Gears and Speed Reduction

2 minute read

Gears and Speed Reduction

Gears

What is the benifit of gears? It can be used to increase torque and decrease speed or decrease torque and increase speed. It is equivalent to an electrical transformer.

By reducing the speed of the load we can also increase encoder resolution.

Note that it will only increase resolution if the encoder is directly connected to the motor.

Specifications

Gears have a pitch circle which is where the two gears interface each other tangantly. The module is the size of the teeth, it is the distance between the tip of the teeth and the pitch diameter. The pitch diameter is given by number of teeth multiplied by the module. The outer diameter is pitch diameter plus 2 module. The face width is the depth, which affects how much force it can handle.

Spur Gear

For two pinion gears, the ratio is given by -B/A (the direction is oppoite). For one ring gear and one pinion gear, the direction of rotation is respected.

Planetary Gear

A good thing about planetary gear is a coaxial shaft. The planetary gear consists of the sun gear, ring gear and planet gears that goes in between. The ratio is given by 1 + R/S. Where R is the number of teeth in the ring, and S is the number of teeth is the sun.

It can be used in very high ratio applications.

There exists an assembly criteria in order to make planetary gears. In particular:

  • (R-S)/2 must be an integer (this ensures that there is integer number of teeth)

  • (R+S)/(N) must be integer (this ensures that teeth aligns correctly for meshing)

Where N is number of planets.

Backlash

There exists a discrepancy between the input and output.

Other Options

Belts and Chains

These are good for positive ratio. A chain is difficult to make and use, but allows high torque. The V-belt is cheap, however they slip easily. For less slippage, a timing belt is used.

Capstan cable Drive

  • Negative Ratio

  • Tensioner Required

  • Very efficient

  • No slip

  • Limited motion range

Design

  • Consider accuracy of manufacturing

  • More teeth gives more efficiency. Usually the minimum is 12.

  • Consider radial distance

  • Avoid cantilever support

  • WD 40 helps in gear assemblies

For accurancy of the tooth module, the waterjet and the 3D printer can handle at least module 2, the object can handle module 1.

  • The load capacity (strength) is given by:

    • Tooth module (size)

    • Face width (depth)

    • Material

    • Number of teeth

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