NOVELTY - A shaft confinement system comprises tappets pitched around a rotor ring. The tappets are adjustable and spaced a small distance away from the outwards-facing surface of the rotor. The tappet clearance is small enough that, when the shaft undergoes a radial vibratory excursion, the outwards-facing surface impacts against the tappet at an amplitude small enough to be non-damaging to the shaft, bearings, ring, tappets and fixed housing.    USE - For limiting the amplitude of transverse vibratory excursions of a rotating shaft (claimed).    ADVANTAGE - The invention permits a shaft to rotate at or close to its critical speed.    DETAILED DESCRIPTION - A shaft confinement system comprises a shaft mounted in journal bearings including a left bearing and a right bearing. The left bearing includes a left bearing shaft portion and a left bearing housing. The right bearing includes a right bearing shaft portion and a right bearing housing. The bearing shaft portions are components of the shaft. The bearing housings are components of a fixed housing of the apparatus. The bearings define an axis of rotation of the shaft. They are structured to constrain the bearing shaft portions against radial movement, relative to the respective bearing housings. The remainder of the shaft other than the left and right bearing shaft portions is termed the un-bearinged portion of the shaft. The left and right bearing housings constrain the left and right bearing shaft portions respectively of the shaft against transverse vibratory excursions but they do not constrain the un-bearinged portion of the shaft against radial excursions. The shaft includes a rotor (20), which is mechanically solid with the shaft. The rotor lies in the un-bearinged portion of the shaft. It is axially spaced on the shaft away from the left and right bearings. The rotor has a ring (29R) with an outwards-facing surface (30), which is concentric with the shaft axis. The outwards-facing surface of the ring is circumferentially continuous. The apparatus includes several excursion-limiting tappets (39). The tappets are pitched around the ring and carried in respective tappet housings (46), which are mounted to a hoop (32R) of a fixed housing. The tappets have respective contact tips. The contact tips are positioned radially outside and clear of the outwards-facing surface of the ring, by respective tappet-clearance distances (36). The tappets are provided with respective operable tappet adjusters (50) and respective operable adjustment locks (52). Each tappet adjuster is effective, when operated, to move the tappet radially towards and away from the outwards-facing surface of the ring and to reduce and increase its tappet clearance (48), which is the distance between its contact tip and the outwards-facing surface of the ring. Each adjustment lock is effective, when operated, to lock the tappet at the adjusted position relative to its tappet housing and at its adjusted tappet clearance. The tappet lies in a condition of adjustment in which, when the shaft is turning slowly about the axis defined by the left and right bearings, its tappet clearance is large enough that the tappet is clear of the outwards-facing surface of the ring. The tappet clearance is small enough that, when the shaft undergoes a radial vibratory excursion, the outwards-facing surface of the ring impacts against the tappet at an amplitude A of the excursion, which is an amplitude that is small enough to be non-damaging to the shaft, bearings, ring, tappets, and fixed housing. The tappet physically and mechanically restrains the outwards-facing surface of the ring and the shaft against an increase in amplitude of the excursions beyond amplitude A.    DESCRIPTION OF DRAWING(S) - The figure shows a close-up of a shaft confinement system of a Francis turbine.    Rotor (20)    Discharge ducts (23)    Ring (29R)    Outwards-facing surface (30)    Hoop (32R)    Inwards-facing surface (35)    Clearance distances (36)    Grooves (37)    Lands (38)    Tappets (39)    Throat (42)    Tappet housings (46)    Holes (47)    Tappet clearance (48)    Washer (49)    Tappet adjusters (50)    Adjustment locks (52)