M. Ibbotson - Professional english in use engineering (794233), страница 25
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gears are momentarily disconnected each time a shift is made.The principle of continually variable transmission(CVT) is entirely different to both manual andautomatic systems. Instead of having a fixednumber of different gear (7) ................................ ,each with different numbers of teeth, CVTuses a special mechanism to continuously andprogressively adjust the gear (8) ............................... .between the input and output (9) ................................ .Therefore, as the vehicle accelerates ordecelerates, continual adjustments are made tothe transmission automatically. This allows theengine to be maintained at a constant speed foroptimum power and fuel efficiencywhile the system is able to continuously(10) ................................
drive to the vehicle's wheels.41.3The Case-IH Puma CVX tractor is equippedwith continuously variable transmission.Make word combinations with gears using words from A and C opposite. Then use theword combinations to complete the sentences (1-4) below. Sometimes there is more than onepossible answer.1 ................................
gears are used to transmit drive through an angle.2 ................................ gears run more quietly than spur gears.3 ................................ gears can generally only transmit drive in one direction.4 ................................ gears allow drivers and followers to revolve in the same direction.Ove,r~otjoa~Think about the transmission system in a machine or vehicle you're familiar with.Describe how drive is transmitted from the input shaft to the output shaft.Professional English in Use Engineering89lfJ Transmission 2-Chains, sprockets and pulleysDrive can be transmitted from one shaft to another, across a distance, by roller chains (orchains). Chains drive, or are driven by, toothed wheels called sprockets.
Pairs of sprocketswith different numbers of teeth can provide different gear ratios. A bicycle is an example of amachine with chain drive.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 in applications where a smooth belt could slip. A combination of several pulleyscan give a mechanical advantage - for example, in cranes to lift heavy loads.
In this case,cables - also called wire ropes - are used instead of belts.Notes: Pulley may refer to pulley wheels and belts together, or to a pulley wheel only.See Unit 33 for more on mechanical advantage.-Conversion between reciprocating and rotary motionThe 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 from its centre.
This increases the turning moment they generate.Bicycle pedals are an example of a simple crank.piston0centre of shaftThe principle of a crankconnecting rodIn an engine, a heavy wheel- called a flywheel- is fitted to the end of the crankshaft.This provides momentum, giving the pistons more constant motion.As well as driving a machine or vehicle, themotion of an engine's crankshaft is used toopen and close the valves in the cylinderhead. Rotary motion is transmitted, oftenvia a toothed belt or chain, to the camshaftat the top of the engine. The cams fixed onthe camshaft cause followers -which areconnected to the valves- to move up anddown as the cam revolves.
As they move,they open and close the valves.Notes: See the following units for more information: Unit 33 (momentum and turning moments).Unit 34 (rotary and reciprocating motion), and Unit 40 (internal combustion engines).See Appendix XI on page 113 for more on mechanisms used with rotary motion.90Professional English in Use Engineering42.1Circle the correct words to complete the text about how chain drive works on bicycles. Lookat A opposite and Appendix XI on page 113 to help you.on a bicycle, the pedals -which provide support for therider's feet- are fitted to a (1) crank I sheave at either side ofthe frame. These are fixed together by a short shaft, called aspindle, which runs through a tube at the bottom of the frame.The tube contains (2) bearings I roller chains, which allow thespindle to turn freely.
A (3) pulley I sprocket, fitted to the shaft,drives a (4) cable I chain, which drives a second (5) crank Isprocket at the rear of the bike. This turns the rear wheel.42.2Complete the text about the advantages of chain drive on bicycles, using the words in thebox. You will need to use some words more than once. Look at A opposite to help you.beltchainpulleysprockettoothedwheelThe advantages of using chain drive on bicycles are:• its ability to transmit drive over a distance- from the centre of the bike to the rear• that a (1) ................................ will not slip as it travels around a (2) ................................ -unlike a(3) ................................ ,which is more likely to slip as it travels around a (4) ................................•42.3that the teeth on a (5) ................................ will force dirt out of the holes in a (6) ................................
,whereas dirt would become trapped between a (7) ................................ and a (8) ................................................................ -a problem which would make a (9) ................................ belt unsuitable forovercoming the problem of slipping.Answer the questions below using words from B opposite and Appendix XI on page 113.1 In an engine, which revolving component generates the up-and-down motion required toopen and close a valve in the cylinder head?2 How can the motion of an engine's crankshaft be transmitted to the camshaft?345678What term is given to a component which presses against a cam and moves with areciprocating action?Which component is heavy and helps an engine to run smoothly?Which two engine components are joined by a connecting rod?What component could be used to connect the end of a crankshaft to a driveshaft that is at20 degrees to the crankshaft?In vehicles, what friction mechanism (controlled by the driver) is used to graduallytransmit drive to the wheels, via the gearbox?In disc brakes, what term is given to the components that are pressed against the brakedisc to generate friction?Ove-r ..f-o t:fou~Think about a machine you're familiar with which uses chain or belt drive, or one whichconverts reciprocating motion to rotary motion (or vice versa).
Describe the mechanismsused, and explain their functions.Professional English in Use Engineering91iiJ Current, voltage and resistance-Electric currentThe photo on the opposite page shows a simple electric circuit (or circuit). A cell provides anelectric current (or current). This flows through wires, which conduct the electricity (provide away for it to travel). The current is used to light a lamp. So, like all circuits, the example includes:•an electrical supply - in this case, the cell•an electrical conductor (or conductor)- an electrical path- in this case, wires•one or more electrical components (or components)- electrical devices (in this case, thelamp) which have a function.Current- measured in amperes, or amps (A) -is the rate of flow of electric charge. Electriccharge is carried by electrons- particles with a negative charge (-),which are normallyattached to atoms.
When an electric current flows through a conductm; the electrons movefrom one atom to another- in the case of a copper wire, from one copper atom to the next.If the number of electrons flowing through a conductor increases, then the amperage, or arnpage(current) increases. When electrons flow, carrying a current, they can be called charge carriers.Notes: In everyday English, cells are called batteries. In technical English, a battery is a number ofcells placed together.Lamps are often called bulbs in everyday English ..:iJVoltage and resistanceThe amount of current (in amps) flowing through a circuit will partly depend on theelectromotive force (EMF) of the electrical supply.
Electromotive force is measured in volts (V),and is generally called voltage. The voltage depends on the 'strength' of the electrical supply. Inthe diagram above, adding a second cell would supply a higher voltage.The amount of current will also depend on electrical resistance (or resistance). This value- in ohms (0) - is a measure of how easily current can flow through the conductors andcomponents in a circuit. For example, a lamp creates resistance because the filament- themetal wire inside it - is very thin.
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