THEORY OF MACHINESLECTURE NO 1

Prepared By:

Bilawal Ahmed

Theory of Machines• Introduction

The subject Theory of Machine may be defined as that branch of Engineering which deals with the study of relative motion between the various parts of machine, and forces which act on them.

(A machine is that device which recieves energy in some available form & utilises it to do some useful work.)

Kinematics• It is that branch of Theory of Machines which deals with

the relative motion between the various parts of machine.• Kinematics is studying the motion of objects(particles/rigid bodies

etc.) but not considering the forces, just examining the motion itself.

Dynamics• It is that branch of Theory of Machines which deals with

the forces and their effects, while acting upon the machine parts in motion.

Kinetics• It is that branch of Theory of Machines which deals with

the inertia forces which arise from combined effect of the mass and motion of the machine parts.

• Kinetics is studying the motion of objects (particles/rigid bodies etc.) and the forces that cause those motions.

Statics It is that branch of Theory of Machines which deals with

the forces & their effects while machine parts are at rest.

Newton’s law • 1st law

Every body continues to be in its state of rest or of uniform motion, unless it is acted upon by an external force.

• 2nd law

The rate of change of change of momentum is directly proportional to the impressed force & takes place in same direction in which the force acts.

• 3rd law

To every action there is equal and opposite reaction.

Momentum• It is the total motion possed by a body.

momentum = mass X velocity

where; m is mass of body

u is initial velocity

v is final velocity

a is constant acceleration = (v-u)/t

t is time in sec required to change from v to u

change of momentum = mv –mu

rate of change of momentum = (mv –mu)/t

= m (v-u)/t = m.a

Mechanics:In its original sense, mechanics refers to the study of the behavior of systems under the action of forces.

Machines:It is a device which receive energy and transform it into some useful work. A machine consists of No:s of parts or bodies

http://www.howstuffworks.com/engine1.htm

I.C Engines(Internal Combustion Engine)

Almost all cars currently use what is called a four-stroke combustion cycle to convert gasoline into motion. The four-stroke approach is also known as the Otto cycle, in honor of Nikolaus Otto, who invented it in 1867. The four strokes are illustrated in Figure 1. They are:Intake strokeCompression strokeCombustion strokeExhaust stroke

1.xps

Figure 1You can see in the figure that a device called a piston replaces the potato in the potato cannon. The piston is connected to the crankshaft by a connecting rod. As the crankshaft revolves, it has the effect of "resetting the cannon." Here's what happens as the engine goes through its cycle:The piston starts at the top, the intake valve opens, and the piston moves down to let the engine take in a cylinder-full of air and gasoline. This is the intake stroke. Only the tiniest drop of gasoline needs to be mixed into the air for this to work. (Part 1 of the figure)Then the piston moves back up to compress this fuel/air mixture. Compression makes the explosion more powerful. (Part 2 of the figure)When the piston reaches the top of its stroke, the spark plug emits a spark to ignite the gasoline. The gasoline charge in the cylinder explodes, driving the piston down. (Part 3 of the figure)Once the piston hits the bottom of its stroke, the exhaust valve opens and the exhaust leaves the cylinder to go out the tailpipe. (Part 4 of the figure)Now the engine is ready for the next cycle, so it intakes another charge of air and gas.Notice that the motion that comes out of an internal combustion engine is rotational, while the motion produced by a potato cannon is linear (straight line). In an engine the linear motion of the pistons is converted into rotational motion by the crankshaft. The rotational motion is nice because we plan to turn (rotate) the car's wheels with it anyway.