Robotics 101
Gears, Pulleys, Sprockets, and Bearings
Information about power transmission and uses for each type.
Why do we need Gears and Pulleys?
• Why do we have gears?
Easy Work Can Tow or Push Heavy Objects!
Wow, How Easy…Keep
On Moving Titanic!
Geek
• Why do we have pulleys?
Nothing To It!
Geek
Easy Work Can Lift Heavy Objects!
Muscles on a Geek?
Come On!
What is Torque?
Torque, moment, or moment of force, is the tendency of a force to rotate an object
about an axis, fulcrum, or pivot. Just as a force is a push or a pull, a torque can be
thought of as a twist to an object. Mathematically, torque is defined as the cross
product of the lever-arm distance and force, which tends to produce rotation.
Torque = Radius X Force
Or Force = Torque / Radius
Gears
• Spur Gears 1:1
Direction -X=Y (Opposite)
X Gear
Y Gear
Teeth
Speed- X=Y
Speed Ratio-
X Pitch Diameter
X Pitch Diameter/Y Pitch Diameter= 1
X Teeth/Y Teeth= 1
X Gear
X Teeth=30
Y Teeth=30
Gears
Speed Ratio- Compare either ratio of # of teeth or diameters as both
have same proportion:
# of teeth/min = (X teeth * RPMx) = (Y teeth * RPMy)
Τx
X Pitch Diameter
Τy
Y teeth RPMx 16
X Gear
X Teeth=30
= X teeth = RPMy = 30 =1.875
Y Teeth=16
Or RPMx/RPMy = 1/1.875 = .533 = 16/30
X PitchDiameter = 1.875 * Y PitchDiameter
X Teeth/Y Teeth= 30/16- Y=1.875X
Speed- RPMy=(1.875)RPMx or RPMx=(.533)RPMy
Torque Ratio- Ty/Tx=.533
(Note: It is the inverse of the speed ratio)
Ty= Tx/1.875 = Tx(.533)
Efficiency of Spur Gears: Approximately 95%
Multiple Stage Gearing
Top View
Motor
Out
d=3x
In
d=3x
d=x
Ratio Gear #1- 3:1
Ratio Gear #2- 3:1
d=x
d=x
Test
•
•
•
•
•
Direction- In direction = Out direction
Speed- Output = Input/(3*3) = Input/9
Speed Ratio- 9:1
Torque Ratio- 1:9
Output Torque= Input Torque X 9
If the motor input RPM (revolutions per minute) is 5400 and input torque is 2 ft.*lbs.
1) What is the output speed in revolutions per second (60 seconds in 1 minute)?
2) What is the output torque in Newton*meters if 1 N-m=0.71ft-lbs.?
Answers
• Answer #1: 5400/9= 600RPM/60= 10 revs per second
• Answer #2: 2ft-lbs*9= 18 ft.-lbs.* 0.71= 12.78 N-m
Gear Types
• Bevel (90° Angle)
• Internal (Planetary)
• Rack and Pinion (Linear Motion)
• Spur (Normal)
For all Gears:
Higher Pitch= More Teeth
Larger Face Width= More Force
Must Match Pressure Angle
Gear Rules and Types
• FIRST Rules:
− Any gear
− Any type
− Any manufacturer
• Plastic (Lighter, Smaller Load)
• Brass (A Little Lighter (mainly due to small width), Good Load)
• Steel (Heavy, Excellent Load, will need to trim down gear on lathe for weight)
Pulleys
• The only difference is the belt connection method
3’’
12’’
Y Pulley
X Pulley
Speed- Y= 4X
Speed RatioXDiameter = 12 = 4:1
YDiameter 3
Torque RatioY= X/4
Efficiency of Pulleys:
Approximately 90%
Pulley Belts & Uses
• Pulley Belts
Timing Belt
V-Belt
Uses:
-Ball Chute (2012)
-Drivetrain (2014, 2015)
-Climbing Obstacles
Problems:
-Slipping
-Belts coming off pulleys
(No Sideways Torque)
Chain and Sprockets
• Chain and Sprockets
− The only difference is the connection method
− Instead of Belts (Pulley)
− Instead of Teeth (Gears)
• Types of Chain
− Plastic
• Light loads, breaks easily
− Steel
• Heavy loads (drivetrain)
• Types of Sprockets
− Nylon
• Light loads
− Aluminum
• Heavy loads (drivetrain)
Plastic Chain StretchesSteel is the way to GO!
Nylon Sprockets CrackAluminum is the way to GO!
Chain and Sprockets
Bicycle Chain
Bearings
• Bearings- What Are They?
Problem!
Bearing Types
Type 1: Softer Material like Bronze or
Aluminum
Type 2: Ball Bearing
Type 3: Roller Bearing
Shaft
Bearing Types
• Sintered Bronze Bearing with optional oil impregnation
Steel Balls
• Ball Bearings
Steel Cylindrical Rollers
Can be
sealed or
unsealed
• Needle Roller Bearings
Clutch
One Direction
Also Available
Bearing Types
• Pillow Block Bearings
− Self Aligning
− Used when you need some compliance in alignment.
Issues to be aware of with bearings
• Misalignment of bearing systems can over-restrict them, causing higher friction and unnecessary force on systems. Be careful of overrestricting things that want to be able to move.
• Dirt, debris, and chips can get into bearings, especially open bearings: it increases friction and resistance. (Therefore protect bearing
services when machining on or near the robot)
• Exceeding the recommended load for a bearing can shorten or even end its life and lead to need for replacement- or may add friction to
the system causing overload on motors causing them to fail.
• Motors have internal bearings you don’t see, so avoid excessive loads on shafts- beware of pounding on motor shaft ends (installing or
removing items on motor shafts). Use proper techniques to avoid damaging internal shaft supports in the motor, by not applying
excessive loads.
• Don’t stand or lean on the robot! No matter how sturdy it “seems”- we have had bent wheel shafts, and gotten frames out of square. In
trying to make weight we lighten up areas where possible, but the robot may be less robust.