Electromagnetic clutch/brakes from Ogura Industrial Corp., Somerset, N.J., start and stop cutting blades in GPS-guided lawn mowers. The McMurtry robotic mowers trim turf at up to 5.5 acres/hr and steer with subinch accuracy.
The rotor in the electromagnetic clutch keys to the engine output shaft. Voltage applied to a coil magnetically couples the armature to the rotor. The armature, with a pulley attached by six leaf springs, matches rotor rpm in about 0.3 sec. The pulley then runs drive belts that turn the blades.
Cutting electrical power to the coil lets the leaf springs pull back the armature from the rotor and into contact with a stationary brake shroud affixed to the clutch backing plate. The arrangement helps slow the blades to a halt in about 5 sec. McMurtry engineers ruled out mechanical brakes, clutches, and linkages because the mowers lack pneumatics or hydraulics to operate the devices.
In one configuration, a diesel-powered mower tows two smaller gasoline-powered wing mowers to extend cut width from 1.5 to 4.2 m. The wings each receive an Ogura GT1 Series double-flux clutch rated at 90 lb-ft of torque. The diesel engine uses a GT2.5 Series double-flux clutch rated at 150 lb-ft of torque.
Double-flux clutches have about 35% higher torque capacities than single-flux types of the same size. That's important because the mowers have limited space. In single-flux electromagnetic clutches, an armature touches the north and the south poles to complete a magnetic circuit. The double-flux design boosts holding power by virtue of a slotted rotor and armature that makes two separate flux paths (four-pole connection).
The clutches manually adjust for wear, which can double or triple life compared with nonadjustable units. A special e-coat resists corrosion better than regular paint, while proprietary bearing grease and close-tolerance bearings extend service life and lower vibration.