Powder metal opens electric door

Most of the requirements for designing the electrically controlled side door on DaimlerChrysler's Voyager Van seemed reasonable: A low weight/ transmitted torque ratio, an efficient reversible transmission, a transmission ratio near 30:1, and a shaft angle of 87.5 degrees . Two specifications did not: design for automated assembly and low cost.

Relying on its experience in both cut metal and powder metal technologies, mG miniGears, in cooperation with Intier Automotive Inc. (Newmarket, Ontario, Canada, a company of the Magna Group), met the challenge. By developing two gearboxes, applied on a frame that assembles onto the door, it was able to offer an integrated solution with significant cost advantages.

The first gearbox connects directly to the electrical driver and electro-magnetic clutch. It comprises two helical gear sets and generates a 9:1 transmission ratio. A flexible cable links it to the second angular gearbox. Offering a transmission ratio of 3:1, this gearbox applies movement through a sprocket to a rack fixed to the car's body.

A pinion and spiral bevel gear, cut from case-hardened steel, transmit the necessary high torque. The spiral bevel gear is designed to maintain the direction of transmitted loads, even when the door is manually operated. Treating the gear surfaces with manganese phosphate ensures proper lubrication for the vehicle's lifetime. Two powder metal bushings, both made from a compound of bronze and graphite for high dimensional accuracy and low friction, make up the spiral bevel pinion bearings.

Powder metal technology is also used to make the ten-tooth sprocket, which press fits to the spiral bevel gear. Only with this process, claims Andrea Scanavini, sales and marketing director for mG miniGears, is it economically possible to achieve a tooth profile allowing a 5-mm center distance variation without compromising the mesh of gears.

The gearbox case, designed for automated assembly, is a light alloy magnesium die-cast part. Casting reliability eliminates need for further mechanical rework and ensures dimensional precision. Remaining components, the bevel shaft and two self-tapping screws, fix the position of the spiral bevel gear/pinion and gearbox respectively.

"We delivered the system in six months," says Scanavini, pointing out that the time interval included rapid prototyping, omologation tests, and tool building. "The cost savings incurred by using the powder metal process," he adds, "made it possible to offer a more complex integrated solution than originally anticipated."