Structures and Mechanisms

Question 1

What is stress?

Answer 1

This is when a member is under compression. When something is under compression it is called a strut.

Question 2

What is strain?

Answer 2

This is when a member experiences a tensile force. A member in tension is called a tie.

Question 3

What is the formula for strain?

Answer 3

Strain = Change in Length/ Original Length

Question 4

Does strain have a unit?

Answer 4

No.

Question 5

What is the formula for stress?

Answer 5

Stress = Force/Area

Question 6

Does stress have a unit?

Answer 6

Yes, it is measured in N/mm2.

Question 7

What is a vector quantity?

Answer 7

A vector quantity has both a magnitude and a direction.

Question 8

Give an example of a vector quantity.

Answer 8

Velocity, weight and force are all vector quantities.

Question 9

What is the formula for the principle of moments?

Answer 9

∑ clockwise moments = ∑ anticlockwise moments

Question 10

What are the formulae for the conditions of equilibrium?

Answer 10

Σ Fh = 0
Σ Fv = 0
Σ M = 0

Question 11

What mechanism allows a motor to be disengaged from a gear system.

Answer 11

A clutch allows a motor to be disengaged from a gear system.

Question 12

What does a bearing do?

Answer 12

A bearing supports a shaft and minimizes the energy lost due to friction.

Question 13

What does a coupling do?

Answer 13

A coupling links an output to a gearbox or generator. A coupling also allows torque to be transmitted easily. They can also be disconnected for repairs.

Question 14

What does a brake do?

Answer 14

A brake reduces the speed of rotation.

Question 15

Give two examples of a brake.

Answer 15

Disk brake, V-brake, water brake, air brake and drum brake.

Question 16

How does a disk brake work?

Answer 16

A disk brake works by pushing brake pads against a brake rotor with a set of calipers.

Question 17

What are the advantages of a compound over a simple gear train?

Answer 17

A compound gear train allows a larger change in speed to be gained in a smaller space.

Question 18

What does a ratchet and pawl do?

Answer 18

A ratchet and pawl stops rotation in one direction.

Question 19

What does a worm and wheel do?

Answer 19

A worm and wheel allows a large change in speed and torque in a small space. It also rotates the direction of motion around 90 degrees.

Question 20

What do bevel gears allow?

Answer 20

Bevel gears allows the direction of motion to be rotated around 90 degrees.

Question 21

What are the advantages of belt drives?

Answer 21

Belt drives allow slippage, which is necessary if there are fragile components in a system. They are also cheap to replace if they break.

Question 22

State the three main types of belts.

Answer 22

V belts, flat belts and toothed belts.

Question 23

State the advantages of chain drives over belt drives.

Answer 23

Chain drives allow a high force to be applied with no slippage.

Question 24

What does a cam do?

Answer 24

A cam changes rotary motion into oscillating or reciprocating.

Question 25

How is a cam used in an engine?

Answer 25

In an engine the cams open and close the valves which let air and fuel in, or exhaust gases out.

Question 26

What three things do all levers have in common?

Answer 26

All levers have a load, an effort and a fulcrum.

Question 27

What does EFL stand for? Give an example as well.

Answer 27

This means that the fulcrum is in the middle. An example of this is a tin opener.

Question 28

What does ELF stand for? Give an example.

Answer 28

This means that the load is in the middle. An example of this is a wheelbarrow.

Question 29

What does LEF stand for? Give an example.

Answer 29

This means that the effort is in the middle. An example of this is a fishing rod.

Question 30

What is the formula for a gear ratio?

Answer 30

Gear ratio = Driven/driver

Question 31

What is a simple gear train?

Answer 31

This is when two or more gears are meshed together.

Question 32

What is a compound gear train?

Answer 32

This is when there are two or more gears on one shaft.

Question 33

What is the formula for the velocity of gears?

Answer 33

Input speed x Input size = Output speed x Output size

Question 34

What is the formula for velocity ratio?

Answer 34

Velocity ratio = speed of input/speed of output

Question 35

What is the formula for torque?

Answer 35

T = Fr

Question 36

Explain what a dynamic force is.

Answer 36

This is when movement is involved. It is more noticeable and structures usually move.

Question 37

Explain what a static force is.

Answer 37

This is when there is no movement involved and structures don't move.

Question 38

Explain what a bending force is.

Answer 38

A bending force is when something curves or flexes. It is often in structures with a force across their length.

Question 39

Explain what a shear force is.

Answer 39

This is when something is cut or torn, i.e: a pair of scissors.

Question 40

Explain what a torsion force is.

Answer 40

Torque is a torsion force, this is a twisting force, i.e: a screw or bolt.

Question 41

Explain what a compression force is.

Answer 41

This when there is a downward weight/pushing force on a member.

Question 42

Explain what a tensile force is.

Answer 42

This is when a member is experiencing a pulling force.

Question 43

What is rotational motion?

Answer 43

This is when something turns in a circle. For example a wheel or a CD.

Question 44

What is linear motion?

Answer 44

This is when something moves in a straight line.

Question 45

What is reciprocating motion?

Answer 45

This is a backwards and forwards motion in a straight line. For example a piston.

Question 46

What is oscillating motion?

Answer 46

This is a backwards and forwards motion, swinging in an arc. For example a swing.

Question 47

What do all mechanisms have in common?

Answer 47

All mechanisms: -involve some kind of motion. - involve some kind of force. - make a job easier to do. - need some kind of input to work. - produce an output.

Question 48

Give the two main types of frame structures.

Answer 48

Cantilevered frame structures and truss frame structures.

Question 49

When analysing a cantilevered frame structure how do you find the forces and direction of the forces in each member?

Answer 49

You use nodal analysis. You choose a node with one known and two unknown forces, and then work out each force. DO NOT USE THE PRINCIPAL OF MOMENTS!

Question 50

When analysing a truss frame structure how do you find the forces and direction of forces in each member?

Answer 50

You treat it as a beam and use moments to find the reaction forces. Then you use resolution of forces the find the remaining forces.

Question 51

Name as many pneumatic actuators as possible.

Answer 51

Push button, plunger, roller, roller trip, solenoid, lever, diaphragm, pilot air, foot pedal, spring.

Question 52

What pneumatic control system is a shuttle valve used in?

Answer 52

OR control.

Question 53

How do you create a simple pneumatic AND control system?

Answer 53

You use two 3/2 valves in series.

Question 54

What is the full name of a 3/2 valve?

Answer 54

A three port, two state valve.