Patent ID: 7971519

Claim:
An apparatus for hydraulically adjusting the blades ( 2 ) of an impeller of an axial-flow fan, comprising: an adjustment cylinder in which is displaceably disposed a piston, wherein said adjustment cylinder is provided with a first chamber and a second chamber on opposite sides of said piston, and wherein each of said chambers is provided with a connection to a control oil line that is respectively divided into two branch lines; a feed line that leads to said branch lines of a first one of said control oil lines, wherein said feed line is divided into two parallel branch lines; a return line that is connected to said branch lines of a second one of said control oil lines, wherein said return line is divided into two parallel branch lines; two redundant four-way valves, each of which is disposed in one of said parallel branch lines of each of said feed line and said return line; and respective seat valves disposed in each branch line of said first one of said control oil lines and in each branch line of said second one of said control oil lines, wherein said seat valves are disposed between the respective connection and the respective four-way valve.