Helical gear can be used for transmitting motion between parallel shafts as well as between perpendicular (or any other angle) shafts. Helical gears used for transmission at any angle are usually called spiral gears. When gear teeth are cut on a spiral that wraps around the cylinder they are designated as helical. Helical teeth enter the meshing zone progressively and therefore have a smoother action than spur gear teeth and tend to be quieter. In addition, the load transmitted maybe somewhat larger or the life of gears may be greater for the same loading, than within equivalent pair of spur gear conversely, in some cases smaller size Helical gears (compared with spur gears) may be used to transmit same loading. The efficiency of the helical gear set, which is dependent on the total normal tooth load (as well as the sliding velocity and friction co-efficient etc.) will usually be slightly lower than for an equivalent spur gear set. For external parallel axis helical gears to mesh they must have same helix angle but be of different hand.

Helix angle form only a few degrees up to about 45° are parallel. As the helix angle increases from zero. In general the noise level is reduced and the load capacity is increased. At angle much above 15° to 20° however drop off. This is due to the fact that the transverse tooth thickness decreases rapidly. Herring Bone or Double Helical gear are frequently used to obtain the noise benefit of single gear without the advantage of thrust loading. The terms double Helical and Herring bone are sometimes used interchangeability actually herring Bone is more correctly used to describe double helical teeth that are cut continuously into a solid blank. The double helical gears may be cut integral with the blank, either in line or staggered, two separate parts may be assembled the double. Herring bone gear having a small groove between two helices. The groove is required for hobbing and shaving or grinding operation. The construction results in equal and opposite thrust relations, balancing each other and imposing no thrust load on the shaft. This types of gears are used only for parallel shaft.

Crossed Helical Gear:

Crossed Helical gear connect skew shafts. Both members may be helical gears or one may be spur gear according to the chosen relationship between the shaft angle and the tooth helices. The efficiency of the helical gears is quiet good though not quiet equal to that of simple spur. This is due to that of simple spur. This is due to the fact that all other things, being equal, the normal (or total) tooth load on a helical gear is higher than that on a spur for an equivalent tangential load.

Advantages of Helical Gear transmission over Spur gear transmission:

1. Helical gear transmission gives comparatively noiseless motion at higher surface speeds.
2. Helical gear transmission gives comparatively smooth transmission at small gears with fewer teeth at large transmission ratio up to 15: 1.

Use of Helical Gears

The teeth of these gears are cut at an angle with the axis of the shaft. Helical gears have an involute profile similar to that of spur gear. However, this involute profile is in a place which is perpendicular to the both elements. The magnitude of the helix angle of pinion and gear is same, however the hand of helix is opposite. A right hand pinion meshes with a left hand gear and vice versa. Helical gears impose radial and thrust loads of the shaft.