Structures and Mechanisms
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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.