 - T8.

Lap connection is a welded joint in which the elements to be connected are located parallel and partially overlap each other (Fig. 13, d). Legend: H1 - H9.

End connection- this is a connection in which the side surfaces of the elements are adjacent to each other (Fig. 13, e). There are no symbols in the standard yet.

Weld seam is a section of a welded joint formed as a result of crystallization of the molten metal of the weld pool.

Weld pool- this is the part of the weld metal that is in a molten state at the time of welding. The depression formed in the weld pool under the action of the arc is called a crater. The metal of the parts being joined that are being welded is called the base metal. The metal intended to be introduced into the weld pool in addition to the molten base metal is called filler metal. Remelted filler metal introduced into the weld pool or deposited onto the base metal is called weld metal. The alloy formed by remelted base or remelted base and weld metals is called weld metal. Depending on the parameters and form of preparation of the welded edges of the parts, the share of participation of the base and deposited metals in the formation of the weld can vary significantly (Fig. 14):

Depending on the share of participation of the base and filler metals in the formation of the weld, its composition may change. The end surfaces of parts that are subject to heating and melting during welding are called weldable edges. To ensure uniform penetration of the welded edges, depending on the thickness of the base metal and the welding method, they are given the most optimal shape, performing preliminary edge preparation. In Fig. 15 shows the forms of edge preparation used for various types welded joints. The main parameters of the shape of prepared edges and joints assembled for welding are e, R, b, a, c - flanging height, radius of curvature, gap, bevel angle, bluntness of edges.

Beading is used when welding thin-walled parts. For thick-walled parts, edges are cut by bevelling them, i.e. performing a straight or curved inclined bevel of the edge to be welded. Unbeveled part of the edge With is called edge dulling, and the distance b between the edges during assembly - a gap. The acute angle b between the plane of the bevel of the edge and the plane of the end is called the angle of bevel of the edge, the angle a between the beveled edges is the angle of cutting of the edges.

The values ​​for the shape parameters of edge preparation and their assembly are regulated by GOST 5264-80. Depending on the types of welded joints, butt and fillet welds are distinguished. The first type of seams is used when producing butt welded joints. The second type of seams is used in corner, T-joint and lap joints.

TO category:

Marking

Basic concepts about clearances and interference

In any mechanism, no matter how complex it is, it is always possible to identify elementary connections that represent a pair of mating surfaces. These surfaces of the parts that make up the units and assemblies must occupy one or another position relative to each other, which will allow them to either make relative movements or remain completely immobile with a certain connection strength. When assembling two parts that fit into each other, a distinction is made between the outer (female) and inner (male) surfaces. One of the dimensions of the contacting surfaces is called the covering dimension, and the other is the covered dimension (Fig. 1, a).

Rice. 1. Types of surfaces of parts (a); gaps in the connection between the hole and the shaft (

For round bodies, the covering surface is the general name of the hole, and the male surface is the shaft. The corresponding dimensions are called the hole diameter and the shaft diameter.

If the surfaces are formed by two parallel planes each, then the connection is called planar with parallel planes. The nature of the mating of two surfaces is called fit. The fit characterizes greater or lesser freedom of relative movement of the parts being connected or the degree of resistance to their mutual displacement. The fits can be with a gap or with an interference fit.

The gap is the positive difference between the dimensions of the hole and the shaft (the size of the hole is larger than the size of the shaft).

The largest gap is the difference between the largest maximum hole size and the smallest maximum shaft size (Fig. 1, b).

The smallest clearance is the difference between the smallest maximum hole size and the largest maximum shaft size.

Let's look at this with an example. Let the shaft size be 30 Godm and the hole size 30+0’027. Then the largest limiting shaft size will be 30-0.02 = 29.98, and the smallest -30-0.04 = 29.96 mm. Admission to in this case will be determined as follows: 29.98-29.96 = 0.02 mm. The largest limit hole size is 30 + 0.027 = 30.027 mm, the smallest limit size is 30 mm, and the tolerance is 30.027-30.00 = 0.027 mm. In this connection, the diameter of the shaft is smaller than the diameter of the hole and, therefore, there is a gap between the hole and the shaft. Largest gap: 30.027-29.96 = 0.067 mm. Smallest gap: 30-29.98=0.02 mm.

Preference is the negative difference between the diameter of the hole and the diameter of the shaft before assembling the parts, which creates a fixed connection after assembly (the size of the hole is larger than the size of the shaft).

The greatest interference is the difference between the largest maximum shaft size and the smallest maximum hole size (Fig. 20,b).

The minimum interference is the difference between the smallest maximum shaft size and the largest maximum hole size. For example, shaft diameter: 35+o!o5i hole diameter: 35+0’0‘7. Then the largest limiting shaft size will be 35.10 and the smallest 35.05 mm. Tolerance 35.10-35.05 = 0.05 mm. Accordingly, the largest limit hole size is 35.027 mm, the smallest is 35 mm. Tolerance 35.027-35 = 0.027 mm. In this connection the shaft size is larger

hole size and therefore there is interference. The maximum interference is 35.10-35 = 0.10 mm; smallest: 35.05-35.027 = 0.023 mm.

Consequently, the degree of strength or mobility of the connection depends on the amount of interference or clearance, i.e., on the nature of the connection of the parts or their fit.

Theorem 8(sufficient condition for integrability). If the function ¦(x) is continuous in the interval, then it is integrable on this interval, i.e. there is an integral.Definition 6. Let the function ¦(x) be defined in the interval . Let's divide this interval into arbitrary parts with points. In each of the resulting partial intervals, where, we choose an arbitrary point. Let's calculate the value of the function and multiply it by the difference. After this we compose the Riemann sum, (1) (sometimes called the integral sum)Definition. A function for which there is a definite integral on an interval is called integrable on this interval. The question naturally arises: under what conditions is a function defined on is integrable on this interval? Without providing evidence, let us consider these conditions.

Theorem 1.If a function is continuous on an interval, then it is integrable on this interval.

Let us formulate a more general theorem on integrability. Theorem 2. If a function is bounded on and continuous on it everywhere except for a finite number of points, then it is integrable on this interval.

16) Properties of a definite integral

I. The value of the definite integral does not depend on the designation of the integration variable, i.e. , where x, t are any letters.

II. A definite integral with the same limits of integration is equal to zero.

III. When rearranging the limits of integration, the definite integral changes its sign to the opposite.

IV. If the integration interval is divided into a finite number of partial intervals, then the definite integral taken over the interval is equal to the sum of definite integrals taken over all its partial intervals.

V. The constant factor can be taken out of the sign of the definite integral.

VI. A definite integral of an algebraic sum of a finite number of continuous functions is equal to the same algebraic sum of definite integrals of these functions.

17. The main theorem of analysis (Barrow's theorem).

Let and be continuous in . Then it is differentiable at this point and its derivative is equal to .

Proof:

The increment at due to continuity at the point is satisfied. Consider . According to the first statement, we obtain. Aiming, we obtain

18. Newton–Leibniz formula.

Theorem 10 (Newton–Leibniz formula).If is any antiderivative of the function ¦(x), then the formula is valid.

Proof.

Once is also an antiderivative for ¦( x), then we take . This equality is valid for anyone. Let's choose. Then . Now . . Means .

Rule. The value of a definite integral of a continuous function is equal to the difference between the values ​​of any antiderivative for it at the upper and lower limits of integration.

Example 19. Find the integrals , , .

Solution. ; ;

19. Ostrogradsky method.

Sometimes, when integrating a proper rational fraction, a method is used, the essence of which is to isolate the rational part of the antiderivative.

Let it have multiple roots (including complex ones). Let's construct a polynomial so that all its roots are simple, and each root is a root of a polynomial. Then , where the roots are the roots of a polynomial with multiplicities one less. In particular, all simple roots will be roots and will not be roots.

Fair ratio (1) , where and are polynomials with undetermined coefficients, the degrees of which are respectively one less than the degrees of the polynomials and . Undetermined coefficients of polynomials and are calculated using differentiation of the equality (1) . Typically, the Ostrogradsky method is used if the polynomial has several roots of high multiplicity.

Example 18. Calculate.

Solution. We believe. Differentiating this equality, we get

Let us equate the coefficients for the same degrees in both sides of the equality (2).

Hence, .

20. Integration of functions of the form , where is a rational function.

By isolating the whole part from a rational fraction - a polynomial, i.e. and representing the fraction in the form of a sum of simple fractions, we see that the integration of the function leads to the calculation of integrals of the following types: a). , is a polynomial. b). , – constant. V). , are constants and the trinomial has no real roots

21. The integral of the form is reduced by substitution to the form considered in the previous paragraph. Differentiating this identity, we have

Where . To find the undetermined coefficients, we write a system of equations, equating the coefficients at the corresponding powers

Where . Hence,

Let's consider the calculation of the integral. Let us first assume that , then . Because , then . The first of the obtained integrals is tabular. To calculate the integral, Abel's substitution is used. In the general case, a change of variable is made in the integral so that the terms with the first degree simultaneously disappear in the newly obtained trinomials. This is achieved, for example, using the fractional linear substitution , if and , if . As a result, we obtain the integral. Let's imagine it in the form . We apply the substitution to the first of these integrals, and the substitution to the second.

23. Improper integrals.

Definite integral called not your own, if at least one of the following conditions is met.

If the interval is finite and the function is Riemann integrable, then the value of the improper integral coincides with the value of the definite integral.