Mechanical Motion

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Mechanical Motion

• AHL Topic 10
• Mechanical Design

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Mechanisms are devices that have been designed to
make jobs easier. They all have certain things in
common They involve some kind of motion. They
involve some kind of force. They must have some
kind of input to make them work. They produce
some kind of output. If we connect mechanisms
together we can build mechanical systems called
machines. Machines help to make our lives
easier and more comfortable.
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Linear motion

• Linear motion is motion in a straight line.
  Steady linear motion is known as velocity
  (uniform motion in a straight line).
• An example of linear motion is the cutting arm of
  a paper guillotine (photo below) as it travels
  from one side of the machine to the other.

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Rotary Motion

• Motion in a circle is called rotary motion. The
  number of complete revolutions made per minute
  (rpm), is called rotary velocity.

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Intermittent Motion

• Intermittent motion is motion which starts and
  stops regularly. For example, in a cinema
  projector the film needs to be moved on one frame
  at a time then held stationary while the light
  projects it onto the screen. This is usually done
  with a Geneva stop as shown here.

Intermittent motion is usually the end result of
a mechanism rather than the starting point for
conversion.
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Oscillating Motion

• Oscillating motion is motion backwards and
  forwards in a circular arc. E.g. playground
  swings (photo) and clock pendulums.

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Reciprocating Motion

• Reciprocating motion is linear motion backwards
  and forwards in a straight in a straight line.
  Sewing machines make use of this type of motion.

Jigsaws and scroll saws which are often used in
school workshops have blades that cut by
reciprocating motion.
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Irregular Motion

• Irregular motion is motion which has no obvious
  pattern to its movement. It is often needed in
  automata to recreate the movements of living
  things.
• Irregular motion is usually created using a cam
  or series of cams

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Linkages

• Linkages are used in mechanical control systems
  to allow forces and motion to be transmitted to
  where they are needed. They consist of a series
  of levers which can change the direction of a
  movement, alter the size of a force or make
  things move in a particular way.
• Reverse Motion Linkages are used to change the
  direction of motion. A single lever with a pivot
  at its centre reverses an input motion without
  affecting the input force. The input force and
  length of transmitted movement can be increased
  or decreased by altering the position of the
  pivot.

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Mechanical Motion in Context
Discuss the following products, what motions do
they use?

• Hydraulic digger

• Bicycle

• Hand drill

• Car Jack

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Torque

• Definition
• Rotational force commonly measured in units of
  Newton metres.

A simple example is to look at what happens when
a nut is tightened by a spanner. A force F
(newtons) is applied by hand at a distance r
(metres) from the centre of the nut. F x r
Torque (Nm) 100 x 0.2 20Nm (newton metres)
If the same force is applied to a longer spanner
at distance r from the centre of rotation, the
torque is greater.
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Ratchet and Pawl

• In mechanical engineering, a ratchet is a device
  that allows linear or rotary motion in only one
  direction, while preventing motion in the
  opposite direction. Ratchets are used in many
  other mechanisms, including clocks, capstans,
  turnstiles, spanners, winders, cable ties, jacks,
  screwdrivers, and hoists. The term is also used
  for a socket wrench.

Ratchets consist of a gearwheel or linear rack
with teeth, and a pivoting spring-loaded finger
called a pawl that engages the teeth.
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Cams

• A Cam is a machine component that either rotates
  or moves back and forth (reciprocates) to create
  a prescribed motion in a contacting element known
  as a follower. The shape of the contacting
  surface of the cam is determined by the
  prescribed motion and the profile of the
  follower.

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Cam followers
As the cam turns, driven by the circular motion,
the cam follower traces the surface of the cam
transmitting its motion to the required mechanism.
Cam follower design is important in the way the
profile of the cam is followed. A fine pointed
follower will more accurately trace the outline
of the cam. This more accurate movement is at the
expense of the strength of the cam follower.
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Conversion of motion
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• Mechanisms can be used to convert one form of
  motion into another

REMEMBER A rack and pinion is a pair of gears
which convert rotational motion into linear
motion.
A bell crank is a type of crank that changes
motion around a 90 degree angle.
Toggle clamp
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Toggle clamps

• Toggle clamps are used to lock things into
  position. They are quick to use and it is
  possible to obtain a large clamping forces.  
• Industry makes considerable use of toggle clamps
  to secure work being machined, pressed or formed.

A toggle clamp usually consists of two links on a
common pivot. The free end of one link is
attached to a fixed pivot. The free end of the
second lever is constrained to move in a straight
line. When a force is applied to the common
pivot, the lower end of the toggle clamp is
pushed downwards against the item being clamped.
The clamp locks in position once the common
pivot is pushed slightly 'over centre', usually
against a stop. Only by applying a force from
the side can the clamp be unlocked. The toggle
principle is used on push chairs to hold them
open in use.
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Parallel Motion

• Parallel linkages are used to make two or more
  parts of a mechanism move together and stay
  parallel to each other as the linkage moves.
• The photo shows a petrol engine cross cut wood
  saw. Rotary motion  from the engine output shaft 
  is transformed into reciprocating motion through
  a crank and slider. This reciprocating motion is
  then transmitted to the saw blade via a parallel
  motion linkage.

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Treadle linkage

• A treadle linkage changes either rotary to
  oscillating motion or oscillating to rotary
  motion. It can be used to power a rotary machine
  by working the treadle by foot.

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Bell Crank

• Bell Cranks are useful for changing the
  direction of motion or transmitting it round a
  corner. E.g. mountain bike brakes.

The photo on the right shows a bell crank linkage
connecting the foot brake pedal in a car to the
front and back drum brakes.
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• To see a range of simple animations and further
  explanations of mechanical motion visit the
  following website
• www.flying-pig.co.uk