Vertical-rack design boosts power-window performance

March 2, 1998 Design News

DESIGN APPLICATIONS From the regional editors

EXCELLENCE IN DESIGN: EDMUND SCIENTIFIC TELESCOPE WINNER

Vertical-rack design boosts power-window performance

Double-helix motor, dual rack-and-pinion drive address requirements for next-millennium automobiles

John Lewis, Northeast Technical Editor

Nashville, TN--In the next century, manual-window cranks on cars might meet the same fate as buggy whips did this century. Whether it's at tolls or fast-food drive-throughs, more drivers choose the convenience of pushing a button to make their windows go up and down.

But how often have you noticed someone with a seized window open the door a crack and reach around to pay a toll? Behind the scenes, engineers designing these mechanisms confront challenges such as lowering cost, size, and weight, while increasing reliability and ease of manufacture and assembly. Moreover, these engineers face new standards that will ensure future window-lift systems are less hazardous to fingers and offer more resistance against forced entries.

For most of us, power-window failure means a trip to the shop. Paul Fenelon of Mark Gears & Systems Inc. took a different approach. When his windows failed in 1989, he rolled up his shirt sleeves and used his experience in research, engineering, and applied materials to solve the problem. Using far-from-perfect parts, he has recently built several prototype double rack-and-pinion window-lift systems that are more efficient than existing systems by a factor of 3:1.

He touts his system is capable of meeting Federal Motor Vehicle Safety Standard No. 118. The standard, drafted to prevent personal injury due to finger pinching and/or head jamming in the window, requires a maximum window-closing force of less than 100N. In addition, OEM standards related to forced-entry performance will require that the distance a window can be forced open decrease. Inherent system advantages include:

70% weight reduction

"If you look at most of today's electric-power window systems," says Fenelon, "it appears that they essentially were derived by replacing the manual-hand crank with a motor, transmission, and drive assembly. The result is an inefficient system that requires oversized torque motors." Of the two basic types of power-window mechanisms in high-volume production today, arm-and-sector systems account for almost 85% of the market. Their complexity, according to Fenelon, means arm-and-sector systems are large and heavy, require numerous parts and fasteners, limit the choice of materials, and encounter operating problems at extreme temperatures.

Because of these disadvantages, cable drives are gaining popularity. More engineers turn to cable drives in their effort to reduce cost, weight, and complexity. Cable drives weigh less but, says Fenelon, often result in warranty issues due to cable wear. That's why in early 1992, shortly after Fenelon began tearing down car doors, he discovered that repetitive-gear stress played a major role in power-window failure. "When the window stops abruptly at the fully open or closed position, the motor stresses the gear teeth while the operator's finger remains on the button. Because the system starts and stops at the beginning and end of each cycle on the same gear teeth," says Fenelon, "failures increase."

Initially he designed new gears and related systems for application in existing window drives. But after patenting a stress-dissipation gear, work began on rack-and-pinion window-lift systems. The result is a double-rack-and-pinion system that reduces gear stress even more. The key to success? Flexibility. "The system works because it's not too rigid, the whole system acts as a shock absorber," says Fenelon.

Double-helix motor. To drive his system, Fenelon visualizes using a 40 in-lb, stall-torque motor with an output shaft on each end. Because each output shaft rotates in the same direction, worm gears with opposite helical angles drive the pinion gears. Each pinion gear includes a flanged cap extending radially from the pinion gear beyond the gear teeth, and overlapping the adjacent rack. These caps maintain relative rack and window position, and reduce the need for window-edge guides below the window plane.

The vertical racks and support braces are integrally molded with top and bottom cross members in the molding process so that the unit comes out of the mold as a complete frame. The unit, assembled on the bench, fits through existing door-panel holes, then mounts to the interior-door panel with only four rivets.

Fenelon is seeking partnerships with suppliers to optimize the design. Before high-volume production, the system requires FEA optimization of gear-tooth profiles, system dynamics, and flexibility. Because existing window-lift motors are oversized for his system, Fenelon also seeks a motor manufacturer to optimize the design and manufacture of a double-ended motor.

To date, working prototypes have been developed and successfully tested for various-size vehicles. Video demonstrations and prototypes were eagerly viewed by major end users. Testing efforts center around OEMs' protocol. Prototypes successfully passed Key life-Door Slam, Key life-Durability, Accelerated Durability, and Vertical Strength (forced entry) requirements. Tests revealed that the product operates at temperatures as low as -40C, with very smooth torque-time traces. "No other system can do that repeatably," claims Fenelon, "most have trouble at -30C and below."

Additional details?Contact Paul Fenelon, Mark Gears & Systems Inc., 13 Inverary, Nashville, TN 37215, FAX: (615) 665-2150