previous concepts

7/28/2019 Previous Concepts

1/6

MovingpartsAngularmotionIf a spanner s used o tighten a bolt' one end remains in thesameposition on the bolt, while the other end turns' This typeof motio.t, where one end of a component pivots while the otherend follows the arc of a circle, is called angular motion' Anexample is a flap on an aircraft wing, which can be moved todifferent positions.The point at rhe centre of angular motion iscalled the axis of rotation (or axis). We say that the motionoccurs about the axis (around it).

ax ls

Anoularmotion

I1

\,

The speedof angular motion is measuredas angular velocity, and increasesn angularvelocity are measuredas angular acceleration.

]| Rotary nd eciprocatngotionl- Whenangularmorion s through360 degrees,t is calleda revolution or rotation). 7hen, m.rrri.rg .t, of machines,.r. ", wheeli, urn throughcomplete evolutions once'severald-.r, i for continuous periods - we say they revolve (or rotate)' The rotational velocity of

revolving parts, or rotating parts, is -."ror.d in revolutions per minute (rpm) - often called,.u, p.rlirrute. The motion of revolving parts is called

rotary motion'Rotary motion generates entrifugal force - that is , force which pushesoutwards, away fromthe axis of rotatio.r. An increase n rotational velocity results n a higher centrifugal force.Some components move backwards and forwards. This type of motion is called reciprocatingmotion (oi reciprocating action) - the component reciprocates. n machines, eciprocatingmotion is often convertJd (changed) o rotry motion. An example is a piston (seeUnit 40)'

E EngineevsI The abbreviation for revolution - rev - is widely used to talkabout engines.For examPle:

I The rotational velocity of an engine is measuredby arev counter.I High and low engine speedsare describedas high revsand low revs.

I Maximum engine speedcan be controlled by a rev limiter'r If an engine's ev limit is exceeded,we say the engine s

over-rewed.r An enginedesignedo run at high speeds a high-rewingengine'I Whenengine peeds increasedmomentaril we say he engines rewed (or rewed up)'I 'sfecansayan engines relryedo a certain peed for example,ewed o 6,000 pm.

D FrictionMoving parts cause riction. Friction is the force of resistancewhen two surfacesslide against.a.h olh.r. Frictional resistance s measuredas the coefficientof friction. This is the forcerequired to overcome riction in order to allow an object made of a specificmaterial to slidealong the surfaceof another specificmaterial'There are rwo types of friction. Static friction - sometimescalled stiction - is the frictionalresistancebetweenfwo surfaces hat are at rest (not moving). It measures he force needed ostart a sliding action. Dynamic friction is the friction betweentwo surfaces hat are alreadysliding againit each other. It measures he force needed o keep a sliding action going. Moreforce is needed o overcome static friction than to overcome dynamic friction.

74 ProfessionolEnglishn Use Engineering


2/6

Enginesnd motorsTypes nd unctionsf enginesndmotorsThe term engine usually refers to petrol engines,dieselenginesand jet engines or lets). n engineering,motor usually means electricmotor - but in generallanguage,'motor' can alsorefer to petrol and dieselengines.Enginesand motors power (or drive) machinesby generatingrotary motion - for example, to drivewheels. n jet engines,compressorsand turbines rotateto generate hrust - pushing force, produced by forcingair from the back of the engine at high velocity.As an engine produces a couple - rotary force - the moving parts of the machine it is drivingwill produce resistance,due to friction and other forces. As a result, torque (twisting force)is exerted on the output shaft of the engine.Torque - calculated as a turning moment, innewton metres - is therefore a measureof how much rotational force an engine can exert.The rate at which an enginecan work to exert torque is the power of the engine,measuredin watts. Although engineersnormally calculate enginepower in watts, the power of vehicleengines s often given in brake horsepower (bhp). This is the power of an engine'soutputshaft measured n horsepower (hp) - a historic measurementof power (seeAppendix III onpage100).Note:See he ol lowing ni ts or more nformat ion: ni t33 ( turningmoments) , n i t34 (rotary

mot ion) , ni t35 (powed, nd Unit41 (shafts) .BrE: etrol ; mE: asol ineBrE: etrol ngine; mE: asol inengineI

f | Internalombustionngines

1 Induction or intakeThe intake valveopens.The mixtureenters he cylinderthrougha port(opening)n thecylinder eadwhilethe oiston movesdownwards.

Petrol and diesel enginesare internal combustion engines.This means they are driven bythe combustion (burning) of fuel in enclosed,sealedspaces alled combustion chambers. npetrol and diesel engines, he combustion chambers are cylinders surrounded by a rylinderblock and closed at the top by a cylinder head. Each cylinder contains a piston. The numberof piston cylinders in an engine varies - engines n small motorcycles have only one, whilesports car enginesmay have twelve.Fuel is supplied to each cylinder from a tank. In most engines, he flow of fuel is generatedby a pump, which forces it - at high pressure through fuel injectors. These vaporize thefuel, allowing it to mix with air. Using this mixture (of fuel and air), most engines unctionas four-stroke engines.This means they work on a cycle of four stages or four strokes.Astroke is an upward or downward movement of a piston.

i,t.t>I;

tt:r.!.pr:rr.il

t:

2 CompressionThe intake valvecloses.The pistonmoves upwards,compressinghemixture.

Thecycle f a four-stroke etrolengineNotes: ee xercise0.2opposi teor an l lustrat ionf a cyl inder.Se eUnit 42 for moreon cam,camshaft, onnecting od and crankshaft.

Professionolnglishn UseEngineering


3/6

44.1 Complete the text about diesel enginesusing words from A and B opposite.Diesel enginesdiffer from (1) ... ... . . ...........ngines n one key respect: hey are not fittedwith a (2) ... ...... ................it each cylinder, to ignite the fuel. This is becausewhen a(3)....-.-..-,........-.........f dieseland air is compressednsidea hot (4 ) ..-, t willexplode spontaneousl without the need for a spark to provide (5) ........--.--..................dieselengine must therefore work in a way which prevents he diesel rom exploding before thepiston is at the top of the cylinder. To achieve his, the engine akes in only air during the(6) ............................tage of the cycle. Therefore, during the (7) .. .srage,onlyai r - and not an air-fuel mixture - is pressurized.lt is only at that last instant, when fullcompression as occurred, hat the (8 ) --... .....-............bove eachcylinder forces vaporzeddiesel nto the combustion chamber,where it ignites.Diesel enginesoperate at lower speeds han petrol engines,making them lesssuitable forhigh-speedapplications. Flowever, hey are more able to (9) .............. heavy vehicles,as they can produce gteatetamounts of (10) .. . han petrol engines.

4O.2 Look at the cross-sectionof an engine, and label it using words and expressions romB opposite.

camshaftr ra l r ro cnr inn

coolingwater

connectingodcrankshaft

On ecylinderof a four-stroke nternalcombustion nqine

Professionol nglishn Use Engineering 87


4/6

Transmssion1GearsGear wheels,or gears,are wheels with teeth (or cogs).The teeth interlock (fit together) with thoseof other gear wheels. flhen one gear wheel revolves, he other revolveswith it - in the oppositedirection - as their teeth meshtogether. Gears are normally fitted to shafts. They transmit rotarymotion from one shaft to another - that is, they transmit drive. Drive, usually supplied by anengine or motor, causesa shaft to revolve. A shaft connected directly to an engine or motor iscalled a driveshaft - or an input shaft. A gear wheel on a driveshaft is called a driver. The secondgear wheel, which mesheswith the driver, is called a follower - the driver drives the follower.An assemblyof several shafts and gear wheels s called a gear rain or transmission system. tbegins with an input shaft and ends with an output shaft. The systemmay contain idler gears(or idlers). Thesechanee he direction of a follower.

I

Gears can provide a mechanical advantage(seeUnit 33) by using different gear ratios. In thediagrams above, the driver has 20 teeth and the follower has 60 teeth. Therefore the driverrotates three times to make the follower rotate once. So the gear ratio is 3:1 (three to one).This means thatif, for example, the input speed that is , the speed of the driver - is 3,000rpm, the output speed(of the follower) will be 1,000 rpm.In some machines,a gearbox is used o provide a number of different gear ratios. Agearbox has a gear selectionsystem,which allows gears o be changed(or shifted) while thetransmission s running. This may be a manual gearbox, where gearsare changedby a person,or an automatic gearbox, which automatically selectsa higher gear or lower gear,as needed.

re Typesfgear heel spur earsrt

Spur gearsare the simplestgears.Theteeth run straight across he wheel.Helical gearshave curved teeth, so thatthey mesh together more smoothly.Bevel gearsallow drive to be transmittedthrough an angle often 90".Crown gearstransmit drive through 90',often to a small gearcalleda pinion.Worm gearstransmit drive through anangle.They also allow a low outputspeed elative o the input speedofthe worm. They can provide a'oneway' drive, as a worm can drive a gearbut a gear cannot drive a worm (themechanismwill lock).

t

worm gears88 Professionol nglishn Use Engineering


5/6

TransmissionChains,procketsndpul leysDrive can be transmitted from one shaft to another, acrossa distance,by roller chains (orchains).Chains drive, or are driven b toothed wheelscalledsprockets. airsof sprocketswith different numbers of teeth can provide different gear ratios. A bicycle is an example of amachinewirh chaindr ive.Belt drive works in a similar way to chain drive. Belts are usually smooth and are fittedaround smooth pulley wheels (or sheaves).However, toothed belts and toothed pulley wheelscan be used n applicationswhere a smooth belt could slip. A combination of severa lpulleysca n give a mechanicaladvantage for example, n cranes o lift heavy oads. n this case,cables alsocalled wire ropes are used nsteadof belts.Notes:Pul leymay efer o pul leywheels ndbelts ogether, r to a pul leywheel nly.SeeUnit33 for moreon mechanicaldvantaoe.

EConvers ionbetweenreciprocat ingandrotarymot ionThe reciprocating linear motion of pistons is converted to rotary motion by connecting rods(or conrods) and a crankshaft. The shape of the crankshaft allows the connecting rods toexert force at a distance rom its centre. This increases he turning moment they generate.Bicyclepedals are an example of a simple crank.

crankprovideseverage p sto

The principle f a crankcrankshat

In an engine,a heavy wheel - called a flywheel - is fitted to the end of the crankshaft.This providesmomentum, giving th e pistonsmore constantmotion.As well as driving a machine or vehicle, themotion of an engine's rankshaft s used oopen and close he valves n the cylinderhead.Rotary motion is transmitted,oftenvia a toothed belt or chain, o the camshaftat the top of the engine.The cams fixed onthe camshaft cause ollowers - which areconnected o the valves to move up anddown as the cam revolves.As they move)they open and close he valves.Notes: eehe ol lowingnitsor morenformation:ni t33 momentumnd urningmoments),Unit 4 (rotary nd eciprocatingotion),ndUnit 0 (internalombustionngines).See ppendixl onpage113formore n mechanismssed ith otarymotion.

90 Professiono/ nglishn L)seEngineering

centre f shaftconnectingod


6/6

ManagingotarymotionThe following mechanismsare often used n machines hat have rotating shaftsand wheels.

allows a shaft, such as a driveshaftor an axle (a shaft to which avehicle'swheelsare fixed) to revolve nsidea holemay require a lubricant - oil or grease to help reduce riction

a universal oint used o connect he ends of rwo shafts hat are at an angle o eachother, allowing drive to be transmitted, hrough the angle,betweentwo shaftsallows drive to be transmitted progressively by friction), from aconstantly revolving driveshaft, o wheels hat are not yet revolvingmay be operatedmanuall or may be an automatically operatedcentrifusal clutch

r used o slow down and stop rotary motion, usually by frictionI in disc brakes,a pair of brake pads s pressedagainst he sidesof ametal brake disc which is fixed to the shafrI in drum brakes,brake pads are pressedagainst he insideof a hollowcvlinder.called a drum

A universaloint

Professionalnglishn Use Engineering I t 3

previous concepts

Documents