Afdeling A: Structures investigation skills
p.28-33
Word
Electrical pylons
Member
Columns
Beams
Arches
Buttresses
Struts
Stays
Guys
Definition
Ties
King and queen posts
Rafters
Tie beams
trusses
Tension
Compression
Shear
Torsion
Bending
Cross-bracing
Structural failure
When does structural failure occur?
______________________________________________________________________________
______________________________________________________________________________
______________________________________________________________________________
When do structures fail?
______________________________________________________________________________
______________________________________________________________________________
______________________________________________________________________________
Name the 3 most likely reasons why structures fail:
1. ________________________________________________
2. ________________________________________________
3. ________________________________________________
Fracture of a member due to lack of strength
A strong material can resist a large force before it fractures or breaks.
A component of a structure will break or fracture when the stress on it becomes so great that it
cannot resist anymore stress.
The strength of a material is therefore a measure of its resistance to a force load on it.
Material choices will, therefore, be determined by the influence of the following forces or stresses:
1. _________________________________
2. _________________________________
3. _________________________________
4. _________________________________
5. _________________________________
External factors that can cause structural members to fracture are:
1.
______________________________________________________________________________
______________________________________________________________________________
2.
______________________________________________________________________________
______________________________________________________________________________
3.
______________________________________________________________________________
______________________________________________________________________________
4.
______________________________________________________________________________
______________________________________________________________________________
5.
______________________________________________________________________________
______________________________________________________________________________
Bending (flexing and buckling) due to lack of stiffness (rigidity)
A rigid structure is a structure that is flexible and easy to bend or change. It collapses easily as
there is no reinforcement through triangulation or gussets.
What are gussets?
______________________________________________________________________________
______________________________________________________________________________
______________________________________________________________________________
______________________________________________________________________________
The following reasons may have an effect on whether a structure will be able to withstand internal
and external forces:
1. _______________________________
2. _______________________________
3. _______________________________
4. _______________________________
Toppling over due to lack of stability (top heavy with a narrow base)
A stable structure is a structure that cannot be easily moved, changed or toppled over when a
force is applied to it.
Stability in a structure depends on:
1. _____________________________
2. ______________________________
3. ______________________________
A structure will be more stable if it has a broader base and a lower centre of gravity. A stable
structure has the ability to withstand a force that was not in the original design specification.
Section C: Mechanical systems and control
p.54 - 61
Word
Mechanical advantage
Wedge
Gears
Gear system
Inclined planes
Spur gears
Driven gear
Idler gear
Gear ratio
Definition
Speed ratio
Velocity ratio
cams
cranks
Cam follower
Reciprocating movement
Simple mechanisms
Simple mechanisms have only one working part.
The wedge
What is a wedge?
______________________________________________________________________________
______________________________________________________________________________
______________________________________________________________________________
What is a wedge used for?
______________________________________________________________________________
______________________________________________________________________________
______________________________________________________________________________
______________________________________________________________________________
Explain how the blade of a knife is an example of two inclined planes.
______________________________________________________________________________
______________________________________________________________________________
______________________________________________________________________________
______________________________________________________________________________
______________________________________________________________________________
What does the mechanical advantage depend on in a wedge?
______________________________________________________________________________
______________________________________________________________________________
______________________________________________________________________________
______________________________________________________________________________
The wheel and axle
Name examples of a wheel and axle:
______________________________________________________________________________
______________________________________________________________________________
What does a wheel consist of?
______________________________________________________________________________
______________________________________________________________________________
The wheel and axle can be used as a force multiplier such as a door knob and a steering wheel.
Gears
What are gears?
______________________________________________________________________________
______________________________________________________________________________
______________________________________________________________________________
In all gear systems, gears change the input movement to give a different output movement. They
have teeth that directly engage with each other. There are many different types of gears and each
type processes movement in a different way.
Spur gears
When spur gears of different sizes engage, we call the smaller gear the pinion or driver gear and
the bigger gear the driven gear. When two or more gears engage in the same way, we call this a
gear system.
Idler gears
Idler gears will always be found between two spur gears.
They synchronize the rotation of the:
____________________________
____________________________
Since a smaller idler will rotate more than the larger gears, it should be made of harder metal. By
using a third gear, two gears will be forced to turn in the same direction.
Gear ratios
Why does a bicycle cycle easier uphill when the gears are changed?
______________________________________________________________________________
______________________________________________________________________________
______________________________________________________________________________
For each full rotation that the driver gear makes, the driven gear makes a quarter turn. If the
driven gear makes a full rotation, the driven gear will rotate at a quarter of the speed of the driver
gear.
How do we calculate the gear ratio?
______________________________________________________________________________
______________________________________________________________________________
______________________________________________________________________________
______________________________________________________________________________
______________________________________________________________________________
Mechanisms that can change the direction of movement
Cams, cranks and cam followers are very important mechanisms in modern machines. They
provide for the conversion of movement in opposite directions. We need cams and cranks to make
modern machines perform multiple tasks in the quickest possible time.
What do we mean by reciprocating movement?
______________________________________________________________________________
______________________________________________________________________________
______________________________________________________________________________
Cam
Cams change the rotary input movement to reciprocating movement of the cam follower. How the
follower must move determines the shape of the cam.
Snail cams convert rotary movement into reciprocating movement. A snail cam will let the follower
move up slowly and then drop suddenly.
Eccentric cams do not have an axis placed centrally (in the middle of the cam). This allows for the
follower to move up and down at a constant rate.
Crank
A crank converts rotary motion into reciprocating motion. It can also be used to convert motion
from reciprocating to rotary motion.
The throw of the reciprocating motion is determined by the distance from the centre of the wheel
to where the lever is connected.