The invention relates to a clamping shaft for a rotationally driven part, comprising a shaft body being rotatable about a shaft axis, at least one fluid chamber being located inside the shaft body and being filled with a predetermined amount of fluid, wherein the fluid chamber is delimited by a piston surface of at least one piston being located inside the shaft body and a cylinder part, being movable relative to each other such that a volume of the fluid chamber is adjustable by moving the piston relative to the cylinder part, wherein the shaft body comprises at least one elastic clamping region delimiting the fluid chamber, and wherein the clamping region has a first diameter if the piston and the cylinder part are in a first relative position, and a second diameter if the piston and the cylinder part are in a second relative position, the second diameter being bigger than the first diameter,

Additionally, the invention relates to a printing cylinder unit of a printing machine comprising such a clamping shaft.

Moreover, the invention relates to a method for operating a clamping shaft of the type mentioned above.

Such clamping shafts and printing cylinder units being equipped therewith are known in the art. The same applies to methods for operating clamping shafts.

The functioning of such clamping shafts is based on selectively pressurizing a fluid, which leads to an elastic expansion of a clamping region of the shaft. In doing so, a printing cylinder or any other rotationally driven part may be clamped to the shaft. The rotationally driven part may be released from the shaft by de pressurizing the fluid, which leads to a contraction of the clamping region diameter.

It is noted that the fluid may be compressible or non-compressible. In the case of a non-compressible fluid, a volume of the fluid chamber will remain substantially constant, wherein the volume change resulting from the volume difference between the first and second relative position substantially equals the volume change resulting from the elastic deformation of the clamping region. In the case of a compressible fluid, the volume of the fluid chamber associated with the second relative position is smaller than the volume of the fluid chamber associated with the first relative position of the piston and the cylinder part.

In this context, the first relative position substantially corresponds to a pressure- relieved state of the fluid, i.e. the fluid is substantially at ambient pressure.

The fluid is often a hydraulic medium, e.g. hydraulic oil.

The fluid usually is pressurized by moving the piston, wherein the piston may be driven manually by actuating a screw or spindle being coupled thereto. In order to do so, a tool, e.g. a screw driver, is used. This process is reversible, i.e. the fluid is de-pressurized by moving the piston in an opposite direction.

It is an object of the present invention to further improve such clamping shafts. Especially, the operation of clamping shafts shall be easy to automate.

The problem is solved by a clamping shaft of the type mentioned above, wherein the clamping shaft further comprises at least one preloading unit, wherein the preloading unit applies a preloading force on the piston and/or the cylinder part such that the piston and the cylinder part are biased towards the second relative position. In other words, the clamping shaft is preloaded towards a clamping state. Consequently, no external force or energy is necessary in order to bring the clamping shaft into a clamping state in which the clamping region is elastically expanded. In order to bring the clamping shaft into a release state, the preloading force has to be actively counteracted by an external force or energy. Thus, the working principle of known clamping shafts is inverted. Since the preloading unit provides a preloading force internal to the clamping shaft, automation of the clamping process is facilitated.

The first relative position and the second relative position may be attained via three possibilities. In a first possibility, the piston is moved and the corresponding cylinder part is stationary. In this context, the cylinder part may be formed integrally with the shaft body or as a separate part. In a second possibility, the piston is stationary and the cylinder part is moved. In a third possibility, both the piston and the cylinder part are moved. In the second and third case, the cylinder part must be formed as a separate part, thus not as a portion of the shaft body.

In a clamping shaft according to the invention, the clamping shaft axis and a piston axis may be coaxial. This variant is especially suitable for clamping shafts rotating at high speed since the coaxial positioning results in a rotationally balanced clamping shaft. Of course, it is also possible to arrange the clamping shaft axis and the piston axis in a non-coaxial manner. This may lead to packaging advantages, i.e. such a clamping shaft only occupies a small installation space.

In principle, the preloading force may be chosen in function of the specific application case. In the fields of printing machines, the fluid may be pressurized to 200 bar and more. The preloading force will be chosen accordingly.

According to an embodiment, the clamping shaft comprises two fluid chambers, each being located inside the shaft body and each being filled with a predetermined amount of fluid, wherein each of the fluid chambers is delimited by a piston surface of a piston being located inside the shaft body and a cylinder part, being movable relative to each other such that a volume of the fluid chamber is adjustable by moving the piston relative to the cylinder part, wherein the shaft body comprises two elastic clamping regions each delimiting one of the fluid chambers, wherein the clamping regions have a first diameter if the corresponding and the corresponding cylinder part are in a first relative position, and a second diameter if the corresponding piston and the corresponding cylinder part are in a second relative position, the second diameter being bigger than the first diameter, and wherein the preloading unit is located between the pistons or between the cylinder parts and applies a preloading force on the pistons or the cylinder parts such that each of the pistons and the corresponding cylinder parts are biased towards the second relative position. Such a clamping shaft may clamp a rotationally driven part in two clamping regions, which leads to a very reliable coupling between the clamping shaft and the rotationally driven part. Since both pistons use a common preloading unit, the clamping shaft is relatively light and compact.

Preferably, the piston axis of the two pistons are coaxial.

The fluid chambers may be fluidically connected via a fluid connection line located inside the shaft body. As a consequence thereof, the pressure in both fluid chambers may be adjusted by using any one of the pistons. This means that it is possible to clamp or release a rotationally driven part form the clamping shaft by just actuating one piston or cylinder part. Consequently, the driven part is coupled to the clamping shaft in a highly reliable way and the clamping shaft is simple to operate. Advantageously, an abutment surface associated with each piston is provided in the shaft body, each piston abutting against the respective abutment surface when in the second relative position. A movement of the piston into the clamping direction is thus limited. Consequently, a maximum clamping force or torque may be adjusted by providing such an abutment surface. Consequently, reliable clamping with a constant clamping force is guaranteed.

According to a variant, the preloading unit comprises a spring assembly. Such a spring assembly may be a disk spring assembly or a coil spring assembly. It is also possible to use gas springs. In the present context, the spring assemblies may also be termed an energy storage since the springs store the energy necessary for providing a second volume of the fluid chamber, i.e. the energy necessary for clamping the rotationally driven part. Such spring assemblies are easy to mount and reliable in operation.

Each of the clamping regions may be formed integrally with the shaft body or each clamping region may be provided by an elastically deformable sleeve provided on the shaft body. Both alternatives allow for reliably clamping the rotationally driven part to the clamping shaft.

The shaft body can comprise at least one bearing interface, by which the clamping shaft is rotatably supportable, the shaft body preferably comprising two bearing interfaces. The bearing interfaces are preferably located at axial ends of the clamping shaft. They may essentially be formed as cylinders or cone portions.

The shaft body also may comprise at least one drive interface by which the clamping shaft is rotationally drivable. The drive interface may be formed as a mounting interface for a gear or a pulley.

In an alternative at least one of the pistons and/or the cylinder part cooperating with the piston comprises an actuation interface by which an external force may be applied to the piston and/or the cylinder part counteracting the preloading force such that at least one of the pistons and the corresponding cylinder part are in the first relative position. In other words, via the actuation interface a force is applied to the piston or the cylinder part counteracting the preloading force. Consequently, the clamping shaft is in a release state in which the rotationally driven part is not coupled thereto. The actuation of the clamping shaft may comprise the application of a pushing force, a pulling force or a torque to the actuation interface. These forms of actuation may easily be automated.

Preferably, the actuation interface is provided in proximity to an axial end of the shaft body. In this context, the actuation interface may axially protrude from the clamping shaft or may be located in a recess provided at an axial end of the shaft body. In both alternatives, the actuation interface is easily accessible for manual or automated actuation.

Furthermore, the problem is solved by a printing cylinder unit of the type mentioned above, comprising a clamping shaft according to the invention. In such a printing cylinder unit a printing cylinder or an adapter may be selectively coupled to the clamping shaft via the clamping mechanism described above. In this context, a printing cylinder designates all kinds of cylinders used in a printing machine, especially a cliche cylinder of a flexographic printing machine.

The problem is also solved by a method for operating a clamping shaft of the type mentioned above, wherein the clamping shaft is in a clamping state if the clamping shaft is not actuated and wherein the clamping shaft is in a release state if the clamping shaft is actuated. Consequently, the functioning principle of known clamping shafts is inverted. As has already been explained above, this facilitates the automation of such clamping cylinders.

The clamping shaft may be actuated by pulling an actuation interface along an axial direction of the clamping shaft, by pushing an actuation interface along an axial direction of the clamping shaft or by turning an actuation interface around an axial direction of the clamping shaft. This actuation activities may be performed manually or automatically, e.g. by a robot or any other specific actuation unit.

The invention will now be explained with reference to two embodiments which are shown in the attached drawings. In the drawings,

- Figure 1 shows a printing cylinder unit according to the invention comprising a clamping shaft according to the invention being operable by a method for operating a clamping shaft according to the invention,

- Figure 2 the printing cylinder of Figure 1 in an enlarged representation, wherein a middle section of the printing cylinder is cut out, - Figure 3 the printing cylinder of Figures 1 and 2 in a further enlarged representation, wherein a middle section of the printing cylinder is cut out, and

- Figure 4 an alternative embodiment of the printing cylinder unit according to the invention in a schematic representation.

Figure 1 shows a printing cylinder unit 10 of a printing machine, which comprises a clamping shaft 12 and a rotationally driven part 14.

In the examples shown the rotationally driven part 14 is a printing cylinder, e.g. a cliche cylinder of a flexographic printing machine or an adapter for such a printing cylinder.

The clamping shaft 12 comprises a shaft body 16 being rotatable about a shaft axis 18.

It may be rotatably supported inside a printing machine via two bearing interfaces 20a, 20b which are located at respective axial ends 16a, 16b of the shaft body 16. The bearing interfaces are formed as cone sections in the example shown.

In order to rotatably drive the clamping shaft 12, the shaft body 16 is equipped with a drive interface 22, which his only shown schematically.

The part 14 is clamped to the clamping shaft 12 via two clamping regions 24a, 24b. In the clamping regions 24a, 24b elastically deformable sleeves 26a, 26b are provided, which delimit a fluid chamber 28a, 28b respectively.

The fluid chambers 28a, 28b are located inside the shaft body 16 and both are filled with a predetermined amount of fluid, e.g. hydraulic oil.

Depending on the pressure of the fluid in the respective fluid chambers 28a, 28b, the sleeves 26a, 26b are deformed to have a first diameter or a second diameter, the second diameter being bigger than the first diameter.

Consequently, the part 14 is clamped to the clamping shaft 12 if the clamping regions 24a, 24b have a second diameter and may be axially and/or rotationally moved with respect to the clamping shaft 12 if the clamping regions 24a, 24b have a first diameter. This change in diameter is achieved by altering the pressure inside the respective fluid chamber 28a, 28b. Thus, the first diameter is achieved if the fluid chambers 28a, 28b are pressurized and the second diameter is achieved if the fluid chambers 28a, 28b are de-pressurized, i.e. are substantially at ambient pressure.

In order to pressurize the fluid chambers 28a, 28b each of them is also delimited by a piston surface of a respective piston 30a, 30b being located inside the shaft body 16 and by a corresponding cylinder part 32a, 32b. Each piston 30a, 30b is movable relative to the corresponding cylinder part 32a, 32b, wherein in the example shown, the pistons are movable along a piston axis, which corresponds to the shaft axis 18. The cylinder parts 32a, 32b are formed as separate parts in the examples shown, but are axially and rotationally fixed inside the shaft body 16. Alternatively, the cylinder parts 32a, 32b may be formed as sections of the shaft body 16.

Thus, in the de-pressurized state of one of the fluid chambers 28a, 28b, the corresponding piston 30a, 30b and cylinder part 32a, 32b are in a first relative position.

In the pressurized state of one of the fluid chambers 28a, 28b, the corresponding piston 30a, 30b and cylinder part 32a, 32b are in a second relative position.

Additionally, an abutment surface 34a, 34b is formed on each of the cylinder parts 32a, 32b, wherein each of the abutment surfaces 34a, 34b is associated with one of the pistons 30a, 30b and the respective piston 30a, 30b abuts against the associated abutment surface 34a, 34b when the corresponding piston 30a, 30b and cylinder part 32a, 32b are in the second relative position.

On the pistons 30a, 30b counter abutment surfaces 36a, 36b are formed.

The clamping shaft 12 further comprises a preloading unit 38, which is formed such that it applies a preloading force on both pistons 30a, 30b such that the pistons 30a, 30b are biased towards the second relative position.

In the example shown in Figures 1 to 3, the preloading unit 38 is used for both pistons 30a, 30b and is located between them. The preloading unit 38 comprises a spring assembly 40, which is an arrangement of disk springs in the example shown.

In order to be able to use the functioning principle of the clamping shaft 12 for different lengths of the clamping shaft 12, piston 30b is connected to the preloading unit via a bar 42. Consequently, the length of the clamping shaft 12 may be adapted by changing the length of the bar 42. The remaining parts of the clamping shaft 12, especially the pistons 30a, 30b and the fluid chambers 28a, 28b do not need to be altered.

Moreover, the fluid chambers 28a, 28b are fluidically connected via a fluid connection line 44.

In the example shown the fluid connection line 44 is an axial bore located inside the shaft body 16.

As will be explained in the following, the fluid connection line 44 makes is possible to put both combinations of a piston 30a, 30b and a cylinder part 32a, 32b in the first relative position by just actuating one of the pistons 30a, 30b. This means that the clamping shaft 12 can be put into the release state by just actuating one single pistons 30a, 30b.

To this end, both pistons 30a, 30b comprise an actuation interface 46a, 46b, which is a realized as an axial end face of the respective piston 30a, 30b. This end face is provided at an axial end of the shaft body 16 and thus is well accessible for actuation.

Thus, if one of the actuation interfaces 46a, 46b is pushed either manually or automatically, the pressure in both fluid chambers 28a, 28b is reduced.

Since the forces resulting from the pressure of the fluid are counterbalanced by the preload forces resulting from the preloading unit 38, both the piston 30a, 30b actually actuated and also the piston 30a, 30b not directly actuated move towards an axial middle of the clamping shaft 12, thereby attaining the first relative position.

In summary, the clamping shaft 12 is in a clamping state, if it is not actuated and it is in a release state if it is actuated by pushing against at least one of the actuation interfaces 46a, 46b. Of course, the release state can also be achieved by pushing on both actuation interfaces 46a, 46b. Figure 4 shows a second embodiment of the printing cylinder unit 10, which differs from the embodiment of Figures 1 to 3 in that a different clamping shaft is used. In the following, only the differences with respect to this first embodiment will be explained. Corresponding parts will be designated with the same references signs that already have been used in Figures 1 to 3, where appropriate the suffixes are omitted in Figure 4.

Essentially, the embodiment of Figure 4 differs from the embodiment of Figures 1 to 3 in that there is only one single piston 30 and one single cylinder part 32 cooperating with the piston 30. Both delimit a single fluid chamber 28 and are movable inside the shaft body 16.

In contrast to the example of Figures 1 to 3, the single fluid chamber 28 is associated with two clamping regions 24a, 24b. More precisely, the single fluid chamber 28 is delimited by two elastically deformable sleeves 26a, 26b. To this end, the fluid chamber 28 comprises an axial bore 48 in the shaft body 16.

The preloading unit 38 biases the piston 30 against the cylinder part 32.

The clamping shaft 12 according to Figure 4 may be operated as follows.

If the clamping shaft 12 is actuated by pushing on either actuation interface 46a of the piston 30 or actuation interface 46b of the cylinder part 32, the piston 30 and the cylinder part 32 will move relative to each other against the preloading force of the preloading unit 38.

In doing so, the fluid inside the fluid chamber 28 will be de-pressurized and the diameter of the sleeves 26a, 26b will be elastically decreased. Consequently, the clamping shaft 12 will attain a release state.

If none of the actuation interfaces 46a, 46b is pushed, the clamping shaft 12 is in its clamping state.