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