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Sealing Science PUBLIC ACCESS

Tracking Down Leaks in the Lab to Keep Them Off the Road.

[+] Author Notes

Associate Editor

Mechanical Engineering 125(02), 36-38 (Feb 01, 2003) (3 pages) doi:10.1115/1.2003-FEB-1

This article reviews an edge-seal gasket that begins its final manufacturing stage stacked together with about a hundred identical paper frames. Each die-cut frame matches, edge for edge and hole for hole, the parts between which it will form a sealing interface. Midway between bolts marks is one of the best places to test gasket performance because that is where bolts in assembly exert the least amount of clamping force. Any gasket leakage is likely to begin where the clamping force is lowest. Thermal cycling and the expansion rates of dissimilar metals are particularly stressful to gaskets in engine service. The thermal tests try to do to the gaskets what engines can do routinely: displace the gasket as the adjoining engine components expand and contract. But with high tack properties, the edge polymer grabs tenaciously to each surface and holds on even as the two components move at different rates.

An Edge-seal gasket begins its final manufacturing stage stacked together with about a hundred identical paper frames. Each die-cut frame matches, edge for edge and hole for hole, the parts between which it will form a sealing interface.

The operator pours a puddle of red, blue, or purple polymer into the wells formed by the openings in the frames. He locks the stack into a special proprietary machine to spread the liquid polymer evenly over the edges of the holes in the gaskets. After the process finishes, the operator removes the stack of paper frames."

The low spring rate at the tips of the polymer sealing edge lets it follow irregular flanges. The structural carrier provides stiffness and shearing resistance.

Grahic Jump LocationThe low spring rate at the tips of the polymer sealing edge lets it follow irregular flanges. The structural carrier provides stiffness and shearing resistance.

Inside each well a colorful coating of polymer now identifies which fluid or joints the gaskets will seal-red for oil or blue for coolant. Purple polymer identifies a seal for bimetallic interfaces. After letting them cure in air for a short time, another operator peels off the gaskets one by one and send them through an oven for final drying and curing.

It is in the paring of individual gaskets from the stack that something nearly magical happens, although Jeff Barrall attributes it to the chemistry of cross-linked polymer chains. Barrall, who manages new business development at Interface Solutions Inc. in Lancaster, Pa., has freed up a couple of hours on a mild December day to walk a small group through the company's R&D and manufacturing operations.

As the group looks on, an operator grasps one corner of a frame and pulls it away from the stack before hanging it on an oven hook. Although the operator gives away no secret in her technique, Barrall assures the onlookers that they couldn't match her ability to produce a ring of polymer that protrudes neatly and evenly above both sides of the gasket's paper plane.

Those rings-without which she'd be holding the commonplace gasket-provide engine manufacturers with a new way to seal troublesome joints, Barrall explains.

Even when it's witnessed firsthand, the precision inherent in separating the gaskets this way can spawn occasional disbelief among the original equipment manufacturers who are buying them. For this reason, Interface Solutions insists on 100 percent inspection of finished product. Using an instrument Barrall calls a Kady gauge, quality control inspectors measure the minimum height of polymer projecting above the plane of each carrier.

Even when a machine pulls individual gaskets from the stack, the consistency of each one remains remarkable.

Edge seal gaskets represent a kind of two-steps-back, one-step-forward thinking in the evolution of seals. What began with fiber-composite paper soon moved on to cork and rubber; then liquid room-temperature vulcanizing, or RTV, silicones; O-rings, and finally, rubber-coated and rubber-edged metal. Interface's edge seal gaskets meld fiber composite and polymers to capture the desirable properties of both. As Barrall explains, they can also save money and time upstream during the prototype stages of an engine component's development.

Midway between bolts marks one of the best places to test gasket performance because that's where bolts in assembly exert the least amount of clamping force. Any gasket leakage-which Barrall classifies as either interfacial (leaking past the flange-gasket interface) or interstitial (leaking through a barrier)-is likely to begin where the clamping force is lowest. The gauge of a gasket-maker's skill is to see how many bolts he can eliminate from an assembly. Another test is how fine—or rather not so fine-the surfaces must be finished on facing parts.

Thermal cycling and the expansion rates of dissimilar metals are particularly stressful to gaskets in engine service, says Brian Lehr, the head of applications engineering at Interface.

Lehr, who's taken leadership of the tour from Barrall, walks the group through the wet laboratory—where standard ASTM and DIN tests probe the characteristics of many materials-and into the engineering lab. There, several thermal cycling experiments are taking place.

In one machine, a lineup of water pumps chills down to -50°C and remains there for two hours. They then heat up to 150°C for another two hours, Lehr explains. This continues over 250 cycles to duplicate the frequency and extremes of cold and hot that an automobile engine might see over a 10-year warranty period. While 1,000 hours of such testing is generally accepted by engine makers as a validation of gasket durability, Lehr says that Interface will often let tests run in the background for as many as 5,000 hours.

The thermal tests try to do to the gaskets what engines can do routinely: displace the gasket as the adjoining engine components expand and contract. But with high tack properties, the edge polymer grabs tenaciously to each surface and holds on even as the two components move at different rates.

To seal, gaskets have to squeeze back against the surfaces sandwiching them. High resistance to what Lehr calls "compressive stress relaxation" is yet another attribute that engineers at Interface seek in formulating their polymers.

And, unlike RTV sealants, the paper carrier that holds the edge polymer prevents the compound from oozing out through the joints.

A transmission shift tower challenges sealing engineers by flexing as a trucker changes gears. Long stretches between bolts add to the challenge.

Grahic Jump LocationA transmission shift tower challenges sealing engineers by flexing as a trucker changes gears. Long stretches between bolts add to the challenge.

Yet, for makers of engines and engine components, perhaps the strongest selling point is the quick and thrifty method that Interface has developed to produce prototypes. "The company has been able to take away a pair of long lead time events from the pro to typing process," Barrall mentioned earlier, as he brought the group through the design department.

Here, the company employs two designers who work up gasket designs from CAD files of the mating surfaces' shapes, materials, and finishes. The designers send finished files next door, where two razor knives suspended over an x-y table cut the designs from actual carrier material. After 25 identical frames are complete, the stack is rushed out to the manufacturing floor, where edge polymer is applied on the production line.

Customers can thus be sure that the gaskets used in validating a new component will be the same ones accompanying it when production parts roll out. And OEMs are spared the long lead time needed for both making the cutting dies and die-cutting the prototype frames. Everything needed for prototyping is housed beneath the Interface roof.

As a result, the company can deliver 25 samples in about a month for $1,200, or, for almost twice that amount, the same number in two weeks once the engineering has been worked out, Barrall explains.

A couple of years ago, Eaton Corp.'s heavy-duty transmission division in Galesburg, Mich., began working with Interface engineers to develop a new sealing solution for a truck transmission shift tower. According to Eaton product engineer Jeff Spitzner, the company wanted to decrease the incidence of gasket failures. Although failures weren't frequent, the company thought that needing to have the gaskets repaired was enough of an inconvenience to truckers that it could present a possible source of dissatisfaction.

Another factor in the company's decision to find a replacement gasket was a California no-leak policy that was hitting some truckers directly in the wallet, Spitzner said.

Since 1995, the company had been sealing the assembly with a gasket that was coated top and bottom with two beads of blue polymer. An investigation into the gasket's installed properties showed that it produced good Fujifilm test results, whereby the sealing pattern on an assembled joint is checked by disassembling it. But, the gasket tended to relax, or "creep," if it was installed below the minimum required clamping force, he said.

Engineers developed an edge seal gasket for the shift tower that worked with a mere 50 psi of flange pressure. Other designs may need 1,000 psi.

Grahic Jump LocationEngineers developed an edge seal gasket for the shift tower that worked with a mere 50 psi of flange pressure. Other designs may need 1,000 psi.

With its edge seal design, Interface offered another solution to sealing the shift tower. Although the new gaskets ended up costing about one-third more apiece than the ones they replaced, the effort has so far saved more than $50,000 in warranty costs, Spitzner said.

Quite a few engines are taken apart at the Interface Solutions' labs, where engineers seek better ways of keeping oil and coolant in place.

Grahic Jump LocationQuite a few engines are taken apart at the Interface Solutions' labs, where engineers seek better ways of keeping oil and coolant in place.

He also recalled another test that demonstrated the tenacity of the Interface design. In that one, a bolt was removed from a power takeoff unit under test and the assembly continued running without leaking for another 1,000 hours.

Spitzner said Interface is the only company he knows of that is applying edge seal polymer onto paper frames. Other manufacturers have been molding rubber edge seals to metal frames for some time, but that technology tends to carry higher tooling costs, he said.

According to Interface's literature, comparable costs for production tooling might be $5,000 for its edge-seal technology, compared with $40,000 for an equivalent rubber-coated metal gasket, or $100,000 for a rubber-edged metal gasket.

Back at the test lab in Lancaster, Lehr brings the group past the collection of engines that lately have come under the scrutiny of the Interface Solutions team. With the exception of head gaskets, every other joint on an engine is a candidate for the company's edge seal technology, he says."

They're often given the troublemakers first, he adds. Which makes sense. Engine manufacturers clearly aren't disposed toward fixing seals that don't leak.

Copyright © 2003 by ASME
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