Helical Gears – excel in quiet operation

SH helical gears

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.

Related links :
“Raw Material” and “Gear Precision Grade” Equivalent Tables
The ABC’s of Gears / Basic Guide – B
Introduction to Gears
Gear Technical Reference
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What are Helical Gears ?

We will re-confirm here the characteristics of helical gears.

Kinds of helical gears :

Having cylindrical pitch surface, gears having outer shapes of round rod, cylinder and circular plate are generically called cylindrical gears. Helical gears are one kind of cylindrical gears. According to the gear classification by shaft positions, helical gears belong to parallel shaft gears. Following are the broad categories of helical gears.

  • General helical gears
  • Herringbone gears
  • Screw gears


Characteristics :

Compared to spur gears, helical gears are stronger and, with less noise and vibration, can transmit rotation and power quietly. They are widely used in automobiles, industrial machinery, speed reducers, etc. Because of the twist of the teeth, they create thrust force. It takes more labor to manufacture. There are right hand and left hand twist, requiring caution in their use.


Helical gears belong to parallel shaft gears and the meshing is almost all rolling contact so that their general efficiency is high at 98-99.5%.

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.