The throttle shaft (2) carries a plastics trip lever (3) at one end (7). The lever end of the shaft has a conical sector (8) roughened by grooves (9). The lever engages the shaft by plastic or elastic deformation. This deformation is obtained by applying ultrasonic pressure to the shaft and the lever or simply by pressing the lever down onto the cone sector. The deformation stage is carried out with the throttle set to a pre-set air throughput and with the lever forced against a housing stop by a spring (5). The adjusted throttle is maintained in this condition by a tool applied to an extension (11) of the shaft. The closing force produces a wide area contact between the housing stop and the trip lever.
