Piston-cylinder unit including a working cylinder; a piston attached to a piston rod and installed with freedom of axial displacement in the working cylinder; and fastening elements, at least one of which is screwed into a threaded bore in the piston rod, where the interior pressure in the working cylinder holds a valve in the bore of the piston rod against an end surface of the fastening element. A vent groove leading to the atmosphere is provided between the end surface and the valve to create a pressure gradient to the working space, the groove being provided between the inner surface of the piston rod and the fastening element.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention pertains to a piston-cylinder unit consisting of a working cylinder; a piston attached to a piston rod and installed with freedom of axial displacement in the working cylinder; and fastening elements, at least one of which is screwed into a threaded bore in the piston rod, where the interior pressure in the working cylinder holds a valve in the bore of the piston rod against an end surface of the fastening element, and where a vent bore leading to the atmosphere is provided between the end surface and the valve to create a pressure gradient to the working space.

2. Description of the Related Art

U.S. Pat. No. 6,223,649 discloses a piston-cylinder unit in which an axially movable piston is provided in the working cylinder, where at least one of the fastening elements is screwed into a threaded bore in the piston rod and holds a valve of a pump rod complex in place in the axial direction, as shown inFIG. 5.

Self-pumping hydropneumatic spring strut units with internal level control are also known (e.g., DE 33 16 069 C2), in which a control sleeve and a snap ring are used to install a valve of a pump-rod complex in the hollow space inside the piston rod, and in which a stopper, which is a component of a fastening element, is used to hold the valve in its axial position in one direction. This pressure valve is held in its axial position in the other direction by the positive pressure in the working space, because this positive pressure is always able to act through a channel on a part of the valve body. A vent bore, which guarantees that the internal pressure is always greater than the atmospheric pressure prevailing behind the valve, is provided for this purpose between the valve body and the stopper. So that this pressure gradient can always be maintained, there must a connection to the atmosphere via the vent bore, which must be located behind the seal. The need to introduce an axially oriented vent bore in the piston rod calls for complicated manufacturing operations.

SUMMARY OF THE INVENTION

The task of the invention is to create a piston-cylinder unit in which a simple and low-cost vent to the atmosphere can be created in the end area of the hollow piston rod, where, in addition, the manufacturing process can also be optimized.

To accomplish this task, at least one groove is provided between the inner surface of the piston rod and the fastening element to serve as the vent bore.

It is advantageous for the vent to be provided in the end area of the hollow piston rod or possibly in the fastening element.

According to an especially favorable embodiment, the groove is located either in the end area of the conical part of the bore of the piston rod or in the outer surface of the fastening element. The advantage here is that the groove can be made while the workpiece is still in the same clamping arrangement as that used for cutting the thread. It does not matter whether the groove is made in the end area of the hollow piston rod or in the end area of the fastening element. The ability to produce the groove in a single clamping of the workpiece makes it possible to shorten the production process, to reduce production costs, and to decrease the setup time. The tooling costs with respect to the clamping elements are also significantly lower.

According to a favorable embodiment, the groove is straight.

According to another essential feature, the groove proceeds at a slant along the inside circumference of the bore and/or along the outside circumference of the fastening element.

According to an embodiment which is very simple in terms of manufacturing, the groove is created by either a swarfless process or by a cutting/grinding process.

DETAILED DESCRIPTION OF THE PRESENTLY PREFERRED EMBODIMENTS

The piston-cylinder unit shown inFIG. 1consists essentially of the outer tube9, in which the working cylinder10is installed. The piston11in the working cylinder10is attached to a piston rod2and is mounted together with the piston rod in such a way that it can move axially back and forth in the working cylinder10. The fastening element4ais connected directly to the outer tube9, whereas the fastening element4bis connected to the piston rod2by way of a pin6, provided with a thread3. The shoulder5on the fastening element4bhas a surface which tapers down to the thread3. The bore1in the piston rod2, which is hollow, has an expanding surface, preferably a conical surface, with which the shoulder5engages. In the hollow space of the piston rod2, there is a valve7, which belongs to a pump-rod complex but is not described in further detail in this embodiment. The internal pressure in the working cylinder brings the valve7into contact with the pin6, because there is a pressure gradient between the atmosphere and the internal pressure in the working cylinder.

FIG. 2shows a spring strut unit, which also has an outer tube9, where the piston rod2, via the thread3, carries the fastening element4b. Tapering and expanding surfaces are provided in this embodiment as well. The connection is the same in principle as that illustrated inFIG. 1.

FIG. 3shows a detailed view of the end area of the piston rod2and of the fastening element4b. The shoulder5has a conical surface which tapers down to the thread3, whereas the bore1expands in the end area so that the shoulder5comes to rest against the piston rod2. To achieve a pressure-tight connection, a seal8is installed in the valve7. Each ofFIGS. 4–6shows a valve body7in the hollow piston rod2, where the end area of the pin6makes contact with the valve7. The seal8seals off the interior space against the end area of the piston rod2. To achieve satisfactory venting in the end area of the piston rod2, a groove12is provided in the conical surface of the piston rod2. This groove12ensures a pressure gradient, because the end area communicates with the atmosphere, whereas the interior space of the working cylinder is sealed off against this area by the seal8. This means that the internal pressure will always press the valve against the pin6.

According toFIGS. 4 and 5, the groove12is provided in the fastening element4b, whereas the groove12inFIGS. 6 and 7is provided into the expanding surface of the piston rod2.

The groove12can be made in the component in question by a cutting/grinding process or by a suitable fabricating process which does not involving actual cutting/grinding.