Mocks Flashcards
What is a mechanism?
- all mechanical systems have mechanisms which transform an input motion and force into an output one
- they’re designed so you can gain a mechanical advantage from using them (make something easier). This involves changing the magnitude and direction of the applied force
- some mechanisms change one type of motion into another
What are the various types of motion?
- linear motion: moving one way in a straight line
- reciprocation motion: moving backwards and forwards in a straight line
- oscillation motion: moving backwards and forwards in an arc
- rotary motion: moving in a circle
What are levers?
- levers move and lift loads by rotating about stationary points called fixed pivots
- there are 3 main types of lever that you need to know about: first order, second order, and third order
What are first order levers?
- first order levers have the pivot between the effort and the load. Pushing down on your side of the lever lifts the load up the other end - the lever has rotated about the pivot
- if the load is closer to the pivot than the effort, a large load can be lifted using smaller effort - the lever gives a mechanical advantage
- as as you move the pivot closer to the load it becomes easier to lift - the magnitude of the effort required decreases
What are second order levers?
- second order levers have the load in the middle
- here the pivot is at one end of the lever and the effort is at the other end. The classic example is a wheelbarrow.
- the closer together the pivot and load are, the easier it is to lift
What are third order levers?
- third order levers have the effort in the middle
- third order levers can be things like fishing rods, cricket bats, and garden spades
- moving the effort and pivot further apart makes it easier to move or lift the load
What are linkages?
- levers can be connected together to form linkages
- Simple linkages can change the magnitude of the force and the direction of motion
- two examples are: push/pull linkages, and bell cranks
What are push/pull linkages?
- push/pull linkages use 2 fixed pivots
- the input and output motions of the linkage are in the same direction. The motion of the link arm is in the opposite direction
- Here, each fixed pivot is in the centre of an arm. Changing the position of these pivots will change the magnitude of the the output force - often the easiest way to see this is by making a model
What is a bell crank/
- a bell crank changes the direction of a force through 90°
- the magnitude of the output force can be changed by moving the fixed pivot so it’s not an equal distance between the two moving pivots
What are gears and gear trains?
- gears are toothed wheels that interlock. They transfer motion from one part of a machine to another
- a gear train is where two or more gears are linked together. They can be used to change the direction of motion or change the magnitude of the input force
- e.g. in a driver gear, turned by hand or a motor, turns the driven gear. Both will turn the opposite directions.
- e.g. if you use a third gear (called an idler) the driver and driven gears will move in the same direction. The idler does not affect the speed of the other two gears.
How can the relationship between the driver and driven gears be described?
- if linked gears are different sizes, the smaller gear will turn faster
- this relationship can be shown by using a gear ratio
- gear Ratio = no. Teeth on driven gear /no. Teeth on the driver gear
- the size of this ratio describes how much the mechanism changes the speed of the gears from the input speed to the output speed
- output speed = speed of driver gear (input speed) / gear ratio
What are rotary systems?
- mechanical systems that work by using rotary motion (e.g. gear trains)
What is a pulley?
- a simple pulley is made up of a wheel with a grooved outer edge and a cable, rope or belt that sits on the groove
- pulleys make lifting a load easier
- one pulley on its own changes the direction of the force required. The same amount of force is needed but pulling down might be easier than lifting something up
- using two or more pulleys together can change the magnitude of the force too - they can make things feel a lot lighter
- e.g. one fixed pulley and one moving pulley (a block and tackle) will mean you only need half the force to lift a load
What is a belt drive?
- a belt drive transfers movement from one rotating shaft to another
- belt drives are used in pillar drills and washing machines
- in a pillar drill, a flexible belt joins two separate pulley wheels - this links the motor to the drill shaft
- this belt can be put in different positions to make the drill turn faster or slower. This works in a similar way to gears - if the wheels are different sizes, the smaller wheel will spin faster
- the diameter of the wheel can be used to calculate the velocity ratio - how fast the driven wheel will spin relative to the driver wheel
- velocity ratio = diameter of the driven pulley wheel / diameter of the driver pulley
- output speed = speed of driver pulley wheel / velocity ratio
What is a cam mechanism?
- cams change rotary motion to reciprocating motion
- it has two main parts - the cam and the follower
- cams come in many shapes and sizes but they always rotate
- changing the size or shape of the cam can be used to change the magnitude of the output motion (the reciprocating motion of the follower)
- the follower rests on the cam as it rotates and follows its shape. It may have a small wheel to reduce friction. The follower moves up and down (reciprocating motion) as the cam turns
What are the different types of cams?
- circular cam: (offset or eccentric) produces a uniform reciprocating motion
- snail cam: for half the follower will not move, then it will gently rise and then suddenly drop. It will only work in one direction
- four-lobed cam: has four lobes (bits that stick out). For each turn of the cam the follower will rise and suddenly fall four times. This cam shape will only work in one direction
- pear cam: for half a turn the follower will not move then it will gently rise and fall
Describe cartridge paper
- high quality and has textured surface
- great for sketching with different drawing materials like pencils, crayons and inks
Describe Lay-out paper
- thin and translucent
- used for general design work
Describe tracing paper
- semi transparent
- used to copy images
Describe grid paper
- square or isometric pattern printed on it
- useful for orthographic and scale drawings
- isometric paper’s good for isometric drawings
Describe bleed proof paper
- used when drawing with felt tips and marker pens
- ink doesn’t spread out/bleed
What are boards
- the weight of paper and board is measured in gsm. Above 200 gsm it’s not paper anymore - it’s a board
- it’s often used in packaging because of its low cost compared to other packaging materials, and it’s high strength to weight ratio
Describe solid white board
- high quality bleached surface, ideal for printing
- used for primary packaging
Describe ink jet card
- used for ink jet printing
- designed so that ink doesn’t bleed
- allows printed image to be sharply defined and of a high quality
Describe corrugated card
- made up of a fluted inner core sandwiched between 2 outer layers which can be printed on
- the flutes add strength and rigidity
- this is useful in a lot of secondary packaging to protect products during transit
Describe duplex board
- has different colour and texture on each side
- often where only one side is seen, so that only one side needs to be smooth for printing
- used for food packaging
Describe foam core board
- is made by sandwiching expanded polystyrene foam between 2 thin layers of card
- it’s stiff, lightweight, and the thin outer card layer can be scored
- good for making models and mounting posters
Describe foil lined board
- has an aluminium foil lining
- used for food packaging, foil keeps flavour in, and air and moisture out
What are softwoods
- grow in colder climates and fast
- cheap and accessible
- have leaves like needles and usually evergreen and have cones
Describe pine
- yellow with brown streaks
- string and cheap but knotty (harder to work with)
- used for telegraph poles, fences, and cheap furniture
Describe larch
- softwood
- attractive yellow to reddish-brown colour
- harder, tougher, more durable than most softwoods
- resistent to rot
- Good for decking and cladding the outside of buildings and fence posts
Describe spruce
- reddish brown colour
- hard and has a good strength to weight ratio
- knotty and not very durable
- used for structural purposes like air rafts, crates, and ship masts
What are hardwoods?
- grow in warm climates and slowly
- more expensive
- broad flat leaves and deciduous
- tighter grain and denser and harder (except balsa)
Describe oak
- light brown
- tough durable and very strong
- attractive grain markings
- finishes well
- used in interior panelling, flooring and furniture
- corrodes steel and fittings
Describe mahogany
- red-brown colour
- durable and easy to work with
- expensive
- good quality furniture
Describe balsa
- white or tan colour
- very low density for a hardwood
- very soft
- easy to cut and shape
- high strength to weight ratio
- modelling