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. Formation of Helical Gears
If several cuts are made on a spur gear perpendicular to its axis and arranged by rotating each cut piece slightly as in a staircase as shown in Figure 3.1, then the meshing becomes smoother due to contacts occurring in phases. Overall the contact ratio increases so that even when the normal spur gear has a small number of teeth and the contact ratio is less than 1, this arrangement (called stepped gear) allows transmission.
Figure 3.1 Stepped Gear
In a stepped gear, if the division is made infinitely narrow and the tooth line is uniformly twisted relative to the shaft direction, in other words, helicoids with a shaft in its center, it becomes a helical gear as shown in Figure 3.2.
Figure 3.2 Helical Gear
Helical gears are able to transmit rotation smoothly with less vibration and noise and therefore are suitable for high speed rotation. However, since the force perpendicular to the tooth surface (normal load) contains a component in the direction of the gear axis, it creates axial thrust requiring a thrust bearing. To balance the thrust forces within a gear, there are gears which combine helical gears with an opposite twist angle called double helical gear or herringbone gear (liking the shape to bones in a herring) as shown in Figure 3.3 and 3.4. In contrast, the regular helical gears are sometimes called single helical gears. Helical gears are often used in marine applications’ large gears.
However, if there is any error in phasing of right and left teeth arrays or if the helical gears are not meshing correctly, the gear drive creates alternating thrust in the axial direction causing vibration. Also, if there are directional errors in the relative tooth lines between the driver and the driven wheels, with helical gears, it is possible to make corrections by adjusting the position of the bearings, but with herringbone gears this is not possible. For these reasons, Maag Gear Company and others recommend using hardened and ground single helical gears for large gears in ships.
Figure 3.3 Herringbone Gear
Figure 3.4 Kinds of Herringbone Gears
Instead of helical gears, circular arc curved tooth as shown in Figure 3.5 is sometimes used.
Figure 3.5 Circular Arc Curved Tooth Gear
Efficiency of Helical Gears
Helical gears are parallel shaft gears and their mesh is almost all rolling contact, thus their general efficiency is high, ranging from 90-99.5%.
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.