Advanced controls eliminate case-hardening contact

Indianapolis -Heat treating crankshafts creates a harder (RC40 or higher) microstructure on the surface of connecting-rod journals, providing smooth engine operation and long life. However, in order to correct for heat-treat distortion and marring, extensive post-grinding is a costly necessity for engine manufacturers.

By finding a way around such marring or distortion, Contour Hardening, Inc., (CHI), saves engine manufacturers a lot of expensive post-processing. CHI's system uses heat treating coils that orbit around a vertically-mounted connecting rod, never touching the shaft. Mannesmann-Rexroth's "don't touch" orbiting control solution with a high resolution, 2-axis servodrive package was a key to the machine's success.

In traditional heat treating machines, as the crankshaft rotates, heavy roller-guided induction heating coils ride on the surfaces to be hardened. Distortion occurs from the weight of the induction-heating heads on the shaft, bending the hot shaft when it's most vulnerable. Marring stems from the cam insert contact.

"Everybody needs RC50 journal surfaces but nobody wants to grind them," quips John Storm, CHI president.

CHI's vertical crankshaft-hardening machine eliminates all contact between the induction coils and the journals. A package of Mannesmann-Rexroth motion controls moves the entire transformer busswork and coil weighing over 2,000 lbs through the required orbital path. It follows the orbiting connecting-rod journals and precisely maintains a 0.160-inch gap on all sides, accurate to 0.001 inch. Any closer and the part overheats and the risk of an electrical "short" increases. Any farther away, and case depth falls short.

To complicate the task, the crankshaft's rotational speed varies between 24 and 32 rpm with each cycle-slower when the coil is between the crankshaft's throws and faster when operating in the open. The motion system generates the circular orbit for the coil heads using essentially linear control logic and elements, which are far simpler and cost less than CNC or analog controls. "It operates like a high precision compound feed mechanism," says Dan Knall, CHI control engineer on the project.

To follow the connecting-rod journal precisely, a Mannesmann-Rexroth drive package moves the induction coil through a circular path, accurate to 0.001 of an inch, with essentially X-Y controls.

CHI created a virtual master, a 1,000-point orbital path generated in Microsoft EXCEL software, that drives all motion controls. Indramat (Hoffman Estates, IL) then loaded it into the servomotors' controllers along with the variable speed commands. "We located the control logic right on the device," says Naser Suleiman, Rexroth-Indramat application engineer.

On each axis, a Rexroth-Indramat DIAX programmable servodrive powers a Rexroth-Star ball screw that moves the transformer, mounted on a table that moves on ball slides. The encoders have a resolution of four million pulses per revolution-ten times higher than conventional servomotors. And to ensure positional accuracy even after power loss, they also feature absolute positional feedback.

The DIAX servodrive is Indramat's third-generation intelligent digital servodrive. Besides onboard control logic, it features a resolution of 2-4 million pulses per revolution, absolute feedback and SERCOS-compatible open-architecture Visual-Motion™software. "It's extremely accurate, easy to program, compatible with distributed networks and you always know where you are-even after a power failure," adds Suleiman.

Speed to market. Thanks to close teamwork among CHI, several Mannesmann- Rexroth companies, and their distributor, Morrell Inc., the new Micropulse™machine was market-ready quickly. "We brainstormed the no-contact control concepts with Morrell one day, then shortly thereafter with a team from Indramat and Star," explains Storm. "Only two months later, we had a working prototype for the motion controls package-and it worked perfectly the first time."

"We needed that speed to meet commitments to our customers and win credibility for follow-on orders," adds Storm. "We may be the industry standard for gear case hardening, but for crankshaft hardening we were the new guys on the block."

Another key to the project's speed-to-market, according to Steve Orlando, Morrell vice president of sales and marketing, was the capability of Mannesmann-Rexroth. "The three Mannesman- Rexroth companies acted as one with our people," adds Storm.

Although Storm was confident the new "no-contact" control scheme would work, it remained to convince the customer. "Obviously the prospect of a solution that would do away with post-heat grinding had great appeal," says Storm, "but the customer needed to be shown that the new idea would work in the real world." They arranged a live demonstration of the orbiting prototype, with both CHI and Mannesmann-Rexroth people on hand for questions.

Controls aside, the heat treating process itself was not an issue. "For decades," says Storm, "our induction hardening process has been recognized high-volume production of case hardened automotive gears."

It was just a matter of transferring that proven technology to the new application. Since gears are symmetrical and the surface to heat-treat is coplanar, gear machines need just one stationary full-circle coil, no orbiting.

Hardening a crankshaft, however, is more complex with nine non-coplanar surfaces, four of them orbiting at variable speed. The finished package for V8 crankshafts is actually a two-machine cell: one four-station machine does the connecting-rod journals and one two-station machine does the main-bearing journals. The main machine uses four slides, the connecting-rod machine uses twelve. All are synchronized with each other. Since only the connecting-rod journals orbit, only the connecting-rod machine needs the "no touch" orbiting controls.

Post-grinding cut. The first "no contact" crankshaft hardening has gone into full operation producing more than 100 V8 crankshafts per hour-all with no distortion or marring, according to Storm. Finished parts emerge smooth and accurate enough to render finish grinding unnecessary. All told, four of the breakthrough crankshaft hardening machines are installed or on order.

Storm says that success so far raises the possibility of grinding the journals in the "green" (RC28) state, thereby cutting grinding requirements by at least half. "The journals will always need grinding to achieve finish, but it's obviously a lot easier to remove a few thousandths of RC28 material than thirty thousandths of RC50 material."

Additional details...Contact Naser Suleiman, Indramat Div. Mannesmann Rexroth, 5150 Prairie Stone Pkwy., Hoffman Estates, IL 60192; Tel: (847) 645-3600; Fax: (847) 645-6201; E-mail: info@indramat.com .