maintain a proper heat balance in the bearing system the unexpected and unobvious result emanating from the invention is believed to reside in the fact that the rotating piston generates a centrifugal force which increases the pressure in the hydraulic fluid progressively in the radial direction toward the periphery of the piston this progressively increasing hydrodynamic pressure is in addition to the hydrostatic pressure in the liquid and it provides stability against unbalanced forces acting on the rotating grinding discs for example when pulp material is unevenly distributed in the grinding space or when fiber bundles or chunks of wood become plugged therein or when the grinding segments have worn down unevenly another unobvious advantage resulting from the invention is the effective stiffening of the rotating shaft that may be attributed to the rotating piston which provides stability in its rotational plane thus for instance an eccentric loading of the rotating grinding disc tends to bend the shaft this tendency is resisted by the hydraulic fluid rotating with the piston around its perimeter which fluid is squeezed into the narrow gap between the piston periphery and the cylinder wall thus wedging the piston in its vertical plane of rotation this self generated wedge effect is illustrated by fig5 and 6 the progressively increasing hydrodynamic force is shown by the arrows in fig5 which counteract the forces f 1 and f 2 exerted on the grinding discs fig6 illustrates how the oil rotating with the end surface of the piston will be squeezed out in a radial direction when travelling from a to b thus further increasing the pressure at b and which pressure tends to force the shaft back to its center of rotation thus counteracting the deflective thrust forces the effect.