Method for manufacturing a piston rod unit

A method of manufacturing cylinder piston device comprising a cylinder (1) having an axis and two end walls and defining a chamber therein, at least one of said end walls being provided with an aperture and sealing means surrounding said aperture, a tubular piston rod member defining a cavity therein and extending in axial direction through said aperture and said sealing means, a piston unit within said chamber and fixed to the inner end of said tubular piston rod member, a terminal member provided on the outer end of said tubular piston rod member, at least one of said piston unit and said terminal member being fixed to said tubular piston rod member by a fastening pin entering into a respective end portion of said cavity, said method comprising the following steps:

• (a) providing a steel-made, tube-shaped piston rod member (3) with a constant initial inner diameter (d.sub.i) and constant initial outer diameter (d.sub.o) over the total axial length thereof;
• (b) providing said fastening pin (4a) to be introduced into said cavity, said fastening pin being made of steel and having
• (b1) an inner, substantially cylindrical end portion (7);
• (b2) an inner end face (7a) at the inner end of said inner end portion (7);
• (b3) an annular axially extending frusto-conically shaped expanding face (8) having a first end with a smaller diameter (d.sub.s) adjacent said inner end portion (7) and substantially corresponding to said initial inner diameter (d.sub.i) of said tube-shaped piston rod member (3), and a second end with a larger diameter (d.sub.1) adjacent the axial end of said expanding face (8) remote from said inner end face (7a), said larger diameter (d.sub.1) being larger than said initial inner diameter (d.sub.i) of said tube-shaped piston rod member;
• (b4) at least one substantially axially directed shoulder face (6b) directed away from said inner end face (7a) being located on the side of said expanding face (8) which is remote from said inner end face (7a);
• (c) introducing said inner end portion (7) into said cavity;
• (d) urging said fastening pin (4a) into said cavity so as to expand said tube-shaped piston rod member (3) along an axial end portion (3a) thereof by said expanding face (8), the outer diameter of said tube-shaped piston rod member (3) being expanded to an expanded diameter (d.sub.ex) along said end portion (3a), said expanded diameter (d.sub.ex) being larger than the initial outer diameter (d.sub.o) of said tube-shaped piston rod member and the expanded inner diameter of said piston rod member corresponding to both the larger diameter second end and the annular axially extending frusto-conically shaped expanding face of said expanding face;
• (e) radially compressing said expanded end portion (3a) of said tube-shaped piston rod member (3) by cold shaping so as to reduce said expanded diameter (d.sub.ex) substantially to said constant initial outer diameter (d.sub.o) and to obtain axial engagement of said steel of said tubular piston rod member (3) with said shoulder face (6b);
• and
• (f) assembling said cylinder, said tubular piston rod member, said piston unit and said one end wall such that at least part of said end portion (3a) of said tubular piston rod member (3) is free to pass through said aperture and said annular sealing means in operation.