General information about admission systems and landings
When assembling 2 parts that fit into each other, a distinction is made between covered And covering surfaces whose meaning is clear by name.
The covering surface is called hole, covered - shaft.
For example, the inner cylindrical surface of the bushing and the surface of the keyway - female surfaces, holes; the outer cylindrical surface of the bushing and the surface of the key - male surfaces, shafts.
The difference between the dimensions of the female and male surfaces (between the dimensions of the hole and the shaft) determines nature of the connection parts or landing, i.e. greater or lesser degree of mobility of parts or degree of strength of connections (for fixed connections).
If the hole size is D larger size shaft d, then the positive difference between them, characterizing the degree of mobility (freedom of relative movement) is called gap S:
S = D – d; Dd; S0. (3.8)
If the shaft size d is greater than the hole size D, then the positive difference between them, characterizing the degree of strength of the connection, is called interference N:
N = d – D; d D; N0. (3.9)
The interference (if necessary) can be expressed as a negative clearance and vice versa:
S= -N;N= -S. (3.10)
Nominal size – basic calculated size, rounded to standard. The nominal dimensions of the hole and shaft in the fit are indicated on the drawing and deviations are calculated from it, which are given in the table of standards for tolerances.
Nominal dimensions (when rounded after calculating strength, rigidity, stability...) are selected according to GOST 6636-69 * “Normal linear dimensions”. The use of only standard linear dimensions leads to a reduction in the standard sizes of workpieces, cutting and measuring tools and reduces the cost of production.
According to GOST, a range of sizes is provided from 0.001 to 20000 mm, based on preferred numbers. Four rows of sizes have been established, increasing in geometric progression with significant values =;
;
;
. The rows are designated Ra5, Ra10, Ra20, Ra40. The largest number of sizes is in the last row, the smallest in the first. When choosing denominations, each previous row must be preferred to the next.
Actual size is the size obtained as a result of measurement with a permissible error.
The dimensions between which the actual size of suitable parts in a batch must be (or be equal) are called limiting - respectively highest limit Dmax, dmax and smallest limit Dmin, dmin.
To simplify, in the drawings and tables, instead of the maximum dimensions, the corresponding maximum deviations are set - upper and lower.
Upper deviation(ES, es) – algebraic difference between the largest limit size and the nominal size of the connection.
ES = D max - d n s; (3.11)
es = d max - d n s, (3.12)
where d n s is the nominal diameter of the connection.
Lower deviation(EI, ei) – algebraic difference between the smallest limit size and the nominal size of the connection:
EI = D min - d n s; (3.13)
ei = d min - d n s. (3.14)
Deviations can be positive, negative or zero.
Dimension tolerance T is the difference between the maximum dimensions:
T D = D max - D min ; (3.15)
T d = d max - d min. (3.16)
Tolerance is always a positive value, so it is indicated in documents without a sign.
Substituting into expressions (3.15) and (3.16) the values of the limiting sizes, expressed in terms of deviations and nominal, we determine:
T D = (ES + d n s) - (EI + d n s) = ES – EI; (3.17)
T d = (еs+ d n s) – (ei + d n s) = еs - ei. (3.18)
The tolerance is equal to the difference between the maximum deviations (with its own sign!).
Tolerance characterizes the accuracy of the size. The smaller the tolerance, the higher the accuracy, the smaller the possible range of size changes in the batch and vice versa. The tolerance value affects the performance properties of the connection and the product, as well as the complexity of manufacturing and the cost of the part. The production of parts with a smaller tolerance requires the use of more precise equipment, precise measuring instruments, devices, and appropriate processing modes, which increases the cost of the product.
When assembling parts (for example, a shaft is connected to a bushing) manufactured within tolerance, depending on random combinations of hole and shaft sizes, different fits can be obtained. They are usually divided into fits with clearance (S), interference (N), and transitional (N-S).
Clearance fit called a fit in which clearances are provided in all joints on the assembly. Similarly defined interference fit.
Transitional is called a fit in which some of the connections on the assembly have gaps, and the rest have interference.
Each fit is characterized by maximum (largest, smallest) gaps or interference, the value of which is determined by the maximum dimensions of the parts.
The smallest gap S min in the connection is formed if a shaft with size d max is installed in a hole with size D min:
S min =D min -d max (3.19)
S min = (EI + d n s) – (еs+ d n s) = EI – еs. (3.20)
The largest gap S max in the connection will be obtained if a shaft with the smallest limit size d min is installed in the hole with the largest maximum size D max:
S max =D max -d min (3.21)
S max = (ES + d n s) - (ei + d n s) = ES - ei. (3.22)
Likewise,
N min = d min - D max = ei – ES = - S max ; (3.23)
N max = d max - D min = eS – EI = - S min. (3.24)
The average clearance or interference is:
S c (N c) =
.
(3.25)
The range of variation of the gap or interference determines the tolerance of the clearance, interference or fit (T S, T N).
Fit tolerance(Т S, T N) – the difference between the maximum clearances or interference:
T S = (T N) = S max (N max) - S min (N min). (3.26)
In this expression, instead of S max, S min, we substitute their values according to (3.20), (3.22):
T S = (ES – ei) – (EI – es) = (ES – EI) + (es – ei) = T D + T d. (3.27)
Thus, the fit tolerance is equal to the sum of the hole and shaft tolerances.
Likewise,
T N = N max – N min = T D + T d . (3.28)
Let's imagine that there is a batch of bushings and shafts that need to be assembled. In this batch of bushings with the largest dimensions Dmax there will be very few (for example, 1 out of 100 pieces), similarly, in the batch of shafts with the smallest dimensions dmin there will also be few (for example, 1 out of 100). It is natural to assume that the assembler, choosing parts without selection and assembling connections, is unlikely to simultaneously take parts with dimensions D max and d min (the probability of this event for our example is 1/1001/100 = 1/10 4). The probability of such an event is very low, so there will be practically no connections on the assembly with a gap equal to S max. For the same reasons, there will be practically no connections on the assembly with a gap equal to S max.
In order to determine the values of the largest
and the smallest
(probabilistic) gaps resulting from the assembly, we approach this engineering problem from the standpoint of probability theory.
We assume that the distribution of parts sizes follows the normal law and the manufacturing tolerance is equal to the range of dimensions during manufacturing, i.e. T = 6. We also assume that parts are not selected during assembly (assembly is random).
It is known that the composition (union) of two normal laws also gives a normal law. Consequently, the distribution of gap values (preferences) follows the normal law.
From the probability theory course it is known that the mathematical expectation of the sum random variables equal to the sum of their mathematical expectations. The actual dimensions of the parts are random variables, the mathematical expectations of which will be close to the average sizes in the batch.
The mathematical expectation of the sum of random sizes is the mathematical expectation of the gap:
M S = M D + M -d . (3.29)
S c = D c - d c , (3.30)
where S c , D c , d c are the average values of the gap, hole and shaft dimensions.
The variance of the sum of independent random variables is equal to the sum of their variances. Dispersion D is the standard deviation squared:
D S = DD + D d; (3.31)
.
(3.32)
Then, taking T = 6, we get:
T S =
.
(3.33)
With probability P = 0.9973, the values of the actual gaps will be within the limits:
Then the largest probabilistic gap will be equal to:
,
(3.35)
and the smallest probabilistic gap:
.
(3.36)
Expressions (3.35) and (3.36) are approximate (the conditions for their derivation were previously specified). These values will be defined more precisely in the section “Dimensional chains”.
To simplify the calculations of tolerances and fits, layout diagrams of tolerance fields are used. Constructions on them are carried out relative to the nominal line, designated 0 - 0. The lines of maximum and nominal sizes are laid off from one boundary.
Consequently, lines of sizes larger than the nominal one will be located above the line 0 - 0, and lines of sizes smaller than the nominal one will be located below.
Upwards from the line 0 – 0 on the selected scale show positive deviations, downwards – negative ones. Two lines of maximum dimensions or maximum deviations of the hole and shaft form two tolerance fields, which are designated in the form of rectangles (the scale of the rectangle is arbitrary along the length). The tolerance zone is the zone of size change enclosed between the lines of the upper and lower deviations (or corresponding dimensions). Tolerance field is a broader concept than tolerance. It is characterized not only by the tolerance value, but also by its location relative to the nominal value. Different (by location) tolerance fields can have the same tolerance.
In clearance fits, the hole tolerance field is located above the shaft tolerance field; in interference fits, the hole tolerance field should be located below the shaft tolerance field. In transitional landings, the tolerance fields must overlap.
Tolerances and fits are established for four ranges of nominal sizes:
small - up to 1 mm;
medium - from 1 to 500 mm;
large - from 500 to 3150 mm;
very large - from 3150 to 10,000 mm.
The mid range is the most important.
Rice. 12.
Table 5 Tolerance values for main hole sizes up to 500 mm
Size range, mm | Qualification tolerance, microns | ||||||||||||||||||
01 | 0 | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 | 11 | 12 | 13 | 14 | 15 | 16 | 17 | |
Until 3 | 0,3 | 0,5 | 0,8 | 1,2 | 2 | 3 | 4 | 6 | 10 | 14 | 25 | 40 | 60 | 100 | 140 | 250 | 4С0 | 600 | 1000 |
3-6 | 0,4 | 0,6 | 1 | 1,5 | 2,5 | 4 | 5 | 8 | 12 | 18 | 30 | 48 | 75 | 120 | 180 | 300 | 480 | 750 | 1200 |
6-10 | 0,4 | 0,6 | 1 | 1,5 | 2,5 | 4 | 6 | 9 | 15 | 22 | 36 | 58 | 90 | 150 | 220 | 360 | 580 | 900 | 1500 |
10-18 | 0,5 | 0,8 | 1,2 | 2 | 3 | 5 | 8 | 11 | 18 | 27 | 43 | 70 | 110 | 180 | 270 | 430 | 700 | 1100 | 1800 |
18-30 | 0,6 | 1 | 1,5 | 2,5 | 4 | 6 | 9 | 12 | 21 | 33 | 52 | 84 | 130 | 210 | 330 | 520 | 840 | 1300 | 2100 |
30-50 | 0,6 | 1 | 1,5 | 2,5 | 4 | 7 | 11 | 16 | 25 | 39 | 62 | 100 | 160 | 250 | 390 | 620 | 1000 | 1600 | 2500 |
50-80 | 0,8 | 1,5 | 2,5 | 4 | 6 | 10 | 15 | 22 | 35 | 54 | 87 | 140 | 220 | 350 | 540 | 870 | 1400 | 2200 | 3500 |
80-120 | 1 | 1,5 | 2,5 | 4 | 6 | 10 | 15 | 22 | 35 | 54 | 87 | 160 | 250 | 400 | 630 | 1000 | 1600 | 2500 | 4000 |
120-180 | 1,2 | 2 | 3,5 | 5 | 8 | 12 | 18 | 25 | 40 | 63 | 100 | 160 | 250 | 400 | 630 | 1000 | 1600 | 2500 | 4000 |
180-250 | 2 | 3 | 4,5 | 7 | 10 | 14 | 20 | 29 | 46 | 72 | 115 | 185 | 290 | 460 | 720 | 1150 | 1850 | 2900 | 4600 |
250-315 | 2,5 | 4 | 6 | 8 | 12 | 16 | 23 | 32 | 52 | 81 | 130 | 210 | 320 | 520 | 810 | 1300 | 2100 | 3200 | 5200 |
315-400 | 3 | 5 | 7 | 9 | 13 | 18 | 25 | 36 | 57 | 89 | 140 | 230 | 360 | 570 | 890 | 1400 | 2300 | 3600 | 5700 |
400-500 | 4 | 6 | 8 | 10 | 15 | 20 | 27 | 40 | 63 | 97 | 155 | 250 | 400 | 630 | 970 | 1550 | 2500 | 4000 | 6300 |
Table 6 Numbering of qualifications according to ST 14475 and their approximate correspondence to accuracy classes according to GOST
The main definitions are given in Fig. 12. Tolerance is defined as the difference between the largest and smallest maximum dimensions or as the absolute value of the difference between the upper and lower deviations. Tolerance is a measure of the accuracy of a given nominal size. The smaller the tolerance, the higher the accuracy of manufacturing the size. The CMEA system for all size ranges establishes 19 qualifications, which are numbered 01, 0, 1, 2, ..., 16, 17. Tolerance values for hole sizes up to 500 mm are given in table. 5. The correspondence of qualifications to accuracy classes is shown in table. 6.
In a car, individual parts fit together. A distinction is always made between the outer female part and the inner male one. Conventionally, the first is called a hole, and the second is a shaft, for example, a keyway is a hole, a key is a shaft, etc.
The fit is the nature of the mating of two parts, determined by the size of the gap or interference. If the hole size is larger than the shaft size, a gap occurs in the mating; if the hole size before assembly is smaller than the shaft size, interference occurs. The gap determines the degree of mobility of the mating parts, the tension - the degree of relative immobility. The greater the interference, the higher the torque transmitted by the fit.
There are two landing systems: the hole system and the shaft system. In the first case, the maximum hole dimensions for a certain diameter and a certain quality for all fits remain constant, and different fits are carried out by changing the maximum dimensions of the shafts. In a shaft system, on the contrary, the maximum dimensions of the shaft remain constant, and fits are carried out by changing the maximum dimensions of the holes. The hole system has a predominant distribution.
All fits are usually divided into three groups: with guaranteed clearance, with guaranteed interference, transitional (Fig. 13).
In the first case, the maximum dimensions of the hole and shaft are chosen so that there is a guaranteed gap in the interface. The difference between the largest limit hole size and the smallest limit shaft size determines the largest gap. The difference between the smallest limit hole size and the largest limit shaft size is the smallest gap. The actual gap will be between the specified limits. The clearance is necessary to ensure mobility of the connection and placement of lubricant. The higher the speed and the higher the viscosity of the lubricant, the larger the gap should be.
In interference fits, the maximum dimensions of the shaft and hole are selected so that the mating guaranteed interference, limited by minimum and maximum values.
Transitional fits can give a small gap or interference. Before the parts are manufactured, it is impossible to say what will be paired. This only becomes clear during assembly. The gap should not exceed the maximum gap value, and the interference should not exceed the maximum interference value. Transitional fits are used if it is necessary to ensure precise centering of the hole and shaft.
Rice. 13.
Table 7
The following conventions are used in the CMEA system:
1) Letters of the Latin alphabet are used, holes are determined by capital letters, shafts - by lowercase letters.
2) The hole in the hole system (main hole) is designated by the letter H and numbers - the quality number. For example, H6, H11, etc.
3) The shaft in the hole system is indicated by a fit symbol and numbers - the quality number. For example, g6, d11, etc.
4) The connection between the hole and the shaft in the hole system is indicated fractionally: in the numerator - the tolerance of the hole, in the denominator - the tolerance of the shaft. Recommended fits in the hole system for sizes from 1 to 500 mm are given in table. 7.
Table 8 Turning precision
In table 8 shows the achieved accuracy during turning under serial and mass production conditions.
The surfaces along which parts are connected during assembly are called mating , the rest - unmatched, or free . Of two mating surfaces, the enclosing surface is called hole , and the covered one is shaft (Fig. 7.1).
In this case, in the designation of hole parameters, capital letters of the Latin alphabet are used ( D, E, S), and shafts – lowercase ( d, e,s).
The mating surfaces are characterized by a common size called nominal connection size (D, d).
Valid part size is the size obtained during manufacturing and measurement with an acceptable error.
Limit dimensions are the maximum ( D max And d max) and minimum ( D min And d min ) permissible dimensions, between which the actual size of a suitable part must lie. The difference between the largest and smallest limit sizes is called admission hole size T.D. and shaft Td .
TD (Td) = D max (d max ) – D min (d min ).
The size tolerance determines the specified boundaries (maximum deviations) of the actual size of a suitable part.
Tolerances are depicted as fields limited by the upper and lower size deviations. In this case, the nominal size corresponds to zero line . The deviation closest to the zero line is called main . The main deviation of the holes is indicated in capital letters of the Latin alphabet A, B, C, Z, shafts – lowercase a, b, c, … , z.
Hole size tolerances T.D. and shaft Td can be defined as the algebraic difference between the upper and lower limit deviations:
TD(Td) = ES(es) – EI(ei).
The tolerance depends on the size and required level of manufacturing accuracy of the part, which is determined quality (degree of accuracy).
Quality is a set of tolerances corresponding to the same degree of accuracy.
The standard establishes 20 qualifications in decreasing order of accuracy: 01; 0; 1; 2…18. Qualities are designated by a combination of capital letters IT with the serial number of qualification: IT 01, IT 0, IT 1, …, IT 18. As the quality number increases, the tolerance for the manufacture of the part increases.
The cost of manufacturing parts and the quality of the connection depend on the correct assignment of quality. Below are the recommended areas of application of qualifications:
– from 01 to 5 – for standards, gauge blocks and gauges;
– from 6 to 8 – to form fits for critical parts, widely used in mechanical engineering;
– from 9 to 11 – to create landings of non-critical units operating at low speeds and loads;
– from 12 to 14 – for admission to free sizes;
– from 15 to 18 – for tolerances on workpieces.
On working drawings of parts, tolerances are indicated next to the nominal size. In this case, the letter specifies the main deviation, and the number specifies the quality of accuracy. For example:
25 k6; 25 N7; 30 h8 ; 30 F8 .
7.2. The concept of plantings and planting systems
Landing is the nature of the connection of two parts, determined by the freedom of their relative movement. Depending on the relative position of the tolerance fields, the holes and the landing shaft can be of three types.
1. With guaranteed clearance S given that: D min ≥ d max :
– maximum clearance S max = D max – d min ;
– minimum clearance S min = D min – d max .
Landings with clearance are designed to form movable and fixed detachable connections. Provide ease of assembly and disassembly of units. Fixed connections require additional fastening with screws, dowels, etc.
2. With guaranteed tension N given that: D max < d min :
– maximum tension N max = d max – D min ;
– minimum interference N min = d min – D max .
Interference fits ensure the formation of permanent connections more often without the use of additional fastening.
3. Transitional landings , at which it is possible to obtain both a gap and an interference in the connection:
– maximum clearance S max = D max – d min ;
– maximum tension N max = d max – D min .
Transitional fits are designed for fixed detachable connections. Provides high centering accuracy. They require additional fastening with screws, dowels, etc.
The ESDP provides for fits in the hole system and in the shaft system.
Landings in the hole system main hole N with different shaft tolerances: a, b, c, d, e, f, g, h(landing with clearance); j S , k, m, n(transitional landings); p, r, s, t, u, v, x, y, z(pressure fit).
Fittings in the shaft system are formed by a combination of tolerance fields main shaft h with different hole tolerances: A, B, C, D, E, F, G, H(landing with clearance); J s , K, M, N(transitional landings); P, R, S, T, U, V, X, Y, Z(pressure fit).
The fits are indicated on the assembly drawings next to the nominal mating size in the form of a fraction: the hole tolerance is in the numerator, the shaft tolerance is in the denominator. For example:
30or30
.
It should be noted that in the designation of the fit in the hole system the letter must be present in the numerator N, and in the shaft system the denominator is the letter h. If the designation contains both letters N And h, for example 20 N6/h5 , then in this case preference is given to the hole system.
One of the most important indicators of machine quality, which significantly affects all criteria of performance and reliability, as well as operational parameters, is the accuracy of manufacturing and assembly of associated parts elements.
Interchangeability. The principle of design and manufacture of parts, which ensures the possibility of correct assembly or replacement during repairs of independently manufactured parts and assembly units without additional processing and fitting while maintaining the appropriate quality and reliability of connections, is called interchangeability .
Interchangeability - characteristic feature modern mechanical engineering. Without it, serial or mass production of machines would be impossible, and the replacement of parts during repairs would be significantly complicated. A distinction is made between complete and incomplete interchangeability. Full interchangeability provides for the correct connection of all mating parts that enter assembly operations. It is ensured by high precision manufacturing of parts. At incomplete interchangeability Correct connection is achieved only among parts manufactured with less precision. For assembly in such cases, a group selection of parts is used (selective assembly) or various compensators and other additional technological methods are used.
The interchangeability of machine parts is ensured by a system of tolerances and fits. General provisions and the basis for constructing a system of tolerances and fits for smooth elements of parts (cylindrical or limited by parallel planes) with nominal dimensions up to 3150 mm are established by the standards: GOST 25346 -89 and GOST 25347 - 82.
The concept of sizes and their deviations. Part parameters are quantified using dimensions. When manufacturing machine parts, it is impossible to obtain perfectly accurate dimensions. At the same time, for the normal operation of the machine, ideal precision in the manufacture of its parts is not necessary. In order for the pairing of parts to meet its intended purpose, its dimensions must be between two permissible limit values.
Size- numerical value of a linear quantity (diameter, length, etc.) in selected units. In mechanical engineering, dimensions are specified in millimeters. Dimensions are actual, maximum and nominal (Fig. 7). Actual size - the size of the element, which is determined by measurement. Limit dimensions - two maximum permissible sizes of an element, between which the actual size must lie. Largest size limit - the largest allowable element size. Smallest size limit - the smallest allowable element size. Nominal size - the size relative to which deviations are determined. Nominal dimensions are selected during design based on strength calculations or design considerations and are indicated on the part drawing or assembly drawing.
Deviation- algebraic difference between the size (actual or limit) and the corresponding nominal size.
Actual deviation - algebraic difference between real and nominal size.
Figure 7 - Limit dimensions and tolerance fields for shaft and hole
Maximum deviation- algebraic difference between the limit and the corresponding nominal size.
There are upper and lower deviations.
Upper deviation- algebraic difference between the largest limit and the corresponding nominal size.
Lower deviation- algebraic difference between the smallest limit and the corresponding nominal size.
Zero line- a line that corresponds to the nominal size, from which the deviation of dimensions is plotted when graphically depicting the fields of tolerances and fits. If the zero line is placed horizontally, then positive deviations are laid up from it, and negative deviations are laid down.
The concept of tolerances and qualifications. Tolerance - the difference between the largest and smallest limit sizes or the algebraic difference between the upper and lower deviations (see Fig. 7).
Tolerance field- a field limited by the largest and smallest limit sizes, which is determined by the tolerance and its position relative to the nominal size.
In a graphical representation, the tolerance field is limited by two lines that correspond to the upper and lower deviations relative to the zero line.
Tolerance fields for the dimensions of mating elements of parts are established for three ranges of nominal sizes: small - up to 1 mm; medium - from 1 to 500 mm and large - from 500 to 3150 mm. The most common in mechanical engineering is the medium range of nominal sizes.
Main deviation- one of two maximum deviations (upper or lower), which determines the position of the tolerance field relative to the zero line. In the standard tolerance system, this is the deviation closest to the zero line.
To meet the requirements for various parts and their fits with certain nominal dimensions, the standard provides a range of tolerances and main deviations that characterize the positions of these tolerances relative to the zero line (Fig. 8).
The placement of the tolerance field relative to the zero line, which depends on the nominal size, is indicated by a letter of the Latin alphabet (or in some cases by two letters) - large for holes and small for shafts.
The larger the size tolerance, the lower the requirements for the accuracy of the part, and the easier and cheaper it is to manufacture. However large details more difficult to manufacture compared to smaller ones with the same deviations from the nominal dimensions. Therefore, the tolerance is assigned depending on the dimensions of the part. In addition, parts with the same nominal size can be manufactured more accurately (with a smaller tolerance range) and less accurately (with a larger tolerance range). The standard provides for 20 qualifications.
Quality(degree of accuracy) - a set of tolerances that meet the same level of accuracy for all nominal sizes.
In order of decreasing accuracy, the grades are designated as follows: 01, 0, 1, 2, 3, ..., 18. The grades 01, 0, 1, 2, 3 and 4 are intended for gauge blocks, gauges, etc.; in qualifications 5-13, tolerances are given for the dimensions of the mating surfaces of parts; in grades 14-18 - for the sizes of non-mating surfaces.
Since the tolerance field is determined by the quality, and its position relative to the zero line is indicated by a letter, the maximum deviations of linear dimensions can be indicated on the drawings of parts by symbols of the tolerance fields. In this case, after the number that indicates the size, there is a symbol of the tolerance field, which consists of a letter and a number, which indicates the quality, for example . The tolerance range and maximum deviations are given in the tables of the standard, and are sometimes placed in parentheses after symbol tolerance fields: . A deviation that is equal to zero is not indicated in the designation. Two maximum deviations can be positive (if two maximum sizes are larger than the nominal), negative (two maximum sizes are less than the nominal) and one is positive, the second is negative (one maximum size is larger and the second is less than the nominal).
Fitting of parts and systems for forming landings. During the assembly of two mating parts, a distinction is made between the female and male surfaces. Although not all such surfaces are cylindrical in shape, they are conventionally called the covering surface hole , and the covered one - shaft .
Figure 8 - Layout of tolerance fields
According to the standard, the following terminology is established:
Ø shaft - a term that is conventionally used to designate the external (male) elements of parts, including non-cylindrical elements;
Ø main shaft - a shaft whose upper deviation is zero;
Ø hole - a term that is conventionally used to designate the internal (encompassing) elements of parts, including non-cylindrical elements;
Ø main hole - a hole whose lower deviation is zero.
By the difference between the dimensions of the hole and the shaft, one can judge the freedom of relative movement of the mating parts or the strength of their fixed connection. The nature of the connection of parts is determined by the concept of “fit”.
Landing- the nature of the connection of two parts, which is determined by the difference in their sizes before assembly.
Nominal fit size - nominal size, common to the hole and shaft that form the connection.
Depending on the size of the mating surfaces of the parts, a gap or interference may occur in the connection.
Gap- the difference between the sizes of the hole and the shaft before assembly, if the size of the hole is larger than the size of the shaft.
Preload- 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.
All landings are divided into three groups: landings with clearance, interference landings and transitional landings.
Clearance fit- a fit that always ensures a gap in the connection, that is, the smallest limiting hole size is greater than or equal to the largest maximum size shaft (the tolerance field of the hole is located above the tolerance field of the shaft). Landings with clearance also include landings in which the lower limit of the hole tolerance field coincides with the upper limit of the shaft tolerance field.
Interference fit- a fit in which interference in the connection is always ensured, that is, the largest maximum hole size is less than or equal to the smallest maximum shaft size (the tolerance field of the hole is located under the tolerance field of the shaft).
Transitional fit- a fit in which both clearance and interference in the connection are possible depending on the actual dimensions of the hole and shaft (the tolerance fields of the hole and shaft overlap partially or completely).
There are two systems for forming landings - the hole system and the shaft system.
Landings in the hole system- fits in which the necessary clearances and interferences are formed by connecting different tolerance fields of the shafts with the tolerance field of the main hole.
Fittings in the shaft system- fits in which the necessary clearances and tensions are formed by connecting different tolerance fields of the holes with the tolerance field of the main shaft.
Fitments in the hole system are formed by changing the placement of the tolerance fields of the shafts relative to the tolerance field of the main hole, and fits in the shaft system are formed by changing the placement of the tolerance fields of the holes relative to the tolerance field of the main shaft.
If the technical documentation shows the size of the connected elements of two parts, then the designation of the fit in the hole system includes the nominal size and tolerance fields of each element, starting from the hole, for example. The tolerance field of the main hole is always indicated by a letter (see Fig. 8).
In the shaft system, the main one is the shaft and its tolerance zone is indicated by the letter. The designation of a fit in a shaft system includes the nominal size common to the two connected elements (hole and shaft), followed by a designation of the tolerance fields for each element, for example.
In mechanical engineering, both systems for forming landings are used. However, the hole system has advantages, since it allows the use of a certain range of cutting tools for machining holes.
Selection of fits for connecting machine parts. In order to have a movable connection of parts, it is necessary to designate a fit with a gap. In this case, you should use: shaft tolerance fields from to - for fits in the hole system and hole tolerance fields from to - for fits in the shaft system. To ensure high accuracy and reliable centering of parts with sufficiently small gaps, fits , , are used. In order to have precise rotation with a small number of revolutions, you should use connections of parts with landings , , . The fit is used to connect low-precision parts with free longitudinal movement or circulation in sliding supports. Free rotation of non-critical parts is achieved by using a fit.
Transitional landings are intended to form fixed connections that are subject to periodic disassembly and reassembly. They can be formed by using shaft tolerance fields when making connections in a hole system and hole tolerance fields if the connection is made in a shaft system. Transitional fits provide a fairly high degree of centering of parts. In such connections, to prevent relative movement of parts, it is necessary to provide fixing devices - pins, keys, etc. Transitional fits are more often used, which are used to connect shafts to hubs gear wheels, pulleys and other parts placed on them, as well as the fit for installing the centering pins.
Interference fits are used to form fixed, permanent connections between parts. They are formed using shaft tolerance fields from to (hole system) and tolerance fields from to (shaft system). Due to tension, these fits provide a fixed connection without the use of additional fastening devices. In such cases, the following plantings are preferably used: ; ; ; .
When assigning tolerance fields for the fit of parts, it is allowed to select them with different qualities: for holes that are more difficult to machine, take a larger tolerance (higher quality) than the shaft tolerance (the difference should not be greater than two qualities). Sometimes non-systemic fits are used, which are formed by a combination of tolerance fields of the shaft and hole, taken from various systems for forming fits, for example Ø.
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