Gears

What is a Gear?A gear is:• A wheel that has teeth

around its edge that interlock with the teeth of other gears.

• Wheels with mating teeth cut in the rim or surface so that one can turn the other without slippage.

• Usually attached to an axle or shaft

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Applications of Gears

Control gears – long life, low noise, precision gearskinematic & stress analysis

Aerospace gears – light weight, moderate to high loadkinematic & stress analysis

Power transmission – long life, high load and speedkinematic & stress analysis

Appliance gears – long life, low noise & cost, low to moderate load kinematic & some stress analysis

Toys and Small Mechanisms – small, low load, low cost kinematic analysis

How a Gear Works

• A gear transfers circular motion from one gear to another.

Gears

Driver Gears: are directly driven by a motor

Driven Gears: mesh with the driver gear

Driven GearDrive Gear

Why Use Gears?1. Change the plane of rotation. 2. Increase or decrease the speed of

applied motion. 3. Increase or decrease the applied

force. 4. Provide a drive without slippage5. Split power6. Move power from one point to

another

Two equal-sized gears

• When 2 interlocking gears have the same number of teeth, they turn at the same speed, but in opposite directions.

Two different-sized gears

• When 2 gears have different numbers of teeth, they turn at different speeds.

• If a small gear turns a large gear, the small gear must turn more than once to cause the large gear to complete one turn. So, the large gear turns more slowly than the small gear.

• Gears can be used to change the speed of motion.

Common Gear Materials

• Cast iron• Steel• Brass• Bronze alloys• Plastic

Different Types of GearsSpur Gears• The teeth meet on a flat surface • They can change the speed and direction of

motion• Ex. Car transmissions

Different Types of GearsBevel Gears• 2 gears connect at an angle• They are often cone shaped• They can change the speed and direction of

motion.• Ex. Egg Beaters

Different Types of Gears

Worm Gears• An axle or shaft has a

screw thread that connects with another gear.

• This system is often used to reduce the speed and change the direction of the motion.

Different Types of Gears

Rack and Pinion Gears• A single gear, called a pinion, meets with a

toothed rack.• The rack may slide or stay in one place.• The system changes circular motion into a

straight line. • Ex. Railroads

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Nomenclature of Spur Gear Teeth

= (tooth spacing)driven gear – (tooth thickness)driver , measured on the pitch circle.

Backlash

Pitch circle gear diam.

Fillet radiusClearance

Base Circle

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Fundamental Law and Involute Curve

Generation of the involute curve

Tangent at the point of contact

rG

rP

rG / rP = constant (constant speed ratio) All common normals have to intersect at the same point P

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Useful Relations

P = N / d

P = diametral pitch, teeth per inch

N = number of teeth

d = pitch diameter (gear diameter)

m (module, mm) = d / N

Metric system

p (circular pitch) = πd / N

Pp = π

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Standard Tooth SpecificationsPressure angle

Two mating gears must have the same diametral pitch, P, and pressure angle, φ.

Pitch line

Line of centers

Line of

action

Base circle

Base circle

Pitch circle

Pitch circle

Pressure angle φ

Standard pressure angles, 14.5o (old), 20o, and 25o