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- ???