embodiments will be described with reference to the accompanying drawings . fig1 shows a piston assembly in a fitted state according to one embodiment . the piston assembly 1 is mounted on the eccentric shaft 46 by means of a rolling bearing . shown in the fitted state are the outer bearing ring 7 of the rolling bearing , the rotary pistons 2 and the fuse body arranged in first and second recesses . the fuse body acts against an undesired movement of the outer bearing ring 7 within the central bore of the circular piston 2 by means of a positive connection . fig2 shows a piston assembly 1 and a fuse body 8 according to another embodiment with wave form shaped end faces 17 of the fuse body 8 . the rotary piston 2 comprises a circular central bore 3 , three curved first constraint surface 4 , an axis of a bore 5 and a first axis of rotation axis 6 . in the present case the axis of the bore 5 and the first axis of rotation 6 . the rotary piston also comprises an inner toothing 20 for interaction with the eccentric shaft not shown . the outer bearing ring 7 and the fuse body 8 , both shown in sectional view , are arranged in the central bore 3 . the fuse body 8 is interrupted by the third recess 16 and comprises wave form shaped end faces 17 . the protrusions of the end face can allow , that a lubricant between the bearing ring and the rotary piston can be replaced . the protrusion of the end face 21 , 21 a are opposite to each other . the protrusion of the end faces 21 , 21 a are provided for contacting or to positively connect with the first and second recess . the fuse body 8 is transformable in an elastically deformed state , in particular for the insertion of the bearing ring 7 in the central bore 3 . fig3 shows an inventive piston assembly 1 according to another embodiment in sectional view and two enlarged details . in the rotary piston 2 is the outer bearing ring 7 fitted and by means of a connecting ring 8 secured against any unwanted axial displacement within the rotary piston 2 . the bearing ring 7 comprises a chamfer 14 to facilitate insertion into the rotary piston 2 . the surface of the bore 9 and outer surface of the bearing ring 11 coincide . it is also shown , that the connecting ring 8 is arranged in both the first recess 10 and second recess 12 . fig4 shows an inventive piston assembly 1 according to another embodiment with wave form shaped end faces of the fuse body 8 in disassembled state . in the surface of the bore 9 a first recess 10 is formed as a circumferential groove . in the outer bearing ring 7 or the outer surface of the bearing ring 11 a second recess 12 is formed as a circumferential groove 12 . the bearing ring 7 comprises two chamfers 14 . fig5 shows the piston assembly 1 according to fig4 , where the fuse body 8 is inserted into the first recess 10 of the surface of the bore 9 . the fuse body 8 is formed as a connecting ring , and is inserted in elastically tensioned state into the first recess 10 . now the outer bearing ring 7 can be inserted into the central bore of the circular piston 2 . fig6 shows the piston assembly 1 according to fig4 and 5 , wherein the outer bearing ring 7 is partially accommodated by the central bore . the outer bearing ring 7 has not yet reached its predetermined position within the central bore . fig7 shows the piston assembly 1 according to fig4 and 6 , wherein the outer bearing ring 7 is arranged in its intended position within the rotary piston 2 . it is not shown that the connecting , ring 8 , after which the outer bearing ring 7 reached its predetermined position has eased with partial displacement in the non - visible second recess . fig8 a and 8b show sectional views of an inventive piston assembly 1 according to another embodiment . fig8 a shows a piston assembly , wherein the bearing ring 7 is disposed in its predetermined location within the cylinder bore . thus , the first recess 10 and second recess 12 are opposite . the surface of the bore 9 and 11 the outer surface of the bearing ring contact each other . the fuse body 8 comprises a rectangular cross section , in particular the fuse body 8 is formed as a segment of a hollow cylinder . the first recess 10 and second recess 12 are each formed as a groove having a rectangular cross - section . the fuse body 8 is positively connected by the first recess 10 and second recess 12 . the fuse body 8 is applied by means of the spring 19 with a force in the radial direction of the circular piston 2 . by the positive fit of the fuse body 8 with the first recess 10 and second recess 12 , the outer bearing ring 7 secured against any unwanted movement of the circular regarding piston 2 . fig8 b shows a rotary piston assembly , according to another embodiment , in a sectional view , wherein the bearing ring 7 has not yet reached its predetermined position within the central bore . thus , the spring - loaded fuse body 8 is essentially completely accommodated in the second recess 12 . upon reaching the predetermined position , the first recess 10 and the second recess 12 would be oriented toward each other . the fuse body 8 would be force in the first recess 10 . fig9 a and 9b show sectional views of a piston assembly with a wire made fuse body 8 according to another embodiment ,. the fuse body 8 extends along the direction along a main extension of the first recess 10 and the second recess 12 . fig9 a shows in a sectional view a rotary piston 2 with a v - shaped first recess 10 , an outer bearing ring 7 with a second also v - shaped recess 12 and a fuse body 8 according to another embodiment . although shown with four circles , consists fuse body 8 only of a single pressure - formed wire with a main direction of extension along the extension of the grooves formed as first recess 10 and second recess 12 . with the circles cross sections a , b , c , d are shown in different positions of the fuse body 8 . so the fuse body 8 touches at different positions along its main direction different constraint surfaces of the first recess 10 and second recess 12 . the space formed by the first recess 10 and second recess 12 accommodates the fuse body 8 in an elastically deformed state . the contact regions of fuse body 8 are forced against the constraint surface of the first recess 10 and the second recess 12 for positively connected transmission of a force to both the rotary piston 2 and the outer bearing ring 7 as well . fig9 b shows the bearing ring 7 with a formed in second recess 12 which is formed as a v - shaped groove . along the second recess 12 , the fuse body 8 extends , which is formed as a pressure - formed wire . along its main direction , the fuse body 8 comprises several contact regions 15 , 15 a , 15 b , 15 c , 15 d . the contact regions 15 b and 15 c of the second constraint surface contact the recess 12 . the contact regions 15 a and 15 d contact constraint surfaces of the first recess , not shown in the rotary piston . the bearing ring 8 is elastically deformed . fig1 a and 10b show sectional views of a piston assembly with elastically deformable fuse body with protrusions of the sidewall and projections on the outer circumferential surface of the fuse according to another embodiment . fig1 a shows a sectional view of a piston assembly , according to another embodiment , with elastically deformable fuse body 8 with protrusions of the sidewall 18 , 18 a , 18 b , 18 c . the outer bearing ring 7 comprises a second recess 12 , which is formed as a groove . in the second recess 12 of the fuse body 8 is inserted . the fuse body 8 comprises a plurality of end face projections 18 , 18 a , 18 b , 18 c , each of them transmits a normal force component to the second recess 12 . thus , the bearing ring 8 is accommodated in the second recess 12 positively connected . it is not shown , that the fuse body 8 is also positively connected in the first recess in the surface of the bore . the fuse body 8 comprises in an elastically deformed state the room of not shown first recess and second recess 12 . with a design according to this embodiment manufacturing costs are reduced because only the end face projections 18 , 18 a , 18 b , 18 c need to have low size tolerances . with this embodiment , material and energy savings are reachable . further , the end face projections allow that a lubricant between the bearing ring and the rotary piston can be replaced . the fuse body 8 is designed as an interrupted connection ring . the connecting ring 7 is completely insertable into the central bore substantially in the second recess 12 prior to insertion of the bearing ring , in particular the fuse body 8 is converted into an elastically deformed state . fig1 b shows a fuse body 8 whose fuse body circumferential surface also comprises a projection 18 a . this projection 18 a is particularly used to save material and machining costs because instead of the entire fuse body circumferential surface only the projection 18 a needs to be made with low size tolerance . further the end face projections can allow that a lubricant between the bearing ring and the rotary piston can be replaced . while the above description has pointed out features of various embodiments , the skilled person will understand that various omissions , substitutions , and changes in the form and details of the device or process illustrated may be made without departing from the scope of the appended claims .