sentences:  With parallel helical gears, each pair of teeth first make contact at a single point at one side of the gear wheel; a moving curve of contact then grows gradually across the tooth face to a maximum, then recedes until the teeth break contact at a single point on the opposite side.In spur gears, teeth suddenly meet at a line contact across their entire width, causing stress and noise.Spur gears make a characteristic whine at high speeds.