UNIT 7 MECHANISMS

Mechanisms are devices that transmit and convert forces and motions from a driving force (input) to an output element. They enable us to carry out certain tasks with greater comfort and less effort.

We can clasify mechanisms in this way:

1. LINEAR MOTION MECHANISMS 1.1. LEVERS 1.1. LEVERS A lever is a rigid bar that it is used with a point of support or a fulcrum

LAW OF THE LEVER: F X f= R x r F:effort, R:load TYPES OF LEVERS: TYPES OF LEVERS:

EXAMPLES TYPE 1 LEVERS SEESAWSCISSORS PLIERS

EXAMPLES TYPE 2 LEVERS LEVERS HANDCART BOTTLE OPENER NUTCRACKER

EXAMPLES TYPE 3 LEVERS EYEBROW TWEEZERS FISHING POLE STAPLER REMOVER

1.2. FIXED PULLEYS A fixed pulley is a wheel that rotates around an axle that is fixed to an immobile surface. A fixed pulley is a wheel that rotates around an axle that is fixed to an immobile surface. A fixed pulley is balanced when the effort, F, is equal to the load, R A fixed pulley is balanced when the effort, F, is equal to the load, R F=R F=R

1.3. Moveable pulleys A moveable pulley has two pulleys, one is fixed while the other can move in a linear direction. A moveable pulley has two pulleys, one is fixed while the other can move in a linear direction. F = R / 2 F = R / 2

Page 103 Page 103 Exercise 2, 3, 5 Exercise 2, 3, 5

2. ROTARY MOTION MECHANISMS 2.1. Friction drive Made up of two or more wheels that are in contact. The motion of the first wheel makes the other wheel turn, transmitting the motion. The relation between the rotation velocities of the wheels depends on the sizes of the wheels

N1: velocity of the primary drive wheel N2: velocity of the output wheel D1: diametre of the primary drive wheel D2: diametre of the output wheel

2.2.Pulleys with belts These are two pulleys or wheels that are a certain distance apart. These are two pulleys or wheels that are a certain distance apart. They rotate simultaneously because of the belt. They rotate simultaneously because of the belt.

2.3. GEAR MECHANISMS AND COGWHEELS Cogwheels are sets of wheels that have teeth called cogs. These teeth mesh together so that one wheel moves the other. Cogwheels are sets of wheels that have teeth called cogs. These teeth mesh together so that one wheel moves the other.

N 1 x Z 1 = N 2 x Z 2 Z 1 /Z 2 = N 2 /N 1 N 1 x Z 1 = N 2 x Z 2 Z 1 /Z 2 = N 2 /N 1 N 1 : velocity of the wheel 1 N 2 : velocity of the wheel 2 Z 1 : number of teeth of the first wheel Z 2 : number of teeth of the second wheel

2.4. GEAR MECHANISMS WITH A CHAIN N 1 x Z 1 = N 2 x Z 2 Z 1 /Z 2 = N 2 /N 1

2.5. PULLEY TRAINS WITH BELTS N 4 /N 1 =(D 1 XD 3 )/(D 2 XD 4 ) N 1 : velocity of the drive pulley N 4 : velocity of the driven pulley D 1, D 2, D 3, D 4 : diametres of the pulleys

2.6. GEAR TRAIN N 4 /N 1 =(Z 1 XZ 3 )/(Z 2 XZ 4 ) N 4 /N 1 =(Z 1 XZ 3 )/(Z 2 XZ 4 ) N 1 : velocity of the drive wheel N 4 : velocity of the driven wheel Z 1, Z 2, Z 3, Z 4 : number of teeth in the wheels

Page 107 Page 107 Exercises 6, 9, 10, 11, 12, 13 Exercises 6, 9, 10, 11, 12, 13

3. MECHANISMS THAT TRANSFORM MOTION 3.1. FROM ROTARY INTO LINEAR MOTION 3.1. FROM ROTARY INTO LINEAR MOTION a) RACK AND PINION L= P X Z X N L: velocity of the movement of the rack P: distance between two of the teeth, in milimetres. Z: number of teeth in the pinion. N: number of rotations per minute of the pinion

3.2. WINCH (AND CRANK HANDLE) 3.2. WINCH (AND CRANK HANDLE) It consists of a crank handle attached to the axle of a cylindrical drum (or winch) It consists of a crank handle attached to the axle of a cylindrical drum (or winch)

3.2. FROM ROTARY INTO RECIPROCATING MOTION a) CRANK-LINK-SLIDER As the wheel rotates, the crank transmits the rotary motion to the link, which moves the slide with a reciprocating motion

Page 111 Page 111 Exercise 14, 18 Exercise 14, 18 Page 112 Page 112 Exercise 1, 4 Exercise 1, 4