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Gears
Gear Basics
Types of Gears
Spur Gear
Spur Gear
Ring Gear
(Internal)
Racks
Types of Gears
Straight Bevel Gears
Miter Gear
Spiral Bevel Gears
Worm Gear
Gear Terminology
Basic Gear Design
Commercially available gears are specified on drawings and are used whenever possible. Stock
gears are available in a wide range of forms and sizes. Gear design includes the selection of the gear
sizes, pitch diameter, center to center distance, size and form of the teeth, shaft diameter, horsepower
rating, speed rations, and appropriate heat treatment and materials.
In actual practice, the designer provides the detail drafter with only a few basic dimensions of the
required gears. The designer may obtain this data from actual engineering calculations or from
manufacturer's catalogs. All other dimensions needed in the construction of the working drawings are
usually worked out by the detail drafter.
Basic Gear Design
CENTER TO CENTER DISTANCES OF TWO FRICTION WHEELS
Basic Gear Design
CENTER TO CENTER DISTANCES OF TWO GEARS
Basic Gear Design
Diametral Pitch (P)
The diametral pitch (P) is a ratio equal to the number of teeth (N) on the gear per inch of pitch diameter (D).
Diametral Pitch (P) = Number of Teeth (N)/ Pitch Diameter (D)
Number of teeth (N)
The number of teeth (N) is the number of gear teeth on the gear or pinion.
Number of teeth (N) = Diametral Pitch (P) x Pitch Diameter (D)
Pitch Diameter (D)
The pitch diameter (D) is an imaginary circle that corresponds to the circumference of the friction gear from which the
spur gear is derived.
Pitch Diameter (D) = Number of teeth (N)/ Pitch Diameter (P)
Center to Center Distance (C to C)
The center to center distance (C to C) is the distance between the centers of the two friction wheels
C to C = (Pitch Diameter of Gear (D1) + Pitch Diameter of Pinion (D2))/2
C to C = ((D1) + (D2))/2
Basic Gear Design
Gear Ratio (M)
When two friction wheels of different diameter are placed together a point of the driver gear will travel the same
distance as a point of the driven gear. Therefore, the number of teeth and the speed of the gears are directly
proportional to its pitch diameter.
M1 = D1 = N1 = RPM2
M2 D2 N2 RPM1
Linear Velocity (V)
Linear velocity is the distance that a given point on the friction wheel travels during a certain time period.
Linear Velocity (V) = (pi) x Pitch Diameter of Gear (D) x RPM
Basic Gear Design
Sample Problem:
Complete the missing
information on the gear
train problem.
M(A) = 2
M(B) = 1
M(C) = 3
M(D) = 2