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Introduction to helical gears
Helical are very similar in many ways but they have several advantages over spur gears and so are worth making though this is a bit more complicated.
The main difference is that whereas the teeth on a spur gear fare straight on a helical gear the teeth form a helix.
One advantage of this is that helical run more quietly. Another advantage is that whereas spur gears always have to run with their axles parallel to each other, two helical gears can work with each other with their axles at any angle to each other.
One problem is that because of the helixes there is a thrust along the axle the gear is one whereas with spur gears there is no end thrust.
This means that there are two cases of helical gear of interest to us. There are helical gear are are used because they are quieter. An example of this would be most of the gears in a car’s gearbox. In this case all of the gears have a helix angle of about 15-20º. This is about the maximum required to get the best quietness without the problem of excessive end thrust.
The other advantage of helical gears is that they can work with their axles at any angle. In practice there is one particular case that is very useful and is still simple to make. This is where the axles are at right angles to each other.
Two gears with shafts parallel
For two helical gears to work with their shafts parallel they must both have teeth with the same helix angle. If the gears have different numbers of teeth then their helix angles will still be the same but their leads will be different. In any case, the helix angle of one will be the opposite of that of the other.
For two gears whose shafts are at right angles
In this case the helix angles of both gears must add up to 90 degrees. The handedness of both must be the same. If the gears have different numbers of teeth then their helix angles will still be the same but their leads will be different. For example, two gears with helix angles each of 45 degrees and both are right hand helices will run together with their axes at right angles to each other.
Making helical gears
Helical gears can be made in a number of ways:
They can be cut on a horizontal milling machine using a standard Brown and Sharp cutter. But in this case the milling table has to be swivelled to create the helix angle. This is what milling machines were traditionally designed to do which is why it is fplaced first here.
They can be made on a vertical milling machine. In this case the cutter is fitted on a stub arbor held in the vertical socket. The helix angle is created by tilting the vertical head. This the way most people would make helical gears.
Helical gears can be made by a process best called rotary copying.
Helical gears can be made by hobbing.