Helical Gears

Helical Gears - Their Teeth Are Twisted Along A Helical Path In The Axial Direction

Helical gears are cylindrical disks which have involute shaped teeth cut into their face at an angle. KHK's stock helical gears are quiet, compact and economical. They are suitable wherever you require high-speed rotation including in machine tools, speed reducers and other industrial machinery. Our helical gears are offered in alloy steel and carbon steel, with many configurations of modules and numbers of teeth. All of the helical gears that we offer allow for secondary operations such as opening the bore, adding of keyways, adding of tapped holes, but only a few allow for the reduction of the hub diameter to be performed. Our offering also includes products which have already had some of these secondary operations completed. These products are identified by a "J" in their part number and they are available within 10 calendar days.

Helical gears belong, by the gear shaft classification, to parallel axes gears and are cylindrical gears in which the teeth are twisted as a helix line.

The shape of a helical gear is given by stacking infinitesimally thinly sliced spur gears and twisting them uniformly around the shaft.  Differentiating by the direction of the twist, the right-hand and the left-hand helical gears are obtained.  Also, for a pair of helical gears, the twist angles are the same but are of the opposite hands.

By slanting the teeth, helical gears can obtain a large contact ratio and the gradual change to the contact surface load which give helical gears their characteristic of smooth transmission of rotation. Consequently, compared to spur gears, helical gears have less vibration, are quieter and are suitable for high speed rotation applications and can transmit high load.

On the other hand, the use of helical gears produces axial thrust, which is a drawback, and requires countermeasures such as installing thrust bearings.  For that reason, in order to eliminate thrust, there exists such items as herringbone gears that combine the right and left hand gears into one body.

On the teeth pitch surfaces of helical gears, there exist two non-interchangeable systems: the normal to tooth system (normal module) and perpendicular to gear shaft system (transverse module). The normal module system has some advantages such as these gears can be made with the same teeth cutting hobs and grinding stones as used for spur gear production.  On the other hand, there are disadvantages such as when replacing them with spur gears, the center distance changes and it is difficult to make the center distance an integer. The transverse module system has the advantage of being able to replace helical gears with spur gears of the same module and the number of teeth without changing the center distance. However, they have the disadvantage of requiring specialized hobs and grinding stones for production.

Incidentally, if using a pair of helical gears, they transmit rotational motion, but by pairing with a gear rack having straight slanted tooth form, they can also be used in linear motion applications.  Also, by meshing helical gears with off-set shafts, they can be used as screw gears.