In this article, the photoelasticity method is used to study the change of forces during gear engagements. The kinetostatic analysis forecasts a change in normal force between involute gear teeth when the point of contact passes the pitch point. This happens because of the action of friction forces between teeth when the torques on the gear wheels are constant. To prove this, measurements of the forces between teeth are implemented with the aid of the photoelasticity method. A plane polariscope and a gear test rig with a closed mechanical circuit are used. The experimental gears are made from epoxy resin. The experiments confirm the theoretical results.