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					TECHNOLOGY STUDY FILE 10

INVESTIGATING THE CRANK-SLIDER

The purpose of this unit is to help you:

• Understand better the crank-slider linkage, how it works and
  what it does.

• Design a crank-slider linkage you might use in project work.

The crank-slider converts movement from circular motion to
oscillatory motion, or the other way around.

It can therefore be used in two ways - either the shaft drives the
piston or the piston drives the shaft (and, in the case of the
petrol engine, both ways at different parts of the cycle).

   In                                                                 Out



                       Out                    In


A CRANK SLIDER MODEL


                              Connecting              Guides
                                 rod

                                                                    Slider
                   Radius



         Crank



                                                         The extreme
                                                         positions of a
                                                         crank-slider
                                                         mechanism
                           Bottom dead centre (BDC)




                            Top dead centre (TDC)




MECHANISMS (VERSION 1.2)                                127
TECHNOLOGY STUDY FILE 10



In each of the examples shown below, there is a crank-slider
mechanism. Make sure you can identify the slider (piston), the
connecting rod (con-rod), the shaft, the crank, top dead centre
(TDC) and bottom dead centre (BDC).




In the pictures, find one example where the piston is driving the
shaft and one where the shaft is driving the piston. Are there
other cases of interest, for example, where the linkage does both?


HOW ARE THE INPUT AND
OUTPUT MOVEMENTS MEASURED?
The oscillations are counted. The rate of oscillation is measured
by counting the number of oscillations per second. This is
measured in Hertz but often referred to as ‘cycles per second’.
The size of the oscillation is measured by ‘amplitude’ which is
half the distance between the TDC and BDC.

Rotations are counted and the rate of rotation is measured by
counting the number of revolutions per second. In industry,
often the number of revolutions per minute are counted. This is
abbreviated to r.p.m.




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TECHNOLOGY STUDY FILE 10



Here is a summary:

  Slider                                     Shaft or Crank
  Input or output                            Output or input
  Oscillatory motion                         Rotational motion
  ...Amplitude (mm)                          ...Radius (mm)
  and                                        and
  Number of                                  Number of revolutions/sec
  oscillations/sec (Hz)


PRACTICAL: THE CRANK-SLIDER
This practical investigation looks at the motion of the crank
slider and how the design affects it. The investigation is carried
out using cardboard engineering. First, cut out and assemble the
parts from the card template (TSF 11). They should look like this:

                               Fix connecting rod
                               with paper fasteners




                            Angle of
                            crank rotation



  Fix crank wheel to base                             Glue follower guides
  with paper fastener                                 to base board


1. Using the rig, rotate the crank steadily and observe the
   motion of the slider.

   Check that:

   • If you input five turns (i.e. turn the crank five times), the
     slider oscillates up and down five times. If you make half a
     turn, the slider makes half an oscillation. The transmission
     ratio is therefore 1:1.

   • The piston moves slowest at TDC and BDC.

   • The piston moves fastest near the ‘centre’, about halfway
     between TDC and BDC.




MECHANISMS (VERSION 1.2)                                  129
TECHNOLOGY STUDY FILE 10



   • Use graph paper to plot a graph showing the motion of the
     slider as the crank rotates. The horizontal axis represents
     the angle the shaft has turned through (in degrees) and the
     vertical axis shows the displacement of the slider (in
     millimetres).
      It should look similar to the graph below which was
      obtained for another crank slider. The TDC and BDC are
      marked on the graph. You can check that the amplitude is
      45 mm for our piston by halving the difference between the
      piston’s displacement at TDC and its displacement at BDC.
             displacement of




                               TDC
                the slider




                               BDC
                                                 1              2             3
                                        rotation of the crank (revolutions)


   • Mark TDC, BDC and amplitude on your graph.
   • Mark the point where you think the slider is moving
     slowest and fastest.

2. Before you try it in practice, guess what will happen to the
   motion of the piston when you increase the radius of the
   crank rotation.

   • What happens to the graph of the slider motion? Make a
     sketch.
                      displacement
                       of the slider




                                             1           2           3
                                       rotation of the crank (revolutions)



   • Try reconnecting the
     conrod to a new
     position in the rig and
     see what happens. Do
     rough sketches of the
     graphs you obtain on
     the same axes so you
     can see what is going
     on:
                                                          Change the radius but keep
                                                          the conrod length the same




MECHANISMS (VERSION 1.2)                                             130
TECHNOLOGY STUDY FILE 10



3. Write a brief conclusion to your research: ‘The effect of
   increasing the shaft radius while keeping the same conrod
   length is....’

   In the next activity, you will use a computer sketch to look
   more carefully at these and other effects.



4. Change the length of the
   conrod. The rig comes with
   five different length
   conrods. You can repeat
   your investigations using
   several different lengths.


                                   Change the conrod length but
                                   keep the radius the same



5. Feel the force for different conrod lengths.

   • Use three conrods of different lengths the longest, shortest
     and one between them. Connect each one in turn. Try to
     get a feel for the force you need to turn the crank in these
     positions:
      - at BDC and TDC;
      - half way between;
      - on either side.




          TDC                            BDC



   • Can you feel any difference? Try taking measurements
     using a force meter. You can also use a different radius for
     the crank.

   • Which crank would you use for maximum movement of
     the slider? Which conrod would you use? Which conrod
     gives the least friction? Why do these two answers give
     you a problem?



MECHANISMS (VERSION 1.2)                          131
TECHNOLOGY STUDY FILE 10




MECHANISMS (VERSION 1.2)   132

				
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