For many months, there has been a debate regarding whether the United States would be better run by one of two political parties. Those on the left insisted that the policies of the right were wrong and those on the right insisted that the policies of the left were wrong. In gearing, left and right both exist and need to be evaluated in your design. Left-hand and right-hand gearing exists in helical gearing, bevel gearing, and worm gearing.
Helical gears are similar to spur gears except for the tooth being skewed in relationship to the bore axis. The benefits of this skew (helix) are that it allows the gear to handle more torque and operate at a lower noise than a spur gear with the same pitch, number of teeth, and face width.
Helical gears can be defined as left-hand or right-hand. To determine the direction of the helix angle, also known as the direction of hand, the gear should be viewed from the hub end through the bore. Looking at the tooth end, follow the trace of the tooth across the face. If the tooth trails counterclockwise, then the gear is said to be left-handed. If the tooth trace is clockwise, then the gear is said to be right-handed. When using a pair of helical gears on parallel shafts, it is a requirement that one of the gears be left hand and the other gear be right hand (Figure 1).
Similarly, if you are using a helical rack and pinion, the pinion and the rack must have opposite helix directions (Figure 2).
The downside to introducing a helix angle to the gearing is that it creates a thrust load. Figure 3 shows how the direction of hand and the direction of rotation affects the direction of thrust forces in a helical gear pair or a helical rack and pinion.
There is a special type of helical gear with a helix angle of forty-five degrees. This type of helical gear is known as a screw gear. Screw gears are unique in that they can be used in left-hand/right-hand pairs, and they can be used in same-hand pairs (Figure 4). When a left-hand and right-hand screw gear are used together, they will operate on parallel shafts. When a right-hand screw gear is mated with another right-hand screw gear, the pair will operate at ninety degrees.
As with traditional helical gears, screw gears are also subject to thrust loads. Figure 5 shows how the direction of hand and the direction of rotation affects the direction of thrust forces in a pair of screw gears.
For bevel gears, the designation of left hand and right hand is referenced by the direction of the spiral angle. To determine the direction of the spiral angle you need to trace the tooth across the face width from the toe to the heel.
As with helical gears, spiral bevel gears will only operate if one gear, typically the pinion, is left-handed and the mating gear is right-handed. Figure 6 shows how the direction of hand and the direction of rotation affects the direction of thrust forces in a spiral bevel gear pair.
As noted in the diagrams, the thrust forces of the pinion will be either direction dependent on the direction of rotation and whether it is the driver or the driven. Conversely, the bevel gear thrust force will always be pulling the gear out of the mesh.
For worm gears, the designation of left hand and right hand is referenced by the direction of the lead angle.
As with helical gears, the direction of the helix is determined by viewing the worm or worm wheel from the hub end through the bore and if the tooth trace is counterclockwise, then the worm is left-handed and if the tooth trace is clockwise then the worm is right-handed. Figure 7 shows how the direction of hand and the direction of rotation affects the direction of thrust forces in a worm gear pair.
As detailed in all the images in this article, gears do not have a preference as to whether they are left or right oriented. Although the torque capacity does not change depending on this orientation, the resulting thrust forces introduced into the system are affected. Please take caution to select the proper thrust bearings for your application when incorporating gears with a helix, spiral or lead angle.
