Gears

Gears were used by humans nearly befor 2,300 years. One of the oldest known account of gears is from the writings of Greek philosopher Aristotle (384~322 BC) in which he describes it. Approximately 100 years later mathematician Archimedes drew a diagram of a hoist that was driven with a set of worm and worm wheel which is shown in fig below. Also Leonardo da Vinci (1451~1519), has drawn a number of gear types that exist today in his numerous drawings.

When gears are used ?

When transmitting power from a source to the required point of application, a series of devices are used including gears, belts, pulleys, chains, hydraulic and electrical systems. Generally if the distances of power transmission is large, gears are not suitable and chains and belts can be used. But when a compact, efficient or high-speed drive is required gear trains offer a competitive and suitable solution. Additional benefits of gear drives include reversibility, configuration at almost any angle between input and output and their suitability to operate in adverse conditions.

Gear

Gears are toothed cylindrical wheels used for transmitting mechanical power from one rotating shaft to another.

Primitive gear trains consisted of teeth or pegs located on discs. The disadvantage of these simple teeth is that the velocity ratio is not constant and changes as the teeth go through the meshing cycle causing noise and vibration problems at elevated speeds.

Primitive Gears - the right side image shows a gear set up found at Ancient gears at the Ctesiphon archeological site in Iraq

The problem is solved by using a profile on the gear teeth, which gives a constant velocity ratio throughout the meshing cycle. Several different geometrical forms are used, but the full depth involute form is primarily used in current engineering practice.

Classification of Gears

  1. Parallel axis gears:

    • spur gears

    • helical gears

    • internal gears

  2. Non-parallel, coplanar gears (intersecting axes)

    • bevel gears

      • Miter Gear

      • Straight bevel gear

      • Spiral bevel gear

    • face gears

  3. Non-parallel, non coplanar gears (non intersecting axes):

    • crossed axis helical

    • cylindrical worm gearing

      • single enveloping worm gearing

      • double enveloping worm gearing

    • hypoid gears

  4. Special gear types:

    • square and rectangular gears

    • elliptical gears

The following figure gives a basic idea how the power transmission is carried out in various types of gears.

Gear & Pinion

In a pair of meshing gear, the smallest one (also having less in number of teeth) is called the pinion and the larger one (also having high number of teeth) is termed as gear. In general a prime mover shaft is connected with pinion and the gear is connected with output shaft.

Gear Materials matches

Standards for Gear Quality

The allowable amounts of variations of the actual tooth form from the theoretical form are specified by the AGMA as a quality number. Detailed charts giving the tolerances for many features are included in AGMA Standard 2000-A88. The quality numbers range from 5 to 15 with increasing precision.

The Intemational Organization for Standardization (ISO) defines a different set of quality numbers in its Standard 1328-1-1995 and 1328-2-1997. These standards differ greatly from the AGMA Standard. One major difference is that the quality numbering system is reversed. Whereas in the AGMA Standard higher numbers indicate greater precision, in the ISO standard lower numbers indicate greater precision.

Gear comparison

The following table compares the various types of gears and its features.