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Gear Drives◦ Rigid means of transmitting power between close
shafts through the meshing action of their teeth.◦ Smallest gear – Pinion.
Can be driven or driving gear◦ Can change both orientation and speed of rotary
motion Examples – car differential, washing machine,
others?◦ Preferred over belts and chains when:
Transmit power without slippage Timing devices (watches)
◦ Disadvantages - Higher costs and lubrication
Gears Characterized by:◦ Number of Teeth (N)◦ Pitch Diameter (D)◦ Circular pitch (p)◦ Diametral pitch (P)◦ Pressure angle (ϕ)
Pitch Diameter (D) ◦ The diameter of the pitch circle. The pitch circle is
the imaginary circle on which the contact point of the gears lie. Where power is transferred.
Circular Pitch (p)◦ The length of the arc between corresponding
points on adjacent teeth. Diametral Pitch (P)
◦ The ratio of the number of teeth per inch of pitch diameter.
Pressure Angle (ϕ)◦ The angle between the line of action and a line
tangent to the pitch circle Standard angles - 20° & 14.5° Line of action – portion of the common tangent to
the base cylinder along which contact between mating teeth occur.
4 types of Gears:◦ Spur Gear
Most common Teeth cut parallel to axis of rotation Good for low to moderate speeds
◦ Helical Gear Teeth not parallel to the shaft Form spiral around the body Allows smoother mating of the teeth High thrust load Reduced bearing and shaft life
Bevel Gears◦ Generally mounted on shafts 90°
Worm Gears◦ Good when a large speed reduction
is needed.◦ Worm can turn the gear but gear
cannot turn the wormo Used to prevent rotation in
one direction
Gear Alignment◦ Critical
Horizontal Vertical Parallel
Speed and Gear Ratios◦ N(2)/N(1) = n(1)/n(2)
N(1) = number of teeth of the driving gear N(2) = number of teeth of the driven gear n(1) = speed of the driving gear in RPM n(2) = speed of the driven gear in RPM
Calculate the driven gear speed of a two gear drive having the following:◦ Driving gear speed – 21 RPM◦ Number of teeth on the driving gear – 40◦ Number of teeth on the driven gear – 20
Driven gear speed - ???
Gear Trains◦ Many gears to achieve desired speed between a
driving component and driven component.◦ Enclosed in a housing - Gearbox
Speed of the last shaft◦ n(3) = [n(1) X N(1)]/N(3)
n(1) = speed of the driving shaft (RPM) n(3) = speed of the last driven shaft (RPM) N(1) = number of teeth of the driving gear N(3) = number of teeth of the last driven gear
◦ Gears installed in a series, speed of last shaft is only dependent on the: Speed of the first shaft Teeth ratio between the first and last gear
Calculate the driven gear speed of the last shaft in a 3 Gear Train having the having the following:◦ Driving gear speed – 21 RPM◦ Second gear speed – 30 RPM◦ Number of teeth on the driving gear N(1)– 40◦ Number of teeth on the second gear N(2) – 20◦ Number of teeth on the third gear N(3) – 10
Driven gear speed of last shaft- ???