The powder metallurgy process.

Gears made by the powder metallurgy process provide a cost effective alternative to conventional steel or cast iron gears that are machine finished. The powder metallurgy (P/M) process yields net-shape, or near-net-shape parts, so that little or no machining is required to obtain a finished part in many cases. Thus the process offers dimensional tolerances and mechanical properties compatible with many applications.

Why P/M for gears?

The P/M process is well-suited for manufacturing spur, helical, and bevel gears. Moreover, some gear racks and spiral face gears have also been produced. Applications include gearmotors for household appliances, plus tractor transmissions, crane drives, and various automotive components, such as oil pumps, balance shaft adjustors, motordriven window lift and seat adjustors, cranking motors, distributors, and headlight activators.

Users often choose P/M for manufacturing gears because it has several process advantages:
• Provides true involute tooth profile and full fillet radius.
• Readily incorporates lightening holes, thereby reducing weight of the part, Figure 1.
• Because the material is porous, it contributes to quiet running gears (porosity dampens sound) and allows them to be self-lubricated (by impregnating with oil).
• Can combine a gear with other mechanical elements, such as cams, ratchets, drive lugs, or other gears, into one piece. Examples are shown in Figures 2, 3, and 4.
• Can make a gear with a radius in a blind corner, eliminating the undercut relief needed with cut gears, and providing extra strength in the radius.
• Requires little or no machining, and material utilization is nearly 100%.
• Can produce a gear with integral mounting shafts, either as short trunnions or by bonding machined steel shafts to the gear during the sintering process.

Limitations

P/M gears have certain limitations regarding strength and size. One such limitation, compared to wrought steel gears, is that gear teeth have approximately 50% lower impact resistance and 33% lower contact fatigue strength due to porosity. Manufacturers can partially offset this limitation by increasing the density of the gear teeth through double pressing and double sintering. Alternatively, hightemperature sintering or case hardening may be used.

The compaction process, which occurs in a vertical direction, produces relatively dense teeth in spur gears because the teeth are parallel to the gear centerline. But other gear types, such as bevel and helical, have teeth oriented at an angle to the centerline. For this reason, the vertically oriented compacting process is less efficient and produces less tooth density in these gears than is possible with spur gears. In such cases, copper infiltration is often used to increase the density (and corresponding mechanical properties) of the gear teeth.

Another limitation of P/M gears is their face width. The amount of powder that can be used in most compaction presses limits the gear face width to well under 3 in. In addition, frictional losses between the powder and the die causes decreased density along the face width, with the lowest density at the mid-point. The larger the face width, the larger this density falloff becomes.

These density differences can cause dimensional variations during sintering and heat treatment. Large density variations lead to distortion, especially with larger gears.

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