Helicon and Spiroid gears have many teeth in simultaneous contact. This, coupled with the fact that each pinion tooth contacts its mating gear tooth along a line perpendicular to the sliding velocity, leads to a number of important advantages.
The number of teeth in contact depends on the number of teeth in the gear member. Generally, 10% of the gear teeth are in simultaneous contact. However, even in the low ratio range, there are two to three times the number of teeth in contact than in worm gears. On higher ratios, there are many times more.
For any gear of a given diameter, a higher ratio means more teeth – which means finer pitch. Since contact between the Helicon or Spiroid pinion and gear extends over the entire length of the pinion, a finer pitch or shorter lead gives a proportionately increased number of teeth in simultaneous contact, and thus, very little sacrifice in capacity. With worm gears, for instance, the upper ratio limit is usually about 80:1. After selecting a pitch sufficient to carry the required load (bearing in mind that, at most, the worm set has two teeth in contact) the gear becomes too large. Therefore, in the case of worm gearing, higher ratios are usually handled by multiple reductions.
Helicon or Spiroid gears, however, do not have an upper limitation. For a given gear diameter, a higher ratio means a shorter lead, and finer pitch, but more teeth in contact. As a results, single reduction ratios of 400:1 are possible.