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a spur-wheel drive gear with helical wheels between intersecting shafts. Most screw gears used have a 90° angle between the shafts. The crossing of the shafts permits both shafts to have power takeoffs along their lengths in different directions, which makes it possible to transmit rotation from one shaft to several others. Screw gears are used chiefly in the kinematic chains of certain devices, since they have a tendency to wear rapidly and jam, even under small loads, because of the point contact of the teeth and the great amount of slippage in gear.