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.