Peculiarities of formation of fits of preferential application in the system of hole and shaft

. The article deals with the issues of the influence of accuracy parameters on the reliability and durability of assembly units of machine units. The accuracy of the connection is determined by the type of fit, formed gaps and tightness. Compliance with accuracy standards significantly reduces the level of external losses from marriage. However, a variety of deviations and tolerances of the elements of parts can lead to a wide range of fits, which is not economically viable. Insufficient accuracy can lead to damage to parts, leakage of liquids and gases, as well as to a decrease in the performance of the machine as a whole. Therefore, machine manufacturers are striving to improve the accuracy of the connection of parts and the use of more precise production methods. Seating systems simplify the manufacturing process and reduce the number of metal cutting tools needed.


Introduction
The reliability and durability of assembly units of various machines is influenced not only by the parameters of strength, rigidity, wear resistance of the applied materials of the surfaces of the parts being joined, but also by the parameters of accuracy [1,2,3]. Joint accuracy is characterized by the type of fit and specific formed gaps and interference [4,5]. Observance of accuracy norms significantly reduces the level of external losses from defects [6].
The variety of basic deviations and standard tolerances of parts can lead to a very wide nomenclature of fits, which is not economical, because for machining holes (drills, countersinks, reamers, broaches) 28 tools of the same name per nominal size would have to be available. The standards therefore establish two equal systems for the formation of fits: the hole system (cH) and the shaft system (ch), in which a constant (reference) position of one of the tolerance fields (hole or shaft) is assumed.
Basic shaft -shaft chosen as a basis for a shaft-basis fit system (h) [7,8].
Basic hole -hole chosen as a basis for a hole-basis fit system (H) [7,8].
Hole-basis fit system -fits where the fundamental deviation of the hole is zero, i.e. the lower limit deviation is zero (Fig. 1, a).
Shaft-basis fit system -fits where the fundamental deviation of the shaft is zero, i.e. the upper limit deviation is zero (Fig. 1, b). 1, 2, 3 -fits, respectively, with a clearance, transition and interference in the hole-basis fit system; 4, 5, 6 -fits, respectively, with a clearance, transition and interference in the shaft-basis fit system; I -basic hole; II -shafts for fits; III -tolerance field of the basic hole; IV -holes for fits; V -basic shaft; VI -tolerance field of the basic shaft

Theoretical position
Two fit systems are necessary not only because of the design features of the machine components [9], but also because of the peculiarities of the manufacturing technology of machine parts and their assembly [10]. For a particular connection, it does not matter in which system the tolerances and landings are assigned, since only the values of clearance or interference characterize the quality of its work. The choice of the system determines the complexity of manufacturing parts and their assembly, and, consequently, the cost of manufacturing the assembly unit and the unit. Preference is given to the hole system, since it is usually much more difficult and expensive to make and measure a hole than to make and measure a shaft of the same size and the same accuracy.
So, shafts of various (and high) accuracy can be processed and measured with universal tools -cutters, grinding wheels, micrometers, etc. And for processing and measuring precise holes, special expensive tools (drills, reamers, broaches, plug gauges) will be required. The number of sets of such tools required to machine holes with the same nominal size depends on the variety of tolerances that can be assigned by the designer.
The shaft system is chosen based on a number of design, technological or operational considerations, but only when it is economically profitable. But the use cases of the shaft system are very limited.
Fits in the shaft system are chosen in the following cases: when shafts made of light-stretched calibrated material (silver) are used without additional mechanical processing of seats; when it is necessary to provide different landings of several parts on separate sections of the shaft (Fig. 2); when standard assemblies or parts made in the shaft system are used in the coupling (for example, fitting the outer rings of rolling bearings into the housing); when, according to the strength conditions, it is impossible to make the shaft stepped; when, according to technological conditions, when, for example, during repair, there is a shaft processed for the repair size (with a decrease) and a hole is made for it and in other justified cases. I -wrist pin; II -connecting-rod; III -piston; 1…3 -connection number To make it even more convenient to assign landings for the designer and the processing of parts for the worker, it was agreed that the tolerance fields of the main parts of the landing systems must satisfy one mandatory condition: one of the maximum dimensions of the main part must match the nominal size. Moreover, for the main hole, this limit size should be the smallest. The tolerance of the main part of the landing system is always directed "into the body" of this part: in the case of the main hole, to increase the limit size compared to the nominal one; in the case of the main shaft -to reduce the limit size compared to the nominal.

Shaping of fits
The design of the connections of parts and the requirements for them may be different. Depending on the purpose of the connection, the structural elements of parts with mating surfaces that have the same nominal size must, during the operation of the mechanism or machine, either provide the possibility of movement of the parts relative to each other.

In the hole-basis fit system
In the shaft-basis fit system The shape of the piston pin To ensure the mobility of the connection, it is necessary that the actual size of the enclosing element of one part (hole) was larger than the actual size of the male element of another part (shaft).
To obtain a fixed connection, it is necessary that the actual size of the male element of one part (shaft) be greater than the actual size of the female element of another part (hole).
ISO 286-2:2010 establishes a basic set of tolerance fields as a combination of some basic deviations and qualifications, comprising 72 tolerance fields for holes and 80 tolerance fields for shafts.
In addition to the basic set, the annex to ISO 286-2:2010 provides an additional set of 34 shaft tolerance fields and 32 bore tolerance fields which is non-preferred.
The basic and supplementary set of tolerance fields gives significantly more tolerance fields than are practically used. Therefore, from the basic set of tolerance fields the preferred application tolerance fields are highlighted, including 10 tolerance fields for bores and 16 tolerance fields for shafts (Table 1). It is allowed to use any tolerance field from the main or additional set. However, in order to prevent unreasonable diversity, the following procedure for selecting tolerance fields has been established: first of all, preferred tolerance fields should be used; if it is impossible to ensure design and technological requirements due to preferred tolerance fields, tolerance fields from the main set should be used; in some technically justified cases, it is allowed to use tolerance fields from an additional set.
Tolerance fields not provided for by the standard are considered special. They are used when technically and economically justified and may be based on other standards for the relevant product (eg rolling bearings), materials (eg plastic products) or processing methods.
In the system of tolerances for linear dimensions, it is theoretically allowed to use any fit in the hole-basis fit or shaft-basis fit system. The standard recommends 68 fits for use, and qualifications from 5 to 12 are used for holes and from 4 to 12 for shafts. Of these, fits of preferred use are highlighted. There are 17 such fits in the hole system, and 10 in the shaft system (Table 2). Thus, in order to reduce the unreasonable diversity and form a given, economically justified, range of cutting tools for making holes, it is proposed to use 4 tolerance fields in the hole-basis fit system -H7, H8, H9 and H11, and in the shaft-basis fit system 9 tolerance fields -H7, F8 , H8, E9, H11, Js7, K7, N7 and P7.

Conclusions
The existing system of tolerances and fits in mechanical engineering is necessary for the standardization of parts in order to ensure their interchangeability. For this, the dimensions of products are performed with a certain accuracy, which is regulated by standards.
Fits are chosen depending on the purpose and operating conditions of the equipment and mechanisms, their accuracy, assembly conditions. At the same time, it is necessary to take into account the possibility of achieving accuracy with various methods of processing the product. Preferred fits should be applied first.