Metrology - the science of measurement, means and methods to ensure 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 the specified accuracy and precision.The regulatory framework for metrology - is standard.In this article, we will consider a system of tolerances and fits, is a subset of this science.
notion of interchangeability of parts
In modern plants tractors, automobiles, machine tools and other machinery not produced units not tens but hundreds and even thousands.With such volumes of production it is important that each manufactured part or assembly in the assembly exactly suited to his seat without additional plumbing adaptations.After all these operations is quite time-consuming, costly and time-consuming that the mass production is not allowed.No less important is the fact that the parts coming to the assembly, allowed the replacement of other general-purpose with them, without prejudice to the operation of all the finished machine.This interchangeability of parts, assemblies and mechanisms called unification.This is a very important moment in engineering, it can save not only costly part in the designing and production of details, and the production, in addition, simplifies repair of the product as a result of its operation.Interchangeability - this property units and mechanisms to hold their places in products without prior selection and perform its core functions in accordance with specifications.
mating parts
two parts, fixed or movable interconnected, called mating.A value for which the joint is usually called matched in size.As an example, the diameter of the holes in the pulley and the corresponding diameter of the shaft.The value at which there is no union, referred to as a free size.For example, the outer diameter of the pulley.To ensure the interchangeability of parts matched the value must always be accurate execution.However, this process is very complex and often impractical.Therefore, in the technique applicable method for producing interchangeable parts when dealing with so-called approximate accuracy.It lies in the fact that for various operating conditions or parts define the tolerances of dimensions that allow flawless operation of these parts in assembly.Such margins calculated for a variety of working conditions, built at a predetermined specific pattern, its name is - "a unified system of tolerances and fits."
concept of tolerance.Characteristic values
calculated data items supplied in the drawing, which made the count abnormalities, called the nominal size.Typically, this value is expressed in integers millimeters.Part size, which is actually obtained by a process called valid.The values range between which the option is called the limit.Of these, the maximum option - it is the largest size limit, and the minimum - the smallest.Deviations - the difference between the nominal value and maximum detail.In the drawings, this option can be designated in numerical form at a nominal amount (the upper value specified above, and the bottom - below).
Example entries
If the figure is set to 40+ 0,15-0,1, it means that the nominal size of the details - 40 mm, the largest limit - 0.15, the lowest - -0.1.The difference between the nominal value and the maximum limit is called the upper deviation between the minimum and - lower.This easily determined by the actual values.From this example it follows that the maximum limit value is equal to the 40 + 0.15 = 40.15 mm, and the smallest: 40-0,1 = 39.9 mm.The difference between the minimum and maximum size limit is called tolerance.Calculated as follows: 40,15-39,9 = 0.25 mm.
gap and tension members
Consider a specific example where the tolerances and fit are crucial.Suppose that we need to part with an opening 40 +0.1 spread on shaft sizes 40-0,1-0,2.Because conditions can be seen that for all embodiments, the diameter is smaller than the hole, and therefore when such a connection is required to occur gap.This land is called mobile, t. To. The shaft will rotate freely in the hole.If the size of the items will be 40 ± 0.2 ± 0.15, while in any condition it will be larger than the diameter of the hole.In this case, the shaft must embed and interference arises in the connection.
Conclusions Based on the above examples, you can draw the following conclusions:
- Clearance is the difference between the actual size of the hole and shaft when the latter more than the former.With this connection parts are free to rotate.
- tightness is called the difference between the actual size of the hole and shaft when the latter is greater than the first.In this connection parts are pressed.
landing and accuracy classes
Landings are divided into fixed (hot pressing, legkopressovaya, dull, tight, tight, tight) and mobile (moving, running, movement, legkohodovaya, shirokohodovaya).In mechanical engineering and instrumentation, there are certain rules that govern admissions and landings.Standard provides certain classes of precision in the manufacture of assemblies using predefined deviations in size.From practice it is known that part of road and agricultural machinery without damage to their functioning can be manufactured with less precision than for lathes, measuring instruments, car.In this regard, tolerances and fit in mechanical have ten different classes of precision.The most accurate of which - a first five: 1, 2, 2a, 3, 3a;The following two are the mean of precision: 4 and 5;and the last three to gross: 7, 8 and 9.
To find out what class of accuracy to be manufactured item, the figure next to the letter, meaning the land, put the figure indicating the setting.For example, labeling C4 means that the sliding type, class 4 minutes;X3 - the type of moves, 3rd class.For all landings of the second class digital signage is not intended, as it is most prevalent.Get detailed information on this parameter can be of the two-volume reference book "Tolerance and landing '(soft VD, 1982 edition).
system and shaft holes
tolerance and landing usually considered as two systems: the hole and shaft.The first is characterized by the fact that it all types with one degree of accuracy and class belong to the same nominal diameter.The holes have a constant value tolerances.A variety of planting in such a system is the result of changing the limit shaft deflection.
The second one is characterized by the fact that all types with one degree of accuracy and class belong to the same nominal diameter.The shaft has a constant value tolerances.A variety of landings carried out by changing the values of tolerances holes.In the drawings of the holes is denoted by the letter A, and the shaft - the letter B. Near the letters placed the sign of accuracy class.
Examples designations
If the figure indicated "30A3", then it means that it is considering the item must be processed by the system holes of the third class of accuracy, if it says "30A", then on the same system, but the second class.If the tolerance and landing are made on the basis of the shaft, then the nominal size indicates the desired type.For example, the item labeled "30V3" corresponds to the processing system of the shaft of the third class of accuracy.
In his book, M. Paley ("Tolerances and landing") explains that the engineering principle is applied more holes than the shaft.This is due to the fact that it requires less equipment and tools.For example, to treat a given nominal diameter of the hole in this system, all landings of this class requires only one scan, for diameter - one limit stopper.Under the system of the shaft for each landing as part of a class requires a separate scan, and a separate tube.
Tolerances and landing: a table of deviations
To determine the accuracy classes and the choice made use of a special reference books.So, tolerances and landing (table example shown in this paper) are usually very small quantities.In order not to write extra zeros in the literature to be marked in micrometers (thousandths of a millimeter).One micron corresponds to 0.001 mm.Typically, in the first column of this table indicates the nominal diameter, and the second - the deviation of the hole.The remaining columns give different values of landings with their respective deviations."Plus" sign next to this value indicates that it should be added to the nominal dimension, "minus" sign - that it should be deducted.
Threads
tolerance and landing threaded connections should take into account the fact that the thread is mated only on the sides of the profile, the exception may be only vapor barrier types.Therefore, the main parameter that determines the nature of the deviations - is the average diameter.Tolerance and fit for external and internal diameter set so as to completely exclude the possibility of jamming the thread crests and troughs.Reduce errors and increase the size of the outer inner values do not affect the process of screwing.However, the deviation in step thread profile angle and lead to jamming of the fastener.
tolerances thread with clearance
most common are tolerance and loose fit.In such compounds, the nominal value of average diameter equal to the largest average value of the thread nut.Deviations made by counting the line perpendicular to the axis of the thread profile.It is defined by GOST 16093-81.The tolerances for the diameter of the threads of nuts and bolts are assigned depending on the desired degree of accuracy (indicated by a number).Adopted the following range of values of this parameter: D1 = 4, 6, 8;q2 = 4, 6, 7, 8;D1 = 4, 6, 7, 8;A2 = 4, 5, 6, 7. Tolerances are established for them.Placing field diameter of the thread with respect to the nominal profile contributes to the definition of the main variations: the upper exterior bolts and lower values for internal variables nuts.These parameters are directly dependent upon the accuracy and the pitch joint.
tolerances, planting and technical measurements
for the production and processing of parts and mechanisms to the settings turner must use a variety of measuring tools.Normally rough measurements and checking the sizes of products using a ruler, callipers and callipers.For more accurate measurements - calipers, micrometers, gauges, and so on. D. What is the lineup, everyone knows, so we will not dwell on it.
calipers - a simple tool for measurement of the external sizes of workpieces.It consists of a pair of curved legs swivel mounted on the same axis.Yet there is a kind of spring caliper, put it to the desired size using the screw and nut.Such a tool more convenient to just a little, that is. A. Maintains a predetermined value.
Nutromer designed to relieve internal measurements.Sometimes conventional and spring type.A device of this instrument is similar to the caliper.The accuracy of the instrument is 0.25 mm.
Caliper - a more precise tool.They can be measured by both external and internal surfaces of workpieces.Turner at work on a lathe using a caliper for measuring the depth of removing undercuts or ledges.This measuring instrument consists of a rod with graduations and jaws and frame with a second pair of jaws.With the frame is fixed by a screw on the rod in the desired position.Measurement accuracy is 0.02 mm.
SHTANGENGLUBINOMER - This device is designed for measuring the depth of the grooves and recesses.In addition, the tool allows you to determine the correct position of the benches along the length of the shaft.The unit of this device is similar to the caliper.
micrometers applied to accurately determine the diameter, thickness and length of the workpiece.They provide readout accurate to 0.01 mm.Measurement object is located between the micrometer screw and fixed heel, adjustment is performed by rotating the drum.
Nutromery used to make precise measurements of internal surfaces.There are fixed and sliding devices.These tools are a rod with measuring ball ends.The distance between them corresponds to the diameter defined by the opening.Measurement range up to bore gauge 54-63 mm, if more heads can be determined by the diameter up to 1500 mm.
