TERMS RELATED TO GEARS

TERMS RELATED TO GEARS

Base Circle.

It is the circle from which involute form is generated. 

Only the base circle on a gear is fixed and unalterable Pitch Circle. It is an imaginary circle most useful in calculations.

 It may be noted that an infinite number of pitch circles can be chosen, each associated with its own pressure angle.

Pitch Circle Diameter (P.C.D.).

 It is the diameter of a circle which by pure rolling action would produce the same motion as the toothed gear wheel. This is the most important diameter in gears.

Module.

 It is defined as the length of the pitch circle diameter per tooth. Thus if P.C.D. of gear be D and number of teeth N, then module (m) = DIN. It is generally expressed in mm.

Diametral Pitch.

 It is expressed as the number of teeth per inch of the P.C.D.

Circular Pitch (CP.).

 It is the arc distance measured around the pitch circle from the flank of one tooth to a similar flank in the next tooth. .-. CP. = nDIN = nm

Addendum.

 This is the radial distance from the pitch circle to the tip of the tooth. Its value is equal to one module.

Clearance.

 This is the radial distance from the tip of a tooth to the bottom of a mating tooth space when the teeth are symmetrically engaged. Its standard value is 0.157 m.

Dedendum.

 This is the radial distance from the pitch circle to the bottom of the tooth
space.
Dedendum = Addendum + Clearance = m + 0.157 m = 1.153 m.

 Blank Diameter.

 This is the diameter of the blank from which gear is out. It is equal to P.C.D. plus twice the addenda.
Blank diameter = P.C.D. + 2m = mN + 2m = m(N + 2). 

Tooth Thickness.

 This is the arc distance measured along the pitch circle from its intercept with one flank to its intercept with the other flank of the same tooth.
Normally tooth thickness = C.P./2 = nm!2

Face of Tooth.

 It is that part of the tooth surface which is above the pitch surface.

Flank of Tooth.

 It is that part of the tooth surface which is lying below the pitch surface.

Line of Action and Pressure Angle.

 The teeth of a pair of gears in mesh, contact each other along the common tangent to their base circles. This path is referred to as line of action. As this is the common generator to both the involutes, the load or pressurebetween the gears is transmitted along this line. The angle between the line of action and the common tangent to the pitch circles is therefore known as pressure angle <|>. The standard values of <)> are 14.5° and 20°.

Base Pitch.

 It is the distance measured around the base circle from the origin of the involute on the tooth to the origin of a similar involute on the next tooth.

pitch point.

It is the length of teeth in axial direction.

Backlash

It is the error in motion that occurs when gears change direction. It exists because there is always some gap between the trailing face of the driving tooth and the leading face of the tooth behind it on the driven gear, and that gap must be closed before force can be transferred in the new direction. The term "backlash" can also be used to refer to the size of the gap, not just the phenomenon it causes; thus, one could speak of a pair of gears as having, for example, "0.1 mm of backlash.