Many factors result in the gears during operation not having optimum contact along the whole tooth. This is partly due to static effects such as manufacturing inaccuracies in the gearbox, on the shafts and gears. And partly due to dynamic effects causing the shafts, teeth, gearbox etc. to deflect during operation. You can partly compensate for these inaccuracies and deflections by making tooth modifications.
In order to compensate for deflection of the teeth and to ease the tooth engagement the tooth profile is typically adjusted by a tip relief and a foot relief or by a smooth arc all over the tooth from bottom to top. A combination of all of these is also possible.
In order to compensate for non-parallelism and deflection of shafts and gearbox or torsion of the shaft, you can adjust the tooth flanks by either end reliefs, a smooth curve along the length of the whole tooth, a tapered tooth or a combination of these.
It must be mentioned that tooth modifications are always a compromise because the surface pressure also increases concurrently with the increased modifications. Therefore, the target is to maximize the engagement ratio during the largest effect imposed on the gear by making some tooth modifications so that the gears barely do not have edge contact.
In almost all cases we have good experience of it being an advantage to use tooth modifications to a limited extent. With our new and very precise CNC-controlled gear grinding machines we can usually offer such tooth modifications without extra costs. In case of very large modifications or very fine tolerances it will have an effect on the cost.