What is tolerance and fit

What is tolerance and fit?

Tolerance and fit is a basic technical course of mechanical specialty. It organically combines tolerance fit and metrology. From the perspective of interchangeability, it studies how to solve the contradiction between use requirements and manufacturing requirements around the two concepts of error and tolerance. The solution to this contradiction is to reasonably determine tolerance fit and adopt appropriate technical measurement means.

Interchangeability

The definition of interchangeability in ISO8402 is: the ability of one entity to replace another entity without change and meet the same requirements.
Interchangeability is an important technical and economic principle of modern production. In the machinery and instrument manufacturing industry, the interchangeability of parts and components means that any one of a batch of parts or components of the same specification can be installed on the machine without any selection or additional repair (such as Fitter repair) to meet the specified performance requirements. In order to meet the interchangeability of parts in mechanical manufacturing, the size of production parts shall be within the allowable tolerance range. This requires a unified standard for the form, size, accuracy and performance of a part. Similar products also need to be reasonably divided according to size to reduce the series of products, which is product standardization.
Interchangeability in machinery and manufacturing industry usually includes the interchangeability of geometric parameters (such as size) and mechanical properties (such as hardness and strength).

Classification

According to the different exchange scope, it can be divided into complete exchange (absolute exchange) and Incomplete Exchange (limited exchange). Complete interchangeability is widely used in mechanical manufacturing. However, in single piece production machines (especially heavy and high-precision instruments), incomplete interchangeability is often used. This is because in this case, complete interchange will lead to processing difficulties (or even inability to process) or high manufacturing costs. Therefore, the accuracy of parts and components is often appropriately reduced in production to facilitate manufacturing. Then, according to the measured size, the matched parts and components are divided into several groups. Make the size difference in each group smaller. Finally, assemble the parts and components of the corresponding group. This not only solves the processing difficulties of parts, but also ensures the accuracy requirements of assembly.

Effect

• (1) In terms of use, for example, the parts of bicycles and watches often used by people, the parts of various equipment used in production, etc. when they are damaged, the repairman can quickly replace them with parts of the same specification to restore the functions of bicycles, watches and equipment. In some cases, the role of interchangeability is difficult to measure by value. For example, on the battlefield, it is absolutely necessary to eliminate the faults of weapons and equipment immediately and continue the battle. At this time, it is absolutely necessary to make the main decisions on the interchangeability of parts and components.
• (2) In terms of manufacturing, interchangeability is a powerful means to improve production level and carry out civilized production. During assembly, there is no need for auxiliary processing and repair, so it can reduce the labor intensity of the assembly workers, shorten the assembly cycle, and enable the assembly workers to work according to the flow operation mode, so as to carry out automatic assembly, so as to greatly improve the production efficiency. During processing, due to the specified tolerance, various parts on the same machine can be processed at the same time. Standard parts with large consumption can also be produced separately by special factories. In this way, high-efficiency special equipment can be used, resulting in computer-aided processing. In this way, the output and quality will be improved and the cost will be significantly reduced.
• (3) From the aspect of design, due to the adoption of the exchange principle to design and produce standard parts and components, the work of drawing and calculation can be simplified, the design cycle can be shortened, and it is convenient to use computer-aided design.

Hole and shaft

The hole and shaft are generalized. The hole is the containment surface and there is no material inside. The hole includes a cylindrical inner surface and a non cylindrical inner surface (a containing surface formed by two parallel planes or tangent planes); The shaft is the contained surface, and there is no material outside. The shaft includes a cylindrical outer surface and a non cylindrical outer surface.

Dimensional terms

Size: given by the designer and composed of numbers and length units (generally mm).
Basic dimension: the hole is D and the shaft is d. when the hole is matched with the shaft, d = D
Actual size: hole Da, shaft Da, obtained through measurement, with measurement error and non true value.
Local actual size: the actual size of different parts is different.

• Limit size: hole Dmax, Dmin, axis Dmax, Dmin
• Maximum material limit (MML): hole Dmin, axis Dmax
• Minimum solid limit (LML): hole Dmax, axis Dmin

Tolerances and deviations terminology

Deviation is the algebraic difference of a dimension minus its basic dimension.
Limit deviation refers to the upper deviation (es, ES) and lower deviation (EI, EI).
Upper and lower deviation of hole: ES=Dmax–D,EI=Dmin–D; The actual deviation of the hole must be between the upper and lower deviations.
Upper and lower deviation of shaft: es=dmax-d,ei=dmin-d; The actual deviation of the shaft must be between the upper and lower deviations.
The deviation can be positive, negative, or zero. Except for zero, the deviation value shall be marked with corresponding “+” or “-“. The deviation affects the fit tightness.
Tolerance is the variation of allowable dimension. Tolerance is absolute and cannot be zero.

• Hole tolerance: TD=|Dmax-Dmin|=|ES-EI|
• Shaft tolerance: Td=|dmax-dmin|=|es-ei|

The limit deviation and tolerance are given by the design and reflect the use requirements.
The tolerance reflects the dimensional manufacturing accuracy. The smaller the tolerance value is, the higher the accuracy is, and the more difficult the manufacturing is. The tolerance zone is determined by the size and position of the tolerance zone. The size of the tolerance zone is determined by the standard tolerance and the position is determined by the basic deviation.

Coordination

1. Definitions
Fit is the relationship between hole and shaft tolerance zone. It is the requirement for a batch of holes and shafts in design, not the tightness of a pair of holes and shafts.
2. Type

• (1) Clearance fit: maximum clearance xMax=Dmax-dmin=ES-ei, minimum clearance Xmin=Dmin-dmax=EI-es;
• (2) Transition fit: maximum clearance Xmax=Dmax-dmin=ES-ei, maximum interference Ymax=Dmin-dmax=Ei-es;
• (3) Interference fit: maximum interference Ymax=Dmin-dmax=Ei-es, minimum interference Ymin=Dmax-dmin=ES-ei.

3. Fit tolerance
The fit tolerance is the sum of the hole and shaft tolerances that make up the fit. Tf=TD+Td。 It reflects the assembly requirements, and the hole axis tolerance reflects the manufacturing requirements.

National standard

1. Standard tolerance series
Tolerance grade: grade 20 (IT01, IT0, IT1, IT2… IT18). The larger the tolerance grade is, the larger the tolerance value is, the lower the accuracy is, and the easier it is to manufacture.
Basic size segments: divided into 13 segments within £ 500mm. The larger the basic dimension, the larger the tolerance value.
2. Basic deviation series
Basic deviation of holes: 28, in capital letters
Basic deviation of axis: 28, represented by lowercase letters
Hole tolerance zone and shaft tolerance zone are represented by basic deviation code and tolerance grade, such as H8, K7, H7, S6, etc.
The basic deviation has nothing to do with the tolerance value. The basic deviation is the deviation close to the zero line, which can be an upper deviation or a lower deviation.
3. Marking method of limit and fit
Mark on the part drawing: f50H7, F50 (), f50H7 () are equivalent.
Mark the fit between the hole and the shaft on the assembly drawing: f50H7 / F6.
4. General tolerance (unspecified tolerance)
It is guaranteed by the processing equipment and does not need to be tested.

Selection principle

Selection principle: economical and meet the use requirements.
1. Selection of benchmark system
Base hole system is preferred;
When matching with standard parts, the selection of reference system is determined by the standard parts: the basic hole system is selected for matching with standard holes, and the basic shaft system is selected for matching with standard shafts;
When the hole (shaft) of the same basic size is matched with multiple shafts (holes), the base hole (shaft) system shall be selected.
2. Selection of tolerance grade
On the premise of ensuring the use requirements, try to choose a lower tolerance level to reduce the cost;
When the tolerance grade is less than it8, the hole fits one level lower than the shaft, such as H7 / F6, P6 / H5, etc;
When the tolerance grade = it8, the hole and shaft can be matched at the same level or one level lower than the shaft, such as H8 / G7, H8 / D8, etc;
When the tolerance grade is > it8, the hole and shaft are matched at the same level; For example: H11 / C11, D9 / H9, etc.
3. Selection of coordination types
If there is relative movement after assembly, clearance fit shall be selected;
If there are positioning accuracy requirements or disassembly requirements after assembly, transition fit shall be selected, and the clearance and interference shall be small;
If load is to be transferred after assembly, interference fit shall be selected.
When selecting tolerance zones, they shall be selected in the order of priority tolerance zones, common tolerance zones and general tolerance zones;
When selecting cooperation, it shall be selected in the order of priority cooperation and common cooperation.

Benchmarking

1. Classification
(1) The base hole system is a system in which the tolerance zone of a hole with a certain basic deviation and the tolerance zone of a shaft with different basic deviations form various fits, which becomes the base hole system. In this system, the position of the tolerance zone of the hole is fixed in the same basic size of the fit, and by changing the position of the tolerance zone of the shaft, various fits are obtained.
The hole in the base hole system is called a reference hole. The national standard stipulates that the lower deviation of the reference hole is 0, and “H” is the basic deviation of the reference hole.
(2) Base axis system basic deviation for a certain shaft tolerance zone, with different basic deviation of the tolerance zone of the hole to form a variety of fit a system called the base axis system. This system is in the same basic size of the fit, the tolerance zone position of the shaft is fixed, by changing the tolerance zone position of the hole, to get a variety of different fits.
The shaft in the base shaft system is called the reference shaft. The national standard stipulates that the upper deviation of the reference shaft is 0 and “h” is the basic deviation of the base shaft system.
2. Selection principle
In general, the base hole system should be used in preference. This can limit the fixed value tool. The number of gage specifications. The base axis system is usually used only in cases with obvious economic effects and in cases where the structural design requirements are not suitable for the use of the base hole system.

Shape tolerance

1. Definition
Shape tolerance is the full amount of variation allowed by the shape of a single actual element. Shape tolerance is expressed by shape tolerance zone. The shape tolerance zone includes four elements such as shape, direction, position and size of the tolerance zone. Shape tolerance items are: straightness, flatness, roundness, cylindricity, line contour, surface contour and other 6 items.
2. Types
(1) Straightness
Straightness is the actual shape of the straight elements on the part to maintain the ideal straight condition. It is also commonly known as the degree of straightness. Straightness tolerance is the actual line to the ideal straight line is the maximum amount of change allowed. That is, given on the drawing, to limit the actual line processing errors allowed by the range of variation.
(2) Flatness
Flatness is the actual shape of the flat elements of the part, maintaining the ideal flat condition. It is also commonly known as the degree of flatness. Flatness tolerance is the actual surface to the plane of the maximum amount of variation allowed. That is, given in the drawing, to limit the actual surface processing errors allowed by the range of variation.
(3) Roundness
Roundness is the actual shape of the elements of the circle on the part, and its center to maintain equidistant situation. That is, the degree of roundness usually said. Roundness tolerance is in the same section, the actual circle on the ideal circle allowed the maximum amount of change. It is also the range of variation allowed for the machining error of the actual circle given on the drawing to limit
(4) Cylindricity
Cylindricity is expressed on the cylindrical surface profile of the part of the points, to maintain its axis equidistant condition. Cylindricity tolerance is the actual cylindrical surface of the ideal cylindrical surface of the maximum amount of variation allowed. Is also given on the drawing, to limit the actual cylindrical surface processing errors allowed by the range of variation.
(5) Line contour degree
Line contour is expressed in a given plane of the part, any shape of the curve, maintaining its ideal shape of the situation. Line contour tolerance refers to the actual contour line of non-circular curves allowable variation. That is, given on the drawing, to limit the actual curve processing error allowed range of variation.
(6) Surface contour degree
Surface contour is to indicate the surface of any shape on the part, maintaining its ideal shape of the condition. The surface profile tolerance is the actual contour line of the non-circular surface, the amount of variation allowed for the ideal contour surface. It is the range of variation given on the drawing to limit the actual surface processing error.

Position Tolerance

1. Definition
The positional tolerance is the full amount of variation allowed in the position of the associated actual element with respect to the datum.
2. Types
(1) Directional tolerance
Directional tolerance is the total amount of variation allowed in the direction of the associated actual element to the datum. These tolerances include parallelism, perpendicularity and inclination.
(2) Positioning tolerance
Positioning tolerance is the full amount of variation allowed in the position of the associated actual element to the datum. These tolerances include coaxiality, symmetry and position.
(3) Runout tolerance
Runout tolerance is a tolerance item given on the basis of a specific inspection method. Runout tolerance can be divided into circular runout and full runout.

Source: China machining solutions provider – Yaang Pipe Industry Co., Limited (www.machinedsgn.com)

(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)

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