Powder Metal for Precision and Performance

The P/M process provides quality and repeatability for gearing manufactured for a wide variety of applications. This article provides insights into how, when, and why it works.

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While the powder metal process is becoming increasingly popular with gear designers because it is a cost-effective alternative to traditional manufacturing processes, the powder metal manufacturing process is ideally suited for the production of cluster and pinion gears used for reduction drives on miniature and fractional horsepower motors.

Fractional horsepower motors are found in any application where incremental motion is precisely controlled by speed, torque, and power. Some typical applications for these motors are business machines like printers or plotters, medical devices, automobiles, motion control and industrial equipment, aviation equipment, and consumer products including appliances and hand and power tools.

Because of the precise function of these motors they require high-quality, close-tolerance gears that provide consistent, predictable performance. While traditional manufacturing processes can produce close-tolerance gears, it is difficult to produce complex shapes or maintain part consistency in high volumes. The precision capabilities of the powder metal process make it a better choice for the high-volume production of gears needed for the fractional horsepower motor industry.

Most pinion and cluster gears used in fractional gear motors are AGMA 6 or higher (Figure 1). The AGMA quality number for a gear is controlled by the standards set by the American Gear Manufacturers Association and is determined by the number of teeth and pitch diameter, diametrical pitch range, tooth-to-tooth composite error tolerance, and total composite error tolerances. The higher the AGMA quality number for the gear, the closer the tolerances.

For example, an AGMA 8 quality gear averages over 20 teeth and up to a 1.999” pitch diameter. The diametrical pitch range is 20 to 200 teeth, tooth-to-tooth composite error tolerance is 0.0010”, and total composite error is 0.0019”. Most gears for fractional gear motors run in a range 32, 48, 60 teeth per inch with pitch diameters of 3/16”dia. to 1.500”dia. It is most desirable to use gears of a higher AGMA quality because closer tolerance gears will run quieter. The powder metal process lends itself to producing these close dimensional tolerances and maintaining at high rates of production.

A difficult tolerance to maintain through traditional manufacturing processes, but one of the features of powder metal compacting, is the ability to maintain the eccentricity of a gear (Figure 2). This is the tolerance between the internal diameter (ID) of the gear and the pitch diameter tolerance referred to as the total composite error. The closer these tolerances are held, the quieter the geared fractional motor will run.

Although there are five basic powder metal manufacturing processes—conventional powder metallurgy (P/M); metal injection molding (MIM); powder forging (P/F); hot isostatic pressing (HIP); and cold isostatic pressing (CIP)—conventional powder metallurgy is the most commonly used of these processes for manufacturing gears.

The P/M process starts with mixing elemental or alloy powders and placing this mixture into a die tooled to the desired gear shape and final part dimensions. The powder is compacted at high pressure to bond the particles, and the part is ejected from the die. The gears are then heated, or sintered, at high temperature in a controlled atmosphere furnace to complete the bonding process. Additionally, the gears can be heat-treated to increase strength and wear resistance or impregnated with oil to make them self-lubricating.

The tooling controls the final shape, tolerances, and part-to-part stability of the gear. With the P/M process, any variation in the design of a gear to enhance the performance of a geared fractional hp motor is incorporated into the design of the compacting tooling. This allows for modifications to improve performance or meet specific requirements without the need for a secondary operation or additional cost per piece. In traditional gear manufacturing processes it is difficult to make modifications to tooth geometry. The P/M process is ideal for making modifications to tooth geometry and tooth profiles economically.

Tooth profiles can be customized by the designer to maximize the tooth contact ratio of the teeth with any custom pitch they need to provide optimized power transmission with other than standard diametrical pitches. The contact ratio can be increased by modifying the shape of the tooth, as shown in figure 3. Tooth profile can also influence the noise level of the gear. Modification to the tooth tip (Figure 3) can reduce the noise of the gear train. The tradeoff is an increase in the backlash of the gear. In many gear applications an increase in backlash is undesirable. In fractional HP motors, however, backlash is not a concern.

The root of the gear tooth is an area of increased stress and can be prone to fracture or breakage. With traditional manufacturing processes it is not possible to produce a full root fillet radius. With P/M the die can be designed to fill in the root area of the tooth providing a true full root fillet radius, which can minimize root stress and reduce the chance of fracture in this area when high loads are applied (Figure 4).

By designing gears for the P/M process the designer has the versatility to add special features to the gear to enhance the performance of the gear, to eliminate the need for subsequent machining operations, or to reduce the assembly time of the final product. The following are some features that can be easily produced using the P/M compacting process:

• A standoff or a boss can be designed to a specified thickness and acts as a precision spacer or bearing surface between two gears. This eliminates the step of adding shims or spacers during the assembly process.

• Lighting holes are often incorporated into a gear’s design to reduce the final weight of the gear. Adding lighting holes also reduces the amount of raw material used to produce the gear. These holes are usually circular in shape but can be modified to eliminate more material as shown in Figure 5.

• The internal diameter (ID) of the gear can be designed for a specific type of shaft. Special configurations such as “D” hole, keyed slot, spline, hex, or square holes are easily formed in the compacting operation and can incorporated into the part, eliminating any second operations.

• The P/M process facilitates the production of complex shapes. This feature of P/M is often taken advantage of by designers to combine two or more gears into a single part. Cluster gears are often produced by combining a reduction gear with a plain gear. This is done when there is a need to decrease the speed of the gear or to increase the torque. A gear can also be combined with another part such as a precision pulley, a cam, or a sprocket. The possibilities are only limited by the designer’s imagination and the requirement of the application.

Although the P/M process is versatile in its very nature, it does have some design guidelines that will optimize the process and reduce cost. It is important for the gear designer to design with the P/M process in mind and to work closely with the P/M manufacturer to reduce development time and to take advantage of design elements that would increase the performance or enhance the quality of the product.

While tooling is a key element in producing the close tolerances needed for gears, the material the gear is produced from and how that material is applied in the production process also determines the characteristics of the gear. Gears can be produced from both ferrous and non-ferrous materials. Gears for fractional HP motors are usually produced from low alloy medium nickel steel for its strength, but most P/M manufacturers have their own propriety blends for their particular process. The porous nature of powder metal ensures a good sound dampening quality for quiet-running motors and allows the material to be impregnated with lubricants.

The material properties of the gear can also be tailored to the specific requirement of the gear design and the application. For example, less powder metal can be applied at the ID of a gear and more powder metal can be applied at the tooth of the gear and compacted at the same compacting tonnage. This produces a lower density at the ID of the gear so that oil can be impregnated for a bearing and a higher density at the gear tooth to increase strength. With a secondary operation, such as heat-treating, the tooth strength and fatigue limits of a gear can match that of wrought materials.

One of the inherent properties of the P/M process is the ability to maintain a high degree of dimensional stability and weight control over high volume runs. With powder compacting, once the tool is proven for accuracy (first article) for the production, the configuration of the gear and the tooth profiles will remain the same for millions of cycles, as the tooling is not subject to the kind of wear seen in gear cutting or grinding. The “part-to-part” and “run-to-run” stability is controlled by the tooling and weight of the part (density), and unlike with machining there is no wear on the tool, and a well made compacting die will not show any measurable wear after 5,000,000 parts. Other tooling components that are likely to wear do so very slowly, in the range of .00005”/100,000 parts, and is very predictable. Figure 6

On today’s compacting equipment, holding the weight of the part to a range of less than +/- ¼ percent of 1 percent is accomplished through SPC (statically process control) as a standard part of QA at compacting. This part-to-part reproducibility—or repeatability, as it is called in the industry—ensures a consistent and reliable product. This quality enhances the ease of assembly, stabilizes the production process, and gives the product a stabilizing quality over its service life. Figure 7

In addition to the other advantages of the P/M process, overall it is a more economical choice. The initial cost of the tooling for a particular gear will depend on the complexity of the part. A higher manufacturing cost for a complex part is also applies to more traditional manufacturing processes. Because the tooling will show little or no wear over runs of millions of parts with P/M, however, the cost per part is relatively low. The P/M process is also more energy and raw-material efficient. Due to net or near-net shape manufacturing, raw material usage is greatly reduced, to less than 3 percent loss. The net shape production of the gear eliminates scrap and the need for secondary operations, which often require additional cooling and heating systems.

The advantages of using a P/M process are numerous, including design versatility, close-tolerances, high-quality gears, and part-to-part repeatability. These are only some of the reasons why many gear designers are turning to this economical manufacturing process.