Introduction 1 What is mechanisms Features of Mechanisms

Introduction • • 1 What is mechanisms Features of Mechanisms Four different types of motion Degree of Freedom

What is mechanisms Mechanisms is a device which transmits energy in the form of movement. A mechanisms changes an input force and movement into an output force and movement. 2

• The combinations of mechanisms build mechanical systems which we call machine. • A machine is a device which does work by converting or transmitting energy. – e. g. bicycle, hand drill • A mechanisms transforms an input motion and force into a desired output motion and force. – e. g. gear, cam 3

Basic Machines Five basic Machines Lever Screw Inclined plane pulley wheel and axle 4

Lever is a rigid rod pivoted about a fixed axis called a fulcrum. 5

Screw mechanisms are used to enable heavy loads to be lifted. 6

An inclined plane is a sloping surface used to gain mechanical advantage when raising a load. 7

A pulley is a wheel with a grooved rim and used as lifting device. 8

Wheels & axles are used to lift heavy load with small effort. 9

Features of Mechanisms Load & Effort Load is the weight of the object need to move Effort is the force applied to the mechanisms Mechanical Advantage = 10 Load Effort

Velocity ratio = Distance moved by the effort Distance moved by the load Efficiency = Mechanical Advantage Velocity Ratio X 100% Efficiency is less or equal to 1 (why? ) Efficiency = 11 Energy Output Energy Input X 100%

Types of Motion Linear Motion The motion in a straight line 12

Rotary motion The motion in the circle 13

Oscillation Motion The motion backwards and forwards in an arc. 14

Reciprocating motion The motion backwards and forwards in a straight line Mechanisms use one of these four basic movements, either on their own or combined. 15

Degree of Freedom Driving & Driven In a mechanisms system, the first original device to generate motion is call driving device. It delivers the energy and motion to the devices connected to it. The device follows the motion of the driving device is called driven device. 16

Degree of freedom of a rigid body is defined as the number of independent movements it has. All motion can be regarded as a combination of translation and rotation. Translation: movement alone or more of the three axes. Rotation: rotation about one or more of the axes. 17

In two dimensions, there 3 degree of freedom. Y X 18

What is the degree of freedom of a rigid body in three dimensions? Y X Z 19

There are six degree of freedom for an unrestrained rigid body in space: Three translating motions along the x, y, z axes and three rotary motions around the x, y and z axes respectively. 20

Kinetic Constraints Rigid body system Two or more rigid bodies in space are collectively called a rigid body system. Kinetic constraints Constraints between rigid bodies that result in the decrease of the degree of freedom of rigid body system 21

Kinetic pair Kinetic constraints between rigid bodies. Low pairs and High pairs. Planar mechanisms Two translational & one rotary motions Lower pairs : Revolute & Prismatic pairs ONLY ONE DEGREE OF FREEDOM 22

y y x Planar revolute pair 23 x Planar prismatic pair

Kinetic Pairs in Spatial Mechanisms Spherical pair DOF=3 Planar pair DOF=3 Cylindrical pair DOF=2 24

Revolute pair DOF=1 Prismatic pair DOF=1 Screw pair DOF=2 25

The influence of kinetic constraints in the motion of rigid body has two intrinsic aspects Geometrical aspect Physical aspect The degree of freedom is important to the mechanisms due to the mechanisms is not a structure which has no motions. Adding kinetic constraints between the rigid bodies will decrease the degree of freedom of the rigid body system 26

Degree of Freedom of Planar Mechanisms Low pair : 1 degree of freedom High pair : 2 degree of freedom Equation for degree of freedom F=3(n-1)-2 L-H Where F=total degree of freedom in the mechanisms n=number of links (including the frame) L=number of lower pairs (1 degree of freedom) H=number of high pairs (2 degree of freedom) 27

Example 1 What is the degree of freedom in this mechanisms? 28

Answer N=4 L=4 (at A, B, C, D) h=0 F = 3 x(4 -1)-2 x 4 -1 x 0=1 29