Crossed Axis Gearing

straight-bevel gears Gears of this type transmit power between two shafts usually at right angles to each other. However, shafts other than 90° can be used. The speed ratio between shafts can be decreased or increased by varying the number of teeth on pinion and gear. These gears are designed to operate at speeds up to 1000 ft/min (300 m/min) and are more economical than spiral-bevel gears for right-angle power transmission where operating conditions do not warrant the superior characteristics of spiral-bevel gearing. When shafts are at right angles and both shafts turn at the same speed, the two bevel gears can be alike and are called miter gears.

spiral-bevel gears Spiral-bevel gear teeth and straight-bevel gear teeth are both cut on cones. They differ in that the cutters for straight-bevel teeth travel in a straight line, resulting in straight teeth, whereas the cutters for spiral-bevel gear teeth travel in the arc of a circle, resulting in teeth that are curved and are called spiral. Figure 4 shows a cross section of a spiral-bevel vertical pump drive.

Spiral-bevel gearing is superior to straight-bevel in that, in the former, loading is always distributed over two or more teeth in any given instant. Recommended maximum pitch line velocity for spiral bevels is about 8000 ft/min (2400 m/min). Spiral bevels are also smoother and quieter in action because the teeth mesh gradually. Because of the curved teeth, spiral-bevel pinions may be designed with fewer teeth than straight-bevel pinions of comparable size. Thrust loads are greater for spiral-bevel gearing than for straight-tooth bevels, however, and vary in axial direction with the direction of rotation and hand of cut of the pinion and gear. Where possible, the hand of spiral should be selected such that the pinion tends to move out of mesh. To assure that the pinion thrust is away from the cone center, out of mesh, the following applies:

Driving Member Hand of Spiral Rotation Direction pinion Left Hand CW

Pinion Right Hand CCW

Reversing the direction of rotation should be avoided.

zerol gears Zerol-bevel gears are cut on conical gear blanks and have curved teeth similar to the spiral bevels, but the teeth are cut with a circular cutter that does not pass through the cone apex. Thus, these are spiral-bevel gears with zero spiral angle (hence the name). Furthermore, the tooth bearing is localized as in spiral-bevel gearing; thus stress concentration at the tips of the gear teeth is eliminated. Zerol-bevel gears are replacing straight-bevel gearing in many installations because their operation is

Gear Gear

Right Hand Left Hand

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