Machinedesign 2514 Easier Convey 0 0

Field Report: Easier to convey

Feb. 1, 2004
A versatile drive replacement package cuts downtime on auto assembly conveyors

Background: The Ford Motor Co.’s Michigan Truck Plant produces the popular Expedition and Lincoln Navigator. Skids holding the vehicles are transported throughout the entire plant on a series of power roll bed conveyors. The conveyors’ drive systems consist of electric motors, speed reducers, chain and sprocket drives, and electric motor brakes that hold the skidmounted bodies in place while the various assembly operations are performed.

Challenge: Ford had 10 different gearmotor packages, which made spare-parts stocking and replacement a chore. The speed reducers’ diverse mounting configurations, shaft sizes, and sprockets, as well as the range of gearmotor brands, horsepower ratios, and frame sizes, made replacement confusing and time-consuming. Another problem with the conveyors stemmed from worn keyways on output shafts (where sprockets mount) and input shafts (where motors mount). Less than half of the power roll bed conveyors had a VFD and without the soft-start capabilities, the keyed shafts took a beating.

The worm gear reducers were also problematic when a malfunction prevented the conveyor from running under its own power or caused a skid to be positioned incorrectly. Interaction of the worm gear sets made it impossible to push the loaded skid by hand in either direction. Backdriving — changing the reducer shaft position by rotating the output shaft — is not a capability of worm gear reducers.

Solution: Ford determined that one basic drive package would fit a broad range of applications if standardized on a single hp size, ratio, and gearbox size, and with only a few mounting modifications. The drive replacement package chosen includes a helicalbevel gearmotor with a hollow shaft and motor brake. It replaces 10 different drive configurations, reducing downtime and replacement. The package is built around a 2-hp right-angle helical bevel gearmotor from Rexnord Industries Inc., Milwaukee. It incorporates a 2-hp high-efficiency EPAct inverter duty electric motor equipped with a Stearns 333-3 armature-actuated electric disc brake.

Instead of using a key and pinion on the motor shaft, the gearmotor uses a press fit, eliminating the possibility of motor shaft keyway failure. On the output shaft, the sprocket uses a Ringfeder shaft-locking device to prevent key failures.

The gearmotor’s hollow double- tapered bushing output shaft design makes it easy to convert to either right or left-hand mounting or interchange with previous drives. And the Class-12 helical bevel gearing, unlike worm gear drives, can be backdriven manually when necessary. Workers simply pull the brake release, and the skid is free to move in either direction.

Although solid-shaft gearboxes are less expensive initially and therefore are used for original installations, versatility makes the hollow-shaft gearboxes less expensive overall as a retrofit. When Ford had 10 combinations of gearboxes, it was necessary to keep 10 spare gearmotors on hand in case of failure. The choice of hollow-shaft outputs means maintenance workers can change shaft sizes, so there’s no need to stock multiple shafts. The hollow output shafts can also be used for right or left-hand mounting, further reducing stock. Now maintenance workers can create their own shaft, slide it in, and put a sprocket on it. They can also do away with the different ratios by changing the sprockets and keeping the same speed within a few feet per min.

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