This invention relates to an insert for use with a disc or impeller mounted on a rapidly rotating hollow shaft such as utilized in high speed turbomachinery, and more particularly to an insert which aids in securing an impeller mounted in overhung fashion to a rotatably driven hollow shaft in high speed turbomachinery such as the high pressure oxygen turbopump of the space shuttle main engine, the insert having a pair of annular surfaces cooperating with receiving surfaces in the impeller to provide a double pilot remaining tight at all speeds to aid in the elimination of whirl instability in such turbomachinery. 
The high pressure oxygen turbopump used in the space shuttle main engine uses duplex ball bearings, i.e., a pair of bearings mounted closely adjacent each other, near each end of the rotor.  A damping seal such as that disclosed in U.S. Pat.  No.  4,545,586, adjacent the pump end of the rotor besides reducing leakage between the fixed and rotating portions also damps rotor lateral motion and thus shares the bearing load in the pump portion of the turbopump.  The duplex bearings are each a pair of ball bearings having an axial preload force applied thereto to avoid ball skidding and wear.  The preload is provided by a spring located between the outer races of each pair of duplex ball bearings.  The ball bearing prevents seal rubbing and the damping seal eliminates instability due to whirl, that is, the orbiting of the rotor shaft due to fluid forces acting to urge the shaft tangentially, when radially from the rotational center, creating an eccentric rotation. 
Due to ball bearing wear the operational time of the high pressure oxygen turbopump of the space shuttle main engine is limited, thereby hindering the space shuttle program.  The excessive ball bearing wear is a result of excessive lateral or side loads.  Such side loads frequently may not be shared equally between the bearing units of one duplex bearing pair because of shaft bending, deformation of the bearing holders and other factors.  The forces on the rapidly rotating impeller may cause the side loads on a single bearing to be in excess of twice the bearing axial preload, which is substantially greater than the generally recommended one-half or one-third loading.  Thus, the balls are forced up and down the shoulders of the bearing races cyclically as the ball train rotates at approximately 43 percent of the shaft speed.  These excursions of the balls cause ball skidding, excessive heating and excessive wear of the bearings.  Additionally, during turbopump start-up and shut-down axial overloads are present which require that the balls have large contact angles to counteract these axial forces.  On the other hand, the large lateral or side loads during operation require small contact angles if over-stress is to be prevented.  Where the outer races are permitted to float and the balls have large contact angles, spinning of the outer races occurs and load shearing by the bearing with the damping seal is limited.  Accordingly, compromises in the bearing design have been made which have resulted in the aforesaid limitations in bearing wear. 
In copending U.S. patent application Ser.  No.  364,743 filed 5-26-89 there is proposed damping seals, damping bearings and support means for the ball bearings of the rotor of such high speed machinery which reduces a substantial number of these problems by preventing tilting and spinning of the outer races; aids in supporting the shaft at low speeds; avoids and counteracts radial loading of the bearings while absorbing axial loads at high speed; and eliminates a substantial amount of the whirl driving forces.  However, there are other whirl drivers in such high speed turbomachinery.  For example, the impeller is rotatably connected to the shaft by a spline, and spline friction is a potential source for tangential forces which result in whirl instability.  This is especially a problem where the impeller projects a substantial amount beyond the end of the shaft such as in the case with a centrifugal impeller of the type used in the high pressure oxygen turbopump of the space shuttle main engine.