• Types of Gears
• Gear Terminology
• Involute Tooth Profile
• Gear Ratios

Types of Gears

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There are many types of gears. Gears mesh with other gears of the same type.

Spur Gears

• Straight teeth
• Meshing gears are on paralell shafts.
• The most common gears used in FIRST.

Helical Gears

• Angular teeth cause each tooth to engage with the matching tooth gradually, resulting in smoother engagement.
• Meshing gears are on paralell shafts.
• Produce side loads

Bevel Gears / Miter Gears

• Gears have teeth on an angle.
• Meshing gears are on perpendicular shafts
• Bevel gears - 2 meshing gears have different tooth count.
  Miter gears - 2 meshing gears have same tooth count.

Worm Gear

Gear Terminology

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Tooth Count: The number of teeth on the gear

Pitch Circle: The circular profile of the meshing surface of the gears

Pitch Diameter: The diameter of the pitch circle. Can be derived using formula (Tooth Count / Diametrical Pitch).

Diametrical Pitch (DP)

• The number of teeth of a gear per inch of its pitch diameter.
• Measure of how big the teeth are.
• Common sizes are 12 (big teeth), 20, 24, 32 (small teeth)
• Meshing gears must have the same diametrical pitch

Pressure Angle

• The angle at a pitch point between the line of pressure which is normal to the tooth surface, and the plane tangent to the pitch surface.
• Most commonly used pitch angles (for spur gears) are 14.5° and 20
• What you need to know: Meshing gears must have the same pressure angle.

The Involute Tooth Profile

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• If gears had straight teeth, the distance from the center of the gear to the point of contact would be continuously changing, resulting at speed being transferred inconsistently as shown in this animation.
• The involute tooth is curved so that as the gear turns, the tooth engages with the meshing tooth in such a way that the speed is transferred inconsistently as shown in this animation.

Gear Ratios

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• Gear ratios allow gears to change the speed output and torque of a transmission.
• Power is fixed (with a given motor).  The faster a gearbox is geared, the less torque it has.
• To find the ratio, divide the number of teeth on the output gear by the number of teeth on the input gear.
• The teeth on the meshing gears will go by at the same rate.  Because the circumference on a smaller gear is smaller, it will rotate more times for every rotation of a large meshed gear.
• Multiply ratios together to get total gearbox ratio.