50a is deformed to take the shape of the end 50b the flattened end 50b no longer passes through the openings in the washer 56 the washer is fixed and the cartridge is complete fig5 d to avoid buckling of the spacer springs fig6 a shows an element 19 with two pins 60 62 around a central zone 61a of a guide 61 fitted with a collar 63 this collar 63 is placed between two bundles of spacer springs 64 65 so that it is still guided in the recess of the projecting portions 11 12 of the pistons 5 6 this ensures low friction which results in a high yield for the actuator in order to increase the stability of the guide 61 the plates 61a 61b are attached to the ends by the parts 61d 6ie the structure of the guide 61 will be better understood from fig6 b 6d there is the collar 63 around the central zone 61a from the centre upwards in fig6 b run two plates 61b 61c enclosing a space 67 the plates 61b 61c do not cover the full width of the guide but leave shoulders 68a 68b for guiding the pin 60 in a first plane at the end of the plates 61b 61c is the part 61d connecting the plates 61b 61c and closing off the space 67 in a perpendicular plane the pin 60 is guided by the plate 69 from the centre downwards in fig6 b here again there is a part 61e providing the connection to the second plate 70 shown in fig6 c fig6 c shows the symmetrical structure of the.