Structures and Mechanisms Flashcards
What is stress?
This is when a member is under compression. When something is under compression it is called a strut.
What is strain?
This is when a member experiences a tensile force. A member in tension is called a tie.
What is the formula for strain?
Strain = Change in Length/ Original Length
Does strain have a unit?
No.
What is the formula for stress?
Stress = Force/Area
Does stress have a unit?
Yes, it is measured in N/mm2.
What is a vector quantity?
A vector quantity has both a magnitude and a direction.
Give an example of a vector quantity.
Velocity, weight and force are all vector quantities.
What is the formula for the principle of moments?
∑ clockwise moments = ∑ anticlockwise moments
What are the formulae for the conditions of equilibrium?
Σ Fh = 0
Σ Fv = 0
Σ M = 0
What mechanism allows a motor to be disengaged from a gear system.
A clutch allows a motor to be disengaged from a gear system.
What does a bearing do?
A bearing supports a shaft and minimizes the energy lost due to friction.
What does a coupling do?
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.
What does a brake do?
A brake reduces the speed of rotation.
Give two examples of a brake.
Disk brake, V-brake, water brake, air brake and drum brake.
How does a disk brake work?
A disk brake works by pushing brake pads against a brake rotor with a set of calipers.
What are the advantages of a compound over a simple gear train?
A compound gear train allows a larger change in speed to be gained in a smaller space.
What does a ratchet and pawl do?
A ratchet and pawl stops rotation in one direction.
What does a worm and wheel do?
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.
What do bevel gears allow?
Bevel gears allows the direction of motion to be rotated around 90 degrees.
What are the advantages of belt drives?
Belt drives allow slippage, which is necessary if there are fragile components in a system. They are also cheap to replace if they break.
State the three main types of belts.
V belts, flat belts and toothed belts.
State the advantages of chain drives over belt drives.
Chain drives allow a high force to be applied with no slippage.
What does a cam do?
A cam changes rotary motion into oscillating or reciprocating.
How is a cam used in an engine?
In an engine the cams open and close the valves which let air and fuel in, or exhaust gases out.
What three things do all levers have in common?
All levers have a load, an effort and a fulcrum.
What does EFL stand for? Give an example as well.
This means that the fulcrum is in the middle. An example of this is a tin opener.
What does ELF stand for? Give an example.
This means that the load is in the middle. An example of this is a wheelbarrow.
What does LEF stand for? Give an example.
This means that the effort is in the middle. An example of this is a fishing rod.
What is the formula for a gear ratio?
Gear ratio = Driven/driver
What is a simple gear train?
This is when two or more gears are meshed together.
What is a compound gear train?
This is when there are two or more gears on one shaft.
What is the formula for the velocity of gears?
Input speed x Input size = Output speed x Output size
What is the formula for velocity ratio?
Velocity ratio = speed of input/speed of output
What is the formula for torque?
T = Fr
Explain what a dynamic force is.
This is when movement is involved. It is more noticeable and structures usually move.
Explain what a static force is.
This is when there is no movement involved and structures don’t move.
Explain what a bending force is.
A bending force is when something curves or flexes. It is often in structures with a force across their length.
Explain what a shear force is.
This is when something is cut or torn, i.e: a pair of scissors.
Explain what a torsion force is.
Torque is a torsion force, this is a twisting force, i.e: a screw or bolt.
Explain what a compression force is.
This when there is a downward weight/pushing force on a member.
Explain what a tensile force is.
This is when a member is experiencing a pulling force.
What is rotational motion?
This is when something turns in a circle. For example a wheel or a CD.
What is linear motion?
This is when something moves in a straight line.
What is reciprocating motion?
This is a backwards and forwards motion in a straight line. For example a piston.
What is oscillating motion?
This is a backwards and forwards motion, swinging in an arc. For example a swing.
What do all mechanisms have in common?
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.
Give the two main types of frame structures.
Cantilevered frame structures and truss frame structures.
When analysing a cantilevered frame structure how do you find the forces and direction of the forces in each member?
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!
When analysing a truss frame structure how do you find the forces and direction of forces in each member?
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.
Name as many pneumatic actuators as possible.
Push button, plunger, roller, roller trip, solenoid, lever, diaphragm, pilot air, foot pedal, spring.
What pneumatic control system is a shuttle valve used in?
OR control.
How do you create a simple pneumatic AND control system?
You use two 3/2 valves in series.
What is the full name of a 3/2 valve?
A three port, two state valve.
