The invention concerns a turbine for the expansion of gas and vapor, comprising a body or case, a fixed external sleeve member, overhanging frontally from a frontal wall of said case, and a turbine shaft (15) extending coaxially and rotating in said external sleeve member (18) with the interposition of a supporting unit (19). The external sleeve member has a head flange to axially hold said supporting unit; the turbine shaft ends with a joint to connect it to a device to be operated. The supporting unit (19) includes ball bearings to support the turbine shaft (15) in said external sleeve member (18), and between said external sleeve member (18) and a contiguous end of said turbine shaft (15) an aggregate (50) is provided to generate an adjustable axial force so as to compensate the axial thrust of the turbine shaft to limit the application of improper axial loads on the ball bearings.