Size tolerance – is called the difference between the largest and smallest limit sizes or the algebraic difference between the upper and lower deviations /2/.
Tolerance is designated by the letter “T” (from lat. tolerance– tolerance):
TD = D max – Dmin = ES – EI – hole size tolerance;
Td = dmax - dmin = es – ei – shaft size tolerance.
For previously discussed examples 1 - 6 (section 1.1), dimensional tolerances are determined as follows:
1) Td = 24.015 – 24.002 = 0.015 – 0.002 = 0.013 mm;
2) Td = 39.975 – 39.950 = (-0.025) – (-0.050) = 0.025 mm;
3) TD = 32.007 – 31.982 = 0.007 – (-0.018) = 0.025 mm;
4) TD = 12.027 – 12 = 0.027 – 0 = 0.027 mm;
5) Td = 78 – 77.954 = 0 – (- 0.046) = 0.046 mm;
6) Td = 100.5 – 99.5 = 0.5 – (- 0.5) = 1 mm.
Tolerance – the value is always positive . The tolerance characterizes the manufacturing accuracy of the part. The smaller the tolerance, the more difficult it is to process the part, since the requirements for the accuracy of the machine, tools, devices, and worker qualifications increase. Unreasonably large tolerances reduce the reliability and quality of the product.
In some connections various combinations size limits holes and shaft, gaps or interference may occur. The nature of the connection of parts, determined by the size of the resulting gaps or interferences, called landing . The fit characterizes greater or lesser freedom of relative movement of the parts being connected or the degree of resistance to their mutual displacement /1/.
Distinguish three groups of landings:
1) with guaranteed clearance;
2) transitional;
3) with guaranteed interference.
If the hole dimensions are larger than the shaft dimensions, then a gap appears in the connection.
Gap – this is the positive difference between the dimensions of the hole and the shaft /1/:
S = D – d 0 – gap;
Smax = Dmax – dmin – largest gap,
Smin = Dmin – dmax – smallest gap.
If before assembly the dimensions of the shaft are larger than the dimensions of the hole, then interference occurs in the connection. Preload – this is the positive difference between the dimensions of the shaft and the hole /1/:
N = d – D 0 – interference,
Nmax = dmax – Dmin – maximum interference;
Nmin = dmin – Dmax – minimum tension.
Fittings in which there is a possibility of a gap or interference are called transitional.
Fit tolerance – this is the clearance tolerance for fits with guaranteed clearance (defined as the difference between the largest and smallest gaps) or the interference tolerance for fits with guaranteed interference (defined as the difference between the largest and smallest interference). In transitional fits, the fit tolerance is the clearance or interference tolerance /1/.
Fit tolerance designation:
TS = Smax – Smin – fit tolerance for fits with guaranteed clearance.
TN = Nmax – Nmin – fit tolerance for fits with guaranteed interference.
T(S,N)=Smax + Nmax – fit tolerance for transitional fits.
For any group of landings, the landing tolerance can be determined by the formula
Tolerance
- Size- numeric value linear magnitude(diameter, length, etc.) in selected units of measurement.
- Actual size- element size established by measurement.
- Limit dimensions- two maximum permissible sizes of an element, between which the actual size must be (or can be equal to).
- Nominal size- the size relative to which deviations are determined.
- Deviation- algebraic difference between the size (actual or maximum size) and the corresponding nominal size.
- Actual deviation- algebraic difference between the real and the corresponding nominal sizes.
- Maximum deviation- algebraic difference between the limit and the corresponding nominal sizes. There are upper and lower limit deviations.
- Upper deviation ES, es- algebraic difference between the largest limit and the corresponding nominal sizes.
Note. ES- upper deviation of the hole; es- upper shaft deflection.
- Lower deviation EI, ei- algebraic difference between the smallest limit and the corresponding nominal sizes.
Note. EI- lower deviation of the hole; ei- lower shaft deflection.
- Main deviation- one of two maximum deviations (upper or lower), which determines the position of the tolerance field relative to the zero line. In this system of tolerances and landings, the main one is the deviation closest to the zero line.
- Zero line- line corresponding to the nominal size, from which deviations of dimensions are plotted when graphic representation fields of tolerances and landings. If the zero line is located horizontally, then positive deviations are laid up from it, and negative deviations are laid down.
- Tolerance T- the difference between the largest and smallest limit sizes or the algebraic difference between the upper and lower deviations.
Note. Admission is absolute value no sign.
- IT standard approval- any of the tolerances established by this system of tolerances and landings.
- Tolerance field- a field limited by the largest and smallest limit sizes and determined by the tolerance value and its position relative to the nominal size. In a graphical representation, the tolerance field is enclosed between two lines corresponding to the upper and lower deviations relative to the zero line.
- Quality (degree of accuracy)- a set of tolerances considered as corresponding to the same level of accuracy for all nominal sizes.
- Tolerance unit i, I- a multiplier in tolerance formulas, which is a function of the nominal size and serves to determine the numerical value of the tolerance.
Note. i- tolerance unit for nominal dimensions up to 500 mm, I- tolerance unit for nominal dimensions St. 500 mm.
- Shaft- a term conventionally used to designate the external elements of parts, including non-cylindrical elements.
- Hole- a term conventionally used to designate internal elements parts, including non-cylindrical elements.
- Main shaft- a shaft whose upper deviation is zero.
- Main hole- a hole whose lower deviation is zero.
- Landing- the nature of the connection of two parts, determined by the difference in their sizes before assembly.
- Nominal fit size- the nominal size common to the hole and shaft making up the connection.
- Fit tolerance- the sum of the tolerances of the hole and shaft making up the connection.
- Gap- the difference between the dimensions of the hole and the shaft before assembly, if the size of the hole larger size shaft
Linear dimensions, angles, surface quality, material properties, technical characteristics
Linear dimensions, angles, surface quality, material properties, technical specifications are indicated:
To eliminate excessive diversity, it is recommended to bring numerical values into conformity (for example, rounding calculated values) with preferred numbers. Based on the series of preferred numbers, developed rows of normal linear dimensions(GOST 6636-69) . Normal linear dimensions, mm:
| 3,2 | 3,4 | 3,6 | 3,8 | 4,0 | 4,2 | 4,5 | 4,8 | 5,0 | 5,3 |
| 5,6 | 6,0 | 6,3 | 6,7 | 7,1 | 7,5 | 8,0 | 8,5 | 9,0 | 9,5 |
| 10 | 10,5 | 11 | 11,5 | 12 | 13 | 14 | 15 | 16 | 17 |
| 18 | 19 | 20 | 21 | 22 | 24 | 25 | 26 | 28 | 30 |
| 32 | 34/35 | 36 | 38 | 40 | 42 | 45/47 | 48 | 50/52 | 53/55 |
| 56 | 60/62 | 63/65 | 67/70 | 71/72 | 75 | 80 | 85 | 90 | 95 |
| 100 | 105 | 110 | 120 | 125 | 130 | 140 | 150 | 160 | 170 |
| 180 | 190 | 200 | 210 | 220 | 240 | 250 | 260 | 280 | 300 |
| 320 | 340 | 360 | 380 | 400 | 420 | 450 | 480 | 500 | 530 |
| 560 | 600 | 630 | 670 | 710 | 750 | 800 | 850 | 900 | 950 |
Note: Dimensions are given below the slash seats for rolling bearings.
Maximum cone angle deviation
The maximum deviation of the cone angle: 1) if the cone is specified by a taper, it is indicated by symbols and a numerical value of the degree of accuracy; 2) if the cone is specified by an angle, it is indicated by symbols and a numerical value of the degree of accuracy.
Shape tolerance and surface arrangement
The shape tolerance and surface location are indicated in the form symbols(graphically with a numerical tolerance value) or text.
| Access group | Type of admission | Sign |
|---|---|---|
| Shape tolerance | Straightness tolerance | |
| Flatness tolerance | ||
| Roundness tolerance | ||
| Cylindricity tolerance | ||
| Longitudinal profile tolerance | ||
| Location tolerance | Parallel tolerance | |
| Perpendicularity tolerance | ||
| Tilt tolerance | ||
| Alignment tolerance | ||
| Symmetry tolerance | ||
| Positional tolerance | ||
| Axis intersection tolerance | ||
| Total shape tolerance and location |
Radial runout tolerance, axial runout, beats in a given direction |
|
| Total radial runout tolerance, full axial runout |
||
| Shape tolerance of a given profile | ||
| Shape tolerance of a given surface |
Quality
Quality is a measure of accuracy. As quality increases, accuracy decreases (tolerance increases).
Tolerance values for main hole sizes up to 500 mm:
| Size, mm | Tolerance, µm for quality | ||||||||||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| 01 | 0 | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 | 11 | 12 | 13 | 14 | 15 | 16 | 17 | |
| Up to 3 | 0,3 | 0,5 | 0,8 | 1,2 | 2 | 3 | 4 | 6 | 10 | 14 | 25 | 40 | 60 | 100 | 140 | 250 | 400 | 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 | 3 | 5 | 8 | 13 | 19 | 30 | 46 | 74 | 120 | 190 | 300 | 460 | 740 | 1200 | 1900 | 3000 |
| 80-120 | 1 | 1,5 | 2,5 | 4 | 6 | 10 | 15 | 22 | 35 | 54 | 87 | 140 | 220 | 350 | 540 | 870 | 1400 | 2200 | 3500 |
| 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 |
See also
Notes
Literature
- A. I. Yakushev, L. N. Vorontsov, N. M. Fedotov. Interchangeability, standardization and technical measurements. 6th ed., revised. and additional.. - M.: Mashinostroenie, 1986. - 352 p.
Links
- The quality and roughness of the surfaces of holes and shafts in the hole system depending on the accuracy class
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Synonyms:See what “Tolerance” is in other dictionaries:
- (RECOGNITION) the fact of recognition of the company's shares on the stock exchange. Setting the stock price. From this moment the shares begin to be listed on the stock exchange. Dictionary of financial terms. Permission Permission is a user attribute that allows access to all sensitive information... Financial Dictionary
Permissible deviation, tolerance, maximum size, allowance; permission, admission, admission Dictionary of Russian synonyms. admission see admission Dictionary of synonyms of the Russian language. Practical guide. M.: Russian language. Z. E. Alexandrova ... Dictionary of synonyms
- (entry) Admission to the market of a new supplier. The new supplier may be a newly established firm or a firm that has previously operated in other markets. Sometimes it is possible to enter a new market by starting from scratch. However… … Economic dictionary
Tolerance (T) size is the difference between the largest and smallest limit sizes or the absolute value of the algebraic difference between the upper and lower deviations.
Tolerance is always positive. It determines the permissible scattering field of the actual dimensions of suitable parts in a batch, i.e., the specified manufacturing accuracy. As tolerances decrease, product quality generally improves, but production costs increase.
To visually represent the dimensions, maximum deviations and tolerances, as well as the nature of the connections, use a graphical, schematic representation of the tolerance fields located relative to the zero line (Fig. 2.1).
Rice. 2.1 Tolerance fields of the hole and shaft when landing with a gap (hole deviations
are positive, shaft deviations are negative)
Zero line- this is a line corresponding to the nominal size, from which dimensional deviations are plotted when graphically depicting tolerances and fits. When the zero line is horizontal, positive deviations are laid up from it, and negative deviations are laid down.
Tolerance field
- this is a field limited by upper and lower deviations. The tolerance field is determined by the tolerance value, and its position relative to the nominal size is determined main deviation.
Main deviation
(Eo) - one of two deviations (upper or lower), determining the position of the tolerance field relative to the zero line. The main deviation is the closest distance from the boundary of the tolerance field to the zero line.
In finished products, parts in most cases are mated along their form-building surfaces, forming connections. Two or more movably or stationarily connected parts are called mating. The surfaces along which the parts are connected are called mating surfaces. The remaining surfaces are called non-mating (free). In accordance with this, the sizes of mating and non-mating (free) surfaces are distinguished.
In the connection of parts included in one another, there is covering and male surfaces.
The covering surface is called hole, covered - shaft(Fig. 2.1). The terms "bore" and "shaft" do not only refer to cylindrical parts. They can be applied to male and female surfaces of any shape, including non-closed ones, such as flat ones (groove and key).
Hole sizes are indicated in any capital letters, for example: A, B, G, B, C, etc., shafts - lowercase: a, b, g, b, c, etc. Limit sizes are indicated with indices max - the largest maximum size, min - the smallest maximum size, for example: A max, B min, a max, b min. The maximum deviations of the holes indicate: upper - ES, lower - EI, shafts - respectively es And ei.
When solving other problems, for example, calculating dimensional chains, maximum deviations can be designated Es- upper deviation, Ei- lower. So for the hole ES = D max - D; EI = D min - D; for shaft es = d max - d; ei = d min - d; for any size Es = A max - A; Ei = A min - A or Es = a max - a; Ei = a min - a.
The tolerances of the dimensions of the female and male surfaces are called, respectively, the hole tolerance ( TA) and shaft tolerance ( Ta).
By degrees of freedom of mutual movement The parts distinguish the following connections:
- A) motionless one-piece connections, in which one connected part is motionless relative to the other during the entire operation of the mechanism: connecting parts by welding, riveting, glue, connections with guaranteed interference (for example, a bronze rim of a worm wheel with a steel hub); the first three types of these connections are not subject to disassembly, and the fourth can be disassembled only when absolutely necessary;
- b) motionless detachable connections, differing from the previous ones in that they allow movement of one part relative to another when adjusting and disassembling the connection during repair (for example, fastening threaded, splined, keyed, wedge and pin connections);
- V) movable joints, in which one connected part moves relative to the other in certain directions during operation of the mechanism.
Each group includes many types of compounds that have their own design features and its scope. Depending on the operational requirements, connections are assembled with different landings.
Landing is the nature of the connection of parts, determined by the size of the resulting gaps or interference.
The fit characterizes greater or lesser freedom of relative movement or the degree of resistance to mutual displacement of the parts being connected. The type of landing is determined by the size and relative position tolerance fields of the hole and shaft. The nominal size of the hole and shaft making up the connection is general and is called the nominal fit size.
If the hole size is larger than the shaft size, then their difference is called clearance ( S), i.e. S = D - d greater than or equal to 0; if the size of the shaft before assembly is greater than the size of the hole, then their difference is called interference ( N), i.e. N = d - D> 0. In calculations, interference is taken as a negative gap.
When calculating the fits, the maximum and average clearances or interferences are determined. Largest ( S max ), smallest ( S min ) and average gap ( S m ), are equal: S max = D max - d min; S min = D min - d max ; S m = 0.5·( S max + S min). Largest ( N max ), smallest tension ( N min ) and average interference ( N m ) are equal: N max = d max - D min; N min = d min - D max ; N m = 0.5·( N max + N min).
The fits are divided into three groups: with clearance, with interference and transitional fits.
Clearance fit
- a fit that provides a gap in the connection (the tolerance field of the hole is located above the tolerance field of the shaft, Fig. 2.2, a.. Fitments with a gap also include fits in which the lower limit of the hole tolerance field coincides with the upper limit of the shaft tolerance field, i.e. .e. S min = 0.
Interference fit
- a fit that ensures interference in the connection (the tolerance field of the hole is located under the tolerance field of the shaft, Fig. 2.2, c.
Transitional fit - a fit in which it is possible to obtain both a gap and an interference fit (the tolerance fields of the hole and shaft overlap partially or completely, Fig. 2.2, b.
Fig.2.2. Schemes of landing tolerance fields: a - with a gap; b - transitional; in - with interference
Fit tolerance
- the difference between the largest and smallest permissible gaps (gap tolerance T.S. in clearance fits) or the largest and smallest permissible interference (interference tolerance TN in interference fits): T.S. = S max - S min; TN = N max - N min.
IN transitional landings the landing tolerance is equal to the sum largest gap and maximum interference, taken by absolute value TS(N) = S max + N max. For all types of fits, the fit tolerance is equal to the sum of the hole and shaft tolerances, i.e. TS(N) = ТD + Td.
In transitional fits, with the largest maximum shaft size and the smallest maximum hole size, the greatest interference is obtained ( N max ), and with the largest maximum hole size and the smallest maximum shaft size - the largest gap ( S max). Minimum clearance in transitional fit equal to zero (S min = 0). The average clearance or interference is equal to half the difference between the maximum clearance and the maximum interference S m( N m ) = 0.5·( S max - N max). Positive value corresponds to the gap S m, negative - interference N m.
The property of independently manufactured parts (or assemblies) to take their place in the assembly (or machine) without additional processing them during assembly and perform their functions in accordance with technical requirements to the operation of this unit (or machine)
Incomplete or limited interchangeability is determined by the selection or additional processing of parts during assembly
Hole system
A set of fits in which different clearances and interferences are obtained by connecting different shafts to the main hole (a hole whose lower deviation is zero)
Shaft system
A set of fits in which various clearances and interferences are obtained by connecting various holes to the main shaft (a shaft whose upper deviation is zero)
In order to increase the level of interchangeability of products and reduce the range of standard tools, tolerance fields for shafts and holes for preferred applications have been established.
The nature of the connection (fit) is determined by the difference in the sizes of the hole and the shaft
Terms and definitions according to GOST 25346
Size— numerical value of a linear quantity (diameter, length, etc.) in selected units of measurement
Actual size— element size established by measurement
Limit dimensions- two maximum permissible sizes of an element, between which the actual size must be (or can be equal to)
Largest (smallest) limit size— the largest (smallest) allowable element size
Nominal size- the size relative to which deviations are determined
Deviation- algebraic difference between the size (actual or maximum size) and the corresponding nominal size
Actual deviation- algebraic difference between the real and the corresponding nominal sizes
Maximum deviation— algebraic difference between the limit and the corresponding nominal sizes. There are upper and lower limit deviations
Upper deviation ES, es- algebraic difference between the largest limit and the corresponding nominal dimensions
ES— upper deviation of the hole; es— upper shaft deflection
Lower deviation EI, ei— algebraic difference between the smallest limit and the corresponding nominal sizes
EI— lower deviation of the hole; ei— lower shaft deflection
Main deviation- one of two maximum deviations (upper or lower), which determines the position of the tolerance field relative to the zero line. In this system of tolerances and landings, the main deviation is that closest to the zero line
Zero line- a line corresponding to the nominal size, from which dimensional deviations are plotted when graphically depicting tolerance fields and fits. If the zero line is horizontal, then positive deviations are laid up from it, and negative deviations are laid down.
Tolerance T- the difference between the largest and smallest limit sizes or the algebraic difference between the upper and lower deviations
Tolerance is an absolute value without sign
IT standard approval- any of the tolerances established by this system of tolerances and landings. (Hereinafter, the term “tolerance” means “standard tolerance”)
Tolerance field- a field limited by the largest and smallest maximum dimensions and determined by the tolerance value and its position relative to the nominal size. In a graphical representation, the tolerance field is enclosed between two lines corresponding to the upper and lower deviations relative to the zero line
Quality (degree of accuracy)- a set of tolerances considered to correspond to the same level of accuracy for all nominal sizes
Tolerance unit i, I- a multiplier in tolerance formulas, which is a function of the nominal size and serves to determine the numerical value of the tolerance
i— tolerance unit for nominal dimensions up to 500 mm, I— tolerance unit for nominal dimensions St. 500 mm
Shaft- a term conventionally used to designate the external elements of parts, including non-cylindrical elements
Hole- a term conventionally used to designate the internal elements of parts, including non-cylindrical elements
Main shaft- a shaft whose upper deviation is zero
Main hole- a hole whose lower deviation is zero
Maximum (minimum) material limit- a term relating to that of the limiting dimensions to which the largest (smallest) volume of material corresponds, i.e. the largest (smallest) maximum shaft size or the smallest (largest) maximum hole size
Landing- the nature of the connection of two parts, determined by the difference in their sizes before assembly
Nominal fit size- nominal size common to the hole and shaft making up the connection
Fit tolerance- the sum of the tolerances of the hole and shaft making up the connection
Gap- the difference between the dimensions of the hole and the shaft before assembly, if the hole size is larger than the shaft size
Preload- the difference between the dimensions of the shaft and the hole before assembly, if the shaft size is larger than the hole size
The interference can be defined as the negative difference between the dimensions of the hole and the shaft
Clearance fit- a fit in which a gap always forms 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. When shown graphically, the tolerance field of the hole is located above the tolerance field of the shaft
Pressure landing - a landing in which interference is always formed in the connection, i.e. The largest maximum hole size is less than or equal to the smallest maximum shaft size. When shown graphically, the tolerance field of the hole is located below the tolerance field of the shaft
Transitional fit- a fit in which it is possible to obtain both a gap and an interference fit in the connection, depending on the actual dimensions of the hole and shaft. When graphically depicting the tolerance fields of the hole and shaft, they overlap completely or partially
Landings in the hole system
— fits in which the required clearances and interferences are obtained by combining different tolerance fields of the shafts with the tolerance field of the main hole
Fittings in the shaft system
— fits in which the required clearances and interferences are obtained by combining different tolerance fields of the holes with the tolerance field of the main shaft
Normal temperature— the tolerances and maximum deviations established in this standard refer to the dimensions of parts at a temperature of 20 degrees C
Interchangeability of smooth cylindrical joints.
Smooth cylindrical joints are divided into movable and fixed.
Movable joints must create a guaranteed minimum gap between the shaft and the hole, ensuring liquid friction, a given bearing capacity bearing and maintaining the specified type of friction as the gap increases.
Fixed connections must ensure accurate centering of parts and transmission of a given torque or axial force during operation due to guaranteed tension or additional fastening of parts with keys, screws, etc. in case of using transitional landings.
Transitional landings– these are fits that can have both small gaps and slight interference. In transitional landings, fixed connections can only be achieved through the use of additional fastening.
Any type of connection (fit) can be obtained by using a system of tolerances formalized in the form of standards. This tolerance system allows for mass production of parts that ensure good assembleability and interchangeability.
Based on the fact that parts up to 500 mm in size are used in tractor, automotive and agricultural engineering, the standard provides for an appropriate system of tolerances and fits within this interval.
Regardless of the type of connection, it must be made in one of two systems: a hole system or a shaft system.
Qualities
Quality, in another way, accuracy class, (from the French gualite - quality) is a set of tolerances that vary depending on the nominal size so that the level of accuracy for all nominal sizes remains the same.
In the ISO system, for sizes up to 3150 mm, 18 qualifications are established: 01;0;1;..16. The CMEA system provides 19 qualifications for sizes from 1 to 10,000 mm (17 have been added).
Quality is characterized by the size tolerance and the difficulty of obtaining the size regardless of the diameter.
The tolerance is established depending on the nominal size and quality. Qualifications are designated by the letters IT and the serial number 01, 0.1, 2..17. For example: IT 5; IT 9; IT 16. Qualifications applied:
IT 01; IT 0; IT 1 - for the production of gauge blocks;
IT 2; IT 3; IT 4 - for calibers;
IT 5…IT 13 - for the formation of landings;
IT 14…IT 17 - for non-critical non-mating surfaces;
Application of precision grades in connections (fittings)
| Quality | Application |
| 5–6 | critical connections in machine tool and engine building (high-precision gears, spindle and instrument bearings in housings and on shafts) |
| 6-7 | connections such as piston - sleeve, gears on shafts, rolling bearings on the shaft and in the housing |
| 7, 8, 9 | precision connections in tractor construction and critical components of agricultural machinery |
| for reduced accuracy requirements, as well as in connections where calibrated shaft material is used | |
| movable joints of agricultural machines with large gaps and significant fluctuations (rough assembly), as well as covers, ring flanges... | |
| 12-13 | motionless welded joints agricultural machinery (ploughs, seeders, etc.) |
Correctly assigning quality is no less important than calculating the dimensions of the part. The purpose of the quality is related to the accuracy and operational purpose of the mechanism, as well as the nature of the required landings.
When choosing manufacturing accuracy (quality), it is also necessary to take into account economic feasibility. Manufacturing parts to extended tolerances does not require large costs and reduces the likelihood of defects, but this reduces the reliability of the design (there is a large variation in clearances and interference) and, as a consequence, the durability of the machine.
Machines generally fail not due to destruction, but due to loss of performance caused by a decrease in the accuracy of assembly of components and assemblies.
Relationship between accuracy and cost of manufacturing parts
For qualifications from 5 to 17, tolerance values are determined based on the tolerance unit i µm, which characterizes the pattern of tolerance changes depending on the diameter. For sizes up to 500 mm
where d avg in mm, i in µm.
The tolerance is expressed by the formula
Where A- the number of tolerance units, constant for a given quality, independent of the nominal size.
The values of the number of tolerance units for qualifications from 5 to 17 are presented in the table.
Table Values of tolerance units for qualifications IT5…IT17
Quality is characterized by the size of the tolerance. When moving from one quality to another, tolerances increase geometrically with a denominator of 1.6.
Changing tolerances when quality changes
Every five qualifications, starting from IT 5, tolerances increase approximately 10 times.
Main deviations
To create fits with various clearances and interferences, CMEA standards establish 27 main deviations for holes and shafts. They are designated by a capital letter of the Latin alphabet for holes and a lowercase letter for shafts. Let us consider in the diagram the position of the tolerance fields of holes and shafts relative to the zero line.
Basic deviations of holes and shafts in the JSO system.
Deviations from A to H (from a to h) are intended to form tolerance fields in fits with gaps; from Js to N (from js to n) - in transitional landings; from P to Zс (from p to z с) - in interference fits. For holes and shafts designated by the letters Js and js, the tolerance field is strictly symmetrical relative to the zero line, and the maximum deviations are equal in magnitude, but have the opposite sign.
Main deviation is the deviation closest to the zero line. For all tolerance fields located above the zero line, the main one is the lower deviation (EI or ei); for tolerance fields located below the zero line - upper deviation (ES or es). The tolerance fields of the same name for holes and shafts are located strictly symmetrically relative to the zero line and their maximum deviations are the same, but opposite in sign (with the exception of transitional fits).
For landings A to H, EIs are known
For landings from J to ZC ES are known
The main deviation of the hole must be symmetrical to the zero line of the main deviation of the shaft, indicated by the same letter. It does not depend on the quality, i.e. it is a constant value for tolerance fields of the same name.
The upper (if the tolerance field is located above the zero line) or lower (if the tolerance field is located below the zero line) deviation is determined by the value of the main deviation and the tolerance of the selected quality.
Concepts - “hole system” and “shaft system”
The standards establish two equal landing systems: the hole system (SA) and the shaft system (SV).
As can be seen from the figure, the main hole in the hole system has a lower deviation EJ equal to zero. This is distinctive feature hole systems.
Formation of landings in the hole system
In the hole system, the hole is the main part and, regardless of the fit, is processed to the nominal size (with tolerance to the body of the part), and different fits are obtained by changing the maximum dimensions of the shaft.
In the shaft system, the shaft is the main part and, regardless of the fit, is processed to the nominal size (with tolerance to the body of the part), and different fits are obtained by changing the limiting dimensions of the hole.
Formation of landings in the shaft system
As can be seen from the figure, the main shaft in the shaft system has an upper deviation es equal to zero. This is a distinctive feature of the shaft system.
In the ISO system of tolerances and fits, a one-sided maximum location of the tolerance field of the main part relative to the nominal mating size is accepted. Therefore, if the tolerances are specified in the hole system, then the lower hole deviation will always be zero (EI=0), and if the tolerances are specified in the shaft system, then the upper shaft deviation will always be zero (es=0) regardless of the fit.
In other words, fits in the CA hole system are fits in which various clearances and interferences are obtained by connecting different shafts to the main hole. These landings are usually designated by the letter “N”.
Fitments in the CB shaft system are fits in which various clearances and interferences are obtained by connecting various holes to the main shaft. These landings are usually designated by the letter “h”.
Selecting a planting system.
The fit is formed by a combination of the tolerance fields of the hole and the shaft. For economic reasons (reducing the unreasonable variety of fits, systematizing cutting and measuring tools for holes, etc.), it is recommended to use two standardized equal fit systems: the SA hole system and the SV shaft system. These systems are equivalent, but are used in industry to varying degrees. For work, it makes absolutely no difference in which system the fit is assigned (with clearance, interference or transitional fit); its specific value is important. From a technical point of view, mounting holes in the system is preferable. Shaft, i.e. the outer surface is much easier to process and control than inner surface– hole. To make holes, a dimensional cutting tool: countersink, broach, reamer, etc. a certain standard size, a complex measuring instrument, which increases the cost of the part. Therefore, the hole system is mainly used.
The shaft system is generally used in three cases:
1) if the shafts are made of calibrated rod material without additional processing of the seats;
