Caterpillar Promotes Innovative Solutions Through Powder Metallurgy
By D. Douglas Graham
Powder metallurgy (P/M) parts, products, and shapes are created when powdered metal is subjected to pressure and heat. Powder metallurgy has a long history, beginning with the ancient Egyptians, who used the process to forge iron tools more than 5,000 years ago. Other ancient peoples also exploited this old-fashioned technology, especially the Incans of Peru, who made jewelry and sacred objects from powdered precious metals.
In modern times, P/M was employed in the production of tungsten filaments for the first electric light bulbs. It also formed the base material for early carbide cutting tools. The automobile industry became a primary consumer of P/M parts in the 1960s and '70s, and remains so to this day. Aircraft turbine engines have also been partly composed of P/M parts since the 1980s.
Today, the process is responsible for sales of approximately $3 billion per year in North America alone. Powder metallurgy parts and pieces are employed in the manufacture of a wide range of products, including automotive parts (a category representing about 70% of the total market), washing machines, lock hardware, off-road equipment, hunting knives, portable radios, postage meters, and fishing reels.
Taking the Lead in P/M
In terms of technical expertise and sophistication, Caterpillar is a leader in the P/M field. Located in Peoria, Illinois, the company has been a major manufacturer of heavy earth-moving equipment since the 1920s.
Caterpillar entered the P/M business in 1971, after it became apparent that the company's suppliers were unable to provide parts to its required design specifications. Rather than shop for another supplier, the company decided to make the pieces themselves to satisfy its internal needs.
In 1994, the company opened a huge manufacturing site in Rockwood, Tenn., dedicated exclusively to the production of P/M parts. Today, this state-of-the-art facility stretches out 150,000 square feet. The plant employs 100 people who make their living working with 13, 12-825-ton compacting presses, 4 sintering furnaces, and 4 heat treating furnaces. Also housed within the facility is a bank of computer numerically controlled (CNC) machines, and a finishing area equipped to do vibratory deburring, resin and oil impregnation, steam oxidation, and packaging and shipping.
The plant also offers its customers bar coding, shot blasting, and tumbling, among other services. Its current output is almost 9 million parts per year.
"Typically, a customer would come to us from outside Caterpillar because its current supplier had fallen short in the area of part quality, or delivery performance," said Product Manager, Mike Kellen. "Our specialty is to make very large and complex parts. This is beyond the pale of most P/M facilities mainly because they lack the equipment or don't have a sufficiently sophisticated manufacturing plant. We have all of that, plus a staff of engineers, metallurgists, and other specialists with years of experience under their belts. Just about anything a customer needs is right here under one roof."
The P/M Process
According to Senior Applications Engineer, Ted Bair, the first step in Caterpillar's powder metallurgy process is to obtain a very fine-grain, homogeneous powder mix. Though the batch is always iron-based, additional alloys are added to create the properties desired by the customer. More often than not, the powder is a mixture of iron, copper, carbon, and die-wall lubricant, an additive that allows the part to be extracted from the die once it has formed.
The powder is poured into a rigid precision die and compacted into shape by exerting 30-50 tons of pressure per square inch. Once the piece is pressed, it is taken through a "sintering" operation, a process in which the piece is subjected to intense heat (about 2050 F) in a controlled atmosphere furnace.
With sintering, a part acquires mechanical properties; without sintering, it may come apart quite readily. Thin-wall pieces, for example, may be lost to rough handling, and heavier parts can break like unfired bricks if dropped on the floor. Sintering metallurgically fuses particles but does not melt them. Mechanical bonds that were set up in the pressing operation are actually transformed into metallurgical bonds after the sintering process is complete, making the part suitable for machining or final use.
After sintering, the part may go through a series of secondary operations. These include traditional machining processes such as drilling and grinding and heat treatment to increase hardness, wear resistance, and strength. Additional operations may include repressing or coining, machining, tumble deburring, steam oxidation, resin or oil impregnation, and plating.
"While the drive to design parts made from P/M is fueled by the desire to reduce costs, there are benefits to powder metallurgy apart from just cost savings," Mr. Bair said. "A P/M part has inherent porosity, which can be a real advantage in certain situations. One that springs to mind involves lubricated sleeve-type bearings. A porous sleeve bearing is much more desirable than a solid one because the porosity can be filled with oil through vacuum impregnation. This eliminates the need for interim lubrication," Mr. Bair explained.
Good News for OEMs
Caterpillar is just beginning to establish its presence as a job shop in the national marketplace, Mr. Bair admits. Currently, it draws most of its clients from business units within Caterpillar itself. However, this promises to change once OEMs understand the strengths that the company brings to contract manufacturing of P/M parts. Not the least of these are the many awards that the company has earned for achievements in the field of powder metallurgy.
In 1989, for example, Caterpillar received the "Ferrous Award of Distinction" in the annual MPIF "P/M Part-of-the-Year" contest. A year later, the firm won the same award for a cover assembly. In 1991, the Caterpillar engine governor control assembly netted the company another Ferrous Award of Distinction. This was followed by yet another of these awards in 1998 for a disc break hub.
"Sinter brazing is one of the things we do very well," Mr. Bair said. "The process involves taking two or more parts from the press and assembling them together. We put a brazing compound through the joint, and as the joined pieces are going through the sintering furnace, they are sintered and brazed in a single operation.
"Sinter brazing affords us the opportunity to make larger, more complex parts than are possible in a single parts operation. The process would attract anyone who has a need for parts in excess of 22 square inches, or pieces that by virtue of their geometry have more cross-sectional changes than can be formed in a single press operation. Right now, we're doing sinter brazing for clients within Caterpillar. We see this process as a major growth opportunity with OEM customers," he said.
Meanwhile, Caterpillar continues to use quality P/M parts internally. A decade ago, the firm's talents were put to use creating a counter balance piston for a Caterpillar agricultural tractor. A counter balance piston is a vital component of the tractor's transmission. Mechanically, it is an extremely complicated device, consisting of a two-piece, sinter-brazed assembly with a complex flange and nine pins on one side. It also has two tabs and complex crown work on the other side.
Machining such a formidable piece would prove very difficult under ordinary circumstances. However, by using P/M production methods, Caterpillar was able to do so less expensively and with far less waste than would have otherwise been possible. The piston has since performed admirably, and remains in production to this day.
On another occasion, Caterpillar wanted to cut costs on the production of governor control levers used on diesel engines. The levers, which functioned as throttle controls, were composed of four different steel parts. The costliest aspect of the old process was the machining of a lever component. Today, the governor control lever is created through a combination of bar stock machining and P/M. By sinter brazing a steel pin to a lever produced through P/M, the company realizes a savings of 73%.
Powder metallurgy has also decreased manufacturing costs on many other tractor components, including track roller and idler caps, which were formerly produced as malleable iron castings. According to the company, manufacturing the caps through a P/M process has given Caterpillar a cost saving of 58%.
"Caterpillar produces hundreds of P/M parts each year and generates annual sales revenues in the millions," Mike Kellen said. "Since opening our P/M facility, we have launched a concentrated effort to attract business outside of Caterpillar, and today we are working actively with OEMs that are not a part of the heavy equipment industry. Our strengths make us attractive to potential customers. These include innovative solutions, cost reductions, quality products, dependability, engineering and application assistance, and a variety of manufacturing services," he concluded.
D. Douglas Graham is a freelance writer based in Columbia, Missouri.
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