Tolerance and planting in engineering

Metrology is the science of measurements, means and methods of ensuring their unity, as well as ways to achieve the required accuracy. Its subject is the allocation of quantitative information about the parameters of objects with specified reliability and accuracy. The normative basis for metrology is standards. In this article we will consider the system of tolerances and landings, which is a subsection of this science.

The concept of the interchangeability of parts

At modern factories, tractors, cars, machine tools and other machines are produced not by units and not by tens, but by hundreds and even by thousands. With such volumes of production, it is very important that each part or assembly produced during assembly accurately approaches its place without additional locksmith fits. After all, such operations are rather laborious, expensive and take a lot of time, which in mass production is not permissible. Equally important is that the parts arriving at the assembly allow replacement for other common purposes, without any damage to the functioning of the entire finished unit. Such interchangeability of parts, units and mechanisms is called unification. This is a very important moment in machine building, it saves not only a costly part for the design and manufacture of parts, but also the production time, in addition, product repair due to its operation is simplified. Interchangeability is the property of nodes and mechanisms to take their places in products without preliminary selection and perform their basic functions in accordance with the technical conditions.

Pairing parts

Two parts, immovable or movably connected to each other, are called interfaced. And the value, by which this joint is carried out, is usually called the mating size. An example is the diameter of the hole in the pulley and the corresponding shaft diameter. The amount by which a connection does not occur is usually called free size. For example, the outside diameter of the pulley. To ensure interchangeability, the interconnected values of the parts must always be precisely executed. However, such processing is very complicated and often inexpedient. Therefore, the technique employs a method for obtaining interchangeable parts when working with the so-called approximate accuracy. It consists in the fact that for different operating conditions the nodes and parts specify permissible deviations in their dimensions, at which the functioning of these parts in the unit is perfectly possible. Such indents, calculated for a variety of working conditions, are built in a given defined scheme, its name is "a unified system of tolerances and landings."

The concept of tolerances. Characteristic of quantities

The calculated part data, supplied in the drawing from which the deviations are counted, is usually called the nominal size. Usually this value is expressed in whole millimeters. The size of the part, which is actually obtained during processing, is called valid. The values between which this parameter oscillates are usually called the limiting values. Of these, the maximum parameter is the largest limit size, and the minimum parameter is the smallest. Deviations are the difference between the nominal and the maximum value of a part. In the drawings, this parameter is usually designated in numerical form at the nominal size (the upper value is indicated above and the lower value is lower).

Example record

If the figure shows the value 40 ^+0.15 _-0.1 , it means that the nominal size of the part is 40 mm, the largest limit is +0.15, the smallest -0.1. The difference between the nominal and the maximum limit is called the upper deviation, and between the minimum - the lower one. From here, the actual values are easily determined. From this example it follows that the maximum limit value will be 40 + 0.15 = 40.15 mm, and the smallest: 40-0.1 = 39.9 mm. The difference between the smallest and largest limit sizes is called tolerance. Calculated as follows: 40,15-39,9 = 0,25 mm.

Gaps and pulls

Let's consider a concrete example, where tolerances and plantings are of key importance. Suppose that we need a piece with a hole 40 ^{+0.1 to be} placed on a shaft with dimensions 40 ^-0.1 _-0.2 . From the condition it can be seen that the diameter for all variants will be smaller than the hole, and so with such a connection there will necessarily be a gap. This landing is usually called movable, because the shaft will freely rotate in the hole. If the part size is 40 ^+0.2 _+0.15 , then under any condition it will be larger than the hole diameter. In this case, the shaft must be pressed in and a tension arises in the joint.

conclusions

Based on the above examples, the following conclusions can be drawn:

• The gap is the difference between the actual dimensions of the shaft and the hole, when the latter are larger than the first. With this connection, the parts have free rotation.
• It is customary to call the difference between the actual dimensions of the hole and the shaft, when the latter is larger than the first. With this connection, the parts are pressed in.

Landings and accuracy classes

Landings are usually divided into fixed (hot, press, lightweight, deaf, tight, dense, tense) and mobile (sliding, running, moving, light-pass, wide-pass). In machine and instrument engineering, there are certain rules that govern the tolerances and plantings. GOST provides for certain classes of accuracy in the manufacture of nodes using specified deviations in size. It is known from practice that the details of road and agricultural machines without harm to their functioning can be made with less accuracy than for turning machines, measuring instruments, cars. In connection with this, tolerances and planting in machine building have ten different accuracy classes. The most accurate of them are the first five: 1, 2, 2a, 3, 3a; The following two relate to the average accuracy: 4 and 5; And the last three to the rude: 7, 8 and 9.

In order to know by what accuracy class the part should be manufactured, in the drawing next to the letter meaning the landing, put a number indicating this parameter. For example, marking C4 means that the type is sliding, class 4; X3 - type running, class 3-rd. For all landings of the second class, the digital designation is not set, since it is the most common. To receive the detailed information on the given parameter it is possible from the two-volume directory "Tolerances and landing" (Myagkov VD, 1982 edition).

Shaft and Hole System

Tolerance and planting are considered to be two systems: a hole and a shaft. The first of them is characterized by the fact that in it all types with one degree of accuracy and class belong to the same nominal diameter. The holes have constant values of limiting deviations. The variety of plantings in such a system is obtained as a result of a change in the limiting deviation of the shaft.

The second of them is characterized by the fact that all types with one degree of accuracy and class belong to the same nominal diameter. The shaft has constant values of limiting deviations. Variety of plantings is realized as a result of changes in the values of the limiting deviations of the holes. In the drawings, the system of holes is usually denoted by the letter A, and the shaft by the letter B. A sign of the accuracy class is placed near the letter.

Examples of notation

If "30A3" is indicated on the drawing, it means that the part in question has to process the holes of the third accuracy class, if "30A" is specified, it means that the same system, but the second class. If the tolerance and fit are made according to the shaft principle, then the nominal size is indicated by the required type. For example, a part with the designation "30B3" corresponds to processing on the shaft system of the third accuracy class.

In his book MA Palei ("Tolerances and planting") explains that in mechanical engineering the hole principle is used more often than the shaft. This is due to the fact that it requires lower costs for tools and tools. For example, in order to process a hole of a given nominal diameter by this system, for all landings of this class only one scan is needed, to change the diameter - one limiting plug. With the shaft system, a separate sweep and a separate plug are needed to ensure each landing within the same class.

Tolerances and plantings: table of deviations

To determine and select the accuracy classes, it is customary to use special reference literature. So, the tolerances and landings (the table with an example is given in this article) are, as a rule, very small values. In order not to write extra zeros, in the literature they are designated in microns (thousandths of a millimeter). One micron corresponds to 0.001 mm. Usually in the first column of such a table indicate the nominal diameters, and in the second - the deviation of the hole. The remaining graphs give different values of plantings with their corresponding deviations. A plus sign near this value indicates that it should be added to the nominal size, the minus sign indicates that it should be subtracted.

Threads

The tolerance and fit of threaded joints must take into account the fact that the thread is mated only along the sides of the profile, except that they can only be vapor-proof types. Therefore, the main parameter, which determines the nature of the deviation values, is the average diameter. The tolerance and fit for the outer and inner diameters are set so as to completely eliminate the possibility of pinching along the valleys and thread tops. The errors in decreasing the outer dimension and increasing the internal value will not affect the screwing process. However, deviations in the thread pitch and profile angle will cause the fastener to jam.

Thread tolerances with clearance

The most common are clearance and landing with clearance. In such joints, the nominal value of the mean diameter is equal to the largest average thread size of the nut. Deviations are taken from the line of the profile perpendicular to the axis of the thread. This is determined by GOST 16093-81. Tolerances for thread diameter nuts and bolts are assigned depending on the specified degree of accuracy (denoted by a number). The following range of values of this parameter is accepted: d1 = 4, 6, 8; D2 = 4, 6, 7, 8; D1 = 4, 6, 7, 8; D2 = 4, 5, 6, 7. Tolerances for them are not established. The placement of the thread diameter fields relative to the nominal profile value helps to determine the basic deviations: the upper ones for external bolt values and the lower ones for the internal values of the nuts. These parameters directly depend on the accuracy and the connection step.

Tolerances, plantings and technical measurements

For the production and processing of parts and mechanisms with given parameters, the turner has to use a variety of measuring tools. Usually, for rough measurements and checking the dimensions of products, rulers, calipers and calipers are used. For more accurate measurements - calipers, micrometers, calibers, etc. What is a ruler, everyone knows, so we will not dwell on it.

Calipers are a simple tool for measuring external dimensions of machined parts. It consists of a pair of rotatable curved legs fixed on one axis. Still there is a spring type of calipers, it is exposed to the necessary size with a screw and nut. Such a tool is slightly more convenient than a simple one, since it preserves a given value.

The inductor is intended for taking internal measurements. There are regular and spring type. The device of this tool is similar to the calipers. The accuracy of the devices is 0.25 mm.

The caliper is a more precise adaptation. It can measure both external and internal surfaces of machined parts. Turner when working on a lathe uses calipers to take measurements of the depth of the undercut or ledges. This measuring tool consists of a bar with divisions and sponges and a frame with a second pair of jaws. With the help of the screw, the frame is fixed on the rod in the required position. The measurement accuracy is 0.02 mm.

Shtangenglubinomer - this device is designed for measuring the depth of grooves and grooves. In addition, the tool allows you to determine the correct position of the ledges along the length of the shaft. The device of this device is similar to a caliper.

Micrometers are used to accurately determine the diameter, thickness and length of the workpiece. They give a reading with an accuracy of 0.01 mm. The measured object is located between the micrometer screw and the fixed heel, adjustment is performed by rotating the drum.

Hematrices serve for accurate measurements of internal surfaces. There are permanent and sliding appliances. These tools are rods with measuring ball ends. The distance between them corresponds to the diameter of the hole being determined. Measuring limits for the caliper are 54-63 mm, with the additional head, diameters up to 1500 mm can be determined.