• DECEMBER 2019
The Mechanics of Bolted Joints
Preloaded Bolted Joint Analysis – Lesson 2
Types of Bolted Joints
Bolted joints can support different load
configurations.
• Two commonly used bolts are partially and fully
threaded bolts.
• Choosing the wrong type of bolt can lead to
joint failure.
• There are two common types of joints:
- Tension joints
- Shear joints
Partially Threaded Bolt
Fully Threaded Bolt
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Tension Joints
F
In a tension joint the bolt is predominantly supporting
tensile loads that pull the plates apart.
• Applied loads F act along the axis of the bolt.
• Due to external tensile loads, the forces acting through
the cross-section of the bolt increases by a fraction of
total applied force, represented by f.
• The clamp force should typically be selected to ensure
there is no plastic deformation of the bolt.
• If the external load applied to the joint is large enough
to overcome the initial compression in the joint
achieved through the bolt preload, then the members
will separate, and the entire load will be carried by the
bolt, which can also greatly reduce the bolt fatigue life
from repetitive loadings.
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P
f+P
f is a fraction of the total applied force, F
Shear Joints
μ
Shear joints experience loads in a direction
perpendicular to the axis of the bolt.
• Performance depends on the friction between the
fastener and the component, as well as the shear
strength of the bolt.
• If the frictional force due to the preload is not
sufficient to counter the external load, then the
component slips, which is often undesirable.
• In some cases, the bolt directly supports the
external load and friction is not needed. Such loads
are called bearing loads.
‐ Here, preload is not important, and the shear strength of
the bold determines the strength of joint.
• Partially threaded bolts are suitable for these joints
as they offer more shear resistance and better
alignment.
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F
μ
μP
Force Balance in Bolted Joints
In both types of joints mentioned, there are several forces acting in the assembly that work to create a
balance between them and maintain a static state.
• Proper designs typically involve ensuring that all of the conditions below are met.
P
Preload = P
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F
P
P
F
Frictional forces in joint μP – F > 0
Forces in bolt f+P< Fyield
Clamp force in joint P - (F-f) > 0
Pressure Cone
A pressure cone of stresses is created in the clamped parts due to the compressive clamping force.
• It’s important to take this cone into account while spacing the bolts.
‐ If external loads exceed compressive load in joint, separation will occur, and bolt will take entire load. Thus,
we want to prevent loss of preload to prevent this from occurring.
‐ If the bolts' pressure cones interfere with each other, the stresses in the clamped part can be very high and the
component might fail.
‐ If the bolts are too far apart, then the contact pressure between connecting parts will be highly non-uniform.
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