Method for manufacturing a piston rod unit and a hollow shaft

The invention relates to a method for producing a piston rod unit, or a shaft, in lightweight construction, with a rod which is hollowly-drilled by means of a deep bore into the rod shank, and the resulting rod opening is closed subsequently by means of a forging process.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a U.S. National Phase of International Patent Application Serial No. PCT/EP2017/000723 entitled “METHOD FOR MANUFACTURING A PISTON ROD UNIT AND A HOLLOW SHAFT,” filed on Jun. 21, 2017. International Patent Application Serial No. PCT/EP2017/000723 claims priority to German Patent Application Serial No. 10 2016 007 593.7 filed on Jun. 21, 2016. The entire contents of each of the above-cited applications are hereby incorporated by reference in their entirety for all purposes.

The invention relates to a method for producing a piston rod unit in, as well as a shaft, which is produced in accordance with such a method.

Piston cylinder units, in particular hydraulic cylinders, are often used in large working machines. A predominant aim of the developers lies with the optimization of the workings machines in terms of overall weight, in order to not only save production costs, but likewise optimize the efficiency of operation of the machines. Although the piston rod units mounted in the hydraulic cylinders contribute only a relatively small part to the overall weight, but especially in large machine types, such as mining machines, significant weight savings can be achieved.

Most of the current developments aiming at a lightweight construction cylinder provide a hollow piston rod construction, but the attachment of the hollow piston rod, which often is a simple pipe element, is effected by means of screw- or weld connections to the piston rod head and the piston. The drawback of this approach lies with the extremely negative effect of these connection types on the dynamic stress resistance of the lightweight construction cylinder. For this purpose, steels tempered and quenched with refractory metals having increased carbon equivalents are in particular used, which achieve an increased strength due to the tempering and quenching of the steel, but can be welded poorly due to the solidified precipitation phase in the steel. For this reason, the use of hollow piston rod constructions, in particular in hydraulic piston rod units for mining machines, has not been possible so far, since the connection techniques used there on the basis of screw- and weld connections cannot be combined with lightweight construction.

The object of the present invention thus is to provide an improved manufacturing method for a piston rod unit, which allows an optimum lightweight construction in particular for mining machines.

This object is achieved by means of a method having the features of claim1. Advantageous configurations of the method are the subject-matter of the dependent claims following the main claim.

According to the invention, it is proposed for the production of the piston rod unit, to first hollowly-drill a piston rod by means of deep-hole drilling in the rod shank in order to ensure the desired weight saving. The resulting rod opening of the deep-hole bore at the rod end is closed-off by means of a forging process afterwards. Using this measure, the resulting closed piston rod is also suitable for use in heavy mining machines since the rod opening closed by the forging process can be connected with the piston of conventional piston rod units in conventional ways and manners. In particular, the resulting piston rod can be connected with the piston through a simple screw-connection. Open-die forging has proven as a suitable forging method.

It is particularly advantageous when the piston rod, including the piston rod head is mechanically formed from a blank. By contract to the prior art, piston rod and piston rod head are to be understood as a one-piece component, possible connections between the two sections of the piston rod are omitted.

In the mechanical processing of the piston rod from a blank, preferably, provision is made for a process so that a material excess remains at the rod end of the piston rod. In particular, a material excess is left in the radial direction at the piston-side end of the tube-shaped piston rod. This material excess, in a corresponding thermo-mechanical stress during the forging process, is ideally used to block the bore openings resulting from the deep bore. The material excess is consequently made to flow, due to the thermo-mechanical stress, in the radial direction of the cylinder. As a result, it is possible to close the bore opening at the rod end without having to employ other connection techniques that have a negative effect on the dynamic stress resistance of the cylinder.

According to an advantageous embodiment of the method according to the invention, a mandrel is inserted into the deep bore prior to the beginning forging process, in particular in the region of the bore opening at the rod end. By means of the inserted mandrel, a precise closing of the bore opening in the forging can be achieved, and thus a complete sealing of the cavity of the piston rod. Moreover, the entering material flow can be controlled in better ways during the forging process. In some aspects, the mandrel may be removed after the forging process.

The inserted mandrel can be in the type of a cylinder with a sharply-tapering end. However, other geometrical shapes are also conceivable.

The mandrel can have a smooth surface structure. Alternatively, the mandrel surface may also provide a non-smooth structure, e.g. with a wave-shaped or another non-smooth profile. For example, the mandrel may have a non-smooth, cone-type surface structure. The quality of the closure can be improved by means of a non-smooth surface structure of the mandrel. As a result, it is ensured that the internal cavity is sealed, during each operating situation, against the intrusion of the non-compressible hydraulic medium.

Advantageously, the bore end, i.e. the bore closing-off that ends in the piston rod shank, is of radial shape. An otherwise edged bore-cross-section at the bore end could lead to a potential notching effect.

The following assembly of the produced piston rod and a piston is for example exclusively based upon a screw connection. As a result, potential weld connections inside the piston rod can be completely dispensed with, which leads to a significantly higher dynamic stress resistance of the entire piston rod unit. Since, by this approach, the stress resistance of the unit is optimized, higher weight savings can be realized in the manufacture in the end.

Besides the method according to the invention, the present invention also relates to a piston rod unit that has been produced in accordance with such a method. The piston rod unit thus provides the same advantages and properties as have been explained in more detail above by means of the method according to the invention. For this reason, a repeated description is omitted.

Furthermore, the invention relates to the use, according to the invention, of the piston rod unit in accordance with the present invention for a mining apparatus, in particular for a mining excavator, ideally as a bucket cylinder.

Moreover, the present invention relates to a mining apparatus, in particular a mining excavator with at least one piston rod unit, which is characterized in that the installed piston rod of the piston rod unit is of hollow design. Advantageously, the piston rod unit has been manufactured in accordance with the method according to the invention. The mining apparatus is thus characterized by the same advantages and properties as have already been explained with reference to the method according to the invention.

The invention proposes a manufacturing method of a hydraulic cylinder in lightweight construction. An advantage of the construction according to the invention consists in that potential weld connections inside the piston rod can be completely dispensed with and required screw connections are reduced to only one connection point between piston rod and piston.FIG. 1shows a completely-assembled hydraulic cylinder in the form of a differential cylinder that has been produced in accordance with the method according to the invention. The hydraulic cylinder includes the cylinder housing10with the piston rod20including piston25displaceably mounted therein. The piston25is screwed to the piston rod end located in the cylinder10by means of a screw connection26. Part of the screw connection is the thread of the piston nut27located inside the piston25.

At the piston rod head22, a boss for mounting the piston rod unit is provided. The same applies to the closed cylinder end. To achieve lightweight construction, the piston rod20comprises a deep bore21, which extends in the longitudinal direction from the piston rod head22to the mounted piston25.

The manufacturing process of the piston rod unit first provides, in a first step, to produce the piston rod20including piston rod head22from a blank in mechanical ways and manners. The piston rod20and the piston rod head22are consequently made from one and the same starting material, and are to be considered as a one-piece end product. Furthermore, during the machining process, a radial material excess30is left at the rod end of the piston rod20.

After that, the deep bore21is made from the rod end in the direction of the rod head22with the bore length B. The inner end23of the deep bore21is of radial shape, in order to thereby prevent a possible notch effect by means of an edged bore cross-section. The resulting intermediate product is shown byFIG. 2. The piston rod has a length L.

The protruding material30is used, after the deep hole drilling21, to close the bore opening at the rod shank. This is effected by means of open-die forging or another, suitable forging process. In the forging process, the protruding material30is made to flow, due to the thermo-mechanical stress, in the radial direction of the cylinder. As a result, it is possible to close the bore opening at the rod shank without having to use other connection techniques that have a negative effect on the dynamic stress resistance.

Prior to the forging, a type of mandrel40can be laid into the bore21(seeFIG. 3), in order to be able to close and seal the bore opening precisely and, on the other hand, be able to control the material flow during the forging in better ways and manners. The proposed method further allows acting more freely both in terms of the material selection and the bore dimensions. This particularly means that the yield in terms of weight reduction can be increased due to the prevented weaknesses that might develop due to previous connection techniques. In large cylinders for the field of mining, up to 400 KG can be saved.

In a further configuration, as shown inFIG. 4, it is also conceivable to configure the geometry of the mandrel40′ in a wave-like or other non-smooth form, in order to thereby increase the quality of the closure. The internal cavity should be sealed against the entering of the non-compressible medium in each operating situation.