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13
th
National Conference on Mechanisms and Machines (NaCoMM07),
IISc, Bangalore, India, December 12-13, 2007 NaCoMM-2007-105
229
On the Anti-Loosening Property of Different Fasteners
S. Saha
1
, S. Srimani
1
, S. Hajra
1
, A. Bhattacharya
2
, and Santanu Das
1*
1
Department of Mechanical Engineering, Kalyani Government Engineering College, Kalyani, India
2
Department of Mechanical Engineering, Thapar University, Patiala, India
*
Corresponding author (email: sdas_me@rediffmail.com)
Abstract
Threaded fasteners are widely used for joining different
mechanical parts temporarily due to its distinct advantages.
However, screw threads have the problem of loosening
under hostile vibrating conditions, which leads to
decreasing of clamping force and finally failure of the
system. To study the anti-loosening phenomenon of the
threaded fasteners, a testing rig has been designed and
fabricated where the clamping force can be continuously
recorded under the application of accelerated known
frequency vibration between two plates of nuts and bolt. In
the present paper, the results obtained on the anti-
loosening property of a number of threaded fasteners are
presented, discussed and the effective one is found out.
Keywords: Threaded fasteners, loosening, anti-loosening
characteristics, vibratory system, accelerated test.
1 Introduction
Threaded fasteners facilitate some machine parts such that
they can be readily assembled or disassembled without
making any damage to either of the components. This is
needed for the purpose of clamping, setting up, servicing,
inspection, overhauling, etc. However, under hostile
vibration condition, threaded fasteners often fail to retain
the tightening torque, thereby causing its loosening
[1][2][3][4][5].
History of evolution of screw fasteners dates back to
few thousand years ago. It is learnt [6] that screw fasteners
were used in the Tigris–Euphrates region in around 1,000
B.C., mainly, for the purpose of water supply. The plate
shaped cross-section of the screw thread was then used.
The People of Greece were also supposed to use screws to
press olives. The application of screws followed was its
use as a feeder. Leonardo da Vinci is credited for creating
and sketching different ideas leading to implementation of
important usage of screw threads. During that time, square
profile screw threads was used [7].
Drastic change followed in its shape of square to
triangular thread increasing the reliability of screw threads.
Wide ranges of screw fasteners were developed and used
in various applications. The advantages realized by the use
of threaded fasteners are easy to assemble and
disassemble, cheap, widely available, ability to generate
very high fastening torque and force by very simple means
and its retention for quite a very long time. However,
hostile vibration conditions tends to cause loosening of
screwed fasteners [1][4][5][8][9][10]. This is a major
drawback of screw fasteners. Loosening of the fasteners
occur mainly when repetitive forces are applied in a plane
perpendicular to the longitudinal axis of the bolt [6][9].
This needs attention to maintain the fastened components
to prevent extensive damage to the assembled parts and
impending fatal accidents.
To prevent loosening, and its consequent problems,
various types of screw fasteners were introduced. Junker
[3] tested cap screws, spring washers and free spinning
locking screws with respect to its anti-loosening
characteristics. Effectiveness of fine screw threads, spring
washers, nylon inserted nuts, double nuts and eccentric
nuts of few sizes to resist loosening were investigated
[2][11]. Test results showed that those popularly known
anti-loosening fasteners did not really possess enough
resistance to loosening. Applications of adhesives in a
fastener can also offer [12] anti-loosening effect in certain
applications. However, unscrewing for disassembly often
poises problems in this case.
Sase et al [5][10][11] developed a new thread profile
of step lock bolt (SLB) to eliminate bolt torsion . This has
steps on the helix, hence the name step lock bolt (SLB).
The stepped part is of zero lead angle, and the portion with
lead angle is the inclined portion. The clamping force is
supported by the step part, which, in the conventional
threads, has a tendency to push out the nut along the flank
angle when friction is overcome. Due to its ability to
prevent torsion by itself, a SLB does not loosen even
though the bottom surface of the bolt and nut slips
.
The anti-loosening property of spiralock internal
thread form system [13] is simple and is discussed in
spiralock webpage, which uses a typically designed 30
0
wedge ramp. This ramp portion clamps and locks any
standard bolt by drawing crests of the bolt thread tightly
against the wedge ramp, thereby preventing transverse
movement of the bolt thread with respect to the nut.
“Wedge locking” of this threaded joint is caused by the
increase in contact friction between the materials due to
enhanced surface area and total elimination of the
transverse motion.
Provision for free spinning systems, metallic friction
locking by distorted threads like ‘cleveloc’ and chemical
locking of fasteners using adhesives can also offer [12]
13
th
National Conference on Mechanisms and Machines (NaCoMM07),
IISc, Bangalore, India, December 12-13, 2007 NaCoMM-2007-105
230
anti-loosening effect in certain applications. A taper-
headed nut has also been proposed by Mondal et al [14].
The present paper contains a report of works aimed at
testing the loosening characteristics of several threaded
fasteners such as nyloc nut, with serrated washer, standard
BSW and metric fasteners, etc., and recommending the
scope of developing an anti-loosening threaded fastener in
future.
2 Mechanism of Loosening and
Desired Anti-Loosening Characteristics
Studying on the loosening mechanism of screwed fasteners
revealed that the relative sliding rotation between nut, bolt
and components joined is the main reason for loosening
[8][9][11][12].
i) The cause for the sliding and consequent loosening is
explained by the fact that the lateral displacement of
fastened element makes the bolt inclined, and hence
increases the tensile stress coming on to the bolt.
ii) Increase of this tensile stress over a limit initiates slip
at the engaged flank surface of the screw thread.
iii) The slip takes place not only in the direction of the
flank but also in the direction of the axis of the screw
thread due to the presence of lead angle.
iv) Differential thermal effects of clamped materials and
fasteners may also induce loosening effect.
From the above discussion, it may be summarized that
loosening can be minimized [6] if,
i) Lead angle is reduced
ii) Flank angle is made as small as possible
iii) Relative slips between the bearing surface of fasteners
and fastened components are reduced by increasing
the friction force.
3 Testing for Anti-Loosening
Performance
3.1 Details of Experimentation
A loosening test rig has been designed and constructed for
performance testing of anti-loosening threaded fasteners as
detailed in reference numbers [14][15] published by the
corresponding author of this paper and schematically
shown in Fig.(1). In this set up, a rocker arm reciprocates,
and once in a revolution, it strikes the plate, which is
clamped by the nut-bolt assembly. Loosening of the
fastener can be detected by the drop in the clamping force,
measured by a load cell (model- SLC 302, Sushma make)
and a digital indicator.
The end of the rocker arm strikes the clamped plate
280 times a minute generating vibration in a plane
perpendicular to the bolt axis for accelerated testing. The
testing is carried out for about 9,000 to 11,000 oscillations.
Same amount of repeated forces are applied through the
mechanism employed, but the magnitude of this force has
not been measured.
Two series of experiments have been conducted with
applying two different initial clamping forces. For the first
case, an initial torque of 1.50 tonnes has been applied,
whereas, for the second set of tests, 0.94 tonne of clamping
force has been applied. For each test, three repetitions have
been carried out for studying the repeatability of each
result obtained.
Though a number of bolts and locking nuts, etc. are
being investigated, lot of opportunities remains to be there
to develop quite effective anti-loosening threaded fasteners
with economical viability. With this idea, a number of tests
are planned for different innovative threaded fasteners. In
this respect, few tests have been conducted first on the
existing screw fasteners, results of which are presented in
this paper. Bolts, both BSW and metric, are commonly
available in the market and made of plain carbon steel;
corresponding nuts are also of the same material.
However, nylon inserted nuts are made of stainless steel
with a nylon ring inserted inside it.
Figure 1: Schematic diagram of Testing Machine
Test results of the fasteners having nylon inserted nut,
and fasteners with serrated washers are presented. These
results are compared with the results obtained from the
tests on standard metric and BSW threaded fasteners.
Interesting results obtained from using a metric (M10) bolt
fitted with a BSW (3/8
”
) nut are also presented.
3.2 Results and Discussion
From the test results shown in Fig.(2), comparison of the
relative effects of loosening in a conventional fastener,
fasteners using a nylon-inserted nut, a simple washer and a
spring washer are studied keeping the clamping force
applied of about 1.50 tonnes. The decrease in the clamping
force with the progress in the number of oscillations are
shown in Fig.(2). Due to experimental difficulties, tests
with 5/8 BSW nylock nut and with M16 inside serrated
washer could not be continued more than 1400 and 2100
cycles respectively.
Standard fasteners, such as metric M16 and BSW-5/8,
exhibit considerably higher loosening tendency than the
13
th
National Conference on Mechanisms and Machines (NaCoMM07),
IISc, Bangalore, India, December 12-13, 2007 NaCoMM-2007-105
231
others. Again, BSW-5/8 fastener shows slower rate of
loosening than the M16 metric one. This may be due to
lesser flank angle (55 degree) of the BSW thread than that
of the metric thread (60 degree). Nylon-inserted nuts are
seen to have higher capability to retain clamping force and
hence, clamping torque than the standard fasteners and
with both inside and outside serrated washers.
In the case of outside serrated washer, small increase
in clamping force is also observed which may be due to
some amount of plastic deformation happening during the
test causing locking effect. Mark of deformation is also
seen on the fastened components. Some of these results
are, to some extent, in line with the earlier results obtained
by Junker [3] and Sase et al [5][10][11]. While nylon-
inserted nuts provide extra frictional gripping, washers
provide large preloads to improve anti-loosening
characteristics.
Results of another set of tests are shown in Fig.(3).
Testing for loosening for M10 bolt with 3/8
”
BSW nut,
fitted with around 0.94 tonne clamping force, along with
nylon-inserted nut, M10 nut-bolt set with or without spring
washer are presented. In the case of metric bolt with BSW
nut, small difference in pitch and nominal diameter
become responsible for providing extra contact force on
the flank portion of the nut and bolt. This, in turn, causes
hike in friction force, thereby resulting in good resistance
against loosening under vibration compared to other types
of fasteners tested. However, in practice, this may not be
used as this may damage the fastener preventing its
repetitive use.
In this case also the nylock nut shows higher
resistance to loosening than the standard M10 metric
fastener and that with spring washer in line with the earlier
results obtained through 1.50 tonnes of initial clamping
force. However, it is seen from Fig.(3) that the Standard
M10 fastener loosens at a faster rate than the M10 nut-bolt
with spring washer, showing little effect of using the
spring washer. Hence, the popular belief of obtaining good
loosening resistance of spring washer is observed not to be
very much effective one. Therefore, use the step lock bolt
and an organic adhesive may be fruitful to provide desired
resistance to loosening.
4 Conclusion
The following conclusions may be drawn from the above
results and discussion.
1. The nylon inserted nuts, imparting higher frictional
grip, when fitted with a standard metric bolt shows
good resistance to loosening compared to the other
fasteners tested at both the initial clamping forces of
1.50 tonnes and 0.94 tonnes respectively.
2. The M10 bolt fitted 3/8
”
BSW nut which have narrow
difference in nominal diameter and pitch value
exhibits considerable anti-loosening property. High
frictional force caused by high contact load between
threaded flanks of the nut and bolt may be the reason
behind. However, some amount of permanent
deformation may damage the nut-bolt mating surfaces.
3. The conventional BSW fasteners have shown lesser
tendency to loosen than the metric nut and bolt which
may be due to lesser flank angle.
4. Other types of fasteners such as with inside and
outside serrated washers have been observed to show
small amount of loosening under vibrating condition.
Acknowledgement
The authors acknowledge the AICTE, New Delhi for the
financial support for this project vide the sanction Ref.
8018/RDII/BOR/R&D(244)/99-2000 Dt. 24/3/2000.
References
[1] J. N. Goodier and R. J. Sweency, “Loosening by
vibration of threaded fastenings”, Mechanical Engineering,
Vol.12, 1945, p.798.
[2] K. Hongo, “Loosening of Bolt and Nut Fastening”,
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Vol.30, No.215, 1964, pp.934-939.
[3] G. H. Junker, “New Criteria for Self Loosening of
Fasteners under Vibration”, SAE International Automotive
Engineering Congress, Paper No.- 690055, 1969, pp.314-
335.
[4] K. Koga, “A discussion on screw-fasteners loosening
caused by impacts”, Japan Society of Mechanical
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[5] N. Sase, S. Koga, K. Nishioka and H. Fuji, “Analysis
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1532.
[6] H. Fuji and N. Sase, “SLB Concept for Screw
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[7] L. da Vinci, Cobex Madrid I Vol. I, U.K., McGraw
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[8] D. B. Dallas, Tool and Manufacturing Engineers Hand
Book, 3
rd
Ed., McGraw Hill Company, p.27, 1975.
[9] Y. Yamamoto and S. Kasei, “Investigations on the
Self-loosening of threaded fasteners under transverse
vibration”, Journal of Japan Society of Precision
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[10] N. Sase, S. Koga, K. Nishioka and H. Fuji,
“Evaluation of Anti-Loosening Nuts for Screw Fasteners”,
Journal of Materials and Processing Technology, Vol.56,
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[11] N. Sase, S. Koga, K. Nishioka and H. Fuji, “An Anti-
Loosening Screw Fastener Innovation and Its Evaluation”,
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[12] www.boltscience.com/pages/vibloose.htm, 2006.
[13] http://www. Spiralock.com, 2006.
[14] M. K. Mondal, N. K. Peyada, S. Patra, S. Pradhan, S.
Banerjee and S. Das, “On the Anti-Loosening
13
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National Conference on Mechanisms and Machines (NaCoMM07),
IISc, Bangalore, India, December 12-13, 2007 NaCoMM-2007-105
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Characteristics of Fasteners”, 20th All India
Manufacturing Technology, Design and Research
Conference, 2002, pp.830-831.
[15] P. Sarkar, P. Mallick, A. Bhattacharya and S. Das,
“Enhancing Anti-Loosening Characteristics of Threaded
Fasteners- Need of Threaded Components under Vibratory
Condition”, Proceedings of the All India Seminar on
Strategies and Techniques for Maintenance of Equipment
and Machinery- Recent Trends, Kolkata, 2004, pp.II18-
II22.
1.42
1.44
1.46
1.48
1.5
1.52
1.54
1.56
0 1000 2000 3000 4000 5000 6000 7000
Number of oscillations
Clamping force, Tonnes
Matric(16)
BSW(5/8)
M(16) with Inside Serrated
M(16) with Outside Serrated
Nylock nut M16
Figure 2: Loosening characteristics of fasteners with about 1.50 tonne clamping force
0.89
0.9
0.91
0.92
0.93
0.94
0.95
0.96
0.97
0 2000 4000 6000 8000 10000
Number of oscillations
Clamping force, tonnes
Matric(10)
M(10) with Spring Washer
Nylock Nut M10
Matric(10) with BSW(3/8) Nut
Figure 3: Loosening characteristics of fasteners with about 0.94 tonne clamping force
