Thursday, April 13, 2023
1:57 PM
WHAT IS BOLT PRELOAD?
A bolt and nut are used together as a compressive force to keep materials connected. The threads
of the nut work with the threads of the screw to apply force to either side of the fastened material.
This tension is called Bolt Preload, which is the compression created as the nut is tightened against
the bolt (or vice-versa).
When a load (weight) is placed on a bolt, it is limited to the amount of load the bolt can handle before
failing. However, when a bolt is tightened against a material, it allows the bolt to distribute the
force through the material, so the bolt itself only holds a portion of the load. This means that a bolt
can hold a significantly higher load when the correct amount of tension is applied. That tension is
known as preload.
Load – The amount of force acting on a fastener assembly
Preload – The amount of tension (compression) needed to distribute a load’s force throughout a fastener
assembly
Working Load – The load placed on the assembly once ready to perform
Bolt Preload – The tension created when the nut is screwed onto a bolt to hold two materials together.
When the tension reaches the optimal preload, the working load (load added after creating the assembly)
placed on a bolt will be distributed into the installation materials, so the bolt does not take the entire load.
The Outcomes of Bolt Preload:
• If the assembly is loose (the preload is not correctly applied), the external load increases the load on
the bolt only. This will result in bolt failure.
• If the assembly is tight (the correct preload has been applied), the load will only cause bolt
deformation by distributing the load through the bolt and the nut.
How Bolt Preload Works
Bolts are incredible tools, but they are actually not as strong as we’d think. When first glancing at a
bolt assembly with a working load attached to it, it appears as though the bolt is holding that entire
load on its own. This is not the case.
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load on its own. This is not the case.
When a bolt has preload, it is able to distribute the working load out across the plate near the head
of the bolt. We will refer to this as the support plate. This means a properly installed bolt assembly
can withstand a much heavier load as it distributes the force out away from itself. When a working
load is applied to a fastener assembly that has not been preloaded, the entire force is placed on the
bolt alone, which makes it much more likely to fail.
Why Is Bolt Preload So Important?
As discussed above, without bolt preload, the entire structure would be totally reliant on the bolt to
hold the weight. When preload is applied, significantly less bolts are needed, as the material
(Support Plate) will play a significantly larger role in holding the working load. This is not a cure-all
however; a working load may still exceed the preload of the bolts which can result in the bolts failing,
the support plate failing, or both.
Easy Ways to Determine Bolt Preload
• Use a Torque Wrench to Reach Optimal Torque
• While this is not a true measure of bolt preload, if a bolt is fastened to its optimal torque it can be
assumed it is close to the correct preload. The reason this method is not readily accepted is because
the torque will be directly affected by the material it is being spun against. A rougher material will
produce more friction, which will make the torque value higher while reducing the preload tension.
The opposite is also true. You can hit optimal torque without hitting the optimal preload (and vice
versa). In the end it depends on the material, which is why this method isn’t exact, but it is a decent
guess.
• Use Preload Indicating Washers
• Preload indicating washers are washers that are designed to spin until a certain amount of load is
applied to them. This way, once the washer is no longer free-spinning, the preload has been met.
This is a much simpler way of determining if the correct compression has been met.
• Use Direct Tension Indicating Washers
• These washers have little bumps that flatten when preload is achieved. Once flattened, a feeler
gauge is used to make sure the bolt shaft is no more than 50% accessible under the washer (ideally
less).
• Use Silicon Direct Tension Indicating Washers
• These washers operate in a similar way to the direct tension-indicating washers above. These
washers have small recesses where a silicon paste is filled in. As the nut tightens against them, the
silicon begins to come out of the sides of the washer. To identify when optimal preload is achieved
on this type of washer, the amount of recesses on the washer minus one need to be exposed out the
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on this type of washer, the amount of recesses on the washer minus one need to be exposed out the
sides. For example: If there are 6 recesses, then a minimum of 5 need to be exposed. 6 recesses –
1 recess = 5 (Preload has been reached)
• Tighten Until Snug, and Then Tighten “X” Degrees
• A less reliable approach is to tighten the nut until snug, and then to torque the nut “X” degrees. For
example, a recommendation of 90 degrees would look like the following: Spin the nut onto the
assembly until snug against the material. Attach the wrench to the nut and spin it a quarter of a turn
(90 degrees). This method will give you a close to accurate preload.
So what does bolt preload do?
Bolt preload ultimately allows a fully tightened bolt to survive in an application where an untightened
or loose assembly would fail very quickly. When tight, the joint provides a conduit for the force to
flow through into the assembly materials themselves. This means the bolt assembly itself is only
under a portion of the force of the working load. While you cannot look at a bolt and see this transfer
occurring, the preload allows bolts to survive in much more rigorous applications.
From <https://www.albanycountyfasteners.com/blog/what-is-bolt-preload/>
Fundamentals of Basic Bolting
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What happens as a bolt is tightened?
Should I tighten the bolt head or the nut?
What is bolt preload and why is it important?
Why do bolts come loose?
What is proof load and how is it different from yield strength and
ultimate strength?
What happens as a bolt is tightened?
When using a traditional wrench to tighten a bolt, the torque applied to
the nut causes it to slide up the inclined plane of the threads. This
relative motion between the nut and the bolt attempts to reduce the
distance between the bearing surfaces of the bolt and nut. This
dimension is the grip length of the bolted joint. When the joint
members within the grip resist, the bolt begins to stretch like a stiff
spring, developing tension and simultaneously compressing the
components together creating the all-important clamp force.
Find out what happens when DTI SmartBolts are tightened in
this video!
Should I tighten the bolt head or the nut?
Either is acceptable, however a torque value defined for tightening the
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Either is acceptable, however a torque value defined for tightening the
head does not necessarily apply to tightening the nut. Tightening the
head vs. the nut can result in different nut factors and therefore
change the torque required to achieve proper preload.
What is bolt preload and why is it important?
Preload is the tension created in a fastener when it is tightened. This
tensile force in the bolt creates a compressive force in the bolted joint
known as clamp force. For practical purposes, the clamp force in an
unloaded bolted joint is assumed to be equal and opposite of the
preload.1 If proper preload, and thus clamp force, is not developed or
maintained, the likelihood of a variety of problems such as fatigue
failure, joint separation, and self-loosening from vibration can plague
the bolted joint leading to joint failure.
1 Usually, but not always. Review Bickford (p. 192, Bickford, 1995) for
more information on the exceptions.
Why do bolts come loose?
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There can be many possible causes for bolts to loosen in
service. When we say “loosen” here we mean lose their tension, or
preload. Here are five major causes:
Vibration which can create relative transverse movement of the bolted
materials leading to self-loosening of the nut.
Relaxation of the bolted joint after tightening due to embedment or
gasket creep.
Elastic interactions occur when multiple bolts are present in a bolted
joint. The additional force applied to the joint members by tightening a
bolt can affect the amount of tension on the other previously tightened
bolts. Elastic interactions can either increase or decrease bolt preload
making it even more difficult to predict.
Temperature fluctuation of the components.
Insufficient initial preload developed at installation
The design of the bolted joint can minimize relaxation and
embedment, and ensuring sufficient preload at installation can reduce
the effects of vibration and likelihood of relative transverse
movement. In other words, properly designed bolted joints that are
properly preloaded should not self-loosen!
What is proof load and how is it different from
yield strength and ultimate strength?
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yield strength and ultimate strength?
Each of these are basic mechanical properties that help define the
expected tensile strength performance of a specific fastener and can
be measured in units of force. In USCS and SI systems, force is
measured in pound-force (lbf) and Newtons (N), respectively. Since
the strength of fasteners is generally quite large, it is also common to
see these forces listed in kilopound-force (klbf) and kilonewton (kN).
Proof load is defined as the maximum tensile force that can be applied
to a bolt that will not result in plastic deformation. In other words, the
material must remain in its elastic region when loaded up to its proof
load. Proof load is typically between 85-95% of the yield strength.
Yield strength can be defined as the tensile force that will produce a
specified amount of permanent deformation (most commonly 0.2%)
within a specific fastener.
Ultimate tensile strength can be defined as the maximum force a
specific fastener must withstand before fracture.
Note: The term strength in this context differs from stress by being defined for a specific
bolt’sstress area and presented in units of force. Worth mentioning is that “strength” is
also commonly used interchangeably with stress and presented in units of pounds per
square inch (psi) for USCS or Megapascals (MPa) for SI. In this case, the value
represents a more general property that can be applied to a variety of stress areas to
derive the limits of applied forces. In other words, the ultimate tensile strength of the
fastener material (MPa) can be presented as the ultimate tensile strength (kN) of a
specific size fastener.
B
ickford, J. H. (1995). An introduction to the design and behavior of
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ickford, J. H. (1995). An introduction to the design and behavior of
bolted joints (3rd ed.) New York, NY: Taylor & Francis Group
Fastenal. (2009). [Illustration of Tensile: Stress-Strain
Relationship] Bolted Joint Design: Mechanical Properties of Steel
Fasteners in Service.
From <https://www.smartbolts.com/fundamentals/#bolt-or-nut>
Understanding Bolt Preload
Materials are held together through the compressive force of a bolt-nut combination.
The nut’s threads act with the screw’s threads to apply force to the fastener, working on
both ends. The resulting tension is referred to as Bolt Preload. To put it simply, it is the
compression produced by the tightening of a nut against a bolt.
The amount of weight that is placed on a bolt is, naturally, limited to how much it is
capable of withstanding before coming apart. However, when tightened against a
material, force is distributed through the material such that the bolt only takes a certain
part of the load upon itself. However, it can hold more with the right preload.
DISTINGUISHING THE TERMS
Load refers to the level of force acting upon a fastener assembly.
Preload is the compression required to distribute the force of a load throughout an
assembly.
Working Load is the load taken by the assembly at the time when it is to perform.
Bolt Preload is a term that refers to the tension created when a nut works with a bolt to
hold together two materials. As the tension hits the optimal preload, the working load
acting on a bolt is distributed across the assembly, in order to prevent the bolt from
bearing the complete load.
THE ROLE OF BOLT PRELOAD CANNOT BE OVERLOOKED
A fully tightened bolt is allowed to survive in a system where a loose assembly would
easily come apart. Upon tightening, the bolted joint serves as a conduit for the force to
distribute itself through the materials. Hence, the assembly bears only a part of the
working load force. Preload is especially useful for rigorous applications where bolts are
unlikely to survive. Bolted joints are strengthened because of the preload.
THE IMPORTANCE OF PRELOAD
If not for bolt preload, all of the burden would be shifted to the bolt to hold the weight.
The introduction of the preload ensures that less number of bolts are needed, with the
support plates playing an important role in holding the load. However, you should be
certain that the working load is less than the preload of the bolts to ensure the support
plates, and consequently, the bolt, doesn’t fail.
Preload is an essential concept in larger mechanical and high-performance instruments
such as Telescopes. It helps raise the natural frequency of a system, safeguarding it
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such as Telescopes. It helps raise the natural frequency of a system, safeguarding it
against external disruptions. It helps you avoid buckling in case of changes in the stress
caused by shifts in the position.
From <https://itafasteners.com/bolt-preload-what-is-it-and-why-its-important.php>
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