The traditional approach to manufacturing power transmission components relies on casting, forging, drilling, hobbing, lathes, mills, grinders and furnaces. The standard technology for making gears and bearings involves a great deal of physical space, energy consumption and metal removal.
Powder metallurgy, however, offers an alternative way to make gears, cams and bearings, and has been offering its advantages for quite a few years. In fact, the first commercial application of the technology was a self-lubricating bearing used in automobiles 80 years ago.
It’s been called a “chipless” manufacturing process because the finished part contains at least 97 percent of the raw powder used in its manufacture. With such little waste, parts can be manufactured at finished dimensions with one-mil tolerances. It’s an efficient, cost-effective, energy-conserving way to manufacture many power transmission components.
Many early powder metallurgy parts were small and of simple design. The technology has advanced, and now it’s possible to produce complex parts weighing as much as 35 lbs. Two notable specialty applications for powder metallurgy are the manufacture of small porous inline filters and the fabrication of parts using metals that are otherwise difficult to machine using standard equipment, such as tungsten, molybdenum and tungsten carbide.
Making metal dust
The technology that underlies the entire powder metallurgy industry is the processes used for generating the finely divided powders that ultimately get pressed into solid shapes. The most popular process is atomization.
It starts with steel, copper or some other non-ferrous material in bulk form. The metal is melted and forced through an orifice. As the melt exits, a high-velocity stream of water, air, steam or other gas directed across the orifice separates the flow into extremely small droplets of molten metal that cool relatively quickly because of the greatly increased ratio of surface area to volume in the dusted metal. The physical properties, feed rate, pressure and temperature of the metal and dispersant are the factors that control the powder’s particle shape and size distribution.
The solidified powder then is heat treated in a reducing atmosphere to clean the particle surface of oxidation. After other processing steps, the powder is ready for pressing. Although starting with the desired alloy melt yields the best properties and strength, it’s possible to blend powders of various characteristics to achieve a specific metallurgy in the finished parts.
Copper powder also can be produced in an electroplating bath. Whereas plating shops strive to achieve a firmly attached, bright, reflective coating of copper on the cathode—and consider anything less to be scrap—making copper dust involves tweaking the relevant electroplating variables to generate a dull, porous, nodular, barely adherent copper plate. The electroplated copper is collected, filtered and dried. After treating it to remove the surface film, it is milled and classified to achieve the desired particle size distribution.
Although copper and copper-based alloys are the most common materials of construction for bearings, iron, steel, stainless steels and aluminum can be used to fabricate gears and other power transmission components.
Making the parts
Converting billions of individual metal particles into one solid gear or bearing requires bringing those particles into intimate contact and then interlocking them into place to form a coherent whole. Several ways are available to achieve this end.
The simplest, most common way, involves compressing those extremely finely divided metals in a die and hydraulic press under pressures ranging from 30 to 50 tons per square in. In the as-pressed condition, the parts are strong enough to be handled carefully and are called “green” because they have not yet achieved their ultimate strength. Subsequent steps provide that.
The standard hydraulic die allows pressing in only one direction. Isotactic pressing, on the other hand, allows pressing in every direction simultaneously to achieve greater and more uniform densities.
Flexible molds completely surround the charge of powder, which is placed in a pressure vessel. The vessel is subsequently filled with oil, water or gas and pressurized to as high as 60,000 psi. When the pressurization cycle is completed, the mold can be separated from the green parts chemically or thermally.
A third method capitalizes on injection molding technology. The powder is mixed with a binder, such as plastic resin, and processed in the molding machine to make green parts. After removing the bulk of the binder using heat or chemicals, the injection molded part, as well as those from the die and isotactic press, is processed further to improve its mechanical strength.
So far, the particles are held together only by mechanical interlinking and cold welds. The process that imparts real strength is called sintering. Those green, as-pressed parts pass through a high-temperature furnace in which the atmosphere is controlled to prevent metal oxidation. The temperature is held below the alloy’s melting point for sufficient time to permit solid-state diffusion to form the metallic bonds that replace the mechanical bonds where particles touch. This results in a strong but porous piece part. At this point, the density is about 95 to 98 percent of the pure metal.
Voids between metal particles, however microscopic they may be, determine density. The fewer and smaller the voids, the lower the porosity, the greater the density and the closer the strength will be to that of the pure metal. For structural parts, the goal is to achieve a void volume significantly less than one percent. Self-lubricating bearings, on the other hand, need to have much greater void volumes to accommodate lubricant.
Other factors that determine the physical properties include the forming pressure, the temperature profile in the sintering oven and the number of times the parts go through the pressing and sintering process.
So why not make the part out of the solid metal right off the bat and achieve the maximum possible strength? An advantage of powder metallurgy is it can easily produce non-circular holes and other odd features that are impossible to achieve using conventional machining practices. But this doesn’t rule out the possibility of using secondary operations, such as electroplating, deburring and machining, to produce undercuts and other features that are impossible to mold in a die. The point is that the powder metallurgy process minimizes the amount of secondary machining a part requires. The smooth surface finish and uniformity of parts coming out of the die permits a high-volume production of near-finished parts.
Oil held within the microscopic pores in a sintered sleeve bearing makes it a self-lubricating bearing. The spinning shaft turning in a stationary sleeve generates heat, which is carried away by the oil circulating in the interconnected passageways. The oil ultimately transfers the heat to the hearing housing where it dissipates to the plant environment.
Getting the oil inside the bearing’s porous structure requires more than simply dropping the freshly sintered parts into an oilcan. They need to be held in an evacuated chamber before being immersed in oil. When the vacuum is released, the inrushing atmospheric pressure easily pushes the lubricant into the very center of the part because there is no air that needs to be displaced first.
Although sintered self-lubricating bearings have some limited degree of ductility, they shouldn’t be installed with a hammer. Such percussive treatment can crack them. Instead, they are press-fit using a special pin that spreads insertion forces over the bearing’s full face.
In many applications, the bearing supports a ferrous shaft, a fact that makes it the sacrificial component when the shaft exhibits excessive roughness, or is out-of-round.
Overloading the bearing is a sure way to ruin it. Its capacity is given by the PV factor, which is the product of the pressure exerted on it and the speed. The pressure is equal to the total force (in lbs.) the shaft exerts on the bearing divided by its effective load-carrying area (length times inside diameter, both measured in inches). The speed is measured in surface feet per minute. The relevant mathematics reduces to a simple formula:
PV = 0.262FN/l
Where PV = the bearing’s PV value
F = force on the bearing (lbs.)
N = shaft speed (rpm)
l = the bearing length (in.)
Selection criteria and bearing specifications generally list four figures: the maximum PV, the maximum pressure (static and dynamic) and the maximum speed. The maximum allowable PV is always far less than the product of the listed maximum speed and pressure. One or both must be reduced significantly below their maximum values if the bearing is to function well.
Self-lubricating bearings, of course, are subject to the standard maintenance caveats regarding vibration, axial loading and misalignment.
Clogging the pores
Similarly, it’s possible to fill the pores with other materials to achieve different ends. For instance, the part can be impregnated with resin to eliminate the porosity completely when the gear or bearing needs to be electroplated. The pores also can be backfilled with metals having a lower melting point, which imparts additional strength and resistance to shock loading.
Gears and camshaft lobes
Power transmission components, such as pinion, bevel (straight and spiral) and face gears, as well as sprockets, are also fabricated using powder metallurgy. The difficulty, however, seems to be the lack of reliability data and methods for predicting the useful life of components involving rolling contact. In contrast to parts fabricated from solid stock, powder metallurgy parts have additional critical variables (density, composition, sintering profiles) that affect the performance.
Gears manufactured properly with powder metallurgy meet AGMA standards. The typical range for the ultimate tensile strength is about 40,000 to 60,000 psi, with AISI 4630 alloy steel running at 160,000 psi and phosphor bronze at 30,000 psi. As a rule, powder metallurgy parts can achieve a tensile strength 75 to 80 percent that of the bulk metal.
For further reading
Stephen Canfield, at the Department of Mechanical Engineering at Tennessee Technological University has posted a description of various gear types at http://gemini.tntech.edu/~slc3675/me361/lecture/grnts4.html.