With a profile shift, the tool profile is shifted outwards by a certain amount during gear cutting. The animation below shows the effects of a profile shift on the tooth form of a gear with 8 teeth. It becomes clear that as the profile shift increases, the undercut becomes smaller and can even be completely avoided.
Even if the tooth shapes differ from each other, the teeth can still mesh with each other. Profile shifted gears (also called corrected gears) can therefore be easily paired with non-profile shifted gears (so-called standard gears) as long as they are manufactured with the same tool and therefore have the same module.
Even if this may not seem so at first glance, a profile shift has no influence on the shape of the tooth flank itself. All profile shifted gears use the same involute for the tooth shape compared to their corresponding standard gears. Only another part of the same involute is used. This becomes clear when the tooth flanks of the gears with different profile shifts are placed on top of each other.
Note that the base circle for constructing the involute is determined solely by the flank angle of the tool profile (standard pressure angle) during gear cutting. And since the angle of the cutting edges does not change with a profile shift, the base circle and thus the involute do not change either.
The radius of curvature of the involute increases with increasing length, i.e. the further away the involute is from the base circle, the larger the radius of curvature is and the less strongly it is therefore curved. The flank shape at this more distant area is rather “flat” than “pointed”. The smaller curvature leads to a larger contact surface of the flanks, which reduces the pressure accordingly (less Hertzian contact stress). This reduces the stress on the flanks and thus increases the flank load-bearing capacity.
The animation below shows the profile shift of a gear with 6 teeth to avoid an undercut. In this case, the thickness of the tip tooth even decreases so much that the involutes taper before the shifted tip diameter is reached. The increase of the tip circle radius by the amount of the profile shift cannot therefore be maintained in this case, the tip diameter is inevitably shortened.
In addition, the tip circle would have to be shortened again to at least 0.2 times the module in order to increase the thickness of the tip tooth. However, such a large reduction of the tip circle would also result in a correspondingly large reduction of the line of action. Involute gears with fewer than 7 teeth should therefore be avoided by any means.
With corrected gears, an extended part of the involute is used as tooth flank compared to standard gears. When meshing with another gear, this further curved part of the involute requires the center distance to be increased by the amount of the profile shift V= x⋅m.
In summary, it can be stated that a profile shift is always applied if:
- An undercut is be avoided,
- The tooth strength must be increased,
- The surface pressure at the flanks is to be decreased, or
- The center distance must be adjusted.