A turbocharger has a shaft formed of a strong material such as steel and an impeller formed of a relatively weaker material such as aluminum alloy. The impeller is mounted on the shaft by means of an internally-threaded insert, for example also of steel, interference fitted into the impeller hub. To prevent loss of the interference fit between the insert and impeller during thermal and speed excursions of the impeller during operation of the turbocharger, a constraining ring is fitted to the hub of the impeller. The ring is of a material having a lower coefficient of thermal expansion than the material of the impeller and surrounds at least a part of that axial length of the impeller which overlies the insert.