The present invention relates to a system for changing or setting the pitch of the blades of a turbomachine propeller or rotor. 
More particularly, although not exclusively, the system is intended to be fitted to turbomachines made up of a pair of coaxial and counter rotating propellers, known in English as “open rotors” or “unducted fans”, the pair of propellers being likened to the fan of turbojet engines. 
There are two structural solutions that are currently being developed, one with the pair of propellers arranged downstream of the turbomachine, the open rotor being qualified as a “pusher”, and the other with the pair of propellers arranged upstream of the turbomachine, the open rotor being qualified as a “puller”. 
Without going into the advantages and disadvantages of these two solutions, which do not form part of the subject matter of the invention here, the pitch of the propeller blades is made adjustable by a pitch change system so as to allow the turbomachine to operate optimally in the various operational phases encountered during flight and on the ground.  Thus, such as system is fitted to each rotor and notably comprises a control actuator that allows action on a transmission device involving link rods and radial arms or shafts which act on the rotary pivot pins or roots of the blades in order to modify the angle at which they are set. 
The actuator of the orientation system is controlled by a hydraulic-fluid feed device which, by means of the orientation system thus fed with fluid, allows the pitch of the relevant propeller to be varied for the following operational phases: 1) in flight, for example between +90° to 30°, 2) on the ground and in reverse between +30° and −30°, and 3) returning to the feathered (90°) position in the event of an in-flight malfunction. 
The actuator of the system is usually a linear-displacement ram that moves under the action of the oilfeed device, and its mobile part is connected to the transmission device via at least one bearing.  Thus, translational movement of the mobile part drives the inner ring of the bearing the rotary outer ring of which, via the rolling elements, transmits the translational movement to the rotary transmission device connected to the rotor of the propeller. 
In order to modify the angular position of the roots of the blades mounted in the conventional way in a polygonal ring of the rotor of the propeller, the transmission device comprises transmission link rods arranged around the bearing or bearings and, connected to these link rods, as many radial arms as there are blades, each radial arm acting directly on the blade roots.  Hence, each radial arm has to pass through cylindrical casings, notably a wall or walls depending on the engine ventilation requirements, that allow oil to return from the rotary casing enclosure to the lubrication oil discharge system.  Problems of sealing between the walls of the casings passed through and all of the radial arms then arise. 
In addition, given the design of these systems, the hydraulic actuator may be highly influenced by the non-axisymmetric aerodynamic loadings applied to the propeller (bending moment applied to the system about an axis perpendicular to the longitudinal axis of the turbomachine) which may go so far as to cause the system to jam. 
It is an object of the present invention to overcome these disadvantages by providing a novel solution.