Mechanisms that change the type of movement

rotational movement.reciprocating movement.

The word "reciprocate" comes from the old Latin words "re" and "pro". "Re" means back and "pro" means forward.

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Figure 1: When you saw wood you make a reciprocating movement.
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Figure 2: A wind pump converts a rotating movement into a reciprocating movement.
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Figure 3: A steam driven locomotive uses a crank-and-slider mechanism to turn the wheels.

When a crank is part of a winch, it changes a big rotational movement with a small force into a small rotational movement with a big force. The longer the crank arm is, the more mechanical advantage it gives.

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Figure 4: A simple hand-powered winch used to lift a bucket of water
slider is attached to the crank by a connecting rod. The slider is normally round, and fits into a round hole. It can only move sideways, not up or down.pivot between the crank and the connecting rod, and another pivot between the connecting rod and the slider. Both pivots change position when the mechanism is working. The crank turns around an axle. The axle never changes position, it just turns.wheel, to make the wheel turn with it. Or the crank can be part of the wheel.

The way the crank works depends only on the distance between the centre of the axle and the centre of the pivot between the crank and the push rod. This distance is called the crank throw. This is shown on Figure 5.

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Figure 5: The different parts of a crank-and-slider mechanism
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Figure 6: Different steps during the operation of a crank-and-slider mechanism
  • How long is the crank throw in the mechanism in Figure 6?
  • How far is the movement of the slider from the furthest position on the left to the furthest position on the right? This can be called the "total sideways movement".
  • If the crank throw was twice as long, how long would the total sideways movement of the slider be?
  • Will the slider ever stand still while the crank is rotating?
    • high pressure. Imagine you are blowing up a balloon. You have to create a high pressure of air in your mouth to make the balloon bigger. The balloon becomes bigger because the high pressure air moves the sides of the balloon outwards. Steam at a high pressure can also move things.Tech1_gr8_ch6_fig7.tiff
    Figure 7
    piston, and the hole inside which the piston moves is called the cylinder.valves to let in the hot steam on the right or the left of the cylinder. The valves have to open and close at the right times. Is there some mechanism that can do this? This is what you will learn about in the next lesson.Tech_English_LG_Grade8-term1-web-resources/image/Tech_English_Gr8_term1_pg76_img1.png
    Figure 8: How a steam engine works

    A crank-and-slider mechanism can be used to change rotational movement into reciprocal movement, or it can be used to change reciprocal movement into rotational movement. In other words, if you move the crank, then the slider will also move. And if you move the slider, the crank will also move.

    cam is a wheel that is not round, or it is a round wheel that turns around an axle that is not at the centre of the wheel.follower. The rotational movement of the cam is changed into the reciprocating movement of the follower. The follower is in a sleeve, so that it can move in one direction only.driven by an axle, so that when the axle rotates, the cam rotates. When the cam rotates, the follower slides on the cam. The position of the follower depends on the angle at which the cam is rotated.

    Do you remember the difference between a driven wheel and a free-turning wheel about which you learnt in the previous chapter?

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    Figure 9: The different parts of a crank-and-slider mechanism, and different shapes of cams
    Tech1_gr8_ch6_fig10a.tif
    Figure 10: The movement of a valve as a cam rotates

    rotation of cam

    starting position

    1 eight of a cam rotation

    2 eights of a cam rotation

    3 eights of a cam rotation

    rotation of cam in degrees

    0°

    45°

    90°

    135°

    distance that valve is open

    0

    2 mm

  • Complete the table above to show how far the valve is open at different positions of the cam in Figure 10. Measure the distance that the valve is open.
  • Which of the pictures above show the valve at its highest position?
  • Which picture shows the valve at its lowest position?
    • 36579.png 

    A cam can convert rotational to reciprocal movement, but not the other way round

    Tech1_gr8_ch6_fig10b.tif 

    4 eights of a cam rotation

    5 eights of a cam rotation

    6 eights of a cam rotation

    7 eights of a cam rotation

    one full cam rotation

    180°

    225°

    270°

    315°

    360°

  • The drawing on the right uses red arrows to show the distance between the centre and the edge of a snail cam at different angles of rotation. Each arrow is rotated with 45° clockwise from the previous arrow.

    Measure the different arrows, from the shortest to the longest and fill in your measurements in the table below.

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    Figure 11: A snail cam

    position

    1

    2

    3

    4

    5

    6

    7

    8

    9

    length

  • Is there a pattern in the lengths of the arrows? How does the pattern work?
  • What happens between position 9 and position 1?
    • 37385.png 

    Some cams are a round wheel, but they do not rotate around the centre of the wheel. These cams are called eccentric cams. They are used on many modern bicycles.

    During a bicycle race, the cyclists sometimes get flat or punctured tyres. They then have to take the wheel off to remove the tyre before they can fix the tyre. This takes a lot of time, and they'll struggle to catch up with the other cyclists again.

    Many years ago, engineers designed a mechanism with which you can quickly take a wheel off a bicycle, without using any tools. This is called a "quick release" mechanism.

    Today, more expensive bicycles use quick release mechanisms on their wheels. They also use a quick release mechanism to make it quick and easy to change the height of the saddle. The photos below and on the right show how a quick release mechanism uses an eccentric cam to lock the saddle at the correct height.

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    Figure 12: A quick-release mechanism is used to clamp theseat post to the frame of this bicycle.
    Tech1_gr8_ch6_fig13.tif
    Figure 13: The sides of the quick release mechanism move or clamp closer together as the eccentric cam is turned by the handle.
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    Figure 14
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    Figure 15: The moving parts inside one cylinder of an engine
  • Make a three-dimensional artistic drawing of the crankshaft of a one-cylinder engine. Use shading to make it more realistic. First make a rough drawing, before you make your final neat drawing.
    • sketchspacesketchspace