GearsGears

Gear TypesGear Types

Rack and Pinion(90% efficiency)

Bevel Gears(70% efficiency)

Spur Gears (90% efficiency)

Helical / Double Helical (80% efficiency)

Worm and Pinion(70% efficiency)

Crown Gear

Equal Wheel Resistance

Differential Gearing

Unequal Wheel Resistance

Epicyclic or Planetary Gears(80% efficiency)

Why use Gears?Why use Gears?In robotics, torque is better than speed. Commercially available motors In robotics, torque is better than speed. Commercially available motors

do not normally have a desirable speed to torque ratio (the main exception do not normally have a desirable speed to torque ratio (the main exception being servos and high torque motors with built in gearboxes). being servos and high torque motors with built in gearboxes).

With gears, you will exchange the high velocity with a better torque. With gears, you will exchange the high velocity with a better torque. This exchange happens with a very simple equation that you can calculate: This exchange happens with a very simple equation that you can calculate:

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*

*

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wheelwheel

robot

input

outputinputoutput

output

inputinputoutput

newnewoldold

Diameterrpm

speed

teeth

teethtorquetorque

teeth

teethrpmrpm

velocitytorquevelocitytorque

Gear BasicsGear Basics

GEAR TRAIN consists of:GEAR TRAIN consists of: Driving gears - attached to the input shaft Driving gears - attached to the input shaft Driven gears - attached to the output shaft Driven gears - attached to the output shaft Idler gears - interposed between the driving and driven gear in order to Idler gears - interposed between the driving and driven gear in order to

maintain the direction of the output shaft the same as the input shaft maintain the direction of the output shaft the same as the input shaft or to increase the distance between the drive and driven gears. or to increase the distance between the drive and driven gears.

Direction of Rotation – odd numbers of gears always rotates in the same Direction of Rotation – odd numbers of gears always rotates in the same direction, and even numbers of gears are counter-rotational. direction, and even numbers of gears are counter-rotational.

GEAR RATIO is the relationship between GEAR RATIO is the relationship between Number of teeth on two gears that are meshed Number of teeth on two gears that are meshed Two sprockets connected with a common roller chain, Two sprockets connected with a common roller chain, Circumferences of two pulleys connected with a drive belt.Circumferences of two pulleys connected with a drive belt.

COMPOUND GEAR TRAIN refers to two or more gears used to transmit COMPOUND GEAR TRAIN refers to two or more gears used to transmit motion.motion.

R

r

D

dgr

Gearing Efficiency Gearing Efficiency Unfortunately, by using gears, you lower your input to output power Unfortunately, by using gears, you lower your input to output power

efficiency. This is due to obvious things such as friction, misalignment of efficiency. This is due to obvious things such as friction, misalignment of pressure angles, lubrication, gear backlash (spacing between meshed gear teeth pressure angles, lubrication, gear backlash (spacing between meshed gear teeth between two gears) and angular momentum, etc.between two gears) and angular momentum, etc.

As you add more gears, the gearing efficiency goes down As you add more gears, the gearing efficiency goes down as is evident from the equationas is evident from the equation

% ficiency GearTypeEf efficiency 1) - gears of (# total

Input

Output

Power

PowericiencyGearingEff

EXAMPLE: For the 6 Spur Gear TrainGearRatio = Daimeter of First Gear/ Diameter of last Gear = # of Teeth on first gear / # of Teeth on last gear = 2:1

Rotational Direstion = Counter-rotational ( Since # of Gears=6 is even)

effienciency_total = 0.9 ^ (5) = 59.1 %

FOR EFFICIENCY, DO NOT USE LONG GEARS TRAINS.INSTEAD USE CHAIN AND SPROCKET OVER LARGER DISTANCES

Robot SpeedRobot Speed

inchesRadius

inchftDiameter

Diameter

Diameterrpm

speed

wheel

wheel

wheel

wheelwheel

robot

3

62/1

3~**60

120~3

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For a 120 rpm For a 120 rpm motor, what size wheel motor, what size wheel should I use if I want should I use if I want my robot’s maximum my robot’s maximum speed to be 3 feet per speed to be 3 feet per second?second?

For the same motor, For the same motor, How would I make my How would I make my robot’s maximum speed 6 robot’s maximum speed 6 feet per second, if 6” feet per second, if 6” wheels are the largest I can wheels are the largest I can fit onto my robot? fit onto my robot?

Put a gear or sprocket on the

motor shaft that is half the size of the

gear or sprocket on the wheel.