Tooth Tips: William Crosher

The following is the second installment on worm and work wheel materials-Part I appears in the August 2006 issue of the magazine.


Bronze Bronze Forgings and Castings
Gear bronzes are designated according to their major alloying element. Only the aluminum and beryllium copper bronzes can improve their mechanical properties by heat treatment because, in other bronzes, the hard delta phase dissolves at 1,000 degrees F. If lower temperatures are used it becomes possible to relieve the segregation of tin in the mass or matrix and not affect the delta phase. To respond favorably to heat treatment the bronze must contain additions of iron and have a high aluminum content.

In correct terminology “bronze” should only be applied to alloys of copper and tin. The general rule is to name all copper alloys with elements other than zinc as bronze. Bronze may also include small quantities of tin, manganese, lead, nickel, aluminum, and silicon. “Brasses” are copper alloys containing up to 50-percent zinc. The more zinc, the harder the brass.

Bronze gear material is available from one of five groups: phosphor or tin, manganese, aluminum, lead, and silicon. Since copper is almost 90 percent of the bronze it must be of the highest grade and purity. The highest standards must also apply for the other elements. The disadvantages of high strength bronzes are that their increased hardness does not permit yielding sufficiently under load. This condition results in high localized temperatures, high unit pressures, and an increased likelihood of scoring. When the bronze material can yield the theoretical line contact broadens into a finite contact area, which reduces the unit pressures. A high tensile bronze material with double the strength rating of a phosphor bronze gear would only have half the wear rating. The worm gear already has an inherent advantage over any other gear in that it can absorb 50-percent higher shock loads. The most important characteristics for power transmission worm gears are friction, pitting, and wear.

Centrifugally Cast Bronze—SAE C90700 (tin bronze)
• 86-89% copper: 9-11% tin: 1-3% zinc: 0.2% lead: 0.02% phosphorus
• Hardness in range 90-120 HB, ultimate tensile stress 50,000 psi

This is the preferred material for heavily stressed and peak loaded worm gears. It has a low coefficient of friction and excellent abrasion resistance. When centrifugally cast to the appropriate specifications it is superior in wear and friction qualities to any other bronze. The structure must be uniform with a fine grain, and the tin must be distributed evenly through the copper matrix. Most centrifugal castings are limited to a minimum diameter of 4”. Below this diameter continuous casting methods are used. The poor wear qualities of such a casting can be improved if the nickel content is increased by 4.0-4.5 percent. German tests indicated a wear life increased by 8.4. (Ref: Phosphor Bronze Castings for Gear Blanks AGMA 6022-C93.)

Aluminum Bronze, Normally Cast—ASTM B148-65T-9D, CDA954000
• 83% min copper: 3-5% iron: 2.5% nickel: 10-11.5% aluminum: 0.5% manganese
• Hardness in range of 190 HB, ultimate tensile stress range 190,000 psi

These bronzes are similar to the manganese bronzes in toughness. They are, however, lighter in weight, and with heat treatment have superior mechanical properties. The ductility is reduced with increased strength. This bronze is selected when speeds are relatively low and there is a need to absorb high stresses. Maybe sand, centrifugally or continuously cast. Frictional qualities are poor. Fatigue strength is maintained even when corrosion is present.

Manganese Bronze, Normally Cast—CDA86300
• Copper 60-66%: tin 0.20%: lead 0.20%: zinc 22-28%: iron 2-4%: nickel 1.0%: aluminum 5.0-7.5%: manganese 2.5-5%
• Hardness in range 225 HB, ultimate tensile strength 225,000 psi

Manganese bronzes are selected for high loads and highly stressed and slow speed worm gears. They are susceptible to corrosion. Frictional properties are extremely poor. Ideally the centers should be < 3.5 inches, and rubbing speeds < 1,000 fpm. Frequently the material is selected for steel mill screw downs. They have the same strength and ductility as an annealed cast steel.

Leaded Bronze
• Copper 78-91%: lead 9-11%: 0.50-1.0% zinc: 0.25% phosphorous
• Ultimate tensile strength 25,000 psi, an elongation of 80%

When the bronze wheel is to operate with a soft steel worm, under virtually no load and low to medium speed, this material can be used.

Silicon Bronze—CDA 92700
Copper 85-88%: tin 9-11%: lead 1.0-2.5%: zinc 0.70%: antimony 0.25%: nickel 1%: sulfur 0.05%: phosphorous 0.25%: aluminum 0.005%: silicon 0.005%
On those rare occasions when non-magnetic properties are required in combination with low loads, such as electrical applications, this is the material of choice.

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is former director of the National Conference on Power Transmission, as well as former chairman of the AGMA's Marketing Council and Enclosed Drive Committee. He was resident engineer-North America for Thyssen Gear Works, and later at Flender Graffenstaden. He is author of the book Design and Application of the Worm Gear.