UNIT 5 STRUCTURES

1. FORCES AND STRUCTURES

1.1. FORCES Gravity

Wind

Mechanical forces

Pressure of gases

Our muscles

Effects of heat

A force is anything that can deform a body or change its state of movement or rest.

1.2. STRUCTURES

A structure prevents a body from breaking or becoming too deformed A structure prevents a body from breaking or becoming too deformed. They may be natural structures or artificial (human-made) structures. Natural structure Artificial structure

The forces that act on a structure are called LOADS Fix or permanent loads don´t vary over time Variable loads are occasional and changeable

2. STRESSES A stress is an internal tension which happens to all bodies subjected to the applying of one or more forces on them

2.1. TRACTION Forces try to STRETCH the body. The body tends to become longer

2.2.BENDING Forces try to bend a body

2.3. COMPRESSION Forces try to compress a body

2.4. TORSION Forces try to twist a body

2.5. SHEAR OR CUTTING OR The forces act like two scissors blades

Page 93 Exercises 6, 7, 8, 11

3.STRUCTURAL CONDITIONS

STABILITY: capacity of a structure to remain upright. The centre of gravity of the structure must be centred over its base

RESISTANCE: capacity of a structure to bear tensions without breaking It depends on the shape and the material

RIGIDITY: all objects become slightly deformed when a force is applied to them

Page 95 Exercises 12, 13, 14, 15

4. TYPES OF ARTIFICIAL STRUCTURES

MASSIVE VAULTED LATTICE TRIANGULATED SUSPENDED MASSIVE VAULTED LATTICE TRIANGULATED SUSPENDED PNEUMATIC ROLLED GEODESIC

4.1.MASSIVE STRUCTURES Carved on rock or built by stacking rock or stone

4.2.VAULTED STRUCTURES A vault is made from a number of arches placed side by side

4.3. LATTICE STRUCTURES These are used in modern blocks or flats

4.4.TRIANGULATED STRUCTURES They´re used to create large spans, and in verttical structures

4.5. SUSPENDED STRUCTURES This type of structure is hung from cables called rods

4.6. PNEUMATIC STRUCTURES This structures are light and can be dismantled

4.7. GEODESIC STRUCTURES They are three-dimensional triangulated structures

Page 99 Exercise 22 Page 100 Exercises 1, 2

UNIT 7 MECHANISMS 5. 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:

5.1. LINEAR MOTION MECHANISMS A. 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:

EXAMPLES TYPE 1 LEVERS SCISSORS SEESAW PLIERS

EXAMPLES TYPE 2 LEVERS LEVERS HANDCART BOTTLE OPENER NUTCRACKER

EXAMPLES TYPE 3 LEVERS EYEBROW TWEEZERS FISHING POLE STAPLER REMOVER

A fixed pulley is balanced when the effort, F, is equal to the load, R B. FIXED PULLEYS 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 F=R

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

Page 103 Exercise 2, 3, 5

5.2. ROTARY MOTION MECHANISMS A. 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

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

C. 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.

N1 x Z1 = N2 x Z2 Z1 /Z2 = N2 /N1 N1 : velocity of the wheel 1 Z1 : number of teeth of the first wheel Z2 : number of teeth of the second wheel

D. GEAR MECHANISMS WITH A CHAIN N1 x Z1 = N2 x Z2 Z1 /Z2 = N2 /N1

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

5.3. MECHANISMS THAT TRANSFORM MOTION A. FROM ROTARY INTO LINEAR MOTION 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

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

B. 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 Exercise 14, 18 Page 112 Exercise 1, 4

Two boys are sitting on a seesaw Two boys are sitting on a seesaw. The first boy weights 42 kg and is 0,70 m from the fulcrum. The second boy is 1,10 m from the same point. How much does this second boy weight?

What is the velocity of rotation of the driven wheel in a cogwheels mechanism if the drive wheel is moving at 20 rpm? The driven wheel has 40 teeth and the drive wheel has 25.