Abstract: The transmission mechanism of the instrument is lodged in two fabricate bridges. Axles are journalled with one end in a one-piece bearing spring, allowing a simple, play free and low-friction bearing and a simple assembling. The bridges are connected together by means of positioning elements and snapped together in elastic holding tongues and held in the desired position. One of the axles can be axially adjusted by means of its bearing screw and thanks to its journalling in the bearing spring, in order to adjust the transmission ratio of the transmission mechanism. The bearing screw of the axle can be adjusted radially too by means of a screw and a flexible tongue in order to adjust the zero position of the tracer contacting the membrane, or to make a zero point correction.