Profile Shift
Profile shift moves the tool reference profile radially out of the blank (positive shift) or into it (negative shift) by the amount x·m during gear cutting. It prevents undercut in gears with few teeth, increases tooth root strength, and allows the center distance between two gears to be specifically adjusted.
Profile Shift Coefficient x
The profile shift coefficient x indicates by how many modules the tool reference profile is shifted. A positive profile shift (x > 0) thickens the tooth root, reduces the fillet, and thereby increases bending strength. It is particularly needed for gears with few teeth (z < 17 at 20° pressure angle) to avoid undercut. A negative profile shift (x < 0) reduces the center distance and can be used when geometric boundary conditions require it.
Effect on Center Distance and Operating Pitch Circle
When two gears with profile shifts (x1 + x2 ≠ 0) are paired, the operating center distance changes relative to the standard center distance. The reference circles remain unchanged (d = m·z), but the operating pitch circles move apart (positive sum) or together (negative sum). The operating pressure angle then deviates from the standard pressure angle of 20°. In practice, profile shift is used deliberately to match gears with a standardized module to a specified center distance.
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