Download - Mechanisms and Movement
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Mechanisms and Movement
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What is a Mechanism?
A mechanism takes an input motion or force and creates a desired output motion or force– Simple machines or a
combination
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What are Simple Machines? Simple machines are very basic
devices that are used to make things work
Often, these simple machines are put together to make more complicated machines
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The Six Machines
Lever Inclined Plane Wheel and Axle Wedge Pulley Screw
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Lever
A lever is an arm that is on a pivot This pivot is called a “fulcrum” The force that is being used to move
the lever is called the “effort” What the lever is moving is called a
“load” The distances between the effort and
load to the fulcrum determine how easy or hard it will be to move the load.
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Classes of Levers
A First Class lever has the fulcrum in the middle, and the effort and load on either end
A Second Class lever has the load in the middle, and the effort and fulcrum on either side
A Third Class lever has the effort in the middle and the load and fulcrum on either side
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Mechanical Advantage
Ratio between load and effort moments
MA>1 = gain in output force Gain in output force = loss in
output motion
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Velocity Ratio
VR = ratio of distance moved by effort and load
MA is gained through loss of VR VR is gained through loss of MA VR>1 means that the effort is
moving a greater distance than the load
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Inclined Plane
The inclined plane is a ramp It is used to make it easier to push
something up and down a distance. How hard is it to move a heavy
brick up a 10 foot high wall? How hard is it to move a heavy
brick up a gradual slope, or inclined plane?
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Wheel and Axle
Wheels are circular, and axles go through their middle to allow them to spin
These simple machines allow for uniform rotation
Off-center wheels are called cams and affect motion depending on their shape
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Wedge
A wedge is a special type of inclined plane
A wedge acts to push apart material– A fork and knife are both wedges
Wedges are used to join things together– Nails
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Pulley
A pulley is used to lift heavy things. A rope or cable is set around a
wheel and axle. By pulling on the rope around the wheel, whatever the other end of the rope is attached to will move. This is a simple pulley.
Many pulleys in sequence are called compound pulleys.
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Screw
A screw is a special inclined plane that is wrapped around a central shaft
Screws are useful to make really strong connections between materials
Screws are also used to slow down a descent without taking up the space of a straight ramp (spiral staircase)
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Kinematics
Kinematics is the study of movement in a mechanical system
Does not regard forces or masses, just movements
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Kinematics The individual elements that are used
in a mechanism are called members or links. One end is fixed to a point.
All links are held together by elements called joints.
A combination is called a “kinematic chain.”
If the system is incapable of movement, it is no longer a mechanism, but rather, a structure
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Motion
There are a number of different types of motion.
These types of motion can be converted into one another
Mechanisms transfer this motion
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Linear Motion
Straight-line motion that occurs in one direction
CD drive on a computer A drawer
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Reciprocal Motion
Back and forth linear motion Piston in a combustion engine Needle in a sewing machine
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Rotary Motion
Movement around a center point or axle
Very common and very efficient
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Oscillating Motion
Back and forth movement in an arc
A clock pendulum Windshield wipers
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Linkages
Linkages transmit motion or force to a desired location– Change direction– Change length– Split motion or force over multiple
paths
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Bell Crank
Changes direction of force 90 degrees
Brakes on a bike Used to ring church bells
– Pulling down on a rope would swing bell
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Reversing Linkage
Changes direction by 180 degrees
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Parallel Linkages
Allows parts to move in a parallel relationship
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Crank and Slider
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Treadle Linkage
Changes oscillating motion to rotary motion
Came from foot-operated pedals for sewing machines and other tools
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Toggle Linkage
“Snaps” in place– Locking pliers– Vice grips– Locking legs of a foldable table
Additional force is needed to unlock the toggle linkage