Gearbox Terminology

By | April 10, 2017


Pitch circle:

 It is the imaginary circle that rolls without slip with a pitch circle of a mating gear .

► Pitch Point:

  It is the point of tangency of two pitch circles and is on the line-of-centers. The pitch point of a tooth profile is at its intersection with the pitch circle.

Circular pitch:

  It is the distance measured on the circumference of the pitch circle from a point on the tooth to the corresponding point on the next tooth. Mathematically,

  circular pitch, pc=∏D/T,


  D= P.C.D.

  T= No. of teeth


  It is the ratio of the pitch circle diameter in millimeters to the number of teeth.

  Mathematically, Module, m=D/T

  The recommended series of the modules in Indian standard are

  1, 1.25, 1.5, 2, 2.5, 3, 4, 5, 6, 8, 10, 12, 16, 20, 25, 32, 40, 50.


  It is the distance between the tooth space and the tooth thickness of mating gear as measured on the pitch circle.

  Purpose of backlash:

   prevent gears from jamming together

   making contact on both sides of their teeth simultaneously.

   Lack of backlash may cause noise, overloading, overheating of the gears and bearings, and even seizing and failure.

  Excessive backlash is objectionable, particularly if the drive is frequently reversing, or if there is an overrunning load as in cam drives. Excess backlash causes noisy operation.

  Backlash does not affect involute action.

Pressure Angle:

  If a tangent is drawn to the involute profile of a tooth at any point on the curve, and if a radius line is drawn through this point of tangency connecting this point with the centre of the gear, then the acute angle included between this tangent and radial line is defined as pressure angle at that point.

  In gear design, the pressure angle is measured at the pitch point…



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