Gear Drives 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

Gear Drives Gears Characterized by: Number of Teeth (N) Pitch Diameter (D) Circular pitch (p) Diametral pitch (P) Pressure angle (ϕ) 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.

Gear Drives Circular Pitch (p) Diametral Pitch (P) Pressure Angle (ϕ) 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.

Gear Drives 4 types of Gears: Spur Gear Helical 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

Gear Drives Bevel Gears Worm Gears Generally mounted on shafts 90° Good when a large speed reduction is needed. Worm can turn the gear but gear cannot turn the worm Used to prevent rotation in one direction

Gear Drives Gear Alignment Speed and Gear Ratios 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

Gear Drives 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 Drives Gear Trains Many gears to achieve desired speed between a driving component and driven component. Enclosed in a housing - Gearbox

Gear Drives 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

Gear Drives 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- ???