Unilateral Tolerance - A unilateral tolerance is a tolerance in which variation is permitted only in one direction from the specified dimension, example, 1.500 +. 005/ -. 000."
Hence when tolerance is given on one side of the basic dimension, it is called Unilateral tolerance.

Wing nut is used for hand vice.

Spring washers are used under nuts to prevent slackness of nuts due to vibration.

When two parallel and coplaner shafts are connected by gears having teeth parallel to the axis of the shaft called spur gears, the arrangement is known as spur gearing.

Another name given to the spur gearing is helical gearing in which the teeth are inclined to the axis of the shaft. There are single and double helical gears. The double helical gears are known as herringbone gears.