Tooth Tips: Nick Sudzum

This month’s installment is the third of six on bevel gearing, which began with the possible failures determined through vibration and will end with new finished products.

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Bevel gears are predominantly manufactured using low carbon alloy materials and then carburized and hardened, or case hardened, with an effective case depth of 10-12 percent of the tooth thickness at the addendum line to create a surface harness of 58-62 R"C" on the Rockwell "C" Scale, as stated in my last installment. There are various types of bevel gearing, such as: spiral bevel; skew bevel; hypoid bevel; straight bevel; and zerols.Figure 1 and Figure 2 indicate the basic nomenclature of bevel gears.

This particular set is shown running at a 90-degree shaft angle with the gear having a right-hand spiral angle, and the pinion with a left-hand spiral angle. The gear set is generated with a 20-degree pressure angle (PA) and a 35-degree spiral angle (SP). Spirals need to be manufactured, sold, and installed as matched sets consisting of a left-hand (LH) and right-hand (RH) component. Spiral bevel matched gear sets will ensure proper mounting distance, backlash, and contact patterns for a smooth rolling action.

Spirals (Figure 3) come with a wide range of ratios, also known as tooth combinations, with the most common system utilized on replacement spiral bevels being that of the 20-degree PA, 35-degree SP angle "Gleason Jobbing System."

This particular set is shown running at a 45-degree shaft angle, which is only one example of an angular spiral bevel gear set. Angular spiral bevels can be manufactured in a wide range of shaft angles above and below 90 degrees.

This set is shown running at a 90-degree shaft angle (Figure 4), with the gear having a right-hand 25-degree mean spiral angle and the pinion with a left-hand 45-degree mean spiral angle. Notice the difference between the hypoid and the spiral bevel, which both are shown running at 90-degree shaft angles. The hypoid pinion (Figure 5), being left hand, is below the centerline of the gear, whereas the spiral pinion is on the centerline of the gear. The rolling action of a hypoid bevel is actually a sliding action where the teeth are sliding through one another to create a rotation, as opposed to the spiral bevel that utilizes a rolling action, essentially lowering the center of gravity on the input (pinion). Hypoid bevels were originally designed for automobile differentials, lowering the center of gravity and enhancing the performance of the automobile. Hypoid bevels are also used throughout the industry in various gearbox drive applications.

This set is shown running at a 90-degree shaft angle (Figure 6), with the gear having a right-hand spiral angle and the pinion with a left-hand spiral angle. The gear set is generated with a 20-degree PA and a zero-degree spiral angle. Zerols utilize a somewhat different geometry than a standard spiral bevel due to the zero-degree spiral angle. Basically, when cutting a zerol bevel, the curvature that you generate is the radius of the cutter diameter being used.

This set is shown running at a 90-degree shaft angle, with the gear having a right-hand spiral angle and the pinion with a left-hand spiral angle. The gear set is generated with a 20-degree PA and a zero-degree spiral angle. Zerols utilize a somewhat different geometry than a standard spiral bevel due to the zero-degree spiral angle. Basically, when cutting a zerol bevel, the curvature that you generate is the radius of the cutter diameter being used.

This particular set is shown running at a 90-degree shaft angle with a 20-degree pressure angle. Straight bevel gears are a form-cut style of gear, which is similar to a tapered spur gear cut on a cone. Straight bevels are typically used where higher gear tolerances are not required and are able to be replaced individually and not necessarily as matched sets, since they are form cut. The straight bevel gear set was the first beveled gear ever designed. All other styles of bevel fears evolved from the straight bevel.

Be sure to read my next installment, which will address obtaining proper gearing through the reverse/inverse engineering of bevel gears.

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