Helical Gears – excel in quiet operation
Helical gears are one type of cylindrical gears with slanted tooth trace. Compared to spur gears, they have the larger contact ratio and excel in quietness and less vibration and able to transmit large force. A pair of helical gears has the same helix angle but the helix hand is opposite.
When the reference section of the gear is in the normal plane, by tilting the hobbing tool, the spur gear hobbing machine and hobbing tool can be used to produce helical gears. Because of the twist of teeth, their manufacturing has the disadvantage of more difficult production.
The helical gears made by KHK can be classified into two groups by the reference section of the gears being in the rotating plane (transverse module) and normal plane (normal module). If the reference section is in the rotating plane, the center distance is identical to spur gears as long as they are the same module and number of teeth. This allows for easy swapping with spur gears. However, in this case, they require special hobbing cutters and grinding stones, leading to higher production cost. On the other hand, if the reference section is in the normal plane, it is possible to use spur gear hobbing tools and grinding stones. However, the same module and number of teeth in spur gears no longer match the center distance of helical gears, and swapping becomes very difficult. In addition, the center distance is usually not an integer.
While spur gears do not generate axial thrust forces, because of the twist in the tooth trace, helical gears produce axial thrust force. Therefore, it is desirable to use thrust bearings to absorb this force. However, combining right hand and left hand helical gears making double helical gears will eliminate the thrust force.
Helical gears are often used in automotive transmissions by replacing spur gears.
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Radial Module and Normal Module
For involute helical gears, there are ones using the normal (perpendicular to teeth) module system and others using radial (perpendicular to shaft) module system which have differing tooth shape reference planes.
As the standard helical gears, KHK offers two line-ups of radial module system KHG series and normal module system SH series and we will list the concrete differences between the two below.
First, the normal module system helical gears have the advantage of being able to use the same tooth cutting tools such as hobs and grinding stones as spur gears. In other words, compared to new production of later discussed radial module helical gears, they can be made more economically. On the other hand, because of the helix angle, compared to the same module and number of teeth spur gears, the pitch circle diameter becomes large, and to replace spur gears with the same module and number of teeth helical gears cannot be done without changing the center distance which is a disadvantage of the normal module system. It is also difficult to maintain the center distance to the easily manageable integer numbers. The advantages and merit of the radial module system is the reverse of the normal module system. The first advantage that can be cited is, because the actual value of the pitch becomes small compared to the normal pitch even with the helix angle, it is possible to replace them with spur gears of the same module and number
of teeth while maintaining the same center distance. As for a disadvantage, because the actual radial pitch changes with each helix angle, it becomes necessary to obtain hobs and grinding wheels for each helix angle which potentially leads to an increase in manufacturing costs.