# crank slider

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```					TECHNOLOGY STUDY FILE 10

INVESTIGATING THE CRANK-SLIDER

• 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

Crank

The extreme
positions of a
crank-slider
mechanism

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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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Here is a summary:

Slider                                     Shaft or Crank
Input or output                            Output or input
Oscillatory motion                         Rotational motion
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.

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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:
the conrod length the same

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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
several different lengths.

Change the conrod length but

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?

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MECHANISMS (VERSION 1.2)   132

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