Welcome to Part 1 of Flexible Assembly’s Fastening Theory educational video.
In this video we will investigate the strength of a fastener, what torque is and how it affects
tightening.
Let’s start off by exploring the threaded fastener.
Its function is to clamp two or more components together by acting as a heavy spring between
the components.
Under heavy loads fasteners start to deform and change shape.
They must be able to withstand the forces upon them to avoid failure.
The total scope of all components being clamped together and the fastener is called the bolted
joint.
The support surface on the fastener which mates to the part it will be clamping is called
the bearing surface.
Pre-load is the state of a bolt’s tensile load, its capacity for being able to withstand
being pulled apart.
While clamp load is the state of a joint’s compressive load, its capacity to withstand
loads while being pushed together.
Both these types of loads, although opposite, rely on the structure of the bolt and joint
to be strong enough to resist deformation from compression or stretching.
Let’s take a deeper look into fastener strength.
Here is a graph where the black line represents fastener strength.
Torque, or stress, is represented on the Y axis while strain is on the X axis.
As stress and strain increase the fastener will eventually reach its limit and fracture.
The yield strength of the fastener is the point of stress which the material it is made
of starts to deform permanently.
The proof load of the fastener is its usable strength where it will return to its original
shape once the load is removed.
This is typically 85-95% of the yield strength.
The ultimate tensile strength is where the fastener is pulled to the point of breaking
and fracturing.
This is where the fastener will fail completely.
While the fastener remains in the elastic range it can deform and safely return to its
original shape once the load is removed, however once it passes into the plastic range it will
remain permanently deformed.
Let’s review by breaking down tightening into 4 phases.
Phase 1 is rundown, where there is no contact between fastener and the components being
clamped.
Phase 2 is draw-down, where the joint is seated and there is contact between all components
being clamped.
Phase 3 is elastic deformation and clamp load build-up.
Clamp-load, also called preload is created when torque is applied and is the primary
goal when tightening.
Phase 4 is plastic deformation.
This is where we hit our yield strength and the fastener begins to deform permanently.
All fasteners have a manufactures rating for their strength.
Making sure you have the right fastener for your job is important to avoid failure.
Now let’s examine torque why we need to measure it when achieving clamp force.
First off, what exactly is torque?
Simply put, torque is a measure of how much force acting on an object causes that object
to rotate on an axis.
There are a few components.
The first is the pivot point, which is generally in the center of our fastener, acting as an
axis.
The second is the force that is applied.
The third is the moment arm, which is the distance from the pivot point to where the
force is applied.
So torque is the cross product of the magnitude of force and the perpendicular distance from
the force to the axis of rotation.
The longer the moment arm is, the more leverage can be used to result in higher torque.
Imagine a door.
The hinge is our pivot point, pushing on the door is our force, and the moment arm is how
close we push on the door in relation to the hinges.
By pushing on the door right next to the hinges we don’t create much torque.
However if we push with the same force on the edge opposite of the hinges we create
much more torque, easily swinging the door open.
A hinge doesn’t tighten under this torsional force, but a threaded fastener does and thus
creates bolt tension and a clamping force.
How is torque measured?
Let’s take a closer look.
The first point we will examine is labeled O.
This is the axis of rotation that our wrench is free to move around.
Next we have P, which is where our hand will be applying force at the end of the wrench.
The distance between O and P is labeled r and represents the moment arm.
When force is applied perpendicularly to the wrench we get our applied force vector which
is labeled as F.
This vector combined with the vector from the wrench creates an angle between them which
is labeled as theta.
The product of multiplying the force, moment arm, sin and theta together is torque.
This shows how if we were to tighten the same bolt with the same amount of force but instead
used a longer wrench our moment arm would be a higher valve, thus creating more torque
through leverage.
Torque is not energy, it’s a cross product of two vectors and must be labeled in such
a way.
We use Nm or lbs. ft. and similar units to do so, but what do those units of measure
mean?
One newton meter is equal to the torque resulting from a force of one newton applied perpendicularly
to a moment arm which is one meter long.
Similarly a lbs. ft. is the same but using one lbs. of force and a one foot moment arm.
How else can we achieve 1 Nm?
Assuming we are applying the force perpendicularly we can use twice the force and half the length
moment arm to reach 1 Nm.
By why measure torque at all when our goal with tightening is to achieve clamp force?
Measuring torque is easy to implement and control since we measure it from the fastener
that we are applying force to.
To measure clamp force would require placing a scale between the mating surfaces, which
can’t be done while tightening without having to later remove that scale during the final
rundown.
Measuring torque can be tricky however since you have to account for many forms of friction,
as we will discover in our next fastening theory video.
Thanks for watching this video, if you have any questions please leave them in the comments
and remember to subscribe to Flexible Assembly so you don’t miss out on Part two of the
series.