In a pair of assembled helical gear pairs, backlash J refers to the clearance between the tooth surfaces of meshing helical gears. The side clearance can be measured on the normal plane or along the meshing line, as shown in the figure, but it should be noted that it is calculated and specified on the end plane or meshing plane, i.e. the tangent plane of the base circle. A single helical gear has no backlash. It has only tooth thickness. The backlash of meshing helical gears is controlled by the center distance of a pair of helical gears and the actual tooth thickness of each helical gear.

The meshing helical gear pair must have a certain backlash and ensure that the non working tooth surfaces will not contact each other. In a fixed meshing, the backlash is changed by the changes of speed, temperature, load and many other factors during operation. Under the condition that the static state can be measured, it is necessary to ensure that there is enough backlash so that the helical gear still has enough backlash under the most unfavorable working environment of loaded operation. The amount of backlash required is related to the size, accuracy, application and installation of helical gears.