What is tolerance?

First, the concept of tolerance and cooperation (1) Interchangeability of parts When assembling the machine in batch production, it is required that a batch of matched parts can be processed according to the requirements of the parts drawing. Without any selection or modification, if a pair is assembled, the working performance requirements of the design can be achieved. This property is called interchangeability. The parts are interchangeable, which can bring convenience to the assembly and repair of the machine, and also provide the modern large-scale production of the machine. (2) Terms related to tolerance In the process of machining parts, the accuracy of the football machine, tool wear, measurement errors, etc., it is impossible to process the dimensions of the parts absolutely accurately. In order to ensure interchangeability, the machining error of the part size must be limited to a certain range. For example, the terminology (axes, similar) of the tolerance is explained. 1, the basic size The dimensions determined at design time, based on the strength and structural requirements of the part. The value should be given priority to the standard diameter or standard length. 2, the actual size By measuring the resulting size. 3, the limit size Two limit values ​​that allow for dimensional changes. It is determined based on the base size. The larger of the two thresholds is called the maximum limit size; the smaller one is called the minimum limit size. 4, size deviation (referred to as deviation) The algebraic difference obtained by subtracting its basic size from a certain size. Dimensional deviations are: Upper deviation = maximum limit size – basic size Lower deviation = minimum limit size – basic size The upper and lower deviations are collectively referred to as limit deviations, and the upper and lower deviations may be positive, negative or zero. The national standard stipulates that the upper deviation code of the hole is ES, the lower deviation code of the hole is EI; the upper deviation code of the axis is es, and the lower deviation code of the axis is ei. 5, dimensional tolerance (referred to as tolerance) Allowable change in size. Dimensional tolerance = maximum limit size – minimum limit size = upper deviation – lower deviation Since the maximum limit size is always greater than the minimum limit size, ie the upper deviation is always greater than the lower deviation, the dimensional tolerance must be positive. The aperture shown in Figure 1a: Basic size = Ø30 Maximum limit size = Ø30.010 Minimum limit size = Ø29.990 Upper deviation ES = maximum limit size – basic size =30.010-30=+0.010 Lower deviation EI = minimum limit size – basic size =29.990-30=-0.010 Tolerance = maximum limit size – minimum limit size =3.010-29.990=0.020 =ES-EI=+0.010-(-0.010)=0.020 If the actual size is between Ø30.010 and Ø29.990, it is qualified. 6, zero line, public relations belt and tolerance map As shown in Figure 1b, the zero line is a reference line used to determine the deviation in the tolerance band diagram, ie the zero deviation line. Usually the zero line indicates the basic size. Mark the “0”, “+” and “—” signs at the left end of the zero line. The deviation above the zero line is positive; the deviation below the zero line is negative. The tolerance zone is an area defined by two straight lines representing the upper and lower deviations. The width and position of the tolerance zone are the two elements that make up the tolerance zone. In order to succinctly explain the above terms and their interrelationships, in practice, the tolerance band diagram is generally indicated. The tolerance zone diagram draws the box in an enlarged view, and the zero line is marked. The width of the box indicates the tolerance tolerance value. The left and right lengths of the box can be arbitrarily determined as needed. To distinguish the tolerance band of the shaft and the hole, the tolerance band of the hole is generally indicated by a diagonal line; the tolerance of the axis is indicated by a dotted point. 7, standard tolerances and standard tolerance levels The standard tolerance is any tolerance listed in the national standard to determine the tolerance band size. The standard tolerance class is the level at which the dimensional accuracy is determined. The standard tolerances are divided into 20 grades, namely IT01, IT0, IT1, IT-18, which represent standard tolerances, and Arabic numerals indicate standard tolerance grades, of which IT01 is the highest, grade is decreasing, and IT18 is the lowest. For a given basic size, the higher the standard tolerance class, the smaller the standard tolerance value and the higher the accuracy of the dimension. The national standard divides the basic size range within 500mm into 13 segments, and lists the standard tolerance values ​​of the basic dimensions of each segment according to different standard tolerance classes. 8, the basic deviation Used to determine the upper or lower deviation of the tolerance zone relative to the neutral position. Generally refers to the deviation from the zero line. As shown in Figure 2, when the tolerance band is above the zero line, the basic deviation is the lower deviation. When the tolerance band is below the zero line, the basic deviation is the upper deviation. According to actual needs, the national standards respectively specify 28 different basic deviations for the holes and axes, as shown in Figure 3. The basic deviation values ​​of the holes and axes can be found in the relevant table. As can be seen from Figure 3: (1) The basic deviation code is represented by the Latin alphabet, the basic deviation code indicated by the uppercase letter, and the lowercase letter indicates the basic deviation code of the axis. Since the basic deviation in the figure only indicates the tolerance band size, one end of the tolerance band is drawn as an opening. (2) The deviation is from A-H to the lower deviation, J-ZC is the upper deviation, and the upper and lower deviations of JS are +IT/2 and —IT/2, respectively. (3) The basic deviation of the axis is offset from a-h, and j-zc is the lower deviation. The upper and lower deviations of js are +IT/2T and -IT/2, respectively. Another deviation of the hole and the shaft can be calculated from the basic deviation and the standard tolerance. 9. The tolerance code of the shaft is composed of the standard tolerance class code and is written in the same number. For example: Ø60H8, indicating the tolerance of the hole with a basic dimension of Ø60, a basic deviation of H, and a standard tolerance class of 8. Another example: Ø60f7, indicating that the basic size is Ø60, the basic deviation is f, and the standard tolerance class is 7th. (3) Relevant terms In machine assembly, the relationship between the basic dimensions of the same, the combined holes and the tolerance band of the shaft is called the fit. Due to the actual size of the holes and shafts, “gap” or “overfill” can occur after assembly. In the cooperation of the hole and the shaft, the difference between the size of the hole minus the size of the shaft is a gap when the algebraic difference is positive, and the interference is negative when it is negative. Type of cooperation There are three types of matching: according to their gaps or interferences: (1) Gap fit The tolerance of the hole is above the public tape of the shaft, and any pair of holes and shafts are matched to have a clearance (including a minimum clearance of zero), as shown in Figure 4a. (2) Interference fit The tolerance of the hole is below the tolerance band of the shaft. Any pair of holes and shafts are matched to have interference (including minimum clearance of zero), as shown in Figure 4b. (3) Over-matching The tolerance band of the hole overlaps the tolerance bands of the shaft, and any pair of holes and shafts may match, which may have a gap or an interference fit, as shown in Fig. 4c. 2, the benchmark system The national standard specifies two benchmark systems, as shown in Figure 4. (1) Base hole system The basic deviation is a system in which the tolerance band of a certain hole and the tolerance band of the basic deviation constitute a kind of cooperation, as shown in Fig. 5a. That is, the tolerance band of the hole is fixed in the same basic size, and the different tolerances are obtained by changing the tolerance band position of the shaft. The hole made by the base hole is called the reference hole. The national standard specifies that the lower deviation of the reference hole is zero, and “H” is the basic deviation code of the reference hole. (2) Base shaft system The basic deviation is a tolerance band of a certain axis and a tolerance band of a hole with different basic deviations constitute a system of various cooperation, as shown in Fig. 5b. That is, the tolerance band position of the shaft is fixed in the same basic size fit, and different tolerances are obtained by changing the tolerance band position of the hole. The hole made by the base axis is called the reference bushing. The national standard specifies that the upper deviation of the reference axis is zero, and “h” is the basic deviation code of the reference axis. It can be seen from the basic deviation series (Figure 3): In the base hole system, the reference hole H is matched with the shaft, a~h (11 types in total) is used for clearance fit; j~n (5 types in total) is mainly used for over-matching; (n, p, r may be excessive fit) Or interference fit); p~zc (12 total) is mainly used for interference fit. In the base shaft system, the reference axis h is matched with the hole, A~H (11 types in total) is used for clearance fit; J~N (5 types in total) is mainly used for over-matching; (N, P, R may be excessive fit) Or interference fit); P~ZC (12 types) are mainly used for interference fit. Second, the choice of tolerance and cooperation The selection of tolerances includes three parts: the reference system, the matching category and the tolerance level. (1) The national standard of the benchmark system stipulates that the base hole system is preferred, because it is generally said that the machining hole is more difficult than the machining axis, and the use of the base hole 限制 can limit and reduce the number of specifications of the fixed value tool and the measuring tool required for the machining. Thereby obtaining better economic benefits. The base shaft system is usually only used for structural design requirements that are not suitable for the use of the base hole system, or where the base shaft system has a significant economic effect. For example, the same shaft is mated with several holes having different tolerance bands (Fig. 6) or cold drawn to make the shaft that is no longer machined. When the shaft is engaged with the hole, the base shaft is used. When the part is matched with the standard, it should be determined according to the reference system selected by the standard part. For example, the coupling of the shaft ring of the rolling bearing and the shaft is made by the base hole; and the cooperation of the seat ring and the body hole is made of the base shaft. (2) Choice of cooperation National standards specify hole and shaft tolerance zones for preferred, common and general use. Priority and common fit should be used as much as possible according to the matching characteristics and function. When the parts have relative rotation or movement, the clearance fit must be selected; when there are no keys, pins and other fasteners between the parts, only relying on the interference between the joint surfaces to achieve the transmission, the interference fit must be selected as the part There is no requirement for relative motion between them, and the coaxiality requirement is high, and when the power is not transmitted by the cooperation, the over-matching is usually selected. (1) Choice of quasi-public relations level Under the condition of ensuring the use of parts, the lower standard tolerance level should be selected as much as possible, that is, the standard tolerance level is large and the tolerance value is large to reduce the manufacturing cost of the parts. Since the machining hole is difficult, when the standard tolerance level is higher than IT8, in the basic size to 500mm, the standard tolerance level of the hole should be selected one level lower than the axis (for example, the hole is 8 and the axis is 7). Machine the hole. Because the higher the tolerance level, the more difficult the machining. When the standard tolerance level is low, the same standard PR level can be selected for the shaft and hole. Usually IT01~IT14 are used for block gauges and gauges; IT5~IT12 are used for fit sizes; IT12~IT18 are used for non-fit sizes. Third, the tolerance and coordination of the note and check the table (1) Cooperating with the annotation in the assembly drawing The matching code consists of the matching hole and shaft tolerance band code, expressed in fractional form, the numerator is the tolerance band code of the hole; the denominator is the tolerance band code of the axis (when the oblique line is used, the oblique part should be in the numerator and denominator The code is highly flush). It can be seen from the above analysis that in the matching code, if the molecule contains H, the base hole is compounded; if the distribution contains h, the base axis is compounded. If the distribution contains H and the distribution also contains h, then the gap fit of the base hole and the reference axis, that is, the minimum gap is zero, is generally regarded as the base hole fit, and can also be regarded as the base shaft fit. In conjunction with the annotations in the assembly drawing, there are three forms: (1) Mark the matching code of the hole and the shaft as shown in Figure 7a. This type of injection is the most widely used. (2) When the part is matched with a standard part or an purchased part, only the tolerance band code of the part can be marked in the assembly drawing. As shown in Fig. 7b, the journal and the rolling bearing ring cooperate, only the journal φ30K6 is injected; the seat hole and the rolling bearing race cooperate, and only the seat hole φ62J7 is injected. (3) Mark the limit deviation of the hole and the shaft as shown in Fig. 8. This type of injection is mainly used for non-standard fits. (2) The tolerance in the part drawing There are three forms of tolerance in the part drawing: 1, labeling tolerance code As shown in Fig. 9a, this type of injection is commonly used in large-scale production in the field. Since it is unified with the inspection tool using the special measuring tool, it is not necessary to note the deviation value. 2, label deviation value As shown in Figure 9b, this type of injection is often used in small batch or single piece production for comparison during processing inspections. Pay attention to the value of the village note deviation; (1) When the upper and lower deviation values ​​are different, the upper deviation is placed on the upper right of the basic size, and the lower deviation is placed on the lower right side and is marked on the same bottom line as the basic size. The deviation number should be smaller than the basic size number, the integer digits before the decimal point should be aligned, and the decimal places should be the same, as shown in Figure ф30. (2) If the upper deviation or the lower deviation is zero, it should be abbreviated as “0”, no “+” or “-” sign in front, no decimal point in the back; another deviation is written in the original position, and its position “0” is aligned, as shown in Figure ф30. (3) If the absolute values ​​of the upper and lower deviation coals are the same, add the “±” sign after the basic size, and fill in only one deviation value, the number of which is the same as the basic size number, such as ф80±0.017. 3. At the same time, the village tolerance tolerance code and deviation value As shown in Figure 9c, the deviation values ​​should be enclosed in parentheses. This type of labeling combines the advantages of the first two forms of labeling and is often used in production where product conversion is more frequent. The national standard stipulates that only one type of labeling can be used for the tolerance of the same part drawing. (2) Checklist of limit deviation values When the basic dimensions, basic deviation codes and standard tolerance classes of the holes or shafts are determined, the upper and lower deviations of the holes or axes can be directly found in the limit deviation table; for the reference parts (reference holes and reference axes) directly from the standard tolerance table Checked in. [Example 10-3] Look up the table to write the axis and hole deviation values ​​of ф30 and ф18. 1. Check the axis and hole deviation values ​​of ф30 It can be seen from the matching code that the basic size of the hole and the shaft is ф30, the hole is the reference hole, and the tolerance level is 7; the matching basic deviation code of the shaft is f, and the tolerance level is 6 level, which is a clearance fit of the base hole. (3) Check the ф30H7 reference hole. In the attached table, the intersection of the horizontal line of the basic size of 24~30 and the column of H7, the upper and lower deviations are found to be μm (ie mm), so ф30H7 can be written as ф30. 0.021 is the tolerance of the reference hole, therefore, It can be found in the standard tolerance table that 21μm (ie 0.021mm) is found at the intersection of the horizontal line >18~30 and the IT7 column. It can be seen that the upper deviation of the reference hole is +