IT 221 Power Transmission

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IT 221 Power Transmission

• Week 2B

2
Describe gear operation in the context of
compound levers
3
Gears and Torque
4
Discuss torque and force relative to gears in mesh

• Torque - Force applied at the tooth times the
  radius to the point of force application
• Units of torque - in-lbs or ft-lbs

5
Discuss torque and force relative to gears in mesh

• Force applied at the tooth pushes the next gear
  through a distance
• One revolution of the gear is the circumference
• C pi x Diameter
• Work F x C
• Time for a revolution determined by rpm
• Usually symbol N
• Power Work / time

6
Discuss torque and force relative to gears in mesh

• Power (in HP) T x N / 5252
• Where T - ft-lbs, N - rpm
• 5252 60 x 550 / (2 x pi)
• 1 HP 550 - ft-lbs/sec

7
Describe the calculation of gear ratio

• Gear ratio is defined as the torque ratio
• Gear ratio Driven/Driving
• Gear ratio 1 / speed ratio
• Speed ratio 1 / torque ratio
• Three ways to calculate
• Number of teeth (most accurate)
• Diameter of gear (must estimate diameter)
• Number of rotations (difficult to get exact)

8
Relation of torque ratio and speed ratio

• Torque and speed are inversely related
• Torque goes up, speed goes down
• To increase torque, driven gear should slow down
• Example problem 3-1

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Relation of torque ratio and speed ratio
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Types of gears

• External - most common
• Internal - ring gears
• Face - side of gear
• Rack - unrolled ring gear

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Types of gears
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Types of gears

• Spur
• Helical
• Herringbone
• Bevel
• Miter
• Rack and pinion
• Worm

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Types of gears

• Spur - Most common (Figure 3-8)
• Involute shape
• Tooth geometry (Figure 3-6)
• Teeth are axial

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Types of gears

• Helical - (Figure 3-8)
• Quiet
• More power delivery
• Teeth at an angle to the axial
• Produces substantial side load

15
Types of gears

• Herringbone - double helical (Figure 3-8)
• Neutralizes side load
• Difficult and expensive to make

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Types of gears

• Bevel gears - face gear (Figure 3-8)
• Provides for angled shafts
• Side load created
• Miter - special case of a bevel
• 45 degree angle

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Types of gears

• Rack and pinion gears (Figure 3-9)
• Unrolled ring gear
• Translational motion from rotational
• Driving gear is called a pinion
• Hold the pinion, rack translates
• Hold the rack, pinion translates

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Types of gears

• Worm - worm and worm gear
• Worm looks like screw
• Threads cause one tooth to move each worm
  revolution
• If 20 teeth, and one thread, Gear ratio 201
• May have more than one thread
• If 20 teeth, and 4 threads, Gear ratio 51

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Worm Gear
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Types of gears

• Other types have been developed for special
  applications
• Hypoid
• Spiroid
• Crossed helical
• Cone Drive
• etc.

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Types of gear sets

• Two gears
• Driving gear is the pinion
• Output gear rotates opposite direction
• Gear ratio - Teeth ratio- Driven/Driving

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Types of gear sets

• Three gears - planar (Figure 3-10)
• Middle gear - idler
• Output gear rotation same as input
• Four planar - simple gear set
• Gear ratio GB / GA x GC / GB x GD / GC
• GD / GA

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Types of gear sets

• Compound
• Parallel shafts (Figure 3-11)
• Center shaft - torque is constant
• Center shaft called the idler shaft
• TB TC
• Gear ratio GB / GA x GD / GC

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Types of gear sets

• Reverted
• Parallel shafts
• Output shaft aligned with the input shaft
• Looks continuous but it isnt
• Center shaft - torque is constant
• Center shaft called the idler shaft
• TB TC
• Gear ratio GB / GA x GD / GC

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Reverted Gear Set
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Types of gear sets

• Planetary -
• Figure 3-17
• Center gear - sun
• Outer gear - ring
• Middle gear(s) - planet
• Planets held together with a carrier

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Planetary gear set

• Case 1 - Hold the ring, drive the sun
• Sun causes the planet carrier to revolve
• Planet must mate with the fixed ring
• If the sun rotates clockwise, planet will precess
  (move) clockwise
• Each tooth of the sun moves the planet one tooth
• Each tooth of the planet moves in the ring one
  tooth
• Speed ratio Sun teeth / Ring teeth

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Planetary Gearsets
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Planetary gear setRing 100 Teeth, Sun 10
teeth, Planet 30 teeth

• Six possible cases
• Hold the sun, drive the ring
• Hold the sun, drive the planet
• Hold the planet, drive the sun
• Hold the planet, drive the ring
• Hold the ring, drive the sun
• Hold the ring, drive the planet

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Types of gear sets

• Epicyclic - Figure 3-20
• Planetary is a special case
• Sun and ring have one tooth difference
• Hold the planet
• One revolution moves the output one tooth

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Types of gear sets

• Harmonic -
• Figure 3-21
• Cam and roller system
• Flexible gear
• Different number of teeth on driven and driving
  members

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Types of gear sets

• Worm - Described earlier
• One of the most common forms of gear set
• Many possibilities for gear ratio
• Used extensively with electric motors
• Often called a gear reducer

33
Describe the advantages of using belt drives

• Inexpensive
• Transfers force over large distances
• Permits directional change
• Serpentine configurations are possible

34
Describe the advantages of using belt drives
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Speed and torque ratio for a belt drive systems

• Force is transmitted by the belt pull
• Friction at the drive pulley delivers the force
• If drive and driven pulleys are different size
• Torque ratio is ratio of driven/driving pulley
  diameters
• Speed ratio is 1/Torque ratio
• Disadvantage - Force transmission is limited
• Belt can slip, stretch

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Types of belts

• Round
• Flat
• V - loading actually on the side
• Cogged

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Types of belts
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Describe the advantages of using chain drives

• Inexpensive compared to gears
• Can transmit force over long distance
• No slip
• Stronger than belts
• Disadvantage
• Not as flexible as belts

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Types of chain

• Roller
• Silent
• Link
• Pintle
• Sidebar
• Block
• Bead

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Types of chain
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Chain and attachments