Simple Machines: Gear Ratios and Velocity Ratios

Simple Machines:
Gears, Velocity Ratios
and Mechanical
What is a simple machine?
 A device which uses
basic mechanisms to
make work easier for the
 Example: Lifting a heavy
box with a rope and pulley
 Simple machines are
found in many
transportation devices
such as the following:
 Cars
 Bicycles
 Trains
What are gears and why
are they so useful?
 Gears are toothed
wheels which
interlock to form
simple machines.
 The tighter the joint,
the less chance of
 Gears range in size
but the important
number is how many
teeth a gear has.
What is a gear ratio?
 A gear ratio is the ratio between the driver gear
(the one being powered) and the driven gear
(the one connected to the driver)
 Gear Ratios are expressed as fractions and can be
written a number of ways.
 1 to 3
 1/3
 1:3
 Always remember that the driven gear will turn in
the opposite direction of the driver gear
What is a pulley?
 A wheel with a grove carved around its
circumference in which a rope or belt is
placed to drive the wheel.
 Pulleys are used in a number of applications
 Elevators
 Cranes
 Automobiles
A Pulley System
Drawbacks of Pulleys
 Pulleys work great as long as the belt which
connects them is tight and both pulleys are
 Pulleys tend to slip when too much strain is placed
on the system.
How do I calculate my
gear ratio?
 A simple equation is used to find the ratio of your gearing
 Number of Teeth on the Driven Gear / Number of Teeth on
the Driver Gear.
 Size of the driver pulley/ Size of the driven pulley
What is Velocity Ratio (VR)?
 When calculating for velocity or speed, it is
best to have a ratio in which the driver gear is
larger than the driven gear.
 For maximum speed you should always have a
larger driver gear/pulley paired with a smaller
driven gear/pulley.
What is Mechanical
Advantage (MA)?
 Reciprocal of velocity
 Mechanical advantage
describes how much
work can be done based
on the input.
 If my system required
me to apply 50 pounds
of force into the system
to lift 100 pounds of
weight, I have used MA.
So which do I want?
 Higher velocity ratios are better when working
on straight and level surfaces which are
 Always remember that with a high VR your system
will have very low power.
 Higher mechanical advantage systems are
better with inclines surfaces that are bumpy.
 With a high MA your system will produce more
power, but will move slower.

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