Influences of Various Factors of Bolt Tightening on LooseningFatigue Failure under Transverse Vibrat

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Influences of Various Factors of Bolt Tightening
on Loosening-Fatigue Failure under Transverse
Vibration

• Shinji Hashimura
• (Kurume National College of Technology, JAPAN)

Paper 2007-01-0807
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Self-Loosening and Fatigue of Bolted Joints
Self-Loosening and Fatigue Test
Typical Loading Types
Axial Vibration
Twist Vibration
Transverse Vibration
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Past Results of Our Researches
1) If a bolt has loosened in the low cycle
region, damage such as crack nucleation at the
root of the first thread of the bolt does not
occur, and loosening is due to bolt rotation.
2) If a bolt has loosened in the high cycle
region, fatigue cracks nucleate at the root of
the first thread in the all experiments. Loosenin
g occurs due to loss of bolt stiffness caused by
crack propagation which then leads to bolt
rotation. 3) 99 of loosening-fatigue life Nf
under transverse vibration is spent in order to
make cracks to nucleate and propagate. 4) The
loosening-fatigue life significantly depends on
the transverse vibration force although the
initial clamping force above a threshold does not
have an influence on loosening-fatigue life.
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Outline
Influences of four different factors of bolt
tightening on loosening-fatigue characteristics
under transverse vibration have been
investigated.
1. Influence of bolt property class
Do the transverse loosening-fatigue
characteristics depend on bolt property class?
2. Influences of plastic region tightening
Do the transverse loosening-fatigue
characteristics improve due to being tightened to
within the plastic region?
3. Influences of Grip Length of Bolted Joint
An axial fatigue characteristic improves with an
increase in the grip length. Does the transverse
loosening-fatigue limit improve?
4. Influences of Engaging Thread Length of Bolted
Joint
An axial fatigue characteristic improves with an
increase in the engaging thread length. Does
the transverse loosening-fatigue limit improve?
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Experimental Apparatus

• Frequency of transverse vibration 5060 Hz
• Lubricant MoS2 Grease

Tightened by wrench
Load cell for transverse vibration
Fi 15 kN
Potentiometer
Air vibrator
Vibrated clamped part
Linear roller
Test bolt
Fixed clamped part
Load cell for clamping force
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Estimation of Nominal Stress at the Root of the
First Thread
Bending moment MB due to constraining the
engaging threads,
C coefficient depending on tightening conditions

Nominal stress at the root of the first thread st
expressed as follows.
C depends on the tightening condition.
The grip length lg35mm The engaging thread
length le10mm
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1. Influence of Bolt Property Class

• Test bolt M10
• Nominal Length la45mm
• Thread length lt27 mm
• Grip length 35mm
• Engaging length 10mm

sw0 Fatigue strength of a smooth specimen sT
True ultimate strength s0.2 Proof stress.
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1. Influence of Bolt Property Class

• Loosening-fatigue life obviously depends on the
  amplitude of transverse vibration force DPt /2.
• Except for DPt /20.25 kN, all bolts had fatigue
  cracks.
• There is a threshold DPtw/2 between 0.25 kN and
  0.35 kN corresponding to the loosening-fatigue
  limit.

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1. Influence of Bolt Property Class
Loosening-fatigue limits do not almost depend on
bolt property class.
The transverse fatigue limits stw can be
estimated using the axial fatigue limits saw in a
practical use because the axial fatigue limit of
bolts is known to a certain degree.
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2. Influence of Plastic Region Tightening
Generally, the axial fatigue limit of a bolted
joint improves if the bolt is tightened to within
the plastic region. Does the transverse
loosening-fatigue limit improve?
Fi 25 kN
Fi 15 kN
Test bolt Commercial hexagon head bolt M10,
Property class 4.8
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2. Influence of Plastic Region Tightening
The transverse loosening-fatigue limit
deteriorates if the bolted joint was tightened to
within the plastic region.
Results of Elastic region tightening.
stw 65 MPa
21 down
stw 51 MPa
Results of Plastic region tightening.
This result is the opposite to the axial fatigue
characteristics.
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3. Influence of Grip Length
Generally, the axial fatigue characteristic of a
bolted joint improves with an increase in the
grip length. Does the transverse
loosening-fatigue limit improve?

• Test bolt Commercial hexagon head bolt M10,
• Property class 10.9
• Initial Clamping Force Fi 15 kN
• Lubricant MoS2 Grease

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3. Influence of Grip Length
The transverse loosening-fatigue characteristics
deteriorates with an increase in the grip length.
lg 25 mm
DPtw/2 0.36 kN
40 down
DPtw/2 0.20 kN
lg 35 mm
This result is the opposite to the axial fatigue
characteristics.
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3. Influence of Grip Length
MB Bending moment at Point B due to gripping
threads,
Grip length
If C does not greatly change, MB increases with
an increase in lg .
The apparent loosening-fatigue limit decreases
with an increase in lg .
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3. Fatigue Morphology of Broken Bolt
The section at the root of the first bolt thread
is subjected to complex and varied stress due to
the cyclic bending moment, cyclic shearing force
and twist torque.
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4. Influence of Engaging Length
Generally, the axial fatigue limit of a bolted
joint improves with an increase in the engaging
thread length. Does the transverse
loosening-fatigue limit improve?

• Test bolt Commercial hexagon head bolt M10,
• Property class 10.9
• Initial Clamping Force Fi 15 kN
• Lubricant MoS2 Grease

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4. Influence of Engaging Length
The apparent transverse loosening-fatigue limit
deteriorates with an increase in the engaging
thread length
DPtw/2 0.30 kN
le 10 mm
33 down
DPtw/2 0.20 kN
le 20 mm
This result is the opposite to the axial fatigue
characteristics.
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4. Influence of Engaging Length
MB Bending moment at Point B due to gripping
threads,
C increases with an increase in le , because the
constraint of the bolt thread increases.
The apparent loosening-fatigue limit decreases
with an increase in le because the apparent
fatigue limit depends on MB
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Improvement Effect by Several ParametersIn each
condition, whether the fatigue limits improve or
not
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Conclusion
The main conclusions obtained in this study are
summarized as follows.
1. Bolt property class has almost no influence on
the loosening-fatigue characteristic of a bolted
joint under transverse. 2. If a bolted joint
which was tightened to within the plastic region
is subjected to transverse vibration, the
loosening-fatigue life and the loosening-fatigue
limit under transverse vibration are reduced.
This result is the contrary to the result of the
axial fatigue characteristics of bolted
joints. 3. The apparent loosening-fatigue limit
decreases with an increase in the grip length
because the bending moment at the root of the
first thread is in proportion to the grip length.
This result is also the contrary to the result of
the axial fatigue characteristics of bolted
joints. 4. If the engaging thread length is long,
the apparent loosening-fatigue limit decreases
although the influence of bolt body length has to
be removed in order to isolate the influence of
the engaging thread length. Once again, this
result is contrary to the result of the axial
fatigue characteristics of bolted joints.
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Thank you very much for your attentions
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3. Fatigue Morphology of Broken Bolt
A bolt is subjected to the complicate stresses by
the bending moment, shearing force and twisting
torque.