Page 67 - CITS - WCS - Mechanical

WORKSHOP SCIENCE - CITS

F  N Kg 
Shear stress (τ ) =   or  
A  cm 2 cm 2 
Eg.
1 Rivets
2 Gudgeon Pin
3 Spring shackle pin
4 Brake rod rivets
5 Chassis rivets
6 Fly wheel holding bolts
7 Swivel pins
8 Gear box shaft
9 Axle shaft
4 Torsional stress: When a shaft is subjected to the action of two equal and opposite couples acting in parallel
planes, then the shaft is said to in torsion. The stress set up by the torsion is known as torsional shear stress.
Eg.
1 Rear axle
2 Crank shaft
3 Coil springs

4 Propeller shaft
5 Starter motor armature shaft
Concept on Center of Gravity and Equilibrium of Forces in Plane
The center of gravity (CG) of an object is the point at which weight is evenly dispersed and all sides are in
balance. A human’s center of gravity can change as he takes on different positions, but in many other objects, it’s
a fixed location.
There are certain conditions for Coplanar Forces to be in equilibrium. These include: The sum of forces must be
zero. The sum of the moments of the forces about a point in the anticlockwise direction is equal to the sum of the
moments of the forces about the same point in the clockwise direction
Equilibrium forces are a set of forces whose resultant is zero. An ‘Equilibrant’ is the force that brings a group of
forces into equilibrium. The equilibrant has the same magnitude as the resulting force but opposite direction. A
force comprises both a magnitude (magnitude) and a direction, making it a vector quantity. For any system of
coplanar forces to be in equilibrium following condition should be satisfied: The algebraic sum of the horizontal
components of all the forces should be zero.
Concept of Moment of Inertia and Torque
Moment of inertia is defined as the quantity expressed by the body resisting angular acceleration which is the sum
of the product of the mass of every particle with its square of a distance from the axis of rotation.
Mass moment of inertia, also known as rotational inertia, is a quantity that is used in measuring a body’s resistance
to a change in its rotation direction or angular momentum. It basically characterizes the acceleration undergone
by an object or solid when torque is applied.
When a bus or a train starts suddenly, the passenger standing inside it falls backward: It happens because the
feet of the passenger being in contact with the floor of the bus come in motion along with the bus but the upper
part of the body remains at rest due to inertia of rest. Hence the passenger falls backward.
Torque is the measure of the force that can cause an object to rotate about an axis. Force is what causes an
object to accelerate in linear kinematics. Similarly, torque is what causes an angular acceleration. Hence, torque
can be defined as the rotational equivalent of linear force.
A doorknob turns because a linear, downward force is applied perpendicular to the knob. A coin spins because it
is pushed with a linear force that is applied at some angle relative to the edge of the coin.
Torque is the product of force (unit N) and perpendicular distance (unit m) of line of force from the axis. Hence SI
unit of torque is Nm.

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CITS : WCS - Mechanical - Exercise 4