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Mechanisms and Movement
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
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
The Six Machines
Lever Inclined Plane Wheel and Axle Wedge Pulley Screw
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.
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
Mechanical Advantage
Ratio between load and effort moments
MA>1 = gain in output force Gain in output force = loss in
output motion
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
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?
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
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
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.
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)
Kinematics
Kinematics is the study of movement in a mechanical system
Does not regard forces or masses, just movements
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
Motion
There are a number of different types of motion.
These types of motion can be converted into one another
Mechanisms transfer this motion
Linear Motion
Straight-line motion that occurs in one direction
CD drive on a computer A drawer
Reciprocal Motion
Back and forth linear motion Piston in a combustion engine Needle in a sewing machine
Rotary Motion
Movement around a center point or axle
Very common and very efficient
Oscillating Motion
Back and forth movement in an arc
A clock pendulum Windshield wipers
Linkages
Linkages transmit motion or force to a desired location– Change direction– Change length– Split motion or force over multiple
paths
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
Reversing Linkage
Changes direction by 180 degrees
Parallel Linkages
Allows parts to move in a parallel relationship
Crank and Slider
Treadle Linkage
Changes oscillating motion to rotary motion
Came from foot-operated pedals for sewing machines and other tools
Toggle Linkage
“Snaps” in place– Locking pliers– Vice grips– Locking legs of a foldable table
Additional force is needed to unlock the toggle linkage