Meaning
Cumulative measurements of circular distance between corresponding points on adjacent teeth reveal inaccuracies in gear geometry. Pitch deviation measures the displacement of a tooth profile from its theoretical position along the pitch circle. Errors in this metric often lead to noise and vibration when the gear rotates under load.
While small variations are expected in moulded parts, large errors indicate problems with the cooling rate or the initial tool machining.
Tooling Precision
Accuracy of the wire EDM or grinding process used to create the mould cavity sets the baseline for the part. Any error in the steel will be replicated in every plastic part. Regular inspection of the tool helps identify wear before it affects the pitch.
Shrinkage Distortion
Uneven contraction of the plastic as it solidifies often pulls the teeth away from their intended positions. This phenomenon is particularly common in resins with high shrinkage rates like polyoxymethylene. Differential cooling, where one side of the gear cools faster than the other, introduces an asymmetric error.
Correcting this requires careful gate placement and balanced cooling channels within the mould. These adjustments ensure that the plastic shrinks toward the center of the gear uniformly.
Transmission Error
Variations in the spacing of teeth cause the driven gear to speed up and slow down during a single revolution. This jerky motion creates high frequency sounds and premature wear on the tooth surfaces.
