Phased chain system squelches transmission noise

Ithaca, NY--When Chrysler's first LH cars went into production, they quickly established themselves as favorites with the automotive press and the car-buying public. It turned out, however, that the transmission used in the initial LH-car production run produced a noise that some customers might find annoying.

Chrysler regarded the sound as unacceptable, and, late in 1991, asked Borg Warner Automotive's Morse Chain Systems Division to find a way to suppress it. By using a system of two phased chains, engineers succeeded in reducing pitch frequency noise by almost 14 dBA and overall transmission noise level by a bit more than 4 dBA, thus solving the problem.

Chrysler uses a North-South engine layout in its LH Series cars. Power moves from the engine, through the transmission's torque converter and gearset, to a chainset. The chainset transmits power to a transfer shaft, which delivers power to a hypoid gearset. That gearset then delivers power to the differential.

Tests demonstrated that the chain's pitch frequency excited a resonance in the transmission, which produced the noise detected by Chrysler. Given that the cars were already in production, the noise had to be eliminated quickly and at minimum cost.

Early in 1992, an engineering team led by Philip Mott, chief engineer at Borg Warner's Morse Chain Systems Division, attacked the problem. The noisy chainset consisted of a randomized 1 ¾-inch-wide rocker joint chain and two sprockets. Reducing chain noise in a conventional fashion would require reducing chain pitch. But chain strength relates directly to chain pitch. To achieve similar transmitted torque capacity, a chain with smaller pitch must become wider than the chain it replaces. Naturally the sprockets that drive the chain also grow. The results: higher cost, greater package size, and increased system weight.

Mott and his colleagues decided to turn away from this conventional approach and instead use two ¾-inch-wide chains and four sprockets. Pairs of sprockets, riveted together, form two sprocket sets that drive the chains. In each set, the sprockets are displaced circumferentially--phased--by ½ pitch. As the phased chain set runs, this construction causes the two chains to generate frequencies that sum to cancel the first-order and all odd harmonics. "The light started going on when we began breaking down misconceptions that we had about our inability to do these things," says Mott.

Conventional thinking held that the new system would cost too much because of the increased part count. Also, conventional thinking said the chains would not share load equally, and necessary tolerances could not be held. In fact system cost came in close to that of the previous design. By November of 1992, the other challenges were overcome, and systems were shipped to Chrysler for testing. In June of 1994, Chrysler put the new chain system into full production.

To ensure equal load distribution across the width of the more narrow chains used in Borg Warner's Gemini™ Phased Chain System, engineers designed new chain guide links. Called ET Guide Links, these components look rather like a slightly spread horseshoe with a fat toe plate. More compliant than a conventional solid guide plate, they allow better load sharing by all the chain's links. Conventional guide plates are thick and stiff. Consequently, more load may be carried by the guide plate and elements close to it than by the rest of the chain. More even load sharing is necessary to extend the life of relatively narrow chains such as those used in the Gemini™.

Ductile iron blanks used to produce sprockets for the Gemini system are machined and hobbed. Four rivets fix the positions of each pair of sprockets in a set, placing each component ½ pitch out of phase with its companion. Finally, the sets undergo inductive heat-treat.

A land some 0.015 to 0.020-inch-high separates the two sprockets in each set, and the rivets pass through this land. Engineers added the land after they observed that prototype sets distorted during heat-treat. The outer circumferences of the sprockets tended to move toward one another and meet. This bowing produced a gap between the two sprockets in each set. By adding the land, engineers al-low the iron components to move a bit without contacting.

Another source of chain noise developed in low-speed, high-torque conditions. Under these conditions, the chainset's tight strand went into resonance at a frequency between 100 Hz and 200 Hz. After experimenting with other approaches, Borg Warner engineers installed a chain snubber consisting of rubber bonded to a steel support. It completely eliminated the moaning noise produced by the chain.

• All industrial and automotive chain drive systems that must operate with minimum noise

The Gemini system's first full-production-volume application was in Chrysler's 1995 model year LH-Series cars. Borg Warner intends to use the system very widely in other automotive applications. From conception to production, the Gemini project required just 24 months of work by a multidisciplinary Product Launch Team at Borg Warner. The team included: Phil Mott; Roger Young, project engineering manager; Dave White, development engineer; John Skurka, NVH engineer; and Roger Wood, manufacturing manager. Presently, the company has the capacity to produce and ship approximately 2,000 Gemini systems per day.

"For principle and mathematics to work out so well is really amazing," says Berthold Martin of Chrysler Corp. He sees other applications for the Gemini system coming in four-wheel-drive cars and trucks. "It has to come eventually because four-wheel-drive applications are becoming more demanding in terms of on-road use of such vehicles, especially vehicles that we export." Martin and Mott co-authored a paper on the Gemini system. They presented it at the SAE International Congress and Exposition in Detroit last spring. Copies of the paper, number ISSN 0148-7191, can be obtained from SAE.

Additional details...Tony Messina, Vice President Sales, Borg Warner Automotive, Box 8024, Sterling Heights, MI 48311, (810) 726-4452.