In Mechanical Design, tolerances and fits may seem like the most basic knowledge in mechanical engineering, but they are at the core of determining part quality, cnc machining costs, and service life. Many rework and failure problems can ultimately be traced back to improper tolerance selection.
For this reason, this assistant has compiled this “91 Questions About Tolerances and Fits,” covering practical questions about basic concepts, tolerance grades, datum system selection, and operating condition matching. It can be used as a quick reference manual and is recommended for future reference.
1. What is tolerance?
Answer: The amount of variation allowed in the dimensions and geometric parameters of a part is called tolerance.
2. What is dimension?
Answer: A numerical value representing a length expressed in a specific unit.
3. What is basic size?
Answer: The size specified by the design.
4. What is actual size?
Answer: The size obtained through measurement.
5. What is limit size?
Answer: The two limit values within which the size is allowed to vary.
6. What are Maximum Material Condition (MMC) and Maximum Material Size?
Answer: Maximum Material Condition refers to the condition in which a hole or shaft has the greatest amount of material within its dimensional tolerance range. The size in this condition is called the maximum material size. It is the general term for the minimum limit size of a hole and the maximum limit size of a shaft.
7. What are Least Material Condition (LMC) and Least Material Size?
Answer: Least Material Condition refers to the condition in which a hole or shaft has the least amount of material within its dimensional tolerance range. The size in this condition is called the least material size. It is the general term for the maximum limit size of a hole and the minimum limit size of a shaft.
8. What is functional size?
Answer: Over the entire length of the mating surface, the size of the largest ideal shaft inscribed in the actual hole is called the functional size of the hole. The size of the smallest ideal hole circumscribed around the actual shaft is called the functional size of the shaft.
9. What is dimensional deviation?
Answer: It is the algebraic difference obtained by subtracting the basic size from a certain size.
10. What is dimensional tolerance?
Answer: It is the amount by which the allowable size may vary.
11. What is the zero line?
Answer: In a tolerance and fit diagram (tolerance zone diagram), it is the reference straight line used to determine deviations, namely the zero-deviation line.
12. What is a tolerance zone?
Answer: In a tolerance zone diagram, it is the area bounded by the two straight lines representing the upper and lower deviations.
13. What is fundamental deviation?
Answer: It is the upper or lower deviation used to determine the position of the tolerance zone relative to the zero line, generally referring to the deviation closer to the zero line. When the tolerance zone is above the zero line, its fundamental deviation is the lower deviation; when it is below the zero line, its fundamental deviation is the upper deviation.
14. What is standard tolerance?
Answer: Any tolerance specified by the national standard for determining the size of a tolerance zone.
15. What is fit?
Answer: It refers to the relationship between the tolerance zones of holes and shafts having the same basic size and being mutually assembled.
16. What is the hole-basis system?
Answer: It is a system in which a hole tolerance zone with a fixed fundamental deviation forms various fits with shaft tolerance zones having different fundamental deviations.
17. What is the shaft-basis system?
Answer: It is a system in which a shaft tolerance zone with a fixed fundamental deviation forms various fits with hole tolerance zones having different fundamental deviations.
18. What is fit tolerance?
Answer: It is the amount of variation in clearance. It is equal to the absolute value of the algebraic difference between the maximum clearance and minimum clearance, and is also equal to the sum of the hole tolerance zone and shaft tolerance zone in the fit.
19. What is a clearance fit?
Answer: The tolerance zone of the hole is completely above the tolerance zone of the shaft, meaning that the fit has clearance (including a fit where the minimum clearance is equal to zero).
20. What is an interference fit?
Answer: The tolerance zone of the hole is completely below the tolerance zone of the shaft, meaning that the fit has interference (including a fit where the minimum interference is equal to zero).
21. What is a transition fit?
Answer: In a hole-and-shaft fit, the tolerance zones of the hole and shaft overlap. When any pair of hole and shaft is fitted together, the result may be clearance or interference.
22. When the hole-basis fit is H11/c11 or the shaft-basis fit is C11/h11, what is the preferred fit characteristic?
Answer: Very large clearance, used for very loose, very slow-moving dynamic fits; exposed components requiring large tolerances and large clearances; and very loose fits requiring convenient assembly. Equivalent to D6/dd6 in the old national standard.
23. When the hole-basis fit is H9/d9 or the shaft-basis fit is D9/h9, what is the preferred fit characteristic?
Answer: A free-rotation fit with very large clearance, used when precision is not the primary requirement, or when there are large temperature variations, high rotational speeds, or high journal pressure. Equivalent to D4/de4 in the old national standard.
24. When the hole-basis fit is H8/f7 or the shaft-basis fit is F8/h7, what is the preferred fit characteristic?
Answer: A rotational fit with relatively small clearance, used for precise rotation at medium speeds and medium journal pressure; also used as a medium locating fit that is relatively easy to assemble. Equivalent to D/dc in the old national standard.
25. When the hole-basis fit is H7/g6 or the shaft-basis fit is G7/h6, what is the preferred fit characteristic?
Answer: A sliding fit with very small clearance, used when free rotation is not desired but free movement and sliding are allowed and precise positioning is required. It can also be used for clearly defined locating fits. Equivalent to D/db in the old national standard.
26. When the hole-basis fits are H7/h6, H8/h7, H9/h9, H11/h11 or the shaft-basis fits are H7/h6, H8/h7, H9/h9, H11/h11, what is the preferred fit characteristic?
Answer: All are clearance locating fits. Parts can be freely assembled and disassembled, while they are generally relatively stationary during operation. The clearance under the maximum material condition is zero, while under the least material condition, the clearance is determined by the tolerance grade. H7/h6 is equivalent to D/d in the old national standard; H8/h7 is equivalent to D3/d3; H9/h9 is equivalent to D4/d4; H11/h11 is equivalent to D6/d6.
27. When the hole-basis fit is H7/h6 or the shaft-basis fit is K7/h6, what is the preferred fit characteristic?
Answer: Transition fit, used for precision positioning. Equivalent to D/gc in the old national standard.
28. When the hole-basis fit is H7/n6 or the shaft-basis fit is N7/h6, what is the preferred fit characteristic?
Answer: Transition fit, allowing relatively large interference for more precise positioning. Equivalent to D/ga in the old national standard.
29. When the hole-basis fit is H7/p6 or the shaft-basis fit is P7/h6, what is the preferred fit characteristic?
Answer: Interference locating fit, namely a small interference fit. It is used when positioning accuracy is particularly important, allowing the best positioning accuracy to meet the rigidity and concentricity requirements of the assembly, while there are no special requirements for the internal hole under pressure and frictional loads are not transmitted by the tightness of the fit. Equivalent to D/ga–D/jf in the old national standard. When H7 is less than or equal to 3 mm, it is a transition fit.
30. When the hole-basis fit is H7/s6 or the shaft-basis fit is S7/h6, what is the preferred fit characteristic?
Answer: Medium press fit, suitable for general steel parts; or for shrink fitting of thin-walled parts. For cast iron parts, it can obtain the tightest fit. Equivalent to D/je in the old national standard.
31. When the hole-basis fit is H7/u6 or the shaft-basis fit is U7/h6, what is the preferred fit characteristic?
Answer: Press fit, suitable for parts that can withstand high pressing forces or shrink fits that should not withstand high pressing forces.
32. When the fundamental deviation of the shaft is a or b, what is the fit characteristic?
Answer: It is a clearance fit and can obtain particularly large clearance. It is rarely used.
33. When the fundamental deviation of the shaft is c, what is the fit characteristic?
Answer: It is a clearance fit and can obtain very large clearance. It is generally suitable for slow, loose dynamic fits. It is used under poor working conditions (such as agricultural machinery), under force-induced deformation, or when a large clearance must be guaranteed for ease of assembly. The recommended fit is H11/c11. Higher-grade fits such as H8/c7 are suitable for close dynamic fits where a shaft operates at high temperatures, such as the exhaust valve and guide of an internal combustion engine.
34. When the fundamental deviation of the shaft is d, what is the fit characteristic?
Answer: It is a clearance fit, generally used for IT7–IT11, and mainly used for loose rotational fits, such as the fit between sealing covers, pulleys, idler pulleys, and shafts. It is also suitable for large-diameter sliding bearing fits, such as sliding supports in turbines, ball mills, rolling forming machines, heavy bending machines, and some other heavy machinery.
35. When the fundamental deviation of the shaft is e, what is the fit characteristic?
Answer: It is a clearance fit, mostly used for IT7–IT9, and generally suitable for support fits requiring obvious clearance and easy rotation, such as long-span and multi-point support. Higher-grade e shafts are suitable for large, high-speed, heavy-load support fits, such as worm generators, large electric motors, internal combustion engines, camshafts, and rocker arm supports.
36. When the fundamental deviation of the shaft is f, what is the fit characteristic?
Answer: It is a clearance fit, mostly used for general rotational fits of IT6–IT8. When temperature effects are not significant, it is widely used for supports lubricated with ordinary lubricating oil or grease, such as the fit between rotating shafts and sliding supports in gearboxes, small electric motors, and pumps.
37. When the fundamental deviation of the shaft is g, what is the fit characteristic?
Answer: It is a clearance fit with very small clearance and high manufacturing cost. Except for precision devices under very light loads, it is not recommended for rotational fits. It is mostly used for IT5–IT7 and is most suitable for precision sliding fits without rotation. It is also used for locating fits such as pins, precision connecting rod bearings, pistons, slide valves, and connecting rod pins.
38. When the fundamental deviation of the shaft is h, what is the fit characteristic?
Answer: It is a clearance fit, mostly used for IT4–IT11. It is widely used for parts without relative rotation as a general locating fit. If there is no temperature deformation effect, it can also be used for precision sliding fits.
39. When the fundamental deviation of the shaft is js, what is the fit characteristic?
Answer: It is a transition fit with completely symmetrical deviation (+IT/2). On average, it is a fit with slight clearance, mostly used for IT4–7. It is used for locating fits where the clearance is smaller than that of an h shaft and slight interference is allowed (such as couplings), and can be assembled by hand or with a wooden hammer.
40. When the fundamental deviation of the shaft is k, what is the fit characteristic?
Answer: It is a transition fit, with an average fit having no clearance. It is suitable for IT4–IT7. It is recommended for locating fits with slight interference, as well as locating fits used to eliminate vibration. It is generally assembled with a wooden hammer.
41. When the fundamental deviation of the shaft is m, what is the fit characteristic?
Answer: It is a transition fit, with an average small transition fit. It is suitable for IT4–IT7 and is assembled using a hammer or press. It is generally recommended for tight component fits. With an H6/n5 fit, it becomes an interference fit.
42. When the fundamental deviation of the shaft is n, what is the fit characteristic?
Answer: It is a transition fit. The average interference is slightly greater than that of an m shaft, and clearance is rarely obtained. It is suitable for IT4–IT7 and is assembled using a hammer or press. It is generally recommended for tight component fits. With an H6/n5 fit, it becomes an interference fit.
43. When the fundamental deviation of the shaft is p, what is the fit characteristic?
Answer: It is an interference fit. When paired with H6 or H7, it is an interference fit; when paired with an H8 hole, it is a transition fit. For non-ferrous parts, it is a relatively light press fit and can be easily disassembled when necessary. For steel, cast iron, or copper-and-steel component assemblies, it is a standard press fit.
44. When the fundamental deviation of the shaft is r, what is the fit characteristic?
Answer: It is an interference fit. For ferrous parts, it is a medium press fit; for non-ferrous parts, it is a light driving fit and can be disassembled when necessary. When paired with an H8 hole, it is an interference fit when the diameter is greater than 100 mm, and a transition fit when the diameter is smaller.
45. When the fundamental deviation of the shaft is s, what is the fit characteristic?
Answer: It is an interference fit, used for permanent and semi-permanent assembly of steel and iron parts. It can produce considerable joining force. When elastic materials such as light alloys are used, the fit characteristic is equivalent to that of a P shaft for ferrous parts. Examples include pressing a ring onto a shaft and valve seat fits. For larger dimensions, thermal expansion or shrink fitting should be used to avoid damaging the mating surfaces.
46. When the fundamental deviation of the shaft is t, u, v, x, y, or z, what is the fit characteristic?
Answer: It is an interference fit, with the amount of interference increasing successively. It is generally not recommended.
47. Under what circumstances should the shaft-basis system be selected?
Answer: Cold-drawn steel that is manufactured according to the tolerance zone of the basic shaft, with a certain tolerance grade (generally grades 8 to 11), and is no longer machined can be directly used as a shaft. In this case, different hole tolerance zone positions can be selected to form various fit requirements. This situation is relatively common in agricultural machinery and textile machinery.
For precision shafts with machining dimensions smaller than 1 mm, machining the shaft is much more difficult than machining a hole of the same grade. Therefore, in the manufacture of instruments, watches and clocks, radio equipment, and electronic products, fine steel wire formed by precision rolling is usually used directly as the shaft. In this case, selecting a shaft-basis fit is more economical than a hole-basis fit.
From a structural perspective, when one shaft mates with several holes at different locations and each has different fit requirements, the shaft-basis system should be considered.
48. How should fits with standard components be selected?
Answer: When mating with a standard component, the standard component should be used as the reference component to determine the fit system.
For example, in a rolling bearing support structure, the fit between the outer ring of the rolling bearing and the housing hole should use the shaft-basis system, while the fit between the bearing inner ring and the journal should use the hole-basis system. The housing hole is manufactured according to J7, and the journal is manufactured according to k6.
49. What tolerance grade should be used for grinding?
Answer: IT1–IT5.
50. What tolerance grade should be used for honing?
Answer: IT4–IT7.
51. What tolerance grade should be used for diamond turning?
Answer: IT5–IT7.
52. What tolerance grade should be used for diamond boring?
Answer: IT5–IT7.
53. What tolerance grade should be used for cylindrical grinding?
Answer: IT5–IT8.
54. What tolerance grade should be used for surface grinding?
Answer: IT5–IT8.
55. What tolerance grade should be used for broaching?
Answer: IT5–IT8.
56. What tolerance grade should be used for precision turning and precision boring?
Answer: IT7–IT9.
57. What tolerance grade should be used for reaming?
Answer: IT6–IT10.
58. What tolerance grade should be used for milling?
Answer: IT8–IT11.
59. What tolerance grade should be used for planing and slotting?
Answer: IT10–IT11.
60. What tolerance grade should be used for rolling and extrusion?
Answer: IT10–IT11.
61. What tolerance grade should be used for rough turning?
Answer: IT10–IT12.
62. What tolerance grade should be used for rough boring?
Answer: IT10–IT12.
63. What tolerance grade should be used for drilling?
Answer: IT10–IT13.
64. What tolerance grade should be used for stamping?
Answer: IT10–IT14.
65. What tolerance grade should be used for sand casting?
Answer: IT14–IT15.
66. What tolerance grade should be used for metal mold casting?
Answer: IT14–IT15.
67. What tolerance grade should be used for forging?
Answer: IT15–IT16.
68. What tolerance grade should be used for gas cutting?
Answer: IT15–IT18.
69. How many methods are there for determining the fundamental deviation?
Answer: There are three methods for determining the fundamental deviation: the experimental method, calculation method, and analogy method.
70. What is the experimental method?
Answer: The experimental method determines the type of fit that satisfies the product's working performance through testing. It is mainly used for important and critical fits in key mechanisms in industries such as aerospace, aviation, national defense, nuclear industry, and railway transportation, where the fit has a significant impact on product performance and there is insufficient experience. This method is relatively reliable. Its disadvantages are that testing is required, the cost is high, and the cycle is long. It is less commonly used.
71. What is the calculation method?
Answer: The calculation method determines the type of fit through theoretical calculations based on usage requirements. Its advantage is that it has a sufficient theoretical basis and costs less than the experimental method. However, because theoretical calculations cannot fully take into account all actual factors in the working environment of machinery and equipment, the design solution is not as accurate as one determined through experiments.
For example, when using the calculation method to determine the fit type of a sliding bearing clearance fit, the allowable minimum clearance can be calculated according to the theory of fluid lubrication, and an appropriate fit type can then be selected from the standards. When using the calculation method to determine an interference fit that relies entirely on interference to transmit loads, the required minimum interference can be calculated according to elastic and plastic deformation theory based on the size of the load to be transmitted. An appropriate interference fit can then be selected, while checking whether the material strength of the parts can withstand the maximum interference generated by the fit.
Because there are many factors affecting fit clearance and interference, theoretical calculations can only be approximate.
72. What is the analogy method?
Answer: The analogy method determines the fit by using a fit that has been verified through production practice in a machine or mechanism of the same type as the design task as a reference, combined with the actual usage requirements and application conditions of the designed product. This method is the most widely used, but it requires designers to have sufficient reference materials and considerable experience. When determining a fit using the analogy method, the following factors should be considered:
Load magnitude. When the load is relatively large, the fit should tend to be selected tighter. That is, the amount of interference in an interference fit should be appropriately increased, the clearance in a clearance fit should be reduced, and a transition fit with a higher probability of obtaining interference should be selected.
Disassembly and assembly conditions and structural characteristics. For fits that are frequently assembled and disassembled, the fit should be looser than a similar fit that is not frequently assembled and disassembled. Fits that are difficult to assemble should also be slightly looser.
Mating length and geometric errors. The longer the mating length, the tighter the actual fit will be compared with a shorter mating length due to the existence of geometric errors. Therefore, an appropriately looser fit should be selected.
Materials and temperature. When the materials of the mating parts are different (with significantly different coefficients of linear expansion) and the operating temperature differs significantly from the standard temperature of +20°C, the effect of thermal deformation should be considered. The effect of assembly deformation should also be considered.
73. Where is tolerance grade 5 used?
Answer: It is mainly used where very small fit tolerances and geometric tolerances are required and the fit characteristics are stable. It is generally used in important areas of machine tools, engines, instruments, and other equipment. Examples include housing holes fitted with Grade D rolling bearings; machine tool spindles fitted with Grade E rolling bearings; machine tool tailstocks and sleeves; journals in precision and high-speed machinery; and precision lead screw diameters.
74. Where is tolerance grade 6 used?
Answer: The fit characteristics can achieve relatively high uniformity, such as holes and journals fitted with Grade E rolling bearings; shaft diameters connected to gears, worm gears, couplings, pulleys, cams, etc.; machine tool lead screw diameters; radial drill columns; outer diameter dimensions of guide components in machine tool fixtures; reference holes of Grade 6 precision gears; and reference shafts of Grade 7 and Grade 8 gears.
75. Where is tolerance grade 7 used?
Answer: Grade 7 precision is slightly lower than Grade 6, and its application conditions are basically similar to Grade 6. It is widely used in general machinery manufacturing. Examples include holes for couplings, pulleys, cams, etc.; machine tool chuck seat holes; fixed drill bushings and replaceable drill bushings in fixtures; reference holes of Grade 7 and Grade 8 gears; and reference shafts of Grade 9 and Grade 10 gears.
76. Where is tolerance grade 8 used?
Answer: It belongs to medium precision in machine manufacturing. Examples include the width-direction dimensions of bearing seat bushings; reference holes of Grade 9–12 gears; and reference shafts of Grade 11–12 gears.
77. Where are tolerance grades 9–10 used?
Answer: They are mainly used in mechanical manufacturing for the outer diameter of bushings and holes; operating components and shafts; idler pulleys and shafts; single keys and splines.
78. Where are tolerance grades 11–12 used?
Answer: The fit precision is very low, and a large clearance may occur after assembly. They are suitable for applications with basically no fit requirements. Examples include the flange and register of a machine tool; sliders and sliding gears; dimensions between machining processes; mating parts produced by stamping; and the connection between a wrench hole and wrench seat in machine tool manufacturing.
79. What are the requirements for linear dimensional tolerance notation?
Answer: The tolerance symbol should have the same height as the basic size digits.
When linear dimensional tolerances are indicated using limit deviations, the upper and lower deviation digits should be one size smaller than the basic size digits. The decimal points of the upper and lower deviations must be aligned, and positive and negative signs must be indicated.
When one deviation is zero, “0” may be used and aligned with the units digit of the other deviation.
The bottom line of the lower deviation should be placed on the same baseline as the basic dimension.
When the numerical values of the upper and lower deviations are equal, the deviation should be written only once, with a “+/-” sign between the deviation and the basic dimension, and the two should have the same font size.
80. What is a conical fit?
Answer: It refers to the relationship formed between the diameters of internal and external cones with the same basic cone due to different combinations. The fit characteristics of a conical fit are formed by specifying the axial positions of the mutually mating internal and external cones to create clearance or interference. Clearance or interference acts in the direction perpendicular to the conical surface, but is specified and measured in the direction perpendicular to the cone axis. For cones with a taper less than or equal to 1:3, the difference between the values specified perpendicular to the conical surface and perpendicular to the cone axis can be ignored. According to the different methods used to determine the relative axial positions of the mating internal and external cones, conical fits are divided into two types: structural conical fits and displacement-type conical fits.
81. What is a structural conical fit?
Answer: A fit obtained by determining the relative axial position of the internal and external cones through the structure itself or structural dimensions.
82. What is a displacement-type conical fit?
Answer: A fit obtained by determining the relative axial position of the internal and external cones by specifying the axial displacement or the magnitude of the axial force that produces the axial displacement.
83. What three items make up the standard tolerance series?
Answer: Tolerance grade, tolerance unit, and basic size ranges.
84. What is general tolerance?
Answer: It refers to the tolerance that can generally be achieved by machine tools and equipment under ordinary workshop process conditions.
85. What does GB/T1804-1992 specify for general tolerances of linear dimensions?
Answer: It specifies four tolerance grades: f, m, c, and v. The letter f represents the fine grade, m represents the medium grade, c represents the coarse grade, and v represents the very coarse grade. Tolerance grades f, m, c, and v correspond to IT12, IT14, IT16, and IT17, respectively.
86. What should be considered for clearance fits?
Answer: The basic hole H (or basic shaft h) and shafts a–h (or holes A–H) of the corresponding tolerance grades form clearance fits, with 11 types in total. Among them, H/a (or A/h) produces the largest clearance, while H/h has the smallest fit clearance.
H/a (A/h), H/b (B/h), and H/c (C/h) fits have very large clearances and are not commonly used. They are generally used in machinery operating under poor working conditions and requiring flexible movement, or in situations involving large deformation under load where a large clearance must be guaranteed because the shaft operates at high temperatures.
H/d (D/h) and H/e (E/h) fits have relatively large clearances and are used for supports requiring low precision and easy rotation. Among them, H/d (D/h) is suitable for relatively loose transmission fits, such as the fit between sealing covers, pulleys, idler pulleys, and shafts. It is also suitable for large-diameter sliding bearing fits, such as sliding bearings in heavy machinery such as ball mills and rolling mills, and is suitable for IT7–IT11. For example, the fit between a pulley and shaft.
H/f (F/h) fits have moderate clearance and are mostly used for general transmission fits of IT7–IT9, such as the fit between rotating shafts and sliding supports in gearboxes, small electric motors, pumps, etc.
H/g (G/h) fits have very small clearance. Except for precision mechanisms under very light loads, they are generally not used as rotational fits. They are mostly used for IT5–IT7 and are suitable for precision fits involving reciprocating swinging and sliding. For example, the fit between a drill bushing and bushing.
H/h fits have a minimum clearance of zero and are used for IT4–IT11. They are suitable for locating fits without relative rotation but requiring centering and guiding. If there is no influence from temperature or deformation, they can also be used for sliding fits. Recommended fits include H6/h5, H7/h6, H8/h7, H9/h9, and H11/h11.
87. What should be considered for transition fits?
Answer: The basic hole H and shafts with the fundamental deviation symbols j–n of the corresponding tolerance grades form transition fits (n forms an interference fit with high-precision holes).
H/j and H/js fits have a higher probability of obtaining clearance. They are mostly used for IT4–IT7 and are suitable for locating fits where the clearance is smaller than h and slight interference is allowed, such as couplings, gear rings and steel hubs, and rolling bearings and housings.
H/k fits have an average clearance close to zero, provide good centering, and result in relatively low contact stress on the parts after assembly. They can be disassembled and are suitable for IT4–IT7, such as the fit of rigid couplings.
H/m and H/n fits have a higher probability of obtaining interference, provide good centering, and result in tight assembly. They are suitable for IT4–IT7.
88. What should be considered for interference fits?
Answer: The basic hole H and shafts with the fundamental deviation symbols p–zc of the corresponding tolerance grades form interference fits (p and r form transition fits with lower-precision H holes).
H/p and H/r fits are interference fits at higher tolerance grades and can be assembled with a hammer or press. They should only be disassembled during major repairs. They are mainly used for locating fits requiring very high centering accuracy, sufficient rigidity of the parts, and resistance to impact loads, and are mostly used for IT6–IT8.
H/s and H/t fits are medium interference fits and generally use IT6 and IT7. They are used for permanent or semi-permanent connections of steel and iron parts. Without auxiliary components, the joining force generated by the interference can directly transmit medium loads. They are generally assembled by pressing, and cold-shaft or hot-fitting methods may also be used, such as the assembly of cast iron wheels and shafts and the fit of columns, pins, shafts, sleeves, etc. pressed into holes.
H/u, H/v, H/x, H/y, and H/z fits are large interference fits. The amount of interference increases successively, and the ratio of interference to diameter is greater than 0.001. They are suitable for transmitting large torques or withstanding large impact loads. They rely entirely on the joining force generated by interference to ensure a firm connection and are usually assembled using hot-fitting or cold-shaft methods. The cast steel wheels and high-manganese steel tires of trains require an H7/u6 or even H6/u5 fit. Because the interference is large, the material of the parts must be good and the strength must be high; otherwise, the parts may be squeezed and cracked. Therefore, caution is required when using these fits, and testing is generally required before production. Before assembly, parts are often selected so that the interference of a batch of components tends to be consistent and moderate.
89. Why is the hole-basis system preferred?
Answer: Because machining holes is more difficult than machining shafts. Changing the size of a hole requires changing the number of cutting tools and measuring tools, whereas changing the size of a shaft does not change the number of cutting tools and measuring tools.
90. How should the type of fit be determined according to usage requirements?
Answer: When the hole and shaft have relative movement or rotation, a clearance fit must be selected. For relative movement, a fit with smaller clearance should be selected; for relative rotation, a fit with larger clearance should be selected.
When there are no connecting components such as keys, pins, or screws between the hole and shaft, and transmission can only be achieved through the fit between the hole and shaft, an interference fit must be selected.
The characteristic of a transition fit is that it may produce either clearance or interference, but the amount of clearance or interference is relatively small. Therefore, when there is no relative movement between parts, high concentricity is required, and power is not transmitted through the fit, a transition fit is often selected.
91. What is the principle for selecting dimensional tolerances and fits?
Answer: The principle of selection is to obtain the best technical and economic benefits while meeting the usage requirements.