Title of Invention: Method for fastening a rebound stop to a piston rod of a vibration damper
Abstract: A method for fastening a rebound stop to a piston rod of a vibration damper positioned at a piston rod and axially supported with a first tool. A second tool component having a press-in surface that presses the tubular portion of the rebound stop into the circumferential groove of the piston rod. The first and the second tool carry out an movement toward one another. At least one tool presses the tubular portion of the rebound stop into the circumferential groove of the piston rod. The pitch angle of the press-in surface varies over the axial longitudinal extension of the press-in surface. As the distance between the tools decreases, the force ratio between the axially directed pressing force and the radially directed pressing force changes such that the axially directed pressing force decreases and the radially directed pressing force increases.
Detailed Description: Part general_description_paragraphs: Part CROSS REFERENCE TO RELATED APPLICATIONS: This is a U.S. national stage of application No. PCT/EP2014/069977, filed on Sep. 19, 2014. Priority is claimed on German Application No. DE102013220627.5, filed Oct. 14, 2013, the content of which is incorporated here by reference.
Part BACKGROUND OF THE INVENTION: 1. Field of the Invention
The present invention is directed to a method for fastening a rebound stop to a piston rod of a vibration damper and to the piston rod/rebound stop assembly produced in so doing. Vibration dampers having a piston rod/rebound stop assembly as component part are well known in the automobile industry. These vibration dampers comprise at least one working cylinder which is at least partially filled with a damping medium and at least one piston arranged inside the working cylinder so as to be axially displaceable with respect to the longitudinal axis of the working cylinder and which divides the working cylinder into two working chambers. The piston is connected on one side to a piston rod which is sealingly guided out of the working cylinder through a piston rod guide and is fixed to a further fastening component part of a motor vehicle. The piston generally has at least one orifice which joins the two working chambers and at least one throttle disk which at least partially covers the orifice on one side.
Conventionally, the end of the working cylinder remote of the piston rod is closed by a cylinder base and is at least indirectly arranged at a component of the motor vehicle that supports the vehicle wheel. When the piston is axially displaced inside the working cylinder, one working chamber becomes smaller and the other working chamber becomes larger. The relative change in size of the two chambers results in a pressure difference between the two chambers which allows the damping medium to flow out of the smaller working chamber through the orifice of the piston into the larger working chamber. The throttle disk inhibits the flow of the damping medium, which causes a damping effect.
2. Description of the Prior Art
In some operating conditions, for example, when driving over potholes, that the piston will move at a greater velocity in direction of the piston rod guide and strike the latter. This can lead to a leak in the piston rod seal or can damage the piston. To avoid this, vibration dampers are outfitted with a rebound stop that is axially fastened to the piston rod and serves to prevent the above-described collision between piston and piston rod guide.
The rebound stop can be fastened to the piston rod in different ways. It can be welded to the piston rod or connected to the piston rod in some other way by bonding. Since the piston rod is a highly precise component part that ideally may not have any deformations after the piston rod is fabricated, the bonding connection between the rebound stop and the piston rod is very inconvenient and entails high costs.
Further, the piston rod can have a circumferential groove as disclosed, for example, in DE 78 10 988 U1. The groove receives a snap ring and secures a rebound stop axially to the piston rod. This fastening method is common. To fasten the snap ring to the piston rod, the snap ring must be opened so that it can pass over the portion of the piston rod preceding the circumferential groove, if possible without contacting this portion. The snap ring then engages in the circumferential groove of the piston rod and secures the rebound stop axially to the piston rod. In addition, a possibility for axial fastening must be implemented for fixing the snap ring to the piston rod so that the snap ring cannot slip out of the groove under heavy loading of the rebound stop. Therefore, the above-mentioned fastening can also be cost-intensive.
A positive-engagement connection of the rebound stop to the piston rod can also be implemented in that the rebound stop is axially fastened to the piston rod through pressing as is disclosed in DE 78 10 988 U1 or DE 85 20 989 U1.
DE 10 2011 089 140 B3 also shows a rebound stop which encompasses around the piston rod in circumferential direction and which is axially fastened to the piston rod between the piston and the piston rod guide. The rebound stop is connected to the piston rod by frictional engagement. According to the constructional variant shown in DE 10 2011 089 140 B3, the piston rod has a circumferential groove, and the rebound stop is pressed into the groove to implement the positive-engagement connection between the piston rod and the rebound stop.
A fastening method having at least two method steps, for example, is known from the prior art. In the first method step, a rebound stop is positioned at a piston rod. The rebound stop encompasses the piston rod in circumferential direction and has a disk-shaped portion and, adjacent thereto, a tubular portion facing the piston rod. The piston rod has a circumferential groove.
The rebound stop is axially supported by the disk-shaped portion at a first tool component and is axially positioned such that the tubular portion of the rebound stop at least partially covers the circumferential groove of the piston rod. In the second method step, a second tool component encompassing the piston rod in circumferential direction is brought into contact with the tubular portion of the rebound stop. The contact surface of the second tool component is sloped, in particular the surface is conical, with a constant pitch, and the cone narrows radially inwardly in direction of the piston rod.
The first tool component and the second tool component then carry out an axial relative movement with respect to the longitudinal axis of the piston rod directed toward one another so that the distance between the two tool components is reduced to a defined final dimension.
In this respect, the second tool component exerts a pressing force directed axially in direction of the disk-shaped portion and a pressing force directed radially inward, i.e., toward the center of the piston rod, on the tubular portion of the rebound stop and presses the latter into the circumferential groove of the piston rod.
A disadvantage of this solution is that as the distance between the tool components decreases an increasingly greater radial pressing force is required to allow an optimal pressing and, therefore, an optimal contact of the tubular portion of the rebound stop in the circumferential groove of the piston rod.
A decrease in the axial pressing force during the decrease in the distance between the tool components is not desirable because this would inevitably lead to rapid tool wear.
Part SUMMARY OF THE INVENTION: It is an object of the present invention to further develop the fastening method mentioned above in such a way that an optimal contact of the tubular portion of the rebound stop in the circumferential groove of the piston rod is achieved and tool wear can nevertheless be minimized.
By changing the pitch angle of the press-in surface with reduction of the distance between the tool components, the force ratio between the axially directed pressing force and the radially directed pressing force changes such that the axially directed pressing force is reduced and the radially directed pressing force is increased.
As a result of a large radially directed pressing force, an optimal contact of the tubular portion of the rebound stop in the circumferential groove of the piston rod is achieved, and the axially directed pressing force during a press-in process is reduced such that the tool is protected.
According to an advantageous constructional variant, the press-in surface of the second tool component can be constructed to be at least partially convex. In this case, the tubular portion of the rebound stop would have a press-in portion which has a circumferential, at least partially concave surface. Further details and advantages of the present invention result from the following description of an embodiment example in conjunction with the drawings.
Part DETAILED DESCRIPTION OF THE PRESENTLY PREFERRED EMBODIMENTS: FIG. 1a), FIG. 1b), FIG. 1c) and FIG. 3 show a fastening method known from the prior art wherein a rebound stop 1 is positioned at and fixed to a piston rod 2. The rebound stop 1 encompasses the piston rod 2 in circumferential direction and has a disk-shaped portion 3 and, adjoining the latter, a tubular portion 4 facing the piston rod 2. The piston rod 2 has a circumferential groove 5. In the embodiment example shown in the drawing, the groove 5 has a contact portion 9 for contacting the tubular portion 4, this contact portion 9 being constructed oblique to the longitudinal axis A of the piston rod 2. Further, the groove has a radius R which serves to prevent a notch effect.
The rebound stop 1 is axially supported by the disk-shaped portion 3 at a first tool component 6 and is axially positioned such that the tubular portion 4 of the rebound stop 1 at least partially covers the circumferential groove 5 of the piston rod 2. A second tool component 7 encompassing the piston rod 2 in circumferential direction is brought into contact with the tubular portion 4 of the rebound stop 1 at the press-in surface 8 thereof. The shape of the press-in surface 8 of the second tool component 7 is defined by a cone-shaped recess 10 in the second tool component 7 and extends obliquely with constant pitch with respect to the longitudinal extension axis A of the piston rod 2, and the cone narrows radially over its entire longitudinal extension in direction of the piston rod 2.
The first tool component 6 and the second tool component 7 then carry out an axial relative movement with respect to the longitudinal axis A of the piston rod 2 directed toward one another so that the distance between the two tool components 6; 7 is reduced to a defined final dimension 11. In so doing, the second tool component 7 exerts a normal force FN on the tubular portion 4 of the rebound stop 1, which normal force FN is directed in direction of the normal vector, i.e., perpendicular to the press-in surface 8, and presses the tubular portion 4 of the rebound stop 1 into the circumferential groove 5 of the piston rod 2.
For better comprehension of the manner of functioning, the normal force FN can be broken up vectorially into a pressing force FA directed axially in direction of the disk-shaped portion 3 and a pressing force FR directed radially inward, i.e., toward the center axis A of the piston rod 2.
Due to the constant pitch angle of the press-in surface 8, the direction of action of the normal force FN and, therefore, also the force ratio between the axially directed pressing force FA and the radially directed pressing force FR is always constant.
In FIG. 3, the force ratio of the forces acting on the tubular portion 4 of the rebound stop 1 are shown by way of example and in a clearer manner at three acting points P1, P2 and P3 along the press-in portion 14. The direction of action of the normal force FN1, FN2 and FN3 acting on points P1, P2 and P3 is always the same because of the constant pitch, so that the respective force ratio of radially directed force FR1, FR2, FR3 to the respective associated axially directed force FA1, FA2, FA3 is always equal.
In order to achieve an optimal contact of the rebound stop 1 at the surface of the contact portion 9 in the groove 5, a very high radially directed pressing force FR must be exerted on the tubular portion 4 and, accordingly, successively increased during the pressing process.
Owing to the constant force ratio between the radially directed pressing force FR and the axially directed pressing force FA, the axially directed pressing force FA increases in proportion to the increase in the radially directed pressing force FR, which highly stresses the tool components 6; 7.
FIG. 2a), FIG. 2b) and FIG. 4 show the fastening method according to claim 1. The press-in surface 8 of the second tool component 7 is constructed such that the pitch angle of the press-in surface 8 varies at least partially over the axial longitudinal extension of the press-in surface 8 so that as the distance between the tool components 6, 7 is reduced the force ratio between the axially directed pressing force FA and the radially directed pressing force FR changes such that the axially directed pressing force FA decreases and the radially directed pressing force FR increases.
The very high radially directed pressing force FR that can be achieved presses the tubular portion 4 of the rebound stop 1 into the circumferential groove 5 of the piston rod 2, which allows an optimal contact of the tubular portion 4 at the contact portion 9 of the groove 5 and at the radius R1 adjoining the latter. The circumferential groove 5 of the piston rod 2 virtually completely takes up the material of the tubular portion 4 of the rebound stop 1 that is displaced from the second tool component 7 by the high radially directed pressing force FR.
FIG. 5 shows an exemplary constructional variant of the second tool component 7. The second tool component 7 has a receiving portion 12 for receiving the piston rod 2, which receiving portion 12 can be constructed as a receiving bore hole 13, and a press-in surface 8 adjoining it. The press-in surface 8 of the second tool component 7 is formed convexly. Further, the press-in surface 8 has a radius R2 which is shown in cross section. Of course, the press-in surface 8 can also be realized without a radius and instead, for example, as a succession of portions with different pitch. This constructional variant is not shown explicitly, but is also possible.
Thus, while there have shown and described and pointed out fundamental novel features of the invention as applied to a preferred embodiment thereof, it will be understood that various omissions and substitutions and changes in the form and details of the devices illustrated, and in their operation, may be made by those skilled in the art without departing from the spirit of the invention. For example, it is expressly intended that all combinations of those elements and/or method steps which perform substantially the same function in substantially the same way to achieve the same results are within the scope of the invention. Moreover, it should be recognized that structures and/or elements and/or method steps shown and/or described in connection with any disclosed form or embodiment of the invention may be incorporated in any other disclosed or described or suggested form or embodiment as a general matter of design choice. It is the intention, therefore, to be limited only as indicated by the scope of the claims appended hereto.


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