Technical Understanding 1 Flashcards
how can structural integrity be achieved by strengthening
- reinforcing or protecting the material
- reinforcing the assembly of parts within the product
how can you protect a product from complete failure if it is over-stressed
- by using sacrificial parts e.g. fuses, elbow pads
- designed to be the weakest so they will break first to protect more significant parts
- usually easy to replace
processes to ensure structural integrity: material reinforcing
- corner protectors e.g. on speaker cabinets
- plain bearing is used to support a shaft
processes to ensure structural integrity: material stiffening
- achieved by adding folds, ribs or webs (Eiffel tower)
processes to ensure structural integrity: material protection
- surface finish: paint, lacquer, galvanising, anodising
- adding stabiliser to a polymer e.g. UV light to uPVC
- preservative
processes to ensure structural integrity: triangulation
- adding cross members to a rectangular frame
- adding corner braces
- adding gusset plates
what are surface finishes and coatings used for
- used to protect against wear, dirt, damage, corrosion and decay
- enhances appearance
vanishing and painting
- attractive and hardwearing finish to woods
- clear or colour, matt, satin or high-gloss finish
- both heat and waterproof
- applied with brush or spray
- can be non-toxic but requires maintenance
finishing timber
- sanded before with glass paper, garnet paper, and wire wool
- sanding sealer is a solvent based product that seals the surface and raises the wood fibres to be sanded
wax/polish and sealants
Timber
- used when a natural appearance on wood is required
- E.G. olive oil - used as a finish when the wood is going to come into contact with food (salad servers)
- Danish or linseed oil - can be used on most woods; it needs 2 to 4 coats for the best protection
- teak oil - for teak, made of linseed oil and silicone to give it a harder wearing surface
-beeswax is used in interior wooden furniture and gives a natural looking finish
Preservative
Timber
- can be applied by spraying or brushing but pressure treatment is preferred
Eg
- tar oil: highly water repellent, has an odour. Uses are bridges, railway sleepers
- immersion treatments: timber is placed in a bath of preservative, heated then cooled
- pressure impregnation: wood is put in a vacuum and pressure is applied until the timber refuses to absorb more
- tantalising : chemicals become chemically fixed into the timber and cannot be removed
Powder coatings
Timber
- more durable on manufactured boards than paint
- impact, chip, temperature and stain resistant
- involves the use of electrostatic charge
Finishes for metals
- painting or lacquering
- needs to be prepared and cleaned
- enamelling - uses: decorative jewellery
- buffing: creates a shine
- polishing: removes deep scratches
Dip coating/plastic coating
Mainly on metals. They are heated then dipped and a gel coating is formed, using powder and a fluidised bed
Electroplating
- coating objects with a thin layer of metal using electricity
- E.G.gold, silver, chromium, tin, nickel and zinc
- done by electrolysis
Galvanising
- zinc is used to cover steel
- consists of: surface preparation, galvanising and final inspection
- steel needs to be perfectly clean
- needs flux: prevents oxidisation
Mechanism definition
A mechanism is a device that transforms an input motion and force into a desired output motion and force
How to work out mechanical advantages of force
MA = output force/input force
Types of motion
Rotary
Linear
Reciprocating
Oscillating
What is a lever
A lever consist of a bar which pivots on a fulcrum. The input force is called effort, the output force is called load
What is a compound lever
Consists of two or more simple levers working together E.G.pliers
What is a linkage
It is an assembly parts used to direct forces and movement where it is needed, it can change the direction of motion and convert between different types of motion
Bevel gears
- transfers direction of 90 degrees, but can be other ages as well
- the 2 gears are different sizes therefore there is not a 1:1 gear ratio
Mitre gears
- 1:1 gear ratio
- pair of identical bevel gears
helical gears
- teeth cut at an angle
- therefore run quieter because the teeth mesh gradually
A rack and pinion
- changes between rotary and linear motion e.g. lifts
worm gears
- 1 continued tooth ‘single thread’
- 90 degrees drive direction
- experience large frictional forces and wear quickly
- self locking
Chain and sprocket drive
- input and output shafts are placed a distance apart, linked by a chain and they both rotate in the same direction
- gear ratio = no. of teeth ion output/ no. of teeth on input
belt and pulley drives
- quieter belt and used in lower load applications
- v shaped belt provides a large contact area and so ensures less slippage
- toothed belts are used when slipping mustn’t occur
- must always have correct tension (too much = high friction losses, too little = chain may slip/jump of sprocket)
screw threads
changes rotary motion into linear
- uses: 3D printers, where accurate linear positioning is required
what is a cam
a cam and followers converts motion to reciprocating motion
- dwell - where no follower motion occurs
- stroke - maximum distance the follower rises
epicyclic gears
4 main components: sun gear, planetary pinions/planet carrier, ring gear
- 1 of 3 must stay stationary
- if power is fed into any 2 components, the whole train rotates 1:1
benefits of epicyclic gears
- the output shaft is in direct line (co-Linear) with inout shaft
- they are very rugged, capable of carrying large torques in a compact passage
- several gear ratios can be obtained by braking different components in the system
uses of epicyclic gears
- car engines
- electric drills
- compact gear box on DC motors
What is a bearing
A component which supports a moving part and allows it to move only in the desired direction
- used to support a drive shaft so it can rotate freely
- The forces on the shaft can be radial (sideways) or axial (pushes the shaft along its axis)
What is a plain bearing
Passing the shaft through a hard wearing metal which offers low resistance, made of polymers e.g. nylon
- lubricated and usually sealed
What is a ball bearing
Consists of metal balls running between two races, handle radial loads
- lubricated and usually sealed
What is a tapered roller bearing
Due to the cone shape, they can support axial and radial loads
A thrust bearing uses - only axial
What are fluid bearings for
For high-performance and high-speed applications
- they can be used to support shaft within a sleeve of high-pressure oil or air. Such bearings provide exceedingly low friction and almost nowhere
Efficiency in mechanical systems
An ideal mechanism transfers power without adding to or subtracting from it. It doesn’t lose energy due to friction, or by generating sound or heat/wear.
However friction is always present so the equations are used to work out the efficiency
What are the efficiencies of a typical gear system
- epicyclic gear train 97%
- three stage compound gear train 94%
- worm drive could be as low as 20%
what are pneumatics
- they are linear actuators in which the driving force is provided by a supply of compressed air
- the stroke of the cylinder is the distance travelled by the pushrod
- cylinders controlled by valves - 3 port and 5 port depending on application
single acting cylinder SAC
- contains a spring
- compressed air is supplied, the piston and pushrod moves outwards until the ends top, air pressure is removed, air supply can escape
- have a strong out stroke but weak in stroke
double acting cylinder DAC
- 2 air ports
- both out stroke and in stroke are under air pressure
uses of pneumatics and hydraulics
- pneumatics in food industries (no problem if it leaks unlike the oil used in hydraulics)
- hydraulics - where accurate control and large force is required
Explain how an open loop control system works in a product such as a microwave oven
- In an open loop system there is no feedback, the
output is not measured/does not feedback to the
input. - no sensors to detect the output (the food cooking), so the input (the heat/time) does not detect if the food has been
cooked.
Explain how the graph shows that this sensor produces a linear output in response to the applied force.
There is a positive correlation relationship between
the two variables, where one variable increases while the
other increases
