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Helical gears – introduction
There are two particular reasons for using helical gears. The first is that they can be used instead of spur gears but will run more quietly. Using two helices like this does produce an end thrust, which does not happen with spur gears. This is greater the greater the helix angle.The second is that they can be used instead of bevel gears to have two gears meshing without their axes being parallel.
A key feature of a helix is the helix angle. This is measured from a line drawn on the surface of the material but parallel to its axis. A spur gear can be seen as being a helical gear with a helix angle of 0°.
A helix can be right or left-handed. A right-handed one has a twist as found on a traditional corkscrew.
Where two helices run with parallel shafts for them to mesh the handedness of one will be the opposite handedness of the other. Whatever the number of teeth on each gear the helix angles must be the same. In general the quietness achieved by using helical gears is greater the helix angle. But most of the benefit is achieve by using a helix angle of about 20°.
Helical gears cost more to produce than spur gears. Where noise has to be minimised, for example, on a car, helical gears might be used. Where noise is not considered to be so important, for example, on a lathe the gears will usually be spur gears.
Where helical gears are used and the shafts are at right angles, one gear will have the same handedness of the other. In this case if the helix angle of each gear are added together they will add up to 90°. Consider two helical gear with a helix angle of 45 degrees. If both are of the same hand, they will mesh when they are at right angles to each other
For a helical gear of a given diameter the number of teeth it has, for a given size of teeth, will depend on the helix angle. This means that for two gears of the same diameter the ratio between them can vary according to the difference in the helix angles.