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In this guide to Tolerance for CNC machining, you'll learn how to create efficient, manufacturable metal and plastic parts for prototyping and low-volume production.
1. What is meant by tolerance?
A: The amount of variation allowed in part size and geometry is called tolerance.
2. What are dimensions?
A: A number that represents a length value in specific units.
3. What are the basic dimensions?
A: Make the design a given size.
4. What are the actual dimensions?
A: It is the size obtained by measurement.
5. What is a limit size?
A: refers to the two limits of allowable dimensional change.
6. What is called the maximum entity state (MMC) and maximum entity size?
A: The maximum solid state is the state in which the hole or shaft has the greatest amount of material within the dimensional tolerances. The size in this state, called the maximum solid size, is a collective term for the minimum limiting size of the hole and the maximum limiting size of the shaft.
7. What is the minimum entity state (LMC) and minimum entity size?
A: The minimum solid state is the state when the hole or shaft has the least material within the dimensional tolerances. The size in this state, called the minimum solid size, is a collective term for the maximum limiting size of the hole and the minimum limiting size of the shaft.
8. What is meant by operational size?
A: The maximum ideal shaft size for the full length of the mating surface, which is connected to the actual hole, is called the operating size of the hole. The size of the smallest ideal bore that is connected to the actual shaft outside the shaft is called the operational size of the shaft.
9. What is meant by dimensional deviation?
A: is the algebraic difference between a certain size and its basic size.
10. What is meant by dimensional tolerance?
A: refers to the amount of dimensional change allowed.
11. What is the zero line?
A: In the Tolerance and Fit Diagram (Tolerance Belt Diagram), a reference line, or zero deviation line, is used to determine deviation.
12. What is a tolerance zone?
A: In a tolerance band diagram, an area defined by two straight lines representing the upper and lower deviation.
13. What is a fundamental deviation?
A: It is used to determine the upper or lower deviation of the tolerance zone in relation to the zero line, generally the deviation near the zero line. When the tolerance zone is above the zero line, its basic deviation is the lower deviation; when it is below the zero line, its basic deviation is the upper deviation.
14. What are standard tolerances?
A: Any tolerance specified in the national standard for determining the size of a tolerance zone.
15. What does it mean to cooperate?
A: It refers to the relationship between bores and shaft tolerance zones of the same basic dimensions, combined with each other.
16. What is the base-hole system?
A: It is a system in which the basic deviation is the tolerance zone of a certain hole and the tolerance zone of an axis with different basic deviations form a fit.
17. What is the base-axis system?
A: It is a system in which the tolerance zone of the basic deviation is a certain axis, and the tolerance zone of the hole with different basic deviation forms various fits.
18. What is meant by fit tolerance?
A: It is the variation of the permissible gap, which is equal to the absolute value of the algebraic difference between the maximum gap and the minimum gap, and also equal to the sum of the matching hole tolerance zones and shaft tolerance zones.
19. What is a gap fit?
A: The tolerance band of the hole is completely above the tolerance band of the shaft, i.e. a fit with clearance (including a fit where the minimum clearance is equal to zero).
20. What is a surplus fit?
A: The tolerance band of the bore is completely below the tolerance band of the shaft, i.e. it has an overfit (including a fit where the minimum overfit equals zero).
21. What is meant by transition fit?
A: In a hole and shaft fit, the tolerance zones of the holes and shafts overlap each other, and any pair of holes and shafts may have a clearance or an overfilled fit.
22. What are the preferred fit characteristics for a base hole fit of H11/c11 or a base hole fit of C11/h11 for a base axis?
A: Large gaps for very loose, slow turning dynamic fits; exposed components requiring large tolerances and large gaps; very loose fits requiring easy assembly. Equivalent to the old national standard D6/dd6.
23. What are the preferred fit characteristics for a base hole fit of H9/d9 or a base hole fit of D9/h9 for a base axis?
A: Free-rotating fits with large clearances are used when accuracy is not a major requirement, or when there are large temperature variations, high speed or large journal pressures. Equivalent to the old national standard D4/de4.
24. What are the preferential fit characteristics for a base hole fit of H8/f7 or a base hole fit of F8/h7 for a base axis?
A: A rotating fit with little clearance for precise rotation at medium speed and medium journal pressure; also used for easier assembly of medium positioning fits. Equivalent to the old national standard D/dc.
25. What are the preferred fit characteristics for a base hole fit of H7/g6 or a base hole fit of G7/h6 for a base axis?
A: A sliding fit with very little clearance is used when free rotation is not desired, but when free movement and sliding are possible and precision positioning is required, and also for positioning fits that require definite positioning. Equivalent to the old national standard D/db.
26. What are the preferred fit characteristics when the base hole fit is H7/h6; H8/h7; H9/h9; H11/h11 or when the base hole fit is H7/h6; H8/h7; H9/h9; H11/h11?
A: All are gap positioning fits, the parts can be freely assembled and disassembled, while the work is generally relatively stationary. The clearance at the maximum solid condition is zero and the clearance at the minimum solid condition is determined by the tolerance class.H7/h6 is equivalent to the old national standard D/d; H8/h7 is equivalent to the old national standard D3/d3; H9/h9 is equivalent to the old national standard D4/d4; H11/h11 is equivalent to the old national standard D6/d6.
27. What are the preferred fit characteristics for a base hole fit of H7/h6 or a base hole fit of K7/h6 for a base axis?
A: Transition fit, for precision positioning. Equivalent to the old national standard D/gc.
28. What are the preferred fit characteristics for a base hole fit of H7/n6 or a base hole fit of N7/h6 for a base axis?
A: A transition fit that allows for more precise positioning with a larger excess. Equivalent to the old national standard D/ga.
29. What are the preferred fit characteristics for a base hole fit of H7/p6 or a base hole fit of P7/h6 for a base axis?
A: Overload positioning fit, i.e. small overload fit, is used when positioning accuracy is particularly important, can achieve the rigidity and neutral requirements of the parts with the best positioning accuracy, while there are no special requirements for the bore with pressure, and do not rely on the tightness of the fit to transmit frictional load. Equivalent to the old national standard D/ga ~ D/jf, where H7 is less than or equal to 3mm for transition fit.
30. What are the preferred fit characteristics when the base hole fit is H7/s6 or the axial base hole fit S7/h6?
A: Medium press-in fit, for general steel parts; or cold shrink fit for thin-walled parts, for cast iron parts to get the tightest fit, equivalent to the old national standard D/je.
31. What are the preferred fit characteristics for a base hole fit of H7/u6 or a base hole fit of U7/h6 for a base axis?
A: Press-in fit, suitable for parts that can be pressed in with a large force or cold shrinkage fit that is not suitable to withstand large press-in forces.
32. What are the fit characteristics when the fundamental deviation of the axis is a; b?
A: It is a gap fit, which allows for a particularly large gap and is rarely applied.
33. What are the fit characteristics when the fundamental deviation of the axis is c?
A: It is a gap fit, which can get a large gap, and is generally suitable for slow, loose moving fits. For use in poor working conditions, when force deformation occurs, or when a large clearance must be guaranteed on the face for ease of assembly. The recommended fit is H11/c11, and its more advanced fits, such as H8/c7, are suitable for tightly moving fits with one shaft operating at high temperatures, such as internal combustion engine exhaust valves and conduits.
34. What are the fit characteristics when the fundamental deviation of the axis is d?
A: It is a clearance fit, which is generally used for IT7～IT11 level, and is used for loose rotation fit, such as seal cover, pulley, idle belt wheel, etc. and shaft fit. They are suitable for large diameter plain bearing mates such as turbines, ball mills, roll forming and some sliding bearings in heavy bending machines and other heavy machinery.
35. What are the fit characteristics when the fundamental deviation of the axis is e?
A: It is a clearance fit, mostly used in IT7～IT9 level, usually applied to the support with obvious clearance and easy to rotate, such as large span, multi-point support, etc. High-grade e axis is suitable for large, high speed, heavy load support, such as worm gear generator, large electric motor, internal combustion engine, concave wheel shaft and rocker arm support, etc.
36. What are the fit characteristics when the fundamental deviation of the axis is f?
A: Gap fit, mostly used for general rotation fit of IT6～IT8 level. It is widely used for the fit of shaft and sliding bearings of common lubricant (grease) lubrication, such as gearboxes, small motors, pumps, etc., when the temperature is not affected much.
37. What are the fit characteristics when the fundamental deviation of the axis is g?
A: It is a clearance fit, the clearance is very small, the manufacturing cost is high, except for the very light load precision device, it is not recommended for rotating fit. Mostly used in IT5～IT7 level, most suitable for precision sliding fit without rotation, also used for pin and other positioning fit, such as precision connecting rod bearing, piston, slide valve and connecting rod pin, etc.
38. What are the fit characteristics when the fundamental deviation of the axis is h?
A: It is a gap fit, mostly used in IT4～IT11 level. Widely used for parts without relative rotation, as a general positioning fit, and also for precision sliding fits if there is no effect of temperature deformation.
39. What are the fit characteristics when the fundamental deviation of the axis is JS?
A: Transition fit, full symmetry deviation (+IT/2). Fits with a slight clearance on average, mostly used in IT 4-7 classes, require less clearance than the h-axis and allow for slightly overloaded positioning fits (e.g. couplings), which can be assembled by hand or with a wooden hammer.
40. What are the fit characteristics when the fundamental deviation of the axis is k?
A: It is a transitional fit, with an average fit without a gap, for IT4-IT7 levels. It is recommended for positioning fits that are slightly overloaded, and for positioning fits that are inverted to eliminate vibration. Usually assembled with a wooden hammer.
41. What are the fit characteristics when the fundamental deviation of the axis is m?
A: Transitional fits, on average, with small transitional fits. Suitable for IT4I-T7 class, assembled by hammer or press, usually recommended for tight assembly fit, and overfit for H6/n5 fit.
42. What are the fit characteristics when the fundamental deviation of the axis is n?
A: It is a transitional fit, the average overload is slightly larger than the m-axis and rarely gets clearance, it is suitable for IT4-IT7 class, it is assembled by hammer or press, it is usually recommended for tight component fit, H6/n5 fit is overload fit.
43. What are the fit characteristics when the fundamental deviation of the axis is p?
A: It is an overfit, with H6 or H7 it is an overfit, with H8 hole it is a transition fit. For non-ferrous parts, it is a lighter press-in fit and easy to remove when needed. Press-in fits are standard for steel, cast iron or copper and steel assembly.
44. What are the fit characteristics when the fundamental deviation of the axis is r?
A: It is an overfilled fit, medium fit for iron parts, light fit for non-iron parts, can be disassembled when needed. For H8 holes, the diameter is overfilled when 100mm or more, and the diameter is transitional.
45. What are the fit characteristics when the fundamental deviation of the axis is s?
A: Overfill fit for permanent and semi-permanent assembly of steel and iron parts. Considerable bonding power can be produced. When elastic materials, such as light alloys, are used, the fit properties are comparable to the P-axis of iron parts. For example, sleeve ring pressed on the shaft, valve seat, etc. For larger sizes, thermal expansion or cold shrinkage is required to avoid damage to the mating surface.
46. What is the fit characteristic when the fundamental deviation of the axis is t; u; v; x; y; z?
A: is a surplus to match, the amount of surplus in turn increases, generally not recommended.
47. Under what circumstances is the base axis system used?
A: Shafts made directly from cold-drawn steel with a certain tolerance class (typically 8 to 11), which is manufactured from tolerance belts of the reference shaft and no longer machined. At this point, different hole tolerance zone positions can be selected to form a variety of different fit requirements. In agricultural and textile machinery, this is the case more often.
Processing size less than 1mm precision shaft is much more difficult than processing the same level of hole, so in the instrumentation manufacturing, watch production, radio and electronic industries, usually using the light-rolled forming of fine steel wire directly to do the shaft, when the choice of base shaft system with better than the base hole system economic efficiency.
From the structural point of view, the circumferential shaft fits several holes in different parts, and each has different fit requirements, this should consider the use of the base shaft system fit.
48. How does it work with standard parts?
A: If it fits with standard parts, the standard parts should be used as the reference parts to determine the fit system. For example, in the rolling bearing support structure, rolling bearing outer ring and housing hole should be used to fit the base shaft system, bearing inner ring and journals should be used to fit the base hole system, housing hole according to J7 manufacturing, journals according to k6 manufacturing.
49. What is the range of tolerance grades to be used for the grinding process?
A: IT1 to IT5 should be taken.
50. What range of tolerance classes should be used for diffraction grinding methods?
A: IT4 to IT7 should be taken.
51. Diamond turning methods, what is the range of tolerance levels that should be taken?
A: IT5 to IT7 should be taken.
52. Diamond boring methods, what is the range of tolerance grades to be taken?
A: IT5 to IT7 should be taken.
53. What range of tolerance classes should be used for round grinding methods?
A: IT5 to IT8 should be taken.
54. What range of tolerance classes should be used for flat grinding methods?
A: IT5 to IT8 should be taken.
55. What range of tolerance classes should be used for broaching methods?
A: IT5 to IT8 should be taken.
56. What is the range of tolerance classes to be taken for the method of finishing turning and boring?
A: IT7 to IT9 should be taken.
57. What is the range of tolerance classes to be used for the method of reaming?
A: IT6 to IT10 should be taken.
58. What is the range of tolerance classes to be used for milling methods?
A: IT8 to IT11 should be taken.
59. What range of tolerance classes should be used for planing and inserting methods?
A: IT10 to IT11 should be taken.
60. What range of tolerance classes should be used for rolling, extrusion methods?
A: IT10 to IT11 should be taken.
61. What range of tolerance classes should be used for roughing methods?
A: IT10 to IT12 should be taken.
62. What range of tolerance classes should be used for rough boring methods?
A: IT10 to IT12 should be taken.
63. What range of tolerance classes should be used for drilling and processing methods?
A: IT10 to IT13 should be taken.
64. What is the range of tolerance grades to be used for stamping methods?
A: IT10 to IT14 should be taken.
65. What range of tolerance classes should be taken for sand casting methods?
A: IT14～IT15 should be taken.
66. What is the range of tolerance classes to be taken for metal-type casting methods?
A: IT14～IT15 should be taken.
67. What is the range of tolerance classes to be used for forging methods?
A: IT15～IT16 should be taken.
68. What is the range of tolerance classes to be used for air cutting methods?
A: IT15 to IT18 should be taken.
69. What are the methods for determining fundamental deviations?
A: There are three methods for determining basic deviations: experimental, computational and analogical.
70. What is the test method?
A: The test method is the application of the test method to determine the type of fit that satisfies the working performance of the product, mainly used in the aerospace, aviation, national defense, nuclear industry and railway transportation industry in some key institutions, important and critical fit that has a large impact on the product performance but lack of experience. The method is more reliable. The disadvantages are the need for experimentation, high cost, long cycle times and low application.
71. What is a method of calculation?
A: The calculation method is to determine the type of fit through theoretical calculations based on the requirements of the use. It has the advantage of being well grounded in theory and less costly than the test method, but because the theoretical calculations do not allow for the full range of practical factors in the working environment of the machine and equipment, the design is not as accurate as that determined by the test method.
For example, when using the calculation method to determine the type of fit of slide bearing clearance, according to the theory of liquid lubrication can calculate the allowable minimum clearance, according to which from the standard to choose the appropriate type of fit; when using the calculation method to determine the type of fit that completely rely on the overload transfer load, according to the size of the load to be transferred, according to the elastic and plastic deformation theory, can calculate the required minimum overload, according to which to choose the appropriate type of fit overload, and at the same time to check whether the material strength of the parts can withstand the maximum overload generated by the type of fit.
Theoretical calculations can only be approximate due to the many factors that affect the fit gap, the excess.
72. What is an analogy?
A: The analogy method is to determine the fit by taking as reference the proven production fit in a machine or mechanism of the same type as the design task, and by combining the actual conditions of use and application requirements of the designed product. The method is most widely used, but requires the designer to have sufficient references and considerable experience. The following factors should be taken into account when determining the fit by analogy.
Force size. When the force is larger, the tendency to choose a tight fit, that is, the amount of excess should be appropriately increased with the amount of excess, reduce the amount of gap with the gap, choose to get the probability of excess of the transition fit.
Dismantling conditions and structural features. For fits that are frequently disassembled, the fit should be looser compared to the same fit for tasks that are not frequently disassembled. Fittings that are difficult to assemble should also be slightly looser.
Combine length and shape position errors. The longer the length of the fit, the tighter the actual formation of the fit compared to a fit with a shorter length of the bond, due to the presence of the form-position error. Therefore, it is advisable to choose the appropriate loose fit.
Material, Temperature. When the material of the phase fittings is different (large difference in the linear expansion coefficient) and the difference between the operating temperature and the standard temperature + 20 ℃, the effect of thermal deformation should be considered. Effects of assembly deformation.
73. When the tolerance level is 5, where should it be used?
A: Mainly used in the case of small tolerance, shape and position tolerance requirements, stable nature of the cooperation, generally in machine tools, engines, instruments and other important parts of the application. For example, with the D class rolling bearing with the hole of the housing; with the E class rolling bearing with the machine spindle, machine tailstock and sleeve, precision machinery and high-speed machinery in the journals, precision screw diameter, etc.
74. When the tolerance level is 6, where should it be used?
A: the nature of the fit can achieve a high degree of uniformity, such as with the E class rolling bearing with the hole, journal; and gear, worm gear, coupling, pulley, cam and other connected shaft diameter, machine screw shaft diameter; rocker arm drilling column; machine tool fixture in the outside diameter of the guide size; 6 level of precision gear reference hole, 7, 8 level gear reference shaft.
75. When the tolerance level is 7, what are the applications?
A: Class 7 is slightly less accurate than class 6, and the application conditions are basically similar to class 6, which is more common in general mechanical engineering. Bores such as couplings, pulleys, cams, etc.; machine chuck seat bores, fixed drill sleeve in fixture, replaceable drill sleeve; 7, 8 gear reference bores, 9, 10 gear reference shaft.
76. When the tolerance level is 8, what are the applications?
A: Medium accuracy in machine building. Such as bearing seat bushings along the width direction dimensions, 9 ~ 12 gear reference hole; 11 ~ 12 gear reference shaft.
77. When the tolerance class is 9 to 10, what are the applications?
A: Mainly used in machinery manufacturing for shaft sleeve outer diameter and holes; maneuvers and shafts; empty shaft pulleys and shafts; single bonds and splines.
78. When the tolerance class is 11 to 12, what are the applications?
A: The accuracy of the fit is very low, and there may be a large gap after assembly, which is suitable for applications where there are basically no fit requirements. e.g. flanges and stops on machine tools; slip and slip gears; inter-process dimensions; pressed mating parts; wrench hole and wrench seat connections in machine building
79.How is the gap fit chosen in the actual design?
↑↑Hinges for crane hooks, flanges with tongue and groove, exhaust valves and ducts for internal combustion engines↑↑
↑↑Matching of pulley and shaft for internal combustion engine spindle↑↑
↑↑Fitting of gear sleeve and shaft, drilling sleeve and bushing↑↑↑
80. How are transition fits chosen in the actual design?
↑↑Matching of the top sleeve of the tailstock of the lathe with the belt wheel and the shaft↑↑
↑↑Matching of rigid coupling joint with worm wheel bronze rim and spoke↑↑
81. How is the surplus fit chosen in the actual design?
A: See figure below.
82. How are the linear dimensional tolerances marked on the part drawings?
A: See figure below.
83. How are the linear dimensional tolerances marked on the assembly drawings?
A: See figure below.
84. How are the linear dimensional tolerances of standard parts marked?
A: See figure below.
85. What are the requirements for linear dimensional tolerance marking?
A: The tolerance code is the same height as the basic dimensional figure. When using the limit deviation to mark the linear dimensional tolerance, the upper and lower deviation numbers are one smaller than the basic dimensional number, and the upper and lower deviation decimal places must be aligned and marked with a positive and negative sign. One of the deviations is zero and can be marked with “0” and aligned with the other deviation by one digit. The lower deviation bottom line is noted on the same bottom line as the base size. When the upper and lower deviations are equal, the deviation is written only once, and a “+/-” sign is written between the deviation and the basic size, with the same font size for both.
86. What is a conical fit?
A: The interrelationship between the same inner and outer cone diameters of the basic cone, as a result of different combinations. The mating characteristics of the cone fits are the formation of gaps or overhangs by the axial positions specified by the interlocking inner and outer cones.
The gap or excess acts in the direction perpendicular to the surface of the cone, but is given and measured in the direction perpendicular to the axis of the cone; for cones with a taper less than or equal to 1:3, the difference between the value given perpendicular to the surface of the cone and perpendicular to the axis of the cone is negligible.
According to the different methods of determining the axial position of the combined inner and outer cone, the cone fit is divided into two types of structural cone fit and displacement cone fit.
87. What is a structural cone fit?
A: A fit obtained by determining the relative axial position of the inner and outer cones by the structure itself or by its dimensions.
88. What is a displacement cone fit?
A: Specify the size of the axial displacement or the axial force that produces the axial displacement to determine the fit obtained by the relative axial position of the inner and outer cones.
89. Which three elements make up the standard tolerance series?
A: Subdivided by tolerance class, tolerance unit and basic dimensions.
90. What are general tolerances?
A: Tolerances that can be achieved by the general machining capacity of the machine tool equipment under normal shop floor process conditions.
91. What are the general tolerances for linear dimensions specified in GB/T1804-1992?
A: A total of four tolerance grades of F, M, C and V are specified, with the letters F for precision grade, M for medium grade, C for rough grade and V for coarsest grade. Tolerance classes f, m, c and v are equivalent to IT12, IT14, lt16 and IT17, respectively.
92. What is a numerical table of limit deviations from the general tolerance of linear dimensions?
A: See figure below.
93. What is a table of values for the limit deviation of the chamfered radius and height?
A: See figure below.
94. What should be taken into account when working with gaps?
A: The reference hole H (or reference shaft h) forms a gap fit with the shaft a to h (or hole A to H) of the corresponding tolerance class, of which there are 11.
The H/a (or A/h) composition has the largest gap and the H/h fit gap is the smallest.
H/a (A/h), H/b (B/h), H/c (C/h) fits, which have large gaps and are not commonly used. Generally used in poor working conditions, requiring flexible action on the machinery, or for the occasion when the force deformation is large and the shaft needs to work at high temperatures to ensure a large gap.
H/d (D/h), H/e (E/h) fits, both of which have large clearance, are used for supports that do not require a high degree of ease of rotation. H/d (D/h) is suitable for looser drive fits, such as seal covers, pulleys and idle pulleys, and shaft fits. It is also suitable for the fit of large diameter plain bearings, such as ball mills, rolling mills and other heavy machinery plain bearings, suitable for IT7 to IT11 class. For example, the fit of the pulley to the shaft.
H/f (F/h) fit, the clearance of this fit is moderate, mostly used for the general transmission fit of IT7 to IT9, such as the gearbox, small motor, pump shaft and sliding support of the fit.
H/g (G/h) fit, this fit has a small clearance, except for the very light load precision mechanism, generally do not need to do the rotating fit, mostly used in IT5-IT7 class, suitable for the precision fit of reciprocating oscillation and sliding. For example, the fit between drill sleeve and bushing.
H/h fit, the minimum clearance of this fit is zero, used in IT4 to IT11 level, suitable for the positioning fit without relative rotation, but with centering and guidance requirements, if no temperature, deformation effect, also used for sliding fit, recommended fit H6/h5, H7/h6, H8/h7, H9/h9 and H11/h11.
95. What should be taken into account when making the transition?
A: The reference hole H and the basic deviation code j to n of the corresponding tolerance grade axis form a transition fit (n and high-precision hole form an overfit fit).
H/j, H/js fits, these two transitional fits have more chances to get clearance, mostly used in IT4 to IT7 class, suitable for positioning fits that require smaller clearance than h and allow a slight overload, such as coupling joints, gear rings and steel hubs, rolling bearings and housing fits, etc.
H/k fit, the average gap obtained by this fit is close to zero, better centering, the parts are subjected to less contact stress after assembly, can be disassembled, suitable for IT4 ~ IT7 level, such as rigid coupling fit.
H/m, H/n fits, these two fits have more chances of getting a surplus, good centering, tighter assembly, suitable for IT4 to IT7.
96. What should be noted in the case of surplus matching?
A: The reference hole H and the basic deviation code p to zc of the axis of the corresponding tolerance grade form a superfluous fit (p and r form a transition fit with the lower precision hole H).
H/p, H/r fits, which are overfilled fits at high tolerance levels, can be assembled by hammering or presses and should only be disassembled during overhaul. It is mainly used for positioning fits with high centering accuracy, sufficient rigidity of parts and impact load, and is mostly used in IT6 to IT8 levels.
H/s, H/t fits, these two fits belong to the medium surplus fit, most use IT6, IT7 level. For permanent or semi-permanent bonding of steel parts. No auxiliary parts are needed, and the bonding force generated by the overload can be used to transfer medium loads directly. General assembly by the pressure method, but also cold shaft or hot sleeve method of assembly, such as cast iron wheel and shaft assembly, column, pin, shaft, sleeve and other pressed into the hole of the fit.
H / u, H / v, H / x, H / y, H / z with, these belong to the large overload with, the amount of overload increased in turn, the ratio of overload and diameter in 0.001 or more. They are suitable for transmitting high torques or bearing high shock intercepts and rely entirely on the bonding force generated by the overload to ensure a strong connection, usually assembled by hot sleeve or cold shaft method. The cast steel wheels of trains are fitted with H7/u6 or even H6/u5 high manganese steel hoops. Due to the large overload, the parts require good material and high strength, otherwise the parts will be squeezed and cracked, so the use of caution, generally after testing before putting into production. A selection is often made before assembly so that the excess of a batch of fittings is consistent and moderate.
97. Why is the base-hole system preferred?
A: Because holes are difficult to machine with axes, changing the size of the hole requires changing the number of tools and gauges. And changing the size of the shaft does not change the number of tools, gauges.
98. How are tolerance levels applied?
A: See figure below.
99. How are the categories of fit determined by the use requirements?
A: When there is relative movement or rotation of the hole or shaft, the clearance fit must be selected. Relative movement selects fits with small gaps and relative rotation selects fits with large gaps.
When there are no keys, pins, screws and other couplings between holes and shafts, the only way to achieve the transmission is by the fit between holes and shafts, the overload fit must be selected.
Transition fits are characterized by the possibility of creating gaps, and also by the possibility of creating surpluses, but the amount of gaps or surpluses is relatively small. Therefore, when there is no relative motion between parts, high concentricity is required, and power is not transferred by the fit, transition fits are often chosen.
100. What are the principles for the selection of dimensional tolerances and fits?
A: The principle of selection is to obtain the best possible techno-economic benefits while meeting the requirements of the use.