Joints are mechanical systems that are used for connecting two or more parts together and bolted
joints are those that are connected using a nut and a bolt which are threaded fasters.
Their main functions are to hold the parts together in an assembly
and to transfer forces from one part to another.
This makes them a crucial aspect of a structure despite being a small component of the design.
They are seen in assemblies of different nature and sizes such as in bridges and building,
in electronic components, automobiles of course and even in hip and orthopedic implants.
Threaded fasteners come in different forms. Two main types are bolts and screws.
They both share the feature that the forces are transferred between the mating threads,
but the main difference lies in their counter threads.
In case of bolts, a nut is used which carries the counter threads. Typically,
the bolt is held at rest using a spanner and the nut is tightened.
In case of screw, it cuts the threads in the material during installation
so there’s no need for a nut or an equivalent.
In this case, the screw is rotated using an electric or manual screw-driver.
In this course our discussion revolves around bolt and nut
assembly but most of the discussion can be extended to screw fasteners too.
Before diving into the mechanics of bolted joints let’s spend some time to
learn the structure of a bolt-nut assembly and the nomenclature.
The nut and the bolt are the two main components that engage via threads cut
into them. The interaction between the threads is where they transfer forces from one another.
The nut and the bolt heads are the two surfaces
between which the mating components are sandwiched.
The portion of the bolt between the nut and the bolt heads is called as
the grip length and this is the load bearing portion of the bolted joint.
Now let’s focus on the threads of the bolt.
The threads of the bolt is where the loads are transferred,
and their dimensions is crucial to designing bolted joints.
Three important terms related to threads are the lead, the pitch and the number of leads.
Lead is the distance advanced by the bolt along its axis for one full rotation.
Pitch is the distance between the crests of two adjacent threads.
Number of leads is the number of ridges that are wrapped around the bolt.
Most bolts have one lead in which case the lead and the pitch are equal.
While we’re discussing about the design of a threaded fastener, it’s worth noting that in
general when we design a bolted joint, we don’t really design the bolt itself.
Instead, we pick one from the existing types that suites the design of our bolted joint.
So what we need is a way of identifying the threaded fasteners from their specifications.
There are two commonly use used way of representing bolts. They are the
ISO metric system and the unified thread standard.
In the ISO metric system, the representation starts with letter M followed by the nominal
diameter of the bolt, the pitch distance and the length of the bolt. All these measures are
represented in millimeters. Here’s a typical example which has a nominal diameter of 8 mm,
a pitch of 1.25 mm with the length of the bolt being 30 mm.
In case of unified thread standard, the major diameter is represented in inches which is
followed by the number of threads per inch, which is nothing but the reciprocal of the pitch
distance. Here’s a typical example for a 6 inch dia diameter bolt that has 32 threads per inch.
In both the cases, the threads are in V shape with
an angle between the threads being equal to 60 degrees.
Looking at the geometry of a typical threaded potion portion,
This angle between two adjacent threads is called as the thread angle.
The axial distance between two adjacent crests of the threads is called as the pitch of the screw.
The bolt generally has two important diameters: the minor and the major diameter.
The minor diameter is the distance between the opposing troughs of the threads and the
major diameter is the distance between the opposing crests of the threads.
So, in case of both the ISO metric and the Unified inch systems, the major diameter
is used to represent the thread diameter and the thread angle 2alpha is 60 degrees.
Another important measure of diameter, which is considered as the effective diameter is called
as the pitch mean diameter which is generally half-way between the minor and the major diameter.
If only the major diameter is known, then one way of approximating the pitch mean diameter is
to use this relation where D is the major diameter and p is the pitch of the screw.
Now that we have learned the nomenclature
of a bolt-nut assembly let us now shift our focus to a bolted joint.
A typical bolted joint consists of two plates
or of components sandwiched between a nut and the a bolt.
The bolt head and the nut remains in contact with
the components and transfer of forces happens between them.
When we tighten the nut by applying torque, it climbs the length of the
bolt and applies compressive forces on the sandwiched plates and presses them together.
As a result, the bolt head and the nut experiences forces in the opposite directions.
All these forces balance each other out and maintains a static state of the system.
If we look at the shank of the bolt, we’ll notice that the
reaction forces are creating a state of pre-stress and it is tensile in nature.
So, when we tighten the bolt there’s a preload present in the bolt.
This load is called as the clamp load, or bolt preload. Since it is in a state of tension,
it is sometimes referred to as bolt pre-tension too.
Since it experiences a tensile preload, there are chances that the bolt shank may
experience some plastic deformation and may even fail when this preload is too high.
The limiting preload at which the bolt starts experiencing
plastic deformation is called as the proof load.
Before we conclude this overview of bolted joints
it’s worth spending some time to learn about the failure of bolted joints.
There are several reasons that may lead to failure of bolted joints and
majority of them are related to the clamp force.
We’ll list five most common issues seen in such joints.
The first reason is due to insufficient clamp force. If the bolt-nut assembly is
tightened properly, it may further loosen during operation and may eventually come
apart which is a failure. Also, some joints rely on the friction force between
the bolt-head and nut and the component surfaces which is directly proportional
to the clamp force. Such joints can fail too due to very little friction.
Another reason for failure is excess clamp load. If the bolts are over tightened,
then shank may experience some plastic deformation which will reduce its stiffness
and may eventually fail in tensile mode if it’s loaded beyond its ultimate strength.
Tension is not the only state of failure in bolts. In some joints that support shear loads, the bolt
may fail in shear mode too when the external shear load is too high. It’s important to make sure that
the shear plane passes through the shank of the bolt and not through threaded section.
Fatigue failure is another failure cause case. This is dangerous as it’ll appear
without any signs. This occurs due to the cyclic loads that the bolts are subjected
too. The fatigue can also be accelerated when the bolts are loosely connected.
Finally, thread stripping is another mode of failure. When there’s too much friction or
adhesion between the threads then they may shear and fail when torque is applied. Since threads are
the crucial load bearing elements of a bolted joint their failure may fail the full system.
This can be avoided by properly lubricating the threads before applying the torque.
These are just some forms of bolt failures, but we can see how the
preload present in the bolt plays a crucial role in their performance and reliability.
with this we can conclude the introduction to the bolted joints.