Metric Worm Gear Pairs

KHK USA - Supplier of Premium Metric Worms & Wormwheels

Metric Worm Gears - Large Speed Reduction With Small Volume

Worm gear pairs are made up of a worm and a worm wheel. These gears allow for a very large reduction ratio in a single pair. They are very quiet and provide smooth transmission of power. Due to the situation of being a friction drive mechanism, worm gear pairs have very poor efficiency. Similar to helical gears, worm wheels are also cylindrical disks which have involute shaped teeth cut into their face at an angle. For worm gear pairs, this is the lead angle and it is identical to the angle cut into the worm. The reduction ratio of a worm gear pair is developed as the ratio of the number of teeth on the worm wheel / the number of starts on the worm. As with helical gears, worm gear pairs must have the same module, pressure angle and direction of lead angle in order to mesh. Our worm gear pairs are offered in many materials, modules, numbers of teeth and reduction ratios. All of the worm wheels that we offer allow for secondary operations such as opening the bore, adding of keyways, adding of tapped holes, and the reduction of the hub diameter to be performed. Most of the worms that we offer allow for secondary operations such as opening the bore, adding of keyways, adding of tapped holes, and the reduction of the hub diameter or shaft 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.

Worm gears are one type of gears that transmit motion between non-intersecting and non-parallel shafts. Their shaft angles are generally 90 degrees. Historically, worm gears were already recorded by Archimedes around 250 B.C.

There are roughly two types of worm gears. One is a worm in cylindrical form meshing with a worm wheel, the pair comprising a "cylindrical worm gear", and the other an hourglass form worm and its mating worm gear called "hourglass worm gear". The worm is generally cylindrical and therefore, unless there is a special need, cylindrical worm gear is simply called worm gear. Also, a worm with two or more threads are called multiple thread worm.

If a worm gear is used, a large speed reduction can be obtained in a compact space compared to using spur gears. Generally, the worm is used to turn the worm wheel. However when the lead angle is especially small, the worm wheel can not turn the worm and it is called its self locking feature. This can be used effectively in some applications but it can not be called perfect preventative for reverse driving. (When the lead angle is large, it is possible to turn the worm with the worm wheel.) In addition, there are many advantages to worm gears such as low noise and vibration, but because of the large sliding at the tooth surfaces (sliding contact), the efficiency is low. (For cylindrical worm gears, the general efficiency is about 30-90%.)  Also, there are disadvantages such as they tend to suffer from tooth surface heat seizure.

Ordinarily the lead angles of the right and left tooth surface of a worm are the same. However in duplex worm gears, they are made so that the tooth thickness of the worm changes continuously in the axial direction so that by shifting the worm axially with shims, it becomes possible to adjust the backlash.

As for the materials for worm gears, for worms: structural carbon steel (S45C, etc.), structural alloy steel (SCM415, etc.), stainless steel and cast iron are used; and for worm wheels: cast iron (FC200, etc.), nonferrous metals such as phosphor bronze and specialty bronze (nickel bronze, aluminum bronze, etc.), engineering plastics such as MC Nylon, etc.  Because the combination of materials for worms and worm wheels affects the allowable transmission force and lubrication methods, the choice of mating materials requires careful consideration. Also, related to the friction of teeth, a harder material for worm than worm wheel is generally used.

The manufacturing methods of worms are roughly divided among cutting, heat treated and ground after cutting and rolling.  And for worm wheels, they can be roughly divided among cutting teeth, cutting teeth after casting, and teeth cutting after the outside rim is cast around the center of the blank.

Worm Gears Categories

The simplest way to obtain a large speed reduction with high torque in a compact space is with worm gear drives. KHK stock worms and worm wheels are available in modules 0.5 to 6 and in speed ratios of 1/10 to 1/120, made in a variety of m0aterials and styles. We also offer stock duplex worms and worm wheels with which you can obtain a very low backlash, high rotational precision system.