Mechanics of Machines ( lecture by Engr. Mehran Waheed )

Mechanics of Machines ( 1^st January 2017 )

Position:-

Describe the existence of a body relative to a convenient reference frame at any particular time.

Displacement Is change in position of an object

D= Xf + Xi

It is a vector quantity with parallel direction & magnitude.

Computation of displacement with respect to time & velocity

d= ½ ( Vf + Vi ) t

Where

d= Displacement

t= time

Vf= final velocity

Vi= initial velocity

Problem # 1.

A car traveled down a road far 45 sec. the car turned onto the street at 20 m/sec & by the end of the street it was traveling at 23 m/sec. HOW LONG IS THE STREET ?

Given data is :-

Vi = 20 m/sec

Vf = 23 m/sec

t = 45 sec

We know that

d = ½ (Vi + Vf ) t

d = ½ ( 20+23 ) 45

d = 967.5 meter

Problem # 2.

A boy through a ball, when he is about 60 fts from home, if the ball leaves his hands at 132 ft/sec & reach home with a speed of 110 ft/sec, HOW LONG DOES IT TAKES THE TO TRAVEL TO HOME ?

Given data is :

D = 60 ft

Vf = 132 ft/sec

Vi = 110 ft/ sec

We know that :

d = ½ (Vi + Vf ) t

t = 2d/(Vi + Vf )

t = 2(60)/132+110

t = 0.496 sec

Graphical Computation of displacement with velocity and time.

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Mechanics of Machines ( 25 December 2016 )

Mechanism of Machines:-

It is a mean of transmitting or controlling a relative motion.

Inversion of Mechanism:-

It means observing the motion of the members of a mechanism by fixing different links as reference frame.

Various Practical Mechanism:-

1.      Actuators are devices that convert some energy into motion.

2.      Cams are devices that convert rotation motion into linear motion.

3.      Gear are devices that transmit torque and adjust rotational velocity.

4.      Levers are devices that transmit & amplify force.

5.      Ratchets are devices used to keep the wheel of car strapped to trailer.

6.      Spring are devices that store & dissipate energy in damped manner.

Hooke’s Joint:-

Is that in which two V-shaped links are connected by a cross arm. It is also called universal joint.

Properties of Hook’s Joint:-

1.      Practically maintenance free.

2.      Shield lubricated needle bearing.

3.      Provide precise positioning.

4.      Maintains low backlash / play.

5.      Operates continuously at high speed.

Types of Hook’s Joint:-

1.      Single Hook’s Joint

2.      Double Hook’s Joint

Disadvantages of Hook’s Joint:-

1.      It cannot be used for constant speed.

2.      Even if the driving shaft is rotating at constant speed, driven shaft rotate with acceleration.