TECHNICAL FIELD

[0001] The present application relates to the manufacturing process of a pair of variable-displacement pistons, each with a hollow body portion and with an actuator arm monolithically adjoining the hollow body portion.

BACKGROUND

[0002] Pistons of this type are commonly used in variable-displacement compressors. The hollow body proton, having a closed piston head opposite the actuator arm, encloses a cavity that is free of piston material and thus reduces the weight and inertia of the piston. The actuator arm extends axially opposite the piston head near the outer diameter of the piston body. At its free axial end remote from the piston body, the actuator arm includes an anti-rotation feature including pair of circumferentially or tangentially extending ears. The free axial end of the actuator arm further includes a central lobe extending along a radial plane radially inward to cover the longitudinal piston axis. Further, on the side of the actuator arm, the piston body bears a central extension with an axial indentation mirrored by a similar, opposite axial indentation in the central lobe. The opposite indentations form shoe pockets for supporting a typically spherical shoe of a swash plate or wobble plate. The displacement of the pistons is varied by changing the angle of the swash plate so that the piston stroke is varied. [0003] Due to the fairly complex shape of the pistons, pairs of variable- displacement pistons are currently manufactured from a pre-shaped blank in a triple- action forging press with a T-shaped action. The term T-shaped action means that in the triple-action forging press, two of the three movable parts are acting from opposite sides and the third movable part is centrally arranged at an angle perpendicular to the two opposite movable parts. The third movable part is a die dedicated to forming the shape of the actuator arms, by which the two pistons of the pair are connected in the platter exiting the press. Such a process is described, for example, in U.S. Patent 6,266,878.

SUMMARY

[0004] It is the object of the present application to provide a method of manufacturing a piston of the above-described type in a less complicated manner.

[0005] According to a first aspect of the invention, a method is provided for manufacturing a pair of variable-displacement pistons each of the variable-displacement pistons having a piston body with a hollow body portion with a closed end and an open end, and an actuator arm adjoining the hollow body portion at the closed end. Each of the variable displacement pistons has a piston head axially adjoining the open end of the hollow body portion. The method comprises the following steps: A first piston body is provided that includes a first hollow body portion with a first open end and a first closed end. The first closed end is adjoined by a first actuator arm monolithically formed with the first hollow body portion. A second piston body is provided that includes a second hollow body portion with a second open end and a second closed end. The second closed end is adjoined by a second actuator arm monolithically formed with the second hollow body portion. A monolithic dual piston head is provided that includes a first head piston head and a second piston head. The two piston heads extend parallel to each other at an axial distance spanned by a recessed hub connecting the first piston head to the second piston head. The first piston head is friction-welded to the first open end of the first piston body and simultaneously, the second piston head is friction- welded to the second open end of the second piston body. Subsequently, separating the first piston head is separated from the second piston head by removing the recessed hub to form a first variable displacement piston and a second variable displacement piston.

[0006] By producing the piston bodies separately, the piston bodies can be formed by shaping a piston body blank into the piston body by unidirectionally forging the piston body blank along a single axis. This applies to each of the first and second piston bodies. This process permits the use of a simple single-action forging press without requiring any specialized equipment.

[0007] According to a further aspect of the invention, alternatively, the piston body blank can be shaped into the piston body by forging the first piston body blank along two axes that are transverse to one another. This reduces the material in the piston body platter that needs to be removed by machining.

[0008] The step of friction-welding the first and second piston heads to the first and second open ends is preferably performed by spin-welding. The spin-welding process is expediently performed by fixedly securing the dual head part, abutting each of the piston heads with the open end of a piston body, and spinning the piston bodies under a simultaneous axial compression force until the open ends are each welded to one of the piston heads.

[0009] For facilitating the welding process, the piston heads of the dual piston head each preferably have an annular rim axially extending away from the recessed hub. The axial rim may an axial length smaller than the axial thickness of the piston head between the recessed hub and the annular rim. Further, the axial rim may an inner rim diameter equal to or smaller than the inner body diameter of the hollow body portion at the open end. Also, the axial rim may an outer rim diameter equal to or greater than the outer body diameter of the hollow body portion at the open end.

[0010] In one or two final steps, the pistons are machined to form undercuts.

[0011] Further details and benefits will become apparent from the following detailed description of the appended drawings. The drawings are provided herewith purely for illustrative purposes and are not intended to limit the scope of the present invention.

BRIEF DESCRIPTION OF THE DRAWINGS

[0012] In the drawings,

[0013] Fig. 1 a shows a side view of a variable-displacement piston;

[0014] Fig. 1 b shows the piston of Fig. 1 a in a different perspective;

[0015] Fig. 2a shows a side view of a first embodiment of a piston body platter formed by a first forging method for making a piston of the type shown in Fig. 1 a;

[0016] Fig. 2b shows the piston body platter of Fig. 2a in a cross-sectional view; [0017] Fig. 2c shows a dual head part for forming piston heads of two pistons of the type shown in Fig. 1 a;

[0018] Fig. 3 shows an alternative embodiment of a piston body platter formed by a second forging method for making a piston of the type shown in Fig. 1 a;

[0019] Fig. 4 shows a step of spin-welding two piston body platters of the type shown in Fig. 2a to a dual head part of the type shown in Fig. 2c; and

[0020] Fig. 5 shows a dual piston assembly after spin-welding.

DETAILED DESCRIPTION OF THE DRAWINGS

[0021] In the following description, the terms "generally longitudinal," generally axial," or "generally parallel" may be used. These terms include a minor taper from the longitudinal, axial, or parallel direction to a degree that still allows for removing forged platters 36 from a die. Accordingly the term "generally" includes any necessary or desirable draft for avoiding a die lock condition and for facilitating removing of the workpiece from the respective dies.

[0022] Referring to Figs. 1 a and 1 b, a variable-displacement piston 10 includes a piston body 12 forming a generally cylindrical hollow body portion 14 and an actuator arm 16 axially adjoining the hollow body portion 14. On the axial end remote from the actuator arm 16, the hollow body portion 14 is closed by a piston head 18. Further, on the axial side of the actuator arm 16, the hollow body portion 14 includes an axial elevation 20 with an indentation 22 shaped as a partial sphere.

[0023] The actuator arm 16 has an axial connector portion 24 that extends generally axially away from the hollow body portion 14 near the outer periphery of the hollow body portion 14. Near the axial end of the connector portion 24 remote from the hollow body portion 14, two lateral ears 26 forming an anti-rotation feature of the piston extend from the connector portion 24 in a generally circumferential or tangential direction with respect to the cylindrical axis X of the hollow body portion 14. A central lobe 28, arranged near the same axial end of the connector portion 24 as the lateral ears 26, extends radially inward to overlap with the cylindrical axis X. The central lobe 28 includes an indentation 30 shaped as a partial sphere that is arranged opposite the indentation 22 in the axial elevation 20 on the hollow body portion 14. The two indentations 22 and 30 and cooperate to form shoe pockets for supporting a typically spherical shoe of a swash plate or wobble plate (not shown). The variable-displacement piston 10 is typically cold-forged from an aluminum blank or an aluminum alloy blank.

[0024] In Figs. 1 a and 1 b, the piston head 18 is shown as unitarily formed with the hollow body portion 14. During the manufacturing process, at least one axial end of the hollow body portion 14 must be open for introducing a die (or for machining the internal cavity 32). Accordingly, the actuator arm 16 may be welded onto a cup-shaped part that includes the piston head 18 and the wall surrounding the cavity 32, or the piston head 18 may be a separate part welded onto the open piston body 12.

[0025] The manufacturing process of this or a similar variable-displacement piston 10 is illustrated in greater detail in Figs. 2a, b, and c, and in Figs 3 through 5. While the drawings show that a circumferential welding bead 34 is formed near the piston head 18, the welding seam may be moved axially closer to the actuator arm 16. Choosing the location of the weld close to the piston head reduces loads on the weld. Side loads on the piston created during compressor operation increase along the piston body toward the catuator arm 16.

[0026] Referring now to Figs. 2a and 2b, a piston body platter 36 is formed by a single-action forging process. "Single action" means that force is only applied in one direction along the single cylindrical axis X indicated by the arrows 38. While two arrows 38 are shown, they represent the force and the counterforce of a single moving die. For example, the die shaping the outside of the piston body 12 and the portions that later form the actuator arm 16 may be stationary, while the cavity 32 inside the piston body 12 may be formed by a moving die.

[0027] Single-action forging presses are standard equipment easily adaptable for producing the shape of the piston body platter 36 shown in Figs. 2a and 2b. To accommodate the requirements for single-action forging, the piston body platter 36 includes a pair of longitudinal ribs 40 that form longitudinal extensions of the anti- rotation ears 26 in order to eliminate any die lock conditions. Similarly, the outline of the central lobe 28 is axially extended toward the elevation 20 on the hollow body portion 14. A cylindrical boss 42 extending axially away from the hollow body portion 14 is formed on the piston body platter 36 adjoining the location of the central lobe 28.

[0028] These excessive portions of material along with any flash and bosses 42 will be removed in one or two final machining processs before or after welding the piston head 18 to the piston body 12. For example, a rough machining step before or after welding the piston head 18 to the piston body 12 may be performed for removing some or most of the excessive material without affecting the structural integrity of the piston body platter 36, and a fine machining step that separates the two pistons 10 from one another may then be applied after the welding step to give the piston 10 its final shape.

[0029] Fig. 2c shows a dual piston head 44 that includes two disc-shaped piston heads 18 coaxially extending parallel to each other and connected at a distance from each other via a central recessed hub 46. The piston heads 18 are preferably slightly hollowed out on the axial side facing away from the recessed hub 46 to form an annular rim 48 that can be friction-welded to the wall 50 of the hollow body portion 14. For example, each of the piston heads 18 may have an axial thickness between the recessed hub 46 and the annular axial rim 48 that is greater than the axial length of the annular rim 48. Further, the axial rim 48 has an inner rim 48 diameter that is equal to or smaller than the inner body diameter inside the wall 50 of the hollow body portion 14. Likewise, the outer diameter of the rim 48 may be at least equal or slightly larger than the outer diameter of the hollow body portion 14 at the open end.

[0030] Fig. 3 shows a piston body platter 56 formed in a dual-action process, which provides an L-shaped action as opposed to the l-shaped action of the piston body platter 36 of Figs. 2a and 2b. Because the forging press applies pressure from two directions (see arrows 38 and 52) that are transverse, in particular perpendicular, to each other, it is possible to eliminate the longitudinal ribs 40 of the platter 36 shown in Figs., 2a and 2b. It may still be advisable to fill the gap between the central lobe 28 and the elevation 20 with material in order to avoid deformations when the platter 36 is removed from the die. Like the platter of Figs. 2a and 2b, the platter 56 includes the axial boss 42 extending outward from the portion that later forms the central lobe 28. [0031] While the platter 36 of Figs. 2a and 2b, before machining, weighs about 150 grams, the platter 56 of Fig. 3 weighs less than 130 grams, in fact, only 124 grams.

[0032] Fig. 4 illustrates a process of spin-welding two piston body platters 36 to the dual piston head 44. The process of Fig. 4 is applicable to both embodiments of the piston body platter 36 shown in Figs. 2a and b as well as to piston body platter 58 in Fig. 3. Spin-welding is a type of friction-welding, in which the two parts to be welded together are pressed together (indicated by straight arrows 38) and simultaneously rotated relative to each other (indicated by curved arrows 54). In the spin-welding process of the variable-displacement piston 10, the dual piston head 44 is fixedly secured against rotation by fixating the recessed hub 46 in a stationary position. On each axial side of the dual piston head 44, one of the previously described piston body platters 36 is rotated by a fixture attached to the cylindrical bosses 42 under a simultaneous force applied in the axial direction toward the dual piston head 44.

[0033] Fig. 5 shows a pre-finished piston arrangement after spin-welding, in which welding beads 34 have formed between the wall 50 of the hollow body portion 14 and the rim 48 of the respective piston head 18 on both axial sides. At the shown stage, two raw pistons 58 are arranged in an orientation relative to each other, in which the portions that will form the piston heads 18 proximately face each other and are still connected via the recessed hub 46.

[0034] In a final step, the bosses 42, the recessed hub 46, the outer welding bead 34, the material between the central lobe 28 and the elevation 20, and any flash will be machined off to form a piston 10 like the one shown in Figs. 1 a and 1 b, including various undercuts. [0035] As mentioned above, while the drawings show the piston heads 18 with only a short rim 48 and the majority of the cavity 32 surrounded by the wall 50 of the hollow body portion 14, it is well within the scope of the present invention to place the welding location closer to the actuator arm 16 so that a larger portion of the cavity 32 is surrounded by the rim 48 of the piston head 18 and a smaller portion of the cavity 32 is surrounded by the wall 50 of the hollow body portion 14.

[0036] While the above description constitutes the preferred embodiments of the present invention, it will be appreciated that the invention is susceptible to modification, variation and change without departing from the proper scope and fair meaning of the accompanying claims.