Mechanisms and Machines
Technology and Design
Department
Mechanisms and Machines
comes
entify different mechanisms and there uses.
nalyse the input, control, output of mechanical syste
nalyse the motions and forces involved.
now about - Mechanical Advantage
- Velocity Ratio and Gear Ratio.
- Efficiency
lculate moments of force.
nderstand and calculate Torque.
lculate input and output speeds.
Mechanisms and Machines
Useful Websites
www.technologystudent
.com
www.dtonline.org
www.howstuffworks.co
m
www.ajkids.com
Mechanisms and Machines
Different types of mechanisms
• Levers
• Linkages
• Gears
• Wheels
•Cranks and ratchets
• Cams
• Chain & Sprocket
Mechanisms and Machines
A mechanism is something that changes an
input motion and force
into an
output motion and force.
Mechanisms and Machines
A machine is something
that uses
mechanisms to do useful
work.
Mechanisms and Machines
All kinds of machines make work easier for us by changing
the direction or size of the applied force. The amount of force
we save by using the machine is called mechanical
advantage.
Mechanisms and Machines
Mechanisms require some type of Motion
(movement)
There are four types of motion:
Linear
Rotary
Reciprocating
Oscillating
Mechanisms and Machines
Mechanisms are activated by forces.
The different types of forces are:
Static - no movement (still force)
Dynamic - moving forces
Compression - squashing force
Tension - pulling force
Bending - compression and tension
Torsion - turning or twisting
Shear - cutting
Equilibrium - all forces are balanced
Mechanisms and Machines
Mechanismscan givean advantagewhen lifting
a load. Thisadvantageiscalled Mechanical
Advantage(MA). It iscalculated asa ratio -
MA = LOAD
EFFORT
Example:
a lever usesan effort of 10N
to lift a load of 50N.
10N
MA = 50N/10N
= 5:1
(or just) = 5
Thislever givescan lift 5 timestheeffort.
50N
Mechanisms and Machines
Efficiency
Efficiency is a comparison of the useful work energy
provided by a machine or system to the work energy
applied to the machine or system.
The formula for efficiency is: output/input = work out/work in
All the parts of a machine or system and how they are
connected together will affect the machine's or system's
efficiency.
Other forces such as friction will affect an object's movement.
Mechanisms and Machines
Velocity Ratio(VR)
Velocity Ratio(VR) is a comparison of the distance
a load moves to the distance travelled by the force
needed to move it.
VR = Distance moved by the Effort = de
Distance moved by the Load
dl
Example: if a lever moves a load 1.0m by pushing
down at the other end by 2.0m, what is the VR?
VR = de
dl
= 2.0m = 2 = 2
1.0m
1
Mechanisms and Machines
Levers
Technology and Design
Department
Mechanisms and Machines
A lever is a rigid beam that can rotate about a fixed
point called the fulcrum. An effort applied to one end
of the beam will cause a load to be moved at the other.
Effort
Load
Fulcrum
Mechanisms and Machines
There are three types(or classes) of lever.
Class1. The Fulcrum is in themiddle.
example: see-saw
Class2. The Load isin the middle.
example: wheel barrow
Class3. The Effort isin the middle.
example: tweezers
FLE
123
F-1
L-2
E-3
Mechanisms and Machines
Therearethree types(or classes) of lever.
Class 2. The Load isin the middle.
example: wheelbarrow
L
F
E
F-1
L-2
E-3
Mechanisms and Machines
There are three types(or classes) of lever.
Class3. The Effort isin the middle.
example: tweezers
This fire extinguisher uses a
third-class lever on its handle F-1
L-2
E-3
Mechanisms and Machines
Torques cause changes in rotational motion. If an object
is at rest, torque exerted on it will cause it to rotate.
When you use a lever, you exert a torque on it and the
lifting bar rotates about the centre.
Mechanisms and Machines
There are two forces that can be applied to the see-saw to
cause it to rotate.
The first force is the weight of person 1 and the second force is
the weight of person 2.
The torque acting on each end is calculated by taking the
weight multiplying by the distance from centre of the see-saw.
Mechanisms and Machines
To calculate the torque, you need to know :
the magnitude of the force,
the direction of the force,
the distance from the application point of the force to the turning
point.
t=Fxd
When more than one force is involved, the torque is calculated
for each separately and then they are added together taking care
to include the proper sign.
Example: m1 = 1 x d1
= F1d1
m2 = 2 x d2
= F2d2
Mechanisms and Machines
Mechanismscan givean advantagewhen lifting
a load. Thisadvantageiscalled Mechanical
Advantage(MA). It iscalculated asa ratio -
MA = LOAD
EFFORT
Example:
a lever usesan effort of 10N
to lift a load of 50N.
10N
MA = 50N/10N
= 5:1
(or just) = 5
Thislever givescan lift 5 timestheeffort.
50N
Mechanisms and Machines
Linkages
Technology and Design
Department
Mechanisms and Machines
A linkage is a mechanism made by connecting
together levers.
To connect the levers together you can use any type
of fastening which allows free movement, for example
screws, pins, paper fasteners, pop rivets etc.
The linkage can be made to change the direction of a
force or make two or more things move at the same
time.
Mechanisms and Machines
Reversemotion linkage
input
control
output
Input and output motionsarein oppositedirections.
Mechanisms and Machines
Push/pull linkage
input
output
control
Input and output motion isin the same direction
Mechanisms and Machines
Parallel motion linkage
input
output
control
The sides stay parallel as they move.
Mechanisms and Machines
Equalising linkage
input
control
Equal outputs from a single input.
output
output
Mechanisms and Machines
Bell-crank lever
input
output
control
A linear input changes to a rotary output.
Mechanisms and Machines
ToggleClamp
input
control
The input motion pushes the output
block to act as a clamp.
output
Mechanisms and Machines
Gears
Technology and Design
Department
Mechanisms and Machines
Gears
Gears are toothed or pegged wheels meshed together to transmit motion
and force. In any pair of gears the larger one will rotate more slowly than
the smaller one, but will rotate with greater force. Each gear in a series
reverses the direction of rotation of the previous gear.
The Gear Train
Gears work in teams. Two gears working together is called a
gear train. The gear on the train to which the force is first
applied is called the driver. The final gear on the train to which
the force is first applied is called the driven gear. Any gears
between the driver and the driven gears are called the idlers.
Notice the arrows on top of the gears. They are showing that the
gears move in different directions.
Mechanisms and Machines
Meshed Gears The diagram below shows five meshed gears. The
first gear that the force is applied is called the driver gear. Notice that
the arrows show how the gears are turning. Every other gear is turning
clockwise. The very last gear is the driven gear. All of the gears in
between are called idlers.
Mechanisms and Machines
Worm and Wormwheel
A gear which has one toothe is
called a worm. The tooth is in
the form of a screw thread. A
wormwheel meshes with the
worm. The wormwheel is a
helical gear with teeth inclined
so that they can engage with
the thread-like worm. The
wormwheel transmits torque
and rotary motion through a
right angle. The worm always
drives the wormwheel and
never the other way round.
Worm mechanisms are very
quiet running.
Mechanisms and Machines
Internal gears
Internal gears have better load-carrying capacity
than external spur gears. They are safer in use
because the teeth are guarded.
Why Do Clocks Have Brass Gears?
Brass gears are often used in clocks where they work well without
any lubricant. Oil causes dust to adhere to the gears and this
causes gear-tooth wear. An advantage of brass gears is that
constant meshing work hardens their teeth. Because of this, the
brass gears in well used old clocks often show little sign of wear.
Mechanisms and Machines
Gears
Simple Gear System
Simple Gear System with Idler Gear
Mechanisms and Machines
Compound Gear System
More complex 'compound' gear trains
can be used to achieve high and low
gear ratios in a compact space by
coupling large and small cogs on the
same axle.
Mechanisms and Machines
Gear Ratio = Velocity Ratio (VR)
driver = 60 teeth
driven = 20 teeth
VR = the number of teeth on the driven gear = N = 20 = 1
the number of teeth on the driving gear. R 60
3
The velocity ratio of a compound gear train is calculated by multiplying
the velocity ratios for all pairs of meshing gears.
VR = n X n X n
r
r
r
Mechanisms and Machines
A rack and pinion mechanism is
used to transform rotary motion
into linear motion and vice versa
Rack and Pinion
A single gear, the pinion, meshes with a sliding toothed rack. This combination
converts rotary motion to back and forth motion. Windshield wipers in cars are
powered by a rack and pinion mechanism. A small pinion at the base of the
wiper meshes with a sliding rack below.
Mechanisms and Machines
Bevel Gears
Gears that mesh at an angle change the direction of rotation.
Bevel Gears
They are used in pairs to transmit rotary motion and torque
where the bevel gear shafts are at right angles (90 degrees) to
each other
Mechanisms and Machines
Driven Gear: the output motion and force are transmitted by this gear
Driver Gear: the input motion and force is applied to this gear
Gear Ratio: the gear ratio is defined as the rotation speed of the output shaft divided
by the rotation speed of the input shaft.
Gear Train: a group of gears working together; They are arranged so that their teeth
closely interlock (mesh).
Gear Wheel: a basic mechanism. A gear is a wheel with accurately machined teeth
round its edge. Its purpose is to transmit rotary motion and force.
Meshed Gears: when the teeth of one gear are engaged with the teeth in the other
Spur Gears: two spur gears of different sizes mesh together; The larger gear is called
a wheel and the smaller gear is called the pinion.
Mechanisms and Machines
Cams
Technology and Design
Department
Mechanisms and Machines
CAMS,
CRANKS
AND
RATCHETS
Mechanisms and Machines
A CAM changes rotary motion
(circular movement) to linear
motion (one that moves in a
straight line). They are found in
many machines and toys.
A CAM has two parts, the FOLLOWER and the CAM
PROFILE. Diagrams one to six show a rotating cam
pushing a follower up and then allowing it to slowly fall
back down.
Mechanisms and Machines
The cam is used to convert rotary motion to
reciprocal motion (backwards and forwards motion)
Mechanisms and Machines
Cams can be shaped in any number of ways and this is determined by
the way the follower is to move. The shape of the cam is called the
PROFILE. Examples of various cam profiles can be seen below.
Pear
Circular/eccentric
Pear shaped cams are used on the
Circular cams or eccentric
shafts of cars. The follower
cams produce a smooth
remains virtually motionless for
motion. These cams are
about half of the cycle of the cam
used in steam engines.
and during the second half it rises
and falls.
Mechanisms and Machines
Heart
Heart shaped cams allow
the follower to rise and fall
with ‘uniform’ velocity.
Drop or Snail
What
type
of
movement do you
think this cam profile
will give ?
Mechanisms and Machines
Different
types of
follower
FLAT
POINT/KNIFE
ROLLER
OFFSET
Mechanisms and Machines
One cycle = One
rotation/revolution
of the cam.
Fall = That part of
the cam that causes
the follower to fall.
Dwell = When the
cam rotates but
the follower does
not rise or fall.
Rise = That part of
the cam that causes
the follower to rise.
Mechanisms and Machines
The Flat Plate Cam / Linear Cam: As the flat plate cam
profile moves to the left the follower drops down the slope and
then eventually rises up at the other end. The flat plate cam
then reverses in the opposite direction and the follower drops
and rises again.
The edge of the flat plate cam can be shaped to give different
vertical movements of the cam follower. Flat plate cams or
linear cams as they are often called are used frequently in
machines which carry out the same repetitive movements.
Mechanisms and Machines
Mechanisms and Machines
QUICK RETURN CRANK MECHANISM
A quick return mechanism such as the one seen opposite is used
where there is a need to convert rotary motion into reciprocating
motion. As the disc rotates the black slide moves forwards and
backwards. Many machines have this type of mechanism and in the
school workshop the best example is the shaping machine.
Mechanisms and Machines
The shaping machine is used to machine flat metal surfaces especially
where a large amount of metal has to be removed. Other machines
such as milling machines are much more expensive and are more
suited to removing smaller amounts of metal, very accurately.
The reciprocating motion of the mechanism inside the shaping machine
can be seen in the diagram. As the disc rotates the top of the machine
moves forwards and backwards, pushing a cutting tool. The cutting tool
removes the metal from work which is carefully bolted down.
Mechanisms and Machines
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.
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.
The cylinder of an internal combustion engine is
another example of a crank and slider mechanism
Mechanisms and Machines
CRANK AND SLIDER MECHANISM
Spark plug
intake valve
The four "strokes" of these engines are as follows.
Intake: The intake valve opens allowing fresh oxygen
rich air mixed with fuel to enter the cylinder.
Compression: The piston is pushed upward by the
flywheel's momentum compressing the air/fuel mix.
chamber
cylinder
Combustion: As the piston reaches the top of its stroke
the spark plug fires igniting the mixture. Due to the
high compression of this mixture (typically around 190
PSI in a typical engine) it is very volatile and it explodes
when the spark is introduced. This pushes the piston
downward and produces power.
Exhaust: After the Air/Fuel mix has been burnt the
remaining chemicals in the cylinder (water and CO2 for
the most part) must be removed so that fresh air can be
brought in. As the piston goes back up after combustion
the exhaust valve opens allowing the exhaust gasses to
be expelled.
Piston
exhaust
valve
Mechanisms and Machines
RATCHET MECHANISMS
A ratchet mechanism is based on a wheel that has teeth cut out of it and a
pawl that follows as the wheel turns. Studying the diagram you will see that
as the ratchet wheel turns and the pawl falls into the 'dip' between the teeth.
The ratchet wheel can only turn in one direction - in this case anticlockwise.
The water well ratchet mechanism allows the person to rotate the handle in an
anticlockwise direction. The bucket of water is heavy and so the person can rest by
taking his/her hands away from the handle. This is because the pawl has fallen into
the 'dip' between the teeth and so the bucket cannot fall back into the well.
Ratchet mechanisms are very useful devices for example, they are used in
mechanical clocks. They are also very useful when using a system, such as the one
shown, to lift heavy weights.
Torque
Levers
Linkages
Pulleys
Cranks
Ratchets
Cams
Mechanisms
Mechanical Advantage
Velocity Ratio
Gear Ratio
Velocity
Moments
Force
Linkage
Bell-crank
Simple gear train
Worm and worm wheel
Internal gears
Lubricant
Chain and Sprocket
Motion
Linear
Rotary
Reciprocating
Oscillating
Static
Load
Effort
Fulcrum or Pivot
Parallel
Equalising
Toggle
Dynamic
Compression
Tension
Bending
Torsion
Shear
Equilibrium
Compound gear train
Axle
Rack and pinion
Bevel gears
Spur gears
Eccentric
Rise
Dwell
Fall
Crank and slider
Compression
Combustion
Exhaust