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LEVERS

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Title: LEVERS


1
LEVERS
2
LEVERS
  • Levers are used to lift heavy weights with the
    least amount of effort. In the example opposite,
    the heavy weight on the left hand side is been
    lifted by the person because of the lever.
  • The longer the 'rod' the easier it is to lift
    the weight. Under normal circumstances the person
    would not be able to lift the weight at all.

3
LEVERS
  • The fulcrum is the place where the rod pivots (or
    rotates).
  • The load is the scientific name for the weight.
  • The effort is quite simply the amount of effort
    used to push down on the rod in order to move the
    weight.

4
  • We use levers in every day life. Bicycle brakes
    work due to the fact that they are based on a
    lever. The diagram opposite shows the fulcrum and
    the effort.
  • Another good example of a lever is a simple door
    handle or a wheel barrow.

5
THREE CLASSES OF LEVERCLASS 1
The workman uses a trolley to move the large
packing case. The fulcrum is the wheel.
6
THREE CLASSES OF LEVERCLASS 2
  • The gardener uses a wheel barrow to lift tools
    and garden waste.
  • The load is in the centre of the barrow

7
THREE CLASSES OF LEVERCLASS 3
  • The fisherman catches the fish which becomes
    the load at the end of the lever.

8
EXERCISES
  • Draw three examples of levers that are used in
    everyday life.
  • Draw your own examples of the three classes of
    lever. Think in terms of examples that you have
    used at home, work or school.

9
QUESTIONS
  • 1. The diagram below shows a lever where an
    effort of 200 N balances a load of 600 N. The
    effort force is 6 metres from the fulcrum. The
    load force is two metres from the fulcrum.

10
  • 2. In the diagram below a crow-bar is used to
    move a 400n load. What effort is required to move
    the load?

11
  • 3. Another crow-bar is used to lever a load of
    120N. The load is 2m from the fulcrum and the
    effort is 6m from the fulcrum. What effort is
    required to move the load ?

12
  • 4. A wheel-barrow is used to lift a load of
    150N. The wheel acts as the fulcrum. Calculate
    the effort required.

13
PULLEY SYSTEMS
14
PULLEY SYSTEMS - 1
  • Pulley systems are used when there is a need to
    transmit rotary motion.
  • The diagram below shows a simple system comprised
    of two pulley wheels and a belt. It is a simple
    mechanical device to winch up and down a rope.
    When the motor is turned on it revolves the
    driver pulley wheel. The belt causes the driven
    pulley wheel to rotate as well, winding out the
    rope.

15
PULLEY
  • Pulley wheels are grooved so that the belt
    cannot slip off. Also, the belt is pulled tight
    between the two pulley wheels (in tension).
  • The friction caused by this means that when the
    driver rotates the driven follows.

16
PULLEY
  • Most pulley wheels have a central shaft on
    which they rotate. To keep the wheel firmly
    attached to the shaft it is usual to use what is
    called a key. The diagrams to the left shows a
    keyed shaft which is pushed through the centre of
    the pulley wheel. A small rectangular key is then
    tapped into position, holding the shaft and the
    pulley wheel together. This fitting means that
    the pulley wheel cannot slip on the shaft

17
PULLEY SYSTEMS - VELOCITY RATIO - 1
The diagram opposite shows a small driver pulley
pulling round a larger driven pulley. The rpm
(revolutions per minute) of the larger driven
pulley wheel will be less than the smaller driver
pulley wheel. The same principle regarding speed
of rotation regarding gears applies to pulley
systems as well.
18
WORKING OUT THE VELOCITY RATIO OF PULLEYS
  • The system shown above has a driver pulley
    attached to a motor.
  • When the motor is switched on the driver
    pulley revolves at 20 rpm. The diameter of the
    driver pulley wheel is 200mm and the driven
    pulley wheel is 400mm.
  • This means for every single revolution of the
    larger driven pulley wheel, the smaller driver
    pulley wheel rotates twice. This is due to
    velocity ratio. The ratio can be worked out
    mathematically in different ways.
  • The two most likely methods are shown NEXT D?

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21
  • This means that the larger pulley wheel (the
    driven pulley wheel) revolves half as fast
    compared to the smaller driver pulley wheel. In
    effect the driven pulley wheel is slower and
    revolves half as many times as the driver.
  • This means if the rpm of the driver pulley
    wheel is divided by 2, the output rpm of the
    driven pulley wheel will be found

22
GEARS AND GEAR SYSTEMS

23
GEARS AND GEAR SYSTEMS
  • Gears can be found in many machines in a workshop
    or factory and at home they are often an
    important part of mechanical devices. In a car
    the gears help the driver to increase and
    decrease speed as he/she changes the gears with
    the gear stick.
  • Can you name and describe a mechanical device
    with gears, that you or your friends use ?

24
  • The gears opposite are called spur gears
    because they mesh together.
  • Gear A is called the driver because this is
    turned by a motor. As gear A turns it meshes
    with gear B and it begins to turn as well. Gear
    B is called the driven gear.

25
GEAR DETAILS
  • The gears above are known as spur gears. The
    circle marked in red shows the outer limit of the
    teeth whilst the green circles are known as the
    pitch circles. The pitch circle of a gear is very
    important as it is used by engineers to determine
    the shape of the teeth and the ratio between
    gears (ratios will be explained later). The
    pitch of a gear is the distance between any point
    on one tooth and the same point on the next
    tooth. The root is the bottom part of a gear
    wheel.  The pitch point is the point where gear
    teeth actually make contact with each other as
    they rotate.

26
GEARS AND GEAR SYSTEMS
  • This is a good example of a gear train. A
    gear train is usually made up of two or more
    gears. The driver in this example is gear A. If
    a motor turns gear A in an anticlockwise
    directionWhich direction does gear B turn
    ?Which direction does gear C turn ?Does
    gear C revolve faster or slower than gear A ?
    - explain your answer.  

27
  • So far you have read about driver gears,
    driven gears and gear trains. An idler gear
    is another important gear. In the example
    opposite gear A turns in an anticlockwise
    direction and also gear C turns in an
    anticlockwise direction. The idler gear is used
    so that the rotation of the two important gears
    is the same.Is the speed of gears A and B the
    same ?

28
DRAWING GEARS
  • It would be very difficult to draw gears if you
    had to draw all the teeth every time you wanted
    to design a gear system.
  • For this reason a gear can be represented by
    drawing two circles.

29
RACK AND PINION
  • A rack and pinion gears system looks quite
    unusual. However, it is still composed of two
    gears. The pinion is the normal round gear and
    the rack is straight or flat. The rack has
    teeth cut in it and they mesh with the teeth of
    the pinion gear. The pinion rotates and moves
    the rack in a straight line - another way of
    describing this is to say rotary motion changes
    to linear motion.

30
RACK AND PINION - DRILLING MACHINE
31
BEVEL GEARS
  • Bevel gears can be used to change the direction
    of drive in a gear system by 90 degrees.
  • A good example is seen as the main mechanism for
    a hand drill. As the handle of the drill is
    turned in a vertical direction, the bevel gears
    change the rotation of the chuck to a horizontal
    rotation.

32
ENLARGED VIEW OF THE BEVEL GEARS OF A HAND DRILL
MECHANISM
A TYPICAL HAND DRILL
33
EXERCISE
34
GEAR WHEELS (SPROCKETS) AND CHAINS
  • Everyone has seen a bicycle or used one and
    noticed that it is driven by a large driver gear
    wheel with pedals attached.
  • Smaller gears at the back are driven round, in
    turn driving round the back wheel. As the back
    wheel turns the bicycle moves forwards.
  • Gears driven by chains are used in machinery,
    motorcycles, in car engines and have many more
    applications.

35
WORM GEARS
36
WORM GEARS
  • The arrangement of gears seen left is called a
    worm and wormwheel.
  • The worm, which in this example is brown in
    colour, only has one tooth but it is like a screw
    thread.
  • The wormwheel, coloured yellow, is like a
    normal gear wheel or spur gear.
  • The worm always drives the worm wheel round, it
    is never the opposite way round as the system
    tends to lock and jam. 

37
  • The picture to the right is a typical set-up
    for a motor and worm gear system.
  • As the worm revolves the wormwheel (spur gear)
    also revolves but the rotary motion is
    transmitted through a ninety degree angle.

38
WIND POWERED GEARS EXERCISE
  • This is a practical exercise based on wind
    power, gears and linkages. The nose of the cat is
    part of a propeller which rotates when the wind
    blows (front view). As it rotates gears at the
    back of the model cat also rotate and a wire
    linkage moves the tail up and down. The
    animation (back view) below shows the arrangement
    of gears at the back of the cat needed to provide
    the right movement. However, this design has
    never been made.1. Make a card model to test if
    the movement will work.2. Make alterations
    where necessary.3. Make a prototype from
    resistant materials and test it outside.

39
Design an alternative wind powered object. The
device must include gears and perhaps other
mechanisms
40
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41
If the cam turns at one rev per minute (rpm) and
the wormwheel has 110 teeth, how fast does the
motor rotate?
  • The system opposite is used to control a set of
    traffic lights. The mechanism involved includes a
    motor which drives a worm gear round, turning a
    wormwheel and the attached cam. As the cam
    rotates it hits a micro switch which turns on a
    light

42
First work out the gear ratio
  • The worm gear always rotates at a faster rate
    than the wormwheel.
  • RPM is 110 X 1 110 rpm (motor rotates 110 times
    per minute)
  • The worm gear and motor rotates 110 times to
    every single rotation of the wormwheel.

43
CRANKS AND CRANK SHAFTS
  • A crank is a device through which rotary motion
    and torque can be applied to a shaft.
  • The simplest device is a crank handle.
  • When a number of cranks are incorporated into a
    shaft, it is called a crankshaft

44
CRANK AND SLIDER MECHANISM
  • This mechanism is composed of three important
    parts
  • The crank which is the rotating disc,
  • The slider which slides inside the tube and
  • The connecting rod which joins the parts
    together.As the slider moves to the right the
    connecting rod pushes the wheel round for the
    first 180 degrees of wheel rotation. When the
    slider begins to move back into the tube, the
    connecting rod pulls the wheel  round to complete
    the rotation.

45
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  • One of the best examples of a crank and slider
    mechanism is a steam train. Steam pressure powers
    the slider mechanism as the connecting rod pushes
    and pulls the wheel round.

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52
CAMS and FOLLOWERS
  • A CAM changes the input motion, which is
    usually rotary motion (a rotating motion), to a
    reciprocating motion of the follower. They are
    found in many machines and toys
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