Gear Deviations - Pitch Deviation
The expression 'error' was generally used for any kind of deviation inside or outside tolerance limits. This has been replaced by the term 'deviation'. Deviation can be defined as 'the difference between the actual dimension and the specified dimension'.
Except for the base pitch deviations, which are measured in base tangent planes, all pitch deviations are measured on a circle concentric to the gear axis at approximately mid-depth of the teeth.
Single pitch deviation fpt:
The algebraic difference between the actual pitch and the corresponding theoretical pitch in the transverse plane, defined on a circle concentric with the gear axis at approximately mid-depth of the tooth.
Normal pitch deviation fpn:
Sometimes, when a portable comparator suitable only for checking in the normal planes is used, the normal pitch deviation instead of the (transverse) single pitch deviation is measured.
Base Pitch deviation fpb:
The algebraic difference between the actual value and the specified value of the base pitch. Pb. If necessary, an appropriate index is added to indicate whether the deviation refers to the normal section (fpbn) or to the transverse section (fpbt).
The result of base pitch measurement is not influenced by any chance eccentricity of the gear teeth.
Mean base pitch deviation fpbm:
The algebraic difference between the arithmetic mean value of the base pitches measured on a number of equi-spaced corresponding flanks around the gear and the specified base pitch pb.
It is often employed to determine the mean profile slope deviation or the base diameter deviation, especially when no profile inspection is undertaken.
Adjacent pitch difference fu:
(no algebraic sign) is the difference between the actual measured values of two consecutive transverse pitches of right or left flanks; it is equal to the difference between the transverse pitch deviations of two consecutive flanks.
Cumulative Pitch Deviations
Cumulative pitch deviation Fpk:
The cumulative pitch deviation over a sector of k pitches is the algebraic difference between the actual length and the theoretical length of the relevant arc. It is equal to the algebraic sum of the single pitch deviations of the same k pitches.
The purpose of tolerance specifications for this kind of deviation is to assure that in service conditions no substantial acceleration and dynamic loads are generated.
ISO quality standards contain formulae giving values for maximum permissible cumulative pitch deviations Fpk for sectors of pitches with k ranging from 2 to z/8 pitches (1/8 of circumference).
Total cumulative pitch deviation Fp:
The total cumulative pitch deviation is the maximum cumulative pitch deviation of any sector of the corresponding flanks of a gear.It is the difference between the actual and the theoretic length of the arc between those two corresponding flanks which - with respect to the theoretic circular division - deviate furthest, whether in the positive or negative sense, from their theoretic position.In inspection practice, the total cumulative pitch deviation is often derived by sequentially adding the transverse pitch deviations fpt: the distance between the highest and the lowest points on the graph thus formed corresponds to the total cumulative pitch deviation .
Pitch sector deviation fpS:
Cumulative pitch sector deviation FpkS:
Total cumulative pitch sector deviation Fps:
For economic reasons and - if comparator measurement is applied - also for reasons of accuracy, the cumulative deviations of large gears are often determined by measurement steps covering a sector of several pitches.
