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Connecting rods that crack by design.

Ford Motor Co. (Dearborn Mich.) uses crankshaft-to-piston connecting rods wit large cracks in them. Ford engineers know all about it. The cracked rods are installed in the modular 4.6-liter V-8 engines that power the 1991 Lincoln Town Car, 1992 Ford Crown Victoria, and 1992 Mercury Grand Marquis models. But far from necessitating major model recalls, the automaker has patented the connecting rod cracking procedure. Indeed, Ford engineers designed the powder-metal-forged rods to crack on purpose. That's because the innovative fracturing process saves machining steps and provides more precise crankshaft bearing geometry than does conventional hot forging, a procedure in which rods are sawed in two.

Auto engine connecting rods transfer the force of the pistons to the crankshafts. In general, they consist of two ring forms--one large and one small--that blend into a central tapered I-beam section. The large ring rotates with the crank pin, the small with the piston pin. Among the keys to good connecting rod performance are high strength and durability, light weight for low reciprocating mass, and tight dimensional tolerances for balance. Another crucial factor is precise circularity of the large crank pin ring, which after splitting is composed of the bearing cap and the rest of the rod, bolted together.

"With cracked powder-forged connecting rods, the crankshaft sees as round a bearing hole as we can economically provide," said Pete Hoag, manager of the crankshaft design team at Ford's Romeo, Mich., engine plant. "The rough irregular mating surface that forms when the bearing cap is physically fractured from the rod virtually eliminates cap shift." Both out-of-round crank bearings and cap shift can increase interference forces within the bearing, which causes wear, he explained.

According to Ford product design engineer David A. Yeager, "Even after long-term tests on production engines, the bearings still looked great. There were no visible wear marks and almost no cap shift."

Besides improving engine performance the innovative technique used to manufacture the connecting rod saved millions of dollars of expenditures on production line equipment. For its efforts, the research team that developed the connecting rod cracking process, which included Yeager, Hoag, and principal research engineer Daniel W. Hall, has received the automaker's Henry Ford Technology Award.

Powder Forgings Ford's cracked connecting rods have their origins in powder-metal-forging techniques developed during the last two decades. Powder forging involves the consolidation of iron powder into a near-final-shape, partially dense preform. The part then achieves full densification after undergoing hot forging. Before powder forging was available, most connecting rods--especially those in high-performance engines--were hot-forged steel.

in hot forging, Yeager explained, steel stock is heated cherry red, placed in a connecting rod-shaped die, and then beaten to shape with a rod-shaped hammer. "But forged steel has variable properties," he noted. "Defects are hard to detect, and they propagate, acting as stress risers. Also, a forged connecting rod is made with an oval crank pin hole, which is then sawed in half to produce the bearing cap. After it's cut, the extra metal that formed the oval is machined away to form a circular hole." The machining operation itself is not simple, Yeager noted.

"We moved into powder-forged connecting rods around 1983, when we discovered that the 1.8-liter four-cylinder engine in the Toyota Camry had them," Hoag said. It turned out that Toyota Motor Corp. had constructed a fully automated powder-forging line in 1981. By mid-1988, Toyota claimed to have produced 10 million such connecting rods. They are used in a number of Toyota models, including the Lexus line. The German automaker Porsche also uses powder-forged connecting rods. Hoag and his colleagues also discovered that Stan Mocarski of Ford's Manufacturing Development Center had worked on powder-forged connecting rods in the early 1970s.

"At first, we were attracted by the near-net-shape aspect of powder forging," Hoag said. "It meant less material waste and would allow us to avoid purchasing the costly broaching equipment that's used to cut off a lot of extra steel stock for hot-forged rods." He added that powder-forged steel is easier to machine and the conformity of the rod's outer surface makes it easier to mount on a fixture.

In 1984, Ford provided tooling funds to Masco Corp. (Taylor, Mich.), the largest forging company in the United States, to develop the techniques required to powder-forge connecting rods on a mass scale. A few years earlier, according to Donald Roskopf, Masco director of corporate research and development, the company had developed expertise in the field when it hired powder-forging guru Art Marquis and purchased experimental powder-forging equipment from Chrysler Corp. when the automaker was experiencing severe financial difficulties.

Production Process

To produce powder-forged connecting rods, measured proportions of high-purity water-atomized soft iron powder, copper, graphite, manganese sulphide, and a wax lubricant are mixed in a blender. The wax is added to facilitate ejection of the compacted powder from the die. The blended powder is compressed on a special powder-metal-forming press to a shape approaching the final connecting rod form, called the preform or green state. At this point the preform is 82 percent of the full theoretical density of steel.

The preform is transferred from the molding press into a rotary furnace with an endothermic (non-surface oxidizing) atmosphere. Heat is applied to vaporize the wax from the preform. Further heating sinters powder particles together. The sintered preforms are rapidly transferred to a prelubricated and preheated 1300 [degrees] F) finish die cavity on a forging press. With one blow of the press, the preform is densified to near full density. The rods are then cooled.

The forgings are then deburred to remove flash. Double disk grinding is used to meet specified thickness dimensions and the bearing surfaces and thrust faces are machined. Cap bolt holes are drilled, tapped, and the caps are bolted onto the rods.

"Powder-forged connecting rods provide greater metallurgical consistency, as well as less dimensional and weight variation," Yeager said. The narrow weight distribution provided by powder forging is especially suited to V-6 and V-8 engines, which require stringent rotational balance. "Consistency of weight is much superior to the forged product," said Hoag. "Powder forging reduced tolerance variability by at least half."

It also allows us to eliminate the weight pad on the small end of the hot-forged rods," Yeager said. Conventional hot-forged rods have balancing bosses (small pads on the surface) that must be machined to a specified size and weight for balance.

By 1986, powder-forged connecting rods were ready to be tried in a production application. "Powder-forged connecting rods were first considered for the 3-liter V-6 engine produced at Ford's Lima, Ohio, plant," Yeager said, "but, because of some remaining uncertainties, the timing for the introduction didn't work out, and it was killed. Then in 1987, the 1.9-liter four-cylinder engine produced at the Dearborn, Mich., plant for the Ford Escort and Mercury Tempo came along. By then, senior management was aware of the possibilities and the benefits."

The connecting rods were and continue to be manufactured on four production lines at the Ridgeway, Pa., plant of Exotic Metals Co., the powder metallurgy division of Masco. To date, Exotic Metals has produced over 10 million powder-forged connecting rods.

"After we successfully applied powder-forged connecting rods to the 1.9-liter engine, we looked for other places in the technology where we could improve performance and cut costs," Hoag said. "One of those places involved splitting the rod from the bearing cap. There are high tooling costs associated with the splitting saw.

"We got the idea for cracking parts from Outboard Marine (Waukegan, I11.) and Mercury Marine (Fond du Lac. Wis.), which had been cracking connecting rods for motorboat engines for some time," Hoag said. In general, the procedures the team investigated involved scribing notches about 0.030 inches wide on the inside surface of the crank pin hole to serve as stress risers to initiate cracks when force is applied.

"We looked into cryogenic cracking, which uses liquid nitrogen to make the rods brittle," Yeager said. "We investigated the wedge method in which a wedge is driven into the metal." The team considered a servo-hydraulic method, patented by MTS Systems Corp. (Minneapolis), which uses a pulsed force that fatigue-fractures the rod.

"We finally focused on a procedure in which the rod is fixtured and half-moon-shaped 'pullers' are inserted into the crank pin hole," Yeager said. "The halves are then pulled apart with over 10,000 pounds of hydraulic force."

"When we told people of our intention to use the cracking method," Hoag recalled, "the typical reaction was, You're crazy; you can't do that.' That's because our connecting rod engineers had struggled for years to grind smooth the joint surfaces created when forged rods are sawed in half. But they were convinced when they saw the testing results."

The connecting rods are cracked by proprietary-technology automated "pulling" machines built by Tri-Way Machine Ltd. (Windsor, Ontario). The machine was installed at Ford's recently retooled Romeo, Mich., engine plant, which produces the 4.6-liter overhead-cam modular V-8 engine used in Ford's luxury cars. The plant has an estimated annual capacity of 535,000 engines. Perfect Surfaces

"Cracking causes the two halves to seat extremely well, with little room for movement," explained Yeager. In fact, the surface perfection that is achieved by mating a cracked cap and rod is so good that it's impossible to tell whether the part was fully separated; a machine checks whether complete cracking occurred.

Cracking saves several machining operations. Previously, after the powder-forged rods were sawed in two, they would travel separately down two production lines. "The cracking procedure saved several million dollars of capital expenditure on a separate automated line to handle the caps," Hoag said. "There's no grinding machine, no machining of the mating surfaces on the split rods and caps. And since each matched set of cap and rod is separated for only 10 seconds, there's little opportunity for misalignment. For example, the whole process of drilling bolt holes is much more forgiving in terms of locating the drill position."

In recent laboratory experiments, according to Hoag, Ford engineers have managed to crack hot-forged connecting rods using a similar technique. A new metal alloy composition is said to be involved.

For the foreseeable future, all new Ford engines, including the new 1.9liter Zeta" engine, will employ cracked powder-forged connecting rods. In addition, both Chrysler and an undisclosed Japanese company are working with Masco to develop the powder-forged connecting rod technology. General Motors Corp. is also said to be investigating the subject in collaboration with Federal Mogul Corp. (Detroit).
COPYRIGHT 1991 American Society of Mechanical Engineers
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Title Annotation:Ford Motor Co.
Author:Ashley, Steven
Publication:Mechanical Engineering-CIME
Date:Feb 1, 1991
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