FORMING THE FUTURE
Proceedings of the
13th International Conference
on the Technology of Plasticity
EDITED BY
Glenn Daehn • Jian Cao • Brad Kinsey
Erman Tekkaya • Anupam Vivek
Yoshinori Yoshida

Development of Anti-Loosening Bolts
Based on Innovative Double Thread
Mechanism
Teruie Takemasu, Toshinaka Shinbutsu, Shuichi Amano, and Jyo Shimura

Abstract Anti-loosening bolted joints were developed with an innovative doublethread mechanism composed of coaxial single and multiple coarse threads. The
number of coarse threads of the multiple-thread was set to 3 (denoted as 3thread
DTB-II), and its thread structure was fundamentally modified as follows: (i) oneof the three multiple-thread grooves was removed for 3thread DTB-II specimens
and one of the two remaining grooves shifted downwards by a half pitch (denoted
3-1thread DTB-IIB) and (ii) the depth of the multiple-thread grooves was reduced
by up to 50% of the thread height (denoted 3-1thread DTB-IIC). FEM thread rolling
simulations were performed using a dedicated die. The two kinds of modified DTB-II
specimens were rolled precisely and the thread heights reached the target value atall cross sections. The forming states in the thread rolling experiments well matched
the FEM simulation states. The performance evaluation tests demonstrated that the
3-1thread DTB-IIC specimens had sufficient tensile strength, and its anti-loosening
performance exceeds the reference level given in DIN25201.
Keywords Bolted joint · Double thread mechanism · Thread rolling · FEM ·Tensile strength · Loosening test

T. Takemasu (B)Happy Science University, 4427-1 Hitotsumatsu-hei, Chosei-mura, Chosei-gun 299-4325, Japan
T. Shinbutsu · S. Amano
Nissei Co. Ltd., 2022 Torisawa, Thomihama-cho, Ohtsuki 409-0502, Japane-mail: t_shinbutsu@nisseiweb.co.jp
S. Amanoe-mail: s_amano@nisseiweb.co.jp
J. Shimura
Suwa University of Science, Toyohira 391-0292, Japane-mail: jyo@rs.sus.ac.jp© The Minerals, Metals & Materials Society 2021
G. Daehn et al. (eds.), Forming the Future, The Minerals, Metals & Materials Series,
https://doi.org/10.1007/978-3-030-75381-8_181

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1 Introduction
It is well known that cyclic stresses or impact loading during service can cause theloosening and fatigue failure of bolted joints, possibly leading to serious accidents[1]. The loosening of bolted joints can be roughly divided into the non-rotational
type, in which the axial force decreases without nut rotation, and the rotational
type, in which return rotation of the nut occurs [2, 3]. Non-rotational loosening
can easily be dealt with by additional tightening of the nut or a design change.
In contrast, rotational loosening is an inevitable issue because it occurs suddenly
in an environment with cyclic loading, severe vibration, and impact forces. Most
commercially available anti-loosening screw parts exhibit macro slip on the nutbearing surface when subjected to a severe transverse vibration load and cannot
completely suppress rotational loosening because most of them only enhance thefrictional force between the bolt and the nut or between the nut and the bearing
surface [4–7]. In comparison, it has been proven that the double-thread bolted (DTB)
joints having two types of coaxial screw thread with different lead angles have an
excellent anti-loosening performance [8]. For such joints, a higher lead nut is first
fastened, and then a lower lead nut is added to form a double nut structure. The
anti-loosening mechanism of DTB is enhanced by a mechanical locking based on
the interference effect of two kinds of nut with different loosening speeds.
DTB joints with a very simple structure composed of a single coarse thread anda single fine thread (denoted DTB-I), have been a major research target [9]. However, as DTB-I has become more widely used, various disadvantages associated withthe fine screw thread have been pointed out by users, including troublesome torquemanagement for each nut, insufficient strength of the fine thread, high manufacturing cost of rolling dies, and low durability. Bolt fasteners, denoted DTB-II, was thenproposed based on an innovative double-thread mechanism composed of a single
coarse thread and a multiple coarse thread [10, 11]. Nut fastening and torque management for DTB-II can be performed very easily since the inner multiple-threadnut, mounted first, is also rotated when the outer single-thread nut is tightened, solving most of the problems encountered with the conventional DTB-I. Thread rolling
experiments and tensile strength tests were conducted on DTB-II specimens with
2, 3, and 4 threads on the multiple-threads, which were denoted 2, 3 and 4thread
DTB-II, respectively, to evaluate the proposed double-thread structure. It is foundthat 3thread DTB-II formed relatively well, although the height of the screw thread
profile of 2 and 4thread DTB-II did not reach the target level. The tensile strengthof these DTB-II specimens tended to decrease with increasing number of multiplethreads, and 3 and 4thread DTB-II were damaged by the shear failure of the screw
thread even with the double-nut structure. Accordingly, to solve these problems, theauthors designed several kinds of modified DTB-II, in which the number of multiplethread grooves superimposed on the single screw thread was selectively reduced tom − 1 (m: number of multiple-threads) or fewer from the m-thread DTB-II, manufactured them by thread-cutting, and carried out tensile strength tests. The shear

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Fig. 1 Diagrams of screw thread structure of a 3thread DTB-II, b 3-1thread DTB-IIA, c 3-1thread
DTB-IIB, d 3-1thread DTB-IIC

fracture of the screw thread of all DTB-II specimens mounted with double nuts was
successfully suppressed by reducing the number of multiple-threads by at least one.
In this study, thread rolling FEM simulations and experiments were conductedon the two types of modified 3thread DTB-II (see below for description) to examine
the material deformation state and forming accuracy. Several performance evaluation
tests were then performed to determine the effect of the modified screw thread profileon the tensile strength and the anti-loosening performance of those DTB-IIs.

2 Methods
2.1 Screw Thread Structure of Modified DTB-II
Figure 1 shows a comparison of the screw thread structure of 3thread DTB-II [10]and those of the three modified types. Figure 2 shows the changes in the threadcross-sectional area ratio, ξ , of each DTB-II, which was calculated by dividing thethread cross-sectional area of three pitches at each angular position of DTB-II bythat of the normal single screw-thread. Figure 1a shows the thread structure of theoriginal 3thread DTB-II, and b shows that of the modified 3-1thread DTB-IIA, inwhich one thread groove was simply removed from the three thread grooves andthe remaining two thread grooves were superimposed onto a single screw thread.
With this modification, the minimum value of the thread cross-sectional area ratio,ξmin , increased to approximately 55%, as shown in Fig. 2. However, since there are
four consecutive small threads at the groove bottom at the 90◦ and 270◦ positions,at which thread shear fracture may initiate, 3-1thread DTB-IIA was excluded fromfurther testing. One of the remaining two thread grooves was shifted downwards bya half pitch to be aligned at an equal pitch, as shown in Fig. 1c; this joint is denoted
3-1thread DTB-IIB or simply DTB-IIB [11]. For 3-1thread DTB-IIA, the complete
screw thread appears at only the 0◦ and 180◦ positions, whereas 3-1thread DTB-IIB
has one or two complete screw threads in three pitches at all cross sections. Theξ value of 3-1thread DTB-IIB changes at the 90◦ cycle, as shown in Fig. 2, whichis half that of 3-1thread DTB-IIA, and ξmin is 70%, which is twice that of 3thread

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Fig. 2 Comparison ofchanges in the threadcross-sectional area ratio, ξ ,of each DTB-II specimens,
which was calculated fromthe thread cross-sectional
area of three pitches at eachangular position

DTB-II. However, a small-thread portion remains at the groove bottom for all angular
positions, which reduces tensile strength. In order to completely remove these small
threads, the depth of the multiple-thread grooves was reduced by up to 50% of thethread height, as shown in Fig. 1d; this joint is denoted 3-1thread DTB-IIC or simply
DTB-IIC. With this modification, ξmin increased up to 85% and improvements intensile strength were expected.

2.2 FEM Simulation Model and Experimental
To reduce cost, DTB-II joints have to be manufactured using a mass production
process. Thread rolling is most suitable for this task because of its productivity andwidespread use. In the thread-rolling of DTB-II, both the single and multiple coarse
threads should be formed simultaneously on a single rod axis through one rolling
process. To realize this process, the authors developed a dedicated roller die (denoted
DTB-II die) with a special groove profile, which has almost the same outline as that of
the thread profile of DTB-II specimens in each corresponding cross section. Threadrolling simulations of M12 DTB-IIB and C were first performed using a 3D FEM
model to examine the forming process of the screw thread. The commercial FEM code
Simufact.forming ver.15 was used. Figure 3a and b show the FEM simulation model,
in which the workpiece is an elasto-plastic cylindrical material and the rolling flat
dies are rigid bodies. During processing, the two flat dies were moved synchronously
in opposite directions (see arrows in the figure) and simultaneously pushed into theworkpiece at the same constant speed. The total amount of indentation of the die,Development of Anti-Loosening Bolts Based on Innovative …

a

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c

b

Fig. 3 a Perspective view of FEM model. b Side view. c FEM mesh of workpiece. (Color figure
online)δmax , was set to a prescribed value for each DTB-II (δmax = 0.56 mm for DTB-IIB andδmax = 0.62 mm for DTB-IIC) until the workpiece was rotated 8 times; then, thedwelling process was performed for two rotations. Coulomb’s friction law wasassumed at the contact interface between the dies and the workpiece, and the coefficient of friction μ was set to 0.2. Figure 3c shows the initial FEM mesh of theworkpiece with a diameter of φ10.8 mm. A semitransparent refinement box wasplaced in the region from an inner diameter of 8 mm to an outer diameter of 14 mm;
this region is divided by hexahedron fine elements with a reference size of 0.4 mm.
The height of the cylindrical workpiece was restricted to the 9-pitch length of a single
screw-thread bolt to reduce calculation time and to avoid the effect of edge droop on
thread forming around the central portion in the axial direction.
The thread rolling experiments of M12 DTB-IIB and C were conducted using amanual rolling machine with a two-roller die plunge feed (FA-16U, Nissei Co., Ltd.)
under a die rotational speed of 62 rpm and a net processing time of 4 s, which included
a 1 s dwelling time. The initial diameter of the workpiece and the final amount of thedie radial feed were decided based on the screw thread forming state. A low-carbon
steel bar made of DIN 17100 was used as the bolt material.
The methods and conditions of the several performance tests are described later.

3 Results and Discussion
3.1 Thread Rolling FEM Simulation and Experiments
Figures 4a and 5b show the equivalent plastic strain distribution, ε, of thread-rolled
DTB-IIB and C obtained from the FEM simulation, and Figs. 4b and 5b show theouter appearance of the samples used in the experiment, respectively. Figure 6a and b

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b

Fig. 4 a Equivalent plastic strain distribution of rolled 3-1thread DTB-IIB using FEM. b Rolled
sample of 3-1thread DTB-IIB used in experiment. (Color figure online)

compare the filling state transition of the material at each representative angular position of the die grooves at each amount of die radial feed δ value obtained from FEM
simulation. The final deformation state of each modified DTB-II in the analysis wellagrees with that in the experiments, and the target screw thread profiles for both cases
are obtained for the entire external surface. The thread forming state during rollingin the FEM simulation also progressed as expected. Therefore, both the multipleand single-thread nuts were fastened easily and smoothly on the thread-rolled DTBII specimens. However, the equivalent plastic strain distribution, ε, of DTB-IIB isvery high at the groove bottom and its maximum value reaches approximately 15.
In contrast, that of DTB-IIC is very low and its maximum value is 7, less than halfof that for DTB-IIB. Hence, in the thread rolling experiments conducted at room
temperature, the material temperature immediately after the processing of DTB-IIB
increased to 110 ◦ C, whereas that of DTB-IIC increased to only about 70 ◦ C. The
reason is that the material filling of the two shallower die grooves located at the centerof the 0◦ position and at the right of the 90◦ position of DTB-IIB completed in theearly deformation stage of δ = 0.28 mm and (surrounded by enclosed space with noroom for overflow) then the material was over-rolled, as shown in Fig. 6a. The material near completely filled grooves flows along the same single screw thread to theunfilled deeper grooves, and this material flow in the circumferential direction causes
severe shear deformation, leading to material surface peeling and shortening tool life.
Many cutting chips appeared in the thread rolling process of DTB-IIB; considerably
fewer chips were observed for DTB-IIC. Since the material deformation of DTB-IIC
advances in the same manner as that of the normal single-thread bolt, in which thematerial mainly rises in the radial direction until 70% of δmax (δ0.47 mm), the thread
forming state does not depend on angular position. Although material flow in thecircumferential direction occurs after the shallow trapezoidal grooves located at thecenter of the 0◦ position and at the right of the 90◦ position of DTB-IIC are filled,
as shown in Fig. 6b, its amount is much smaller than that for DTB-IIB, and there isno portion around the groove bottom where is excessively high. These results indicate that the thread rolling formability of DTB-IIC is greatly improved over that of
DTB-IIB because of the good thread profile balance in the circumferential direction.

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a

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b

Fig. 5 a Equivalent plastic strain distribution of rolled 3-1thread DTB-IIC using FEM. b Rolled
sample of 3-1thread DTB-IIC used in experiment. (Color figure online)aDieradialfeed

Angular position
0

45

90

0.14
mm
0.28
mm
0.42
mm
0.56
mm

b

Die
radialfeed

Angular position
0°

45°

90°

0.16
mm
0.31
mm
0.47
mm
0.62
mm

Fig. 6 Comparison of material filling state in various typical sections of die groove using FEM
simulation between a 3-1thread DTB-IIB and b 3-1thread DTB-IIC

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3.2 Tensile Strength Tests
The tensile strength tests of M12 rolled 3-1thread DTB-II specimens (denoted DTBIIs) and a conventional rolled single-thread bolts were conducted using a hydrolicuniversal material testing machine (UH-300kN made by Shimadzu Corp.) equippedwith a special rig with a self-aligning function in the axial direction. A test specimen
mounted with double nuts or a single screw nut was placed in the rig and extended
at a constant speed of 2 mm/min. The under-head full length of the bolt specimen
was 110 mm and the length of the screw portion was 60 mm. The failure mode of all
modified DTB-IIs mounted with double nuts was the breaking of the base material
and the maximum loads were almost the same as that for the normal single-threadbolt. Here, the non-shear fracture condition of the screw thread was determinedby mounting only a single screw nut with different heights, namely normal height(10 mm), 1.25 times normal height (12.5 mm), and 1.5 times normal height (15 mm).
Figure 7a–c show the appearance of the fractured DTB-IIB and C with a singlenut structure, and Fig. 7d compares the load-stroke curves obtained from the tensile
tests. All these tests were conducted three times under the same conditions; the same
results were obtained in each trial. DTB-IIB was damaged by the shear fracture of
the screw thread with the 1.25-times-normal-height single nut, and was fractured
via necking of the base material with the 1.5-times-normal-height single nut. For
DTB-IIC, breaking of base material with the 1.25-times-normal-height single nut
and damage by shear fracture with the normal-height-single nut were observed. The
maximum load reached approximately 90% (47 kN) in the case of base material
breaking. Since the minimum and maximum values of the axial tension of the JIS
M12 standard single-thread bolt are 14 and 25 kN [12], respectively, it is possible touse DTB-IIC even with the normal single-nut structure. Accordingly, DTB-IIB andC have remarkably improved tensile strength compared with that of 3thread DTB-II
in the previous study [10]. The axial load of DTB-IIC is almost equal to that of
the normal single-thread bolt because of the elimination of the small threads at thegroove bottom.

3.3 Vibration Loosening Tests
Comparative vibration loosening tests were conducted using the conventional M12
DTB-I and modified M12 3-1thread DTB-IIB and C with a double-nut structure.
A Junker test bench (J120T, Vibrationmaster), shown in Fig. 8a, was employed toanalyze the self-loosening behavior of secured bolted joints [13]. This apparatus
can specify the state in which a bolted joint loses its axial preload when subjected toshear loading by transverse vibration. The transverse displacement and the oscillation
frequency were set to 1 mm and 12.5 Hz, respectively, which are the most extreme
conditions for this apparatus. The initial axial preload, P0 , of the modified DTBIIs was set by adjusting only the tightening torque of the outer single nut to be

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Fig. 7 Tensile strength test results of a 3-1thread DTB-IIB with 1.25-times-normal-height sigle
nut, b 3-1thread DTB-IIB with 1.5-times-normal-height single nut, c 3-1thread DTB-IIC with 1.25times-normal-height single nut. d Stroke-load curves obtained from tensile tests

Fig. 8 a Junker vibration loosening test bench. b Axial tension versus number of load cycles inJunker’s vibration loosening tests

fastened later. The proper P0 for a bolted joint with a nominal size of M12-1.75 isprescribed to be approximately 20 kN in JIS strength class 4.8. However, since theanti-loosening performance of the modified DTB-IIs was so high that the axial force
hardly decreased when tightening was conducted with the specific axial load, thebolt head shank exhibited shear fracture during testing. P0 was thus set to a slightly
lower value, within the range of 17–18 kN. A test was considered to have completed
when the total number of oscillations exceeded 2000 cycles (about 160 s) or when
it became apparent that the bolted joint had loosened. Figure 8b shows the variationin the axial load of the conventional DTB-I and modified DTB-IIs with the numberof load cycles in the Junker vibration test. Here, DTB-Ia denotes the case where theinner nut was rotated back according to the regular instruction, and DTB-Ib denotes

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the case where that procedure was omitted. The final residual rate of the axial load
was approximately 15% for DTB-Ia and 10% for DTB-Ib. In contrast, those forthe two DTB-IIs were larger than 85%, which is considerably higher than thosefor DTB-I and exceeds the reference value of 80% given in DIN25201. A detailed
comparison of the changes of the axial load indicates that the rates of both DTB-Ia
and DTB-Ib suddenly drop by about 4 kN immediately after the starting point; then,that of DTB-Ia continues to decrease with a nearly constant slope, whereas that of
DTB-Ib rapidly decreases for about 300 cycles and then decreases very gradually.
The rates of both DTB-IIs are almost constant after an initial decrease of about 1 kN.
The reason for this significant difference is as follows. (i) The loosening speed ratio
between the inner nut and the outer nut of DTB-IIs is 3:1, which is larger than thatof DTB-I (2:1). (ii) The outer nut of DTB-IIs with a single coarse screw thread canhold the axial load of a bolted joint more strongly than can DTB-I with a single finescrew thread.

4 Conclusions
1. The rolling formability of the modified 3-1thread DTB-II specimens was considerably improved. Since the material deformation of DTB-IIC advanced in
the same manner as that of the normal single thread bolt up to 70% of the total
amount of die radial feed, the over-rolled area was greatly decreased aroundthe thread groove portion and the material temperature immediately after theprocessing was much lower compared with that for DTB-IIB (70 ◦ C vs. 110 ◦ C).
2. The tensile strengths of DTB-IIB mounted with a 1.25-times-normal-heightsingle-thread nut and DTB-IIC mounted with a normal-height single-thread nut
reached the range of 14–25 kN prescribed by JIS.
3. It is found that both DTB-IIB and DTB-IIC have outstanding anti-loosening
performance, as determined using Junker vibration loosening test. The final
residual axial loads were kept at more than 80% after the prescribed 2000 cycles
by adjusting only the tightening torque of the outer single nut to be fastened
later.

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