Gears, Pulleys, Sprockets, and Bearings
Information about power transmission and uses for each type.
Robotics 101
Why do we need Gears and Pulleys?
• Why do we have gears?
• Why do we have pulleys?
Geek
Easy Work Can Tow or Push Heavy Objects!
Easy Work Can Lift Heavy Objects!
Nothing To It!
Geek
Wow, How Easy…Keep On Moving Titanic!
Muscles on a Geek?Come On!
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 ForceOr Force = Torque / Radius
What is Torque?
Gears
• Spur Gears 1:1
Direction -X=Y (Opposite)
X Gear Y Gear
X Teeth=30 Y Teeth=30
Speed- X=YSpeed Ratio-X Pitch Diameter/Y Pitch Diameter= 1X Teeth/Y Teeth= 1
Teeth
X Gear
X Pitch Diameter
Y Gear
Y Pitch Diameter
GearsSpeed Ratio- Compare either ratio of # of teeth or diameters as both have same proportion: # of teeth/min = (X teeth * RPMx) = (Y teeth * RPMy)
= X teeth = RPMy = 30 =1.875 Y teeth RPMx 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%
X Gear
X Pitch Diameter
X Teeth=30Y Teeth=16
Y Pitch Diameter
Τx
Τy
Multiple Stage Gearing
Top View
d=xd=x
d=3x
d=3x
Ratio Gear #1- 3:1Ratio Gear #2- 3:1
Motor
d=x
Out
In
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 TeethLarger Face Width= More ForceMust 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
12’’3’’
Speed- Y= 4XSpeed Ratio-
XDiameter = 12 = 4:1 YDiameter 3
Torque Ratio-Y= X/4
Efficiency of Pulleys:Approximately 90%
X Pulley
Y Pulley
Pulley Belts & Uses
• Pulley BeltsV-Belt
Timing 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 Stretches- Steel is the way to GO!Nylon Sprockets Crack- Aluminum 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 AluminumType 2: Ball BearingType 3: Roller Bearing
Shaft
Bearing Types
• Sintered Bronze Bearing with optional oil impregnation
• Ball Bearings
• Needle Roller Bearings
Clutch One DirectionAlso Available
Steel Balls
Steel Cylindrical Rollers
Can be sealed or unsealed
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 over-restricting 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.