A thread is a surface formed by the helical movement of an arbitrary flat contour along the side surface of a cylinder or cone.
The most effective design solutions that increase thread fatigue resistance include the development of variants of asymmetric threads, threads with a variable step on the length of the screwing of the threaded pair, and an increase in the radius of rounding of the thread cavity. Under the terms of interchangeability and due to technological difficulties, practical implementation in high-strength bolts was achieved only by increasing the radius of rounding of the thread depression.
GOST 9150-81 for metric threads provides for the manufacture of a bolt thread with a rounded depression radius R = (0.1 0.144) Р, where Р is the thread pitch. Abroad and in some branches of the domestic industry, for the manufacture of fasteners from high-strength and titanium alloys, a thread according to the ISO 5855 standard, is used, the main profile of which for bolts is presented in fig. 1. The radius of rounding of the hollow of the thread of the bolt R according to ISO 5855 is equal to (0.180 0.150) P Further increase of the radius of rounding of the hollow of the thread is not possible, as it leads to a decrease in the static strength of the threaded connection. Calculations show that when the relative radius of the thread depression changes from 0.1P to 0.2P, the stress concentration factor in the working part of the thread changes from 7.75 to 5.35.
A thread according to the ISO 5855 standard only due to the increased radius of the cavity gives, compared to a metric thread according to GOST 9150-81, an actual increase in low-cycle fatigue by 30 and 43%, multi-cycle fatigue by 1.5-5.2 and 1.3-2, 7 times, respectively, for bolts made of titanium alloy VT16 and alloy structural steel 16ХСН. It should be borne in mind that the increase in the radius of rounding of the depression of the thread on the bolts required an increase in the inner diameter of the nut from 0.25 M to 0.312 N, where N is the theoretical height of the thread profile (Fig. 2).
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Fig. 1. Basic (1), maximum (2) and minimum (5) thread profiles of high-strength bolts
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Fig. 2. Basic thread profile of high strength connections
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This is necessary to exclude damage to the depression of the bolt thread of accuracy class 4h6h by the nut during screwing, which is certainly unacceptable in a high-strength threaded connection. To make fuller use of the design possibilities of increasing the fatigue resistance of high-strength bolts M l2 and higher, the use of nuts with an incomplete profile thread from the support end (Fig. 3) allows. At the same time, the low-cycle fatigue of bolts increases by at least 20%, the fatigue limit by 65-70%. Thread manufacturing technology has a significant impact on the fatigue resistance of threaded parts. Obtaining a thread according to optimal technical processes can increase the cyclic durability, the maximum amplitude of the alternating voltages of the cycle, increase thermal stability and corrosion resistance up to dozens of times.
Most researchers note an increase in the fatigue resistance of a rolled thread compared to a polished and cut one. This is facilitated by the continuous arrangement of metal fibers parallel to the profile of the thread, strengthening it (Fig. 4). For example, when transitioning from tapping to pumping threads on Mb bolts made of VT16 titanium alloy, the minimum value of low-cycle fatigue increases from 2600 to 6800 cycles, and the fatigue limit increases by a factor of 2. The influence of manufacturing technology is manifested through a number of factors of the quality of the surface layer of the thread, among which compressive residual stresses, their distribution in the surface layer of the hollow of the thread play a primary role. Their role is especially important in rolling threads, strengthening by surface plastic deformation and chemical-thermal methods.
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Fig. 3. Nut for high strength bolts |
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Fig. 4. Location of the metal fibers of the knurled thread |
It is known that the heat treatment of bolts with a rolled thread reduces their cyclic durability to the level of bolts with a cut thread. The strengthened surface layer loses its advantages, since compressive residual stresses in it are removed during heating, and tensile stresses may appear in the surface layers due to uneven cooling during hardening. At the same time, phase transformations occur, which are unevenly distributed across the cross-section, which sharply reduces fatigue resistance. Thus, to obtain the maximum fatigue resistance of the bolts on the thread, the rolling of the thread should be carried out after the complete heat treatment of the bolts. For alloy steel 16ХСН this gives an increase in cyclic durability by 5 times, fatigue limit by 1.6 times; for corrosion-resistant steel 03Х11Н10М2Т-ІЛ increase in cyclic durability by 1.76 times, endurance limits by 1.22 times; for heat-resistant steels 13X11H2V2MF-Shy 10X11H23T3MP, increase in cyclic durability by 4 and 2.5 times, endurance limits by 1.6 and 1.34 times, respectively.
The creation of thread compressive residual stresses in the cavity can be implemented not only by rolling or rolling the thread, but also by other PPD methods. Cyclic durability and endurance limit are increased after strengthening the thread with microbeads. This type of strengthening creates stable residual compressive stresses with a small degree and shallow depth of the riveted layer. Blowing with micro-balls can become an indispensable method of strengthening the thread in the manufacture of bolts from high-strength steels σв = 1850 MPa and higher, such as 03Н18К9М5Т-ЕЛ and others, when the rolling of the thread after strengthening heat treatment is complicated due to the low stability of the thread-forming tool.
The fatigue resistance of the thread is significantly affected by the chemical and thermal treatment and the condition of the coating. Nitriding, cementation, anodic oxidation lead to the appearance of compressive residual stresses in the surface layers of the thread, which increases the fatigue resistance of the bolts. Moreover, this effect can vary quite widely depending on the processing modes, the initial state of the surface, etc.
CLASSIFICATION OF CARVING
All threads can be divided into the following groups:
1) in shape– cylindrical, in which the top of the profile lies on a cylindrical surface, and conical, in which the top of the profile lies on a conical surface;
2) by location – an external thread, located on the outer surface of the part (screw, pipe, etc.) and an internal thread located on the inner surface of the part (nut, coupling);
3) according to the shape of the profile – triangular, trapezoidal, stable, round;
4) by the number of events – one-way and multi-way;
5) according to the direction of cutting – right and left;
6) by size – metric and inch;
7) by appointment – general and special purpose.
BASIC CARVING STANDARDS
SAE - (Society of Automotive Engineers) – American standard, inch thread (60°);
JIC - (Joint Industrial Counsil) - American standard, inch thread (60°);
JIS - (Japanese Industrial Standard) - Japanese standard;
DIN - (Deutsche Institut fur Normung) - German standard, metric thread (60º);
BSP - (British Standard Pipe) - English standard;
BSPT - pipe conical (Tapered) thread (55°);
BSPP - pipe cylindrical (Parallel) thread (55°);
BSW - ( British Standard Whitworth) - English standard, Whitworth cylindrical thread (55°);
UN - (Unified thread) - American standard (60°);
UNF- small (Fine) inch thread;
UNC - large (Coarse) inch thread;
UNS - special thread;
NPT - (National Pipe Tapered) - American standard (60°);
NPTF - pipe conical thread for fuel (fuel);
NPSM - pipe cylindrical (Straight Mechanical) thread;
SF - (Straight Flange) - straight flange connection ORFS - flange connection with sealing Pro-Ring;
GOST 9150-81 - metric thread (60°) - analogue of ISO;
GOST 25229-82 - metric conical thread (60°);
GOST 6111-52 - conical inch thread (60°) - analogue of NPTF;
GOST 6357-81 - pipe cylindrical thread (55°) - analogue of BSPP;
GOST 6211-81 - pipe conical thread (55°) - analogue of BSPT;
GOST 633-80 - pipes are smooth, highly hermetic and with planted; outward ends, and couplings to them (NKT).