What are tolerances and fits? What are the basics of tolerance and fit?

Tolerance fit is the size data of the fit relationship, for example, for a hole of 40, fit with a shaft of 40:

1. When it needs to be able to rotate, it is called gap fit.

1. Need a very large gap, or agricultural machinery: you can choose H11 C112, need a slightly smaller gap: choose H9 D9

3. Need a very small gap: choose H8 F7

2. When there is no need to rotate (including the fit between the bearing and the shaft), it is called a transition fit.

1. Close fit, used for positioning: H7 JS6

2. Coordination between bearing and shaft: H7 K6

3. When the shaft and hole need to be completely fixed together, it is called interference fit.

1. The shaft of interference fit should be smaller than the hole, and it needs to be assembled with a press or temperature difference method.

Fourth, the letters in front of the match are ...... by a, b, and cThe X, Y, Z, and A levels have the largest clearance, while the Z level clearance is negative (not only is there no clearance, but its shaft is smaller than the hole).

Fifth, the letter is followed by the accuracy level, the smaller the number, the higher the accuracy.

6. The basic size is the benchmark value of the design, and the shaft and hole that fit with each other should be the same benchmark value.

7. Tolerance is a series of mating forms based on the basic size.

8. Non-rigid interference fit, you can choose a large interference fit, such as: H7 Z6 (which needs to be assembled with a press) 9. The design sequence is: first to determine the basic size, and then choose the form of fit.

What are the three elements of fit tolerance and fit tolerance, if you want to learn machinery, you can understand it.

It refers to the relationship between the hole and the shaft tolerance zone of the same basic size, which determines the size of the clearance or interference and its change, and reflects the mating properties.

The basics of tolerances and fits are as follows:

1. The limit size refers to the two limit values that allow the size to change.

2. The minimum solid state refers to the state when the hole or shaft is within the dimensional tolerance and has the least material. The size in this state is called the minimum solid size, which is a collective term for the maximum limit size of the hole and the minimum limit size of the shaft.

3. In the tolerance and fit ** (referred to as the tolerance zone diagram), determine a datum line of deviation, that is, the zero deviation line.

4. The base hole system is a system in which the tolerance zone of the basic deviation is a certain hole, and the tolerance zone of the shaft with different basic deviations forms a kind of cooperation.

5. In the fit between the hole and the shaft, the tolerance zone of the hole and the shaft overlaps each other, and any pair of holes and shafts are matched, which may have clearance or interference fit.

<>1. Clearance fit: The tolerance zone of the hole is completely above the tolerance zone of the shaft, that is, the fit with clearance, including the fit with the minimum clearance equal to zero.

2. Interference fit: The tolerance zone of the hole is completely below the tolerance zone of the shaft, that is, the fit with interference, including the fit with the minimum interference equal to zero.

3. Transition fit: In the fit between the hole and the shaft, the tolerance zone of the hole and the shaft overlaps each other, and any pair of holes and shafts are matched, which may have clearance or interference fit.

The fit tolerance is the amount of variation in the allowable clearance, which is equal to the absolute value of the algebraic difference between the maximum and minimum clearances, and is also equal to the sum of the hole tolerance zones and shaft tolerance zones that fit with each other.

Fit tolerance refers to the amount of variation that allows clearance or interference. It is the allowable value given by the designer to the degree of change in the fit tightness according to the requirements of the performance of the mating part of the machine. It reflects the degree of change in the tightness of the fit, indicates the accuracy of the fit, and is an important comprehensive index to evaluate the quality of the fit.

Numerically, it is an absolute value that has no positive or negative signs, and cannot be zero. Its numeric value is expressed by the formula as:

For clearance fit tf = xmax—xmin for interference fit tf = ymin—ymax for transition fit tf = xmax—ymax If the maximum and minimum clearance and interference are converted into the above equation by the limit size or limit deviation of the hole and shaft, respectively, the common formula for the fit tolerances of the three types of fits is as follows

tf = th +ts

th=｜ dmax- dmin ｜=es-ei｜ts=｜ dmax- dmin ｜=es-ei｜

Selection principle: economical, to meet the requirements of use.

1. Selection of benchmark system.

Preferential selection of base hole system;

When matched with the standard parts, the selection of the benchmark system is determined by the standard parts: the base hole system is selected for matching with the standard hole, and the base shaft system is selected for matching with the standard shaft;

When the hole (shaft) of the same basic size is matched with a multi-piece shaft (hole), the base hole (shaft) system should be selected.

2. Selection of tolerance grade.

On the premise of ensuring the use requirements, try to choose a lower tolerance level to reduce costs;

When the tolerance grade=IT8, the hole and the shaft can be matched at the same level, or the hole can be matched one level lower than the shaft, for example: H8 G7, H8 D8, etc.;

When the tolerance class is IT8, the hole and the shaft are matched at the same level; Example: H11 C11, D9 H9, etc.

3. Selection of matching types.

If there is relative movement after assembly, a gap fit should be selected;

If there is a requirement for positioning accuracy or disassembly after assembly, a transition fit should be selected, and the gap and interference should be small;

If the load is to be transferred after assembly, an interference fit should be selected.

When selecting tolerance zones, the selection is carried out in the order of priority tolerance zones, common tolerance zones, and general tolerance zones;

When selecting the fit, the selection is carried out in the order of priority fit and commonly used fit.

Fit tolerance is a form of tolerance commonly used in machinery manufacturing. In machinery manufacturing, due to the influence of various factors, including the accuracy of processing equipment, the accuracy of processing technology, the characteristics of materials, etc., there are often certain size differences between different parts of the same part, or between different parts. In order to ensure the interchangeability of the parts and the precision of the mechanical assembly as much as possible, the tolerance control of the parts is required.

Fitting tolerance refers to the tolerance set in order to ensure the accuracy of the fit between parts in mechanical manufacturing. It refers to the maximum clearance or minimum coincidence between two parts, i.e., the range of mating clearances or mating clearances. Fit tolerances are generally divided into two types: machining tolerances and assembly tolerances.

Machining tolerances refer to the tolerances set to ensure the dimensional accuracy of parts when manufacturing. The size of the machining tolerance depends on factors such as the precision of the processing equipment, the accuracy of the machining process, and the characteristics of the material. Machining tolerances can be divided into several types, such as basic deviations, limit deviations, and tolerance zones.

The basic deviation is the minimum difference between the standard size and the actual size, the limit deviation refers to the maximum deviation of the part size, and the tolerance zone refers to the range between the upper and lower limits of the part size.

Assembly tolerance refers to the tolerance set in order to ensure assembly accuracy during the assembly process of parts. The size of the assembly tolerance depends on the relative positional accuracy between the parts and how they fit together. Assembly tolerances can be divided into several types, such as axial tolerances, radial tolerances, and angular tolerances.

Axial tolerance refers to the distance tolerance between the axes of two parts, radial tolerance refers to the radial distance tolerance between two parts, and angular tolerance refers to the angular tolerance between two parts.

The design and control of fit tolerances is a very important part of machine building. On the one hand, reasonable fit tolerances can be coded to ensure the interchangeability and assembly accuracy of parts, improve the reliability and work efficiency of mechanical equipment, on the other hand, unreasonable fit tolerances will lead to a decrease in the fit accuracy of parts, and even affect the performance and life of the entire mechanical equipment. Therefore, in machinery manufacturing, it is necessary to reasonably set the fitting tolerance according to the specific situation, and carry out strict tolerance control and inspection to ensure the normal operation of machinery and equipment.

Fit tolerance refers to the sum of the tolerances of the holes and the vibratory shafts that make up the fit. It is the amount of movement that allows for a gap or interference.

There are three types of tolerance fits: gap fit (formerly known as dynamic fit), transition fit and interference fit (formerly known as static fit).

1. Clearance fit - there is obvious clearance between the shaft and the hole, and the shaft can rotate in the hole.

2. Interference fit - there is no gap between the shaft and the hole, the shaft and the hole are tightly connected together, and the shaft will not be able to rotate alone.

3. Transition fit - between the gap fit and the interference fit, there is a possibility of gap, there is a possibility of interference, such a fit can be used as a precision positioning fit.

Machining errors exist objectively, and any high-precision machine has machining errors, but the errors are relatively small. Interchangeability requires tolerances, which are the limits on the range of machining errors, or the tolerance for machining errors.

It can be seen that for the hole shaft fit of the same diameter 12, the same fit tolerance of 12(-0,, when the base hole system and the base shaft system are selected, the tolerance of the hole and the shaft is not the same. The advantage of the base hole system is: the hole is more difficult to process than the shaft, we can process the hole first, and then take different shafts to cooperate with him, and the transition interference gap can be processed at will.

If you choose the base shaft system, you will first process the shaft, the shaft tolerance is h0 h18, the maximum diameter of the shaft is 12, and the minimum tolerance is 12+ (the tolerance is negative).In this way, the shaft is machined, and then the hole of 12 is processed, and the transition fit, clearance fit or interference fit can be selected as needed. If the shaft is 12h6, it is 12(0,, interference fit.

Holes can be fitted with 12p7, which is 12(, and their tolerance fits 12p7h6= 12(-0, where -0 = the minimum size of the maximum size of the hole axis and the maximum size of the hole shaft 0.

The above content reference: Encyclopedia - Fit tolerance.