Transcript Mechanisms Presentation 2012

Mechanisms
Dean Hackett March 2012
Types of motion
•
•
•
•
Linear
Rotary
Reciprocating
Oscillating
Simple (Basic) Machines
• Two ‘families’
– Inclined plane
– lever
Simple Machines
•
•
•
•
•
•
Inclined plane
Wedge
Screw
Lever
Wheel and axle
Pulley
Work = Force x distance
Classes of Lever
• Class 1
• Class 2
• Class 3
Types of Linkage
• Parallel
• Reverse motion
• Bell crank
• Treadle
• Crank slider
Have a think...
Lever Mechanisms
Load
xN
Effort
10N
4m
1m
Fulcrum
Load
yN
Effort
10N
1m
Fulcrum
4m
Mechanical Advantage
Load
Mechanical Advantage =
Effort
Load
40N
Effort
10N
4m
1m
Velocity Ratio
Distance moved by
Effort
Velocity Ratio =
Distance moved by
Load
Load
40N
Effort
10N
4m
200mm
1m
50mm
Efficiency
Efficiency =
V.R.
Load
40N
Effort
10N
4m
200mm
M.A.
1m
50mm
x 100%
Lever Mechanism
What load can this person lift?
Rotary Motion
A Pulley Mechanism uses rotary motion to transmit rotary
motion between two parallel shafts.
Discuss...
• How do you attach a pulley to a shaft?
Mechanisms using Rotary Motion
Pulley mechanisms can be used to increase or
decrease rotary velocity
Velocity
Ratio
Distance moved by
Effort
Velocity Ratio =
Distance moved by
Load
Distance moved by
the driver pulley
Velocity Ratio =
Distance moved by
the driven pulley
Diameter of Driven Pulley
Velocity Ratio =
Diameter of Driver Pulley
Velocity Ratio
Pulley Shaft Rotary Velocities can be calculated using the
following formula
rotary velocity of driven pulley x diameter of driven pulley =
rotary velocity of driver pulley x diameter of driver pulley
rotary velocity of driven = rotary velocity of driver x
diameter of driver pulley
diameter of driven pulley
What is the rotary velocity of the driven pulley shaft?
rotary velocity of driven = rotary velocity of driver x
diameter of driver pulley
diameter of driven pulley
=
450 x
30
90
= 150 revs/min
revs/min
Pulleys and
Belts
Vee pulley and section through a vee pulley and
belt
A section through a
grooved pulley and
round belt
Stepped cone pulleys provide a range of shaft speeds
Flat belts
and
pulleys
A section through a flat
pulley and belt
Jockey pulley in use
Flat belt in use on a
threshing machine
Chains and
sprockets
Bicycle
chain and
sprockets
Graphical symbols
number of teeth on the driven sprocket
Velocity Ratio =
number of teeth on the driver sprocket
=
12
36
=
1:3
Example
Pulleys and Lifting Devices
The
pulley is
a form
of Class
1 lever
Movable single pulley
Pulleys
Distance moved by
Effort
Velocity Ratio =
Distance moved by
Load
Velocity Ratio = the number of rope sections that support the
load
Two Pulley System
Distance
moved by
Effort
Velocity Ratio =
Distance
moved by
Load
2x
Velocity Ratio =
x
Velocity Ratio =
2:1
Four Pulley System
Distance
moved by
Effort
Velocity Ratio =
Distance
moved by
Load
4x
Velocity Ratio =
x
Velocity Ratio =
4:1
Cams
Cams
Uses
Pear shaped cams are used in valve
control mechanisms
Cams used
in a four
cylinder
engine
http://www.youtube.
com/watch?v=OXd1
PlGur8M&feature=re
lated
Cam motions
Types of cam follower
Types of cam follower
Springs are used to keep the follower in
contact with the cam
Cam Profiles
Displacement graph for a pear
shaped cam
Displacement Graphs
Bearings
Thrust
Bearings
Bearings
Bearings
•
•
•
•
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•
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Bronze
Nylon
PTFE
Air
White metal
Cast Iron
Sintered
Gears
Gears
Gears are not only
used to transmit
motion.
They are also used
to transmit force.
Gears
Number of teeth on the
driven gear
Mechanical Advantage =
Number of teeth on the
driver gear
Velocity Ratio = Gear Ratio =
Number of teeth on the
driven gear
Number of teeth on the
driver gear
Gears
Gears
Gear Ratio =
Product of teeth on the driven gears
Product of teeth on the driver gears
Gears
Gears
Gears
Gears
http://www.youtube.
com/watch?v=9NoQ
m0wnK_c&feature=r
elated
http://www.youtube.
com/watch?v=K4Jhr
uinbWc&NR=1
Basic Gear
Geometry
http://www.sdpsi.com/D190/PDF/D190T25.PDF
The inclined plane
The inclined plane
The inclined
plane

Effort required to pull trolley up slope
sin = 1/100 = 0.01
F = effort E
F = 1000 x sin
F = 1000 x 0.01
M.A. = 1000/10
= 100
F = 10N
E = 10N
Follow link to see effects of steeper incline:
http://lectureonline.cl.msu.edu/~mmp/applist/si/plane.htm
The screw thread
Screw thread terms
Screw thread forms
Screw thread forms
Screw thread forms
B.S.
PD7308
Newton’s Laws
• First Law
– A body continues in its state of rest or uniform
motion in a straight line unless compelled by
some external forces to change that state.
(sometimes know as the law of inertia)
Newton’s Laws
• Second Law
– Rate of change of momentum is proportional
to the applied force and takes place in the
direction in which the force acts.
(Continued force means continued
acceleration)
Newton’s Laws
• Third Law
– To every action there is an equal and
opposite reaction