Practical Design approaches
Gears/Sprockets
Gear ratios
• Suppose you want to design a mobile robot
that will move at 12 ft per second and 4 feet
per second. You have decided on the high rate
of speed for general maneuvering around a
playing field and the low rate of speed for
pushing matches you might find yourself in
while on the field. What type of gear train will
you have to use?
• Before designing a gear train you will need
some data and will have to make some
assumptions.
• What is the motor speed?
• What is your motor power output?
• What is your wheel diameter?
• Each motor has a set of specifications
including:
– a max speed or free speed
– Stall torque
– Maximum power
Gear rod
Cut Gear rod
Gear Assembly
Sprockets
25 pitch sprockets
Gear/Sprocket combination
Worm gears
Worm gears meshed
Gear Sizes
Planetary gears
Pictures from Howstuffworks.com
Gears vs Sprockets
Sprockets:
Require less alignment (more slop)
Transmission of power over longer distances
85-95% efficient in transmission of power
Gears:
More compact transmissions
90-95% efficient in transmission of power
Require high degree of alignment
Our original question
Suppose you want to design a mobile robot
that will move at 12 ft per second and 4 feet
per second. You have decided on the high
rate of speed for general maneuvering
around a playing field and the low rate of
speed for pushing matches you might find
yourself in while on the field. What type of
gear train will you have to use?
Assumptions for this example
• Assume:
– free motor speed of 5000 rpm
– Wheel diameter of 6 “
Linear movement per wheel rotation
• Circumference of wheel
=2pR = 2p3” = 18.85 ”
• Linear movement = 18.85 ”/rev
RPM required to achieved desired speeds
Low Speed
– (4ft/sec) x (12in/ft) x (60 sec/min) = 2,880in/min
– (2,880in/min) x (rev/18.85 in) = 152.79 rpm
– Round off to 153 rpm
High Speed
– (12ft/sec) x (12in/ft) x (60 sec/min) = 8,640in/min
– (8,640in/min) x (rev/18.85 in) = 458.36 rpm
– Round off to 458 rpm
Gear reductions needed
Low Speed
5,000rpm/153rpm  32.68:1  33:1 reduction
High Speed
5,000rpm/458rpm  10.92:1  11:1 reduction
Gear ratios
Ratios are the ratio of teeth between two sprockets or gears.
•
Given a 12 tooth gear and a 60 tooth gear, the gears will turn together because their teeth
have meshed:
–
–
–
–
If the 12 tooth gear is driving the 60 tooth gear, for every 5 turns of the 12 tooth gear the 60 tooth
gear will turn once.
This results in a 5:1 reduction in the speed of the shaft the 60 tooth gear is on with respect to the
shaft the 12 tooth gear is on.
If the 60 tooth gear is driving the 12 tooth gear, for every 1 turns of the 60 tooth gear the 12 tooth
gear will turn five times.
This results in a 1:5 increase in the speed of the shaft the 12 tooth gear is on with respect to the
shaft the 60 tooth gear is on.
Gear Ratios
• If I want a reduction of greater than 5:1 (such
as the 33:1 ratio in our example) then I have
two choices:
1) Drive a 330 tooth gear/sprocket with a 10
tooth gear/sprocket.
2) Create a gear train with multiple reductions
Option 1
Drive a 330 tooth gear/sprocket with a 10 tooth
gear/sprocket
• If you use a 330 tooth gear, its diameter will be almost 50%
larger than a 60 tooth gear.
• The largest sprocket generally available is around 125 teeth
and has a diameter of almost 10 “ and is available in 25 pitch
size only
Option 2
Create a gear train with multiple reductions
– For example a 5:1 and a 20:3 combination
– 12 tooth to 60 tooth combined with a 9 tooth to
60 tooth
– (12/60) x (9/60) = .03 = 1/33.33  33:1 approx
Driven gear/sprocket of first reduction is on the
same shaft and coupled to the driving
gear/sprocket of the second reduction.
Home work
• None this week
• All homework to be submitted no later than
Dec.9th