TECHNOLOGY

Optimized Thread Geometry to Improve
Connection Strength
優化螺紋形狀以提高接合強度
This article analyzes a systematic approach to increasing the strength and durability of fastening connections basedon optimization on thread geometry. It is shown that small diameter threads come with a large pitch. Threads of large
diameter with a reduced level of defects are fasteners with a small pitch, which, as the main one, is much larger in absolutevalue. Optimization in the form of unification is proposed to bring the variety of thread geometry to a rational minimum toimprove the performance of the fastening connection.

The appearance of carving is considered thebeginning of the industrial revolution. Now, threaded
connections of fasteners, due to their simplicity,reliability and versatility, are widely used for almost
any industrial equipment. At the same time, it ispromising for improvement in various fields, including
new industries. Improving the design of a threaded
connection can be aimed at increasing the strengthcharacteristics without reducing the ductility of thefastener metal due to two great advantages of threads.

Advantages of Threads
Firstly, the detachable connection of parts andelements makes it possible to design and manufacturecomplex machines and structures with expanded
production of their modifications. Secondly,
dismantling and simple replacement of worn, damaged
and destroyed fasteners ensures quick restoration offunctionality and extends the service life of machines
and structures while reducing the costs associated with
their use and maintenance.

Systematic Approach
in Absence
H o w e v e r, d e s p i t e t h e s e a d v a n t a g e s o fthreads and long-standing use with expandingstandardization, a systematic and well-founded
approach to increasing the strength of fasteners
under static and variable loads is practicallyabsent. This is due to the fact that the potential

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Fastener World no.207/2024 惠達雜誌of material science to increase the strength of fastening joints is, as arule, limited by the technological deformability of materials. When both
the design possibilities of thread profile geometry have a more promisingpotential for their strength optimization, it logically follows from an analyticalconsideration of the inconsistency and irrationality of standards for thegeometry of fastening threads.
Used for several hundred years and standardized since the 19th century,
fastening threads are the same from a theoretical point of view, since they
have common construction principles and are characterized by almost thesame set of geometric parameters. A screw thread in technology is a group ofalternating projection and depressions of a certain extent on the surface of abody of revolution, which is the main geometric element of a threaded fastener.
At the same time, threads for connecting bolts, screws, studs and nuts can beconsidered as an inevitable design defect such as an inclined screw notch,
which has a certain helix angle and can be subject to the combined effects
of a combination of shear and pullout stresses. Typically, the value of sucha structural defect for a fastener made of high-strength steel is taken to be
the working height of the thread profile, calculated from the pitch and radiusof curvature of the thread root. It is obvious that, for example, a high andoverloaded protrusion in the form of an equilateral triangular profile can leadto the formation of a crack in the thread root and failure of the fastener undertension or bending conditions, since the thread of the connection is in a complex
stress state and is characterized by a high stress concentration, especially inthe first turns, the most likely sources of deformation and destruction. The
influence of stress concentration is taken into account by the theoretical stressconcentration coefficient in relation to the maximum tensile stress in themost loaded zone of the thread to the nominal stress, taking into account theinternal and average diameters of the thread. It can be noted that calculationsof the theoretical stress concentration factor in the thread of a connection, forexample, for the main connector of a nuclear power reactor vessel, are carriedout provided that its minimum value is not less than 4.0.

TECHNOLOGY
Systematic Assessment in Absence
In numerous reviews and studies of sets of fastener performance characteristics for resistance to deformation and fracture under
stationary and variable loading of the connection, there is still no systematic assessment of the influence of thread geometry taking into
account the unfavorable load distribution along the turns of the connection. It can also be noted that the practice of thread geometry,
which has existed for about 200 years, in particular, the inch and metric threads of Whitworth and Sellers, respectively, has not undergone
fundamental changes, despite the development of screw-cutting lathes and other industrial technologies, starting from the late 18th andearly 19th centuries. For example, in Germany at the end of the 19th century and in the USA in the second half of the 20th century, thenumber of national standards was very large: 11 thread systems with 274 varieties and about 8 thousand standards (taking into accountspecial-purpose fasteners, up to 2 million varieties that did not become the subject of system optimization of thread geometry).
According to GOST 24705-2004 (ISO 724: 1993), for a
metric profile thread for a nominal diameter, for example, M20
or M48, instead of 3, 4 or 5, as many as 6 options are given fora practically ranked series of large, medium and small thread
pitches. For these threads with the so-called normal thread
pitch, the ratio of diameter to thread pitch is in the range of8.0…9.6. Also, according to the standard, the diameters M140
and M180 for the case of a main or large pitch of 8.0 mm (which is practically not used and at the same time is the firstdesignated nominal pitch) give a ratio of diameter to pitchin the range of 17.5…22.5 , which is approximately 2 times
more than for threads of smaller diameter M20 and M48. This
indicates that the designation of the geometric parameter of
the thread as large or main, as well as small, is not objective
enough and is not justified in any way. Because standard
large diameter threads with the largest pitch are essentiallyfine pitch threads and are less defective than smaller
diameter threads. But the M20 and M48 threads have avery large pitch geometry and are more defective due tothe larger relative height of the thread profile. It is notdifficult to see that the lack of consistency and unjustified
diversity of thread geometry within the framework of anon-evolving standard contradicts the logic of ensuring thestrength and performance, including resistance to brittle
fracture, of a threaded connection.
In the past, mainly in the 19th and 20th centuries, fastening
threads were designed for stationary and moderate loading
conditions and the use of coarse pitches was structurally andtechnologically acceptable. Currently, fastening threads havebecome more often used under higher static tensile loads, aswell as under conditions of low- and high-cycle fatigue loading,
not only in air, but also in various corrosive environments. Thisinitiated the development of fastening materials that combine
increased strength with a simultaneous increase in corrosion
resistance through alloying and the use of surface strengthening
and protective coatings. At the same time, despite this, thelikelihood of a metallurgical, power or corrosion defect suchas a small crack in the thread due to significant defects inthe thread itself and high uneven load distribution along theturns, including the growth of this crack taking into accountthe height of the thread profile, remains significant. It should
be noted that the overload of the first working thread canbe up to two times higher than the load of the free thread.
A small thread pitch instead of a large one due to thereduction of stress concentration, uneven distribution ofload along the threads and their more effective unloading
by adjacent threads, allows increasing the resistance offastening threads to static, cyclic and brittle fracture.

Unified Thread Geometry
Options Await Development
For example, the transition from M12x1,75 stud threads with athread radius of 0.14mm to the M12x1.0 version with 0.12 mm madeof 38KhN3MFA steel with a yield strength of 900 N/mm2 made it
possible to increase the resistance of the nut connection to low-cycle
fatigue based on 10,000 cycles before failure by 25% based on stresses
in the net cross-section of the stud. In terms of the number of cycles, thedurability of the fastening joints increased by 2.5 times at a maximum
cycle voltage of 700 N/mm2. Photo 1 shows fragments of the fractureof the studs, where it is clear that for a pitch of 1.0 mm, the formationof a viscous and neck-shaped fracture surface took place. This type offracture is close to the classic static axisymmetric fracture of a smoothcylindrical sample. A small pitch, as it is more technologically advanced,
in production conditions allows to reduce the costs of cutting tool wear
and waste, as well as increase its durability when rolling threads. The
use of small steps should be limited by the resistance to destruction of
the threaded connection by “chain” shear under conditions of collapse orcutting of thread turns under static loads. Thus, we can conclude that it
is advisable to develop new and unified thread geometry options toimprove the performance of the fastening connection. The reliabilityof more loaded and maneuverable machines and structures will beensured through system optimization of a set of thread geometry
parameters.

Photo 1: An example of the influence of metric thread geometryon the resistance to low-cycle fatigue failure of a M12 stud-nut
fastening connection: 1- pitch thread 1,0mm; 2- pitch thread 1,75mm.
The material of the studs and nuts is steel 38KhN3MFA with a yieldstrength of 900N/mm2.

Copyright owned by Fastener World / Article by Vladimir Igorevich Gorynin惠達雜誌 Fastener World no.207/2024

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