Double helix takes the whine out of synchronous belts

March 2, 1998 Design News

DESIGN APPLICATIONS From the regional editors

AT THE NATIONAL DESIGN ENGINEERING SHOW

Double helix takes the whine out of synchronous belts

Allows synchronous belt drives to operate at higher speeds without violating OSHA standards

Charles J. Murray, Senior Regional Editor

Lincoln, NE--Most engineers in the power transmission field know the main drawback of synchronous belts: they're noisy. Synchronous belts are so noisy that they must often be enclosed by sound insulation structures to protect nearby workers. They're so noisy that at high speeds, they can violate OSHA workplace regulations.

Now, however, engineers from Good-year Tire & Rubber Co. have developed a product that dramatically reduces the gear tooth whine associated with synchronous belts. Designed for a wide range of applications, the Eagle Pd synchronous drive belt cuts noise, particularly at higher operating speeds, where noise reduction is critical. In tests at 3,500 rpm, it typically lowered noise from 92 dB to 73 dB. At 1,750 rpm, it cut noise from 80 dB to 62 dB when compared to conventional synchronous belts.

Such reductions are considered extraordinary in the power transmission industry, where engineers have long struggled to cut two or three decibels from the operating noise of synchronous belts. "On average, we're looking at noise reductions in the range of 10-20 decibels," notes Michael J. W. Gregg, chief engineer for Goodyear Tire & Rubber in Lincoln, NE. "These are very, very large reductions."

The Eagle Pd accomplishes that by employing a gear tooth design that is unlike any other on the market today. The key is an offset double helix pattern where the belt's tracks are 180 degrees out of phase. In essence, its teeth take on the appearance of a series of broken Vs. Ultimately, the frequency of the noise generated by one side of the belt is exactly 180 degrees out of phase with the frequency of the noise on the other side, resulting in a noise cancellation effect.

The double helix design further reduces noise by gradually rolling the belt's teeth through gear sprockets. Because its teeth are engaged gradually and continuously, rather than suddenly, the buzzing and whining noises often associated with tooth engagement are reduced.

Gregg says that engineers could also have reduced tooth engagement noise by employing a single helix, rather than a double. But the single helix, he says, must be held in place by sprocket flanges, which can generate unacceptable side forces. In contrast, the double helix is self-tracking, and therefore needs no flange to hold it in place. Result: better wear characteristics and no flange noise.

Ironically, the helix concept used in the Eagle Pd is more than 30 years old. It has not been employed on a commercial product up to now, however, because it was too labor intensive for volume production. Up to now, the problem has been that the helical elements of the belt interlocked with the male mold used during molding, thus preventing the belts from being easily removed.

During a five-year development period, however, Goodyear engineers developed proprietary production methods which enabled them to solve those problems. The technique, Gregg says, is unlike anything previously used in the industry. "To build our system, we had to go outside the envelope of conventional thinking," he says.

To enhance other mechanical characteristics of the belt, Goodyear engineers also developed special materials for the teeth and facing fabric. The teeth are made from Hibrex, a thermosetting elastomer. And the belt's facing fabric is coated with a proprietary polyolephin called Pliogard. As a result, Eagle Pd's tooth tensile strength approaches that of urethane, which is approximately 3,000-4,000 psi. The greater tensile strength, in turn, contributes to the belt's ability to offer approximately twice the power of conventional rounded or curvilinear toothed belts.

Eagle Pd's noise and power characteristics make it ideal for use in high-speed industrial drive applications that employ timing belts. In the machine tool industry, for example, many new products must be covered with foam-lined metal enclosures to contain drive belt noise. The Eagle Pd not only eliminates the need for such enclosures, it also cuts costly work area monitoring, training, and testing.

For that reason, Eagle Pd has seen early use on spindle-based machines and cylinder boring systems. It has also been employed in applications requiring high torque and high horsepower. There, Pd's material advantages enable it to withstand the larger loads without frequent replacement or retensioning. Other applications include automotive cam drives, CNC machinery, and heavy industrial drives.

The new belt has also been used at an automotive test track at a major theme park, where it reduced peak electric vehicle drive noise from 116 dB to 95 dB at 6,000 rpm. Such applications are representative of the impact of the new design, Gregg says. Whereas, noise once limited the speed of many synchronous belt applications to 3,000 ft/min, Eagle Pd has allowed some of those same applications to run at 15,000 ft/min. "That's the main advantage of this belt," Gregg says. "Because it's quieter, you can now run your applications at much higher speeds than were possible before."

Additional details?Contact Eagle Hot Line, The Goodyear Tire & Rubber Co., P.O. Box 83248, Lincoln, NE 68501, (800)-809-8666.