Chapter 20 Belts, Chains, and Gears

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Engineering Drawing and Design
Sixth Edition
Chapter 20
Belts, Chains, and Gears
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Chapter 20Belts,Chains, and Gears
Janis Christie
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20-1 Belt Drives

• Flat belts
• V-belts
• Selecting a belt drive

PhotoDisk
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Types of flat belts for power transmission

• Conventional
• Plain flat belt without teeth, grooves, or
  serrations
• Grooved or serrated (poly-V)
• Basic flat belt with a longitudinally ribbed
  underside

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Types of flat belts for power transmission

• Positive Drive (timing belts)
• Basic flat belt with evenly spaced teeth on the
  bottom surface

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Materials used for conventional belts

• Leather
• Rubberized fabric or cord
• Nonreinforced rubber or plastic
• Reinforced leather
• Fabric

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V-Belt

• Uses wedging action to transmit power
• V-belts are the basic workhorse of industry

American Beltrite Rubber Co.
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Definitions

• Sheaves The grooved wheels of pulleys
• Idler pulleys Grooved sheaves or flat pulleys
  that do not serve to transmit power. Usually
  used as belt tighteners.

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20-2 Chain Drives

• Basic types of chain drives
• Sprockets
• Design of roller chain drives

Whitney
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Types of power-transmission chains

• Detachable

• Pintle

• Offset-sidebar

Rich King

• Roller

Drives PRC
Whitney
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Types of power-transmission chains

• Double-pitch

• Inverted tooth (silent)

• Bead (slider)

Ramsey
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Chain Terminology

• Roller width

• Roller diameter

• Pitch

• Roller

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Chain Terminology

• Pin

• Roller link plate

• Pin link plate

• Sprocket

Headco
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Design of a roller chain drive is based on

• Average horsepower to be transmitted.
• Revolutions per minute of the driving and
  driven members.
• Shaft diameter.
• Permissible diameters of the sprockets.

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Design of a roller chain drive is based on

• Load characteristics, whether smooth and steady,
  pulsating, heavy-starting, or subject to peaks.
• Lubrication, whether periodic, occasional, or
  copious.
• Life expectancy.

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Designing roller chain drives

• More information on designing roller chain drives
  can be found on pages 751-761 of the Engineering
  Drawing and Design textbook.

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Quick Quiz
?

• What is another name for positive drive belts?

• Timing belts

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20-3 Gear Drives

• Spur gears

Nick Rowe
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Definitions

• Gears Rolling cylinders or cones having teeth on
  their contact surfaces to ensure positive motion.
• Pinion The smaller of two gears working
  together.

American Precision Gear
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Definitions

• Involute The curve traced by a point on a taut
  string unwinding from a circle the form of the
  gear that best produces constant angular velocity.

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Gear function

• The function of a gear is to transmit motion,
  rotating or reciprocating, from one machine part
  to another and where necessary to reduce or
  increase the revolutions or a shaft.

Daisuke Morita
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Definition

• Spur gears Gears that connect parallel shafts.

Lawrence Laury
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Simplified representations of involute gear teeth

• Teeth on a gear are not normally shown on working
  drawings.
• Two methods for drawings in which teeth should be
  represented

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Working drawings of spur gears

• Since teeth are cut to shape, gear-blank
  dimensions are shown on driving, and gear tooth
  information is given in a table.

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Working drawings of spur gears

• Front view
• Teeth need not be shown.
• Phantom lines represent outside and root circles.
• A center line represents the pitch circle.

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Working drawings of spur gears

• Section view
• Root and outside circles are shown as solid lines.

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Diametral pitch and module

• Diametral pitch
• Is a ratio of the number of teeth to a unit
  length of pitch diameter.
• Is used for inch-sized gears
• Module
• Is the length of pitch diameter per tooth
  measured in millimeters.
• Is used for metric gears

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20-4 Power-Transmitting Capacity of Spur Gears

• Selecting the spur gear drive

Jonnie Miles
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Selecting the spur gear drive

• It is difficult and expensive to determine the
  best gear set for a particular application.
• The most economic procedure is to select standard
  stock gears with an adequate load rating for the
  application.
• More information about selecting spur gear drives
  appears on pages 767-769 of the Engineering
  Drawing and Design textbook.

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20-5 Rack and Pinion
Poli Hi Solidur
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Definitions Rack

• Rack A straight bar having teeth that engage the
  teeth on a spur gear.

Poli Hi Solidur
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20-6 Bevel Gears

• Working drawings of bevel gears

Daisuke Morita
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Definitions

• Bevel gears Gears that connect shafts whose axes
  intersect.

• Miter gears Bevel gears having the same
  diametral pitch or module, pressure angle, and
  number of teeth.

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Quick Quiz
?

• What is the term for the smaller of two gears
  working together?

• Pinion

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20-7 Worm and Worm Gears

• Working drawings of worm and worm gears

The Studio Dog
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Definition

• Worm gears Gears that connect shafts whose axes
  do not intersect.

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20-8 Comparison of Chain, Gear, and Belt Drives

• Comparative advantages and disadvantages are
  discussed on pages 775-776 of the Engineering
  Drawing and Design textbook.