a pressure relief valve 164 the solenoid valve 146 and line 144 back to the hydraulic fluid reservoir 122 the rotor 18 is now in its raised position shown in fig3 solenoid valve 158 is returned to its second i e central position as illustrated in fig6 this locks the jack 46 in its raised position a sensor not shown is provided to indicate that the trolley 102 is in position with its forks 100 engaged beneath the cam plate 78 solenoid valve 158 is adjusted to its first position i e downwardly in fig6 the weight of the rotor 18 then forces hydraulic fluid out of the cylinder space 50 via a fluid flow restrictor 166 the conduit 160 the solenoid valve 158 and the conduit 144 back to the hydraulic fluid reservoir 122 at the same time the solenoid valve 154 is adjusted upwardly in fig6 into its third position hydraulic fluid flows via a conduit 170 into the cylinder space 44 the pressure of fluid in the conduit 170 opens a check valve 172 and hydraulic fluid is forced out of the cylinder space 36 via the valve 172 a non return valve 174 the solenoid valve 154 and the conduit 144 into the reservoir 122 the clamp up member is thus fully raised to release the rotor.