Bellows accumulator, in particular pulsation damper

A bellows accumulator, in particular a pulsation damper, includes a bellows (3) arranged in an accumulator housing (1) and separating two media chambers (27, 28) from each other. Bellows folds (19) of the bellows can be moved at least partially along the inner wall (35) of the accumulator housing (1). The outside diameter of the bellows folds (19) is selected to be slightly smaller than the associated diameter of the inner wall (35) of the accumulator housing (1) in such a way that spaces (37, 41) are formed, which spaces together form a hydraulic damper for at least one medium.

FIELD OF THE INVENTION

The invention concerns a bellows accumulator, in particular a pulsation damper, comprising a bellows arranged in an accumulator housing and separating two media chambers from each other. The bellows folds of the bellows can be moved at least partially along the inner wall of the accumulator housing.

BACKGROUND OF THE INVENTION

Bellows accumulators of this kind are prior art, compare for example WO 2011/079890 A1. Such bellows accumulators are preferably used in hydraulic systems as pulsation dampers in order to reduce or dampen pressure spikes that occur in hydraulic fluids. In applications where pressure spikes occur with high frequency and the bellows are subjected to high vibrational loads, the known bellows accumulators are at their limit. In particular in systems where safety is critical, such as hydraulic actuators for wing flaps or stabiliser components in aircraft, the known bellows accumulators are not able to meet the stipulated operational safety requirements.

SUMMARY OF THE INVENTION

Based upon this problem, an object of the invention to provide an improved bellows accumulator that can be used specifically as a pulsation damper and that is characterized by a favourable operating characteristic and by a particularly effective damping effect.

This object is basically met according to the invention by a bellows accumulator having, as a particular feature of the invention, the outer diameter of the bellows folds selected to be slightly smaller than the associable diameter of the inner wall of the accumulator housing in such a way that interspaces are formed and form together a hydraulic damper for at least one medium. With the chosen, small difference in diameter and the correspondingly small width of the gap formed between the outside of the bellows and the inner wall of the housing, a plurality of damping throttles is formed between subsequent fold gaps that correspond to the number of bellows folds. In the instance of a bellows movement, where the volumes of the interspaces change, parts of the medium flows through, which parts are is located inside the gaps and inside the interspaces. Particularly in the instance of pulsations that take place at high frequencies and relatively low amplitudes, such as is the case with hydraulic fluid supplied by screw compressors, hydraulic damping is achieved on the outside of the bellows in addition to the damping caused by the volume changes of the bellows.

In advantageous exemplary embodiments the size of the interspaces to achieve the hydraulic damping effect is chosen such that, under consideration of the medium in the gap, a sliding guide of the bellows folds is achieved in the accumulator housing in the gap between the outer diameter of the bellows and the inner diameter of the housing. A kind of slide bearing is formed by a lubricating film of the medium if the gap is small. Advantageously, a sliding guide of this kind can facilitate a movement of the bellows parallel to the longitudinal axis of the accumulator housing and impedes it in the direction transverse to the longitudinal axis. Whilst the above-described, known solution provides a sleeve that covers the outside of the bellows to protect it from excessive vibrational loads that occur in operation, which sleeve is attached to the moving end of the bellows so as to provide circumferential guidance for the bellows, the sliding guide provided by the invention acts as a hydrostatic slide bearing. This hydrostatic slide bearing provides the bellows with protection against excessive vibrational loads without burdening the bellows with an additional moveable mass, such as the sleeve in the known solution, which ensures an optimal response and damping characteristic at high frequencies.

In advantageous exemplary embodiments the medium space between the outside of the bellows and the associated inside of the bellows housing may contain a medium of high viscosity, such as a phosphoric acid ester oil (HFD-R). The medium on the inside of the bellows may be a process gas such as nitrogen gas. When using these types of media, the bellows accumulator according to the invention is particularly suitable for aeronautical engineering applications where corresponding safety regulations apply for the media used, such as low flammability, cold environment operating limits and such like.

The size or the dimension of the gap respectively may, advantageously, be between 3.0 mm and 0.15 mm, particularly preferred between 2.0 mm for a low hydraulic damping effect and 0.25 mm for an extremely high hydraulic damping effect. With the dimension of the gap chosen in this manner, the gap between the sliding guide of a corresponding bellows fold is transformed into damping spaces at the assignable inner wall of the accumulator housing at least when the bellows is extended. The damping spaces are delimited by two adjacently located bellows folds.

In advantageous exemplary embodiments the height of the bellows in its extended state may be one to one and a half times of the internal diameter.

The bellows may, with particular advantage, be made from a metallic material and/or the ends of the folds may have tapered cross-sections and/or the bellows does not require any additional guiding facilities in the vicinity of its sliding guide inside the accumulator housing.

In a preferred embodiment the one end of the bellows is attached to a moveable end body. The other end of the bellows is secured to a retaining ring that is fixed to the housing.

Particularly advantageously the arrangement may be such that the moveable end body closes the inside of the bellows off with respect to the one medium connection of the accumulator housing, is circular in shape and is provided with protruding guide members for making contact with inner wall parts of the accumulator housing, where media passages extend between the guide members of the end body. Despite the lateral guide provided for the moveable end of the bellows, the fluid connection to the media space between the outside of the bellows and the inside of the accumulator housing remains intact.

Other objects, advantages and salient features of the present invention will become apparent from the following detailed description, which, taken in conjunction with the drawings, discloses preferred embodiments of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The invention is now explained with reference to the attached drawings by way of examples of pulsation dampers, which are particularly intended to reduce or dampen pressure spikes, which occur at high frequencies in the hydraulic fluid of the hydraulic system of an aircraft.FIG. 1depicts an accumulator housing1of a first exemplary embodiment comprising a main housing part2, which may contain the metallic bellows3. The main housing part2is made in the form of a circular-cylindrical pot with a bottom5, which is closed except for a centrally located fluid connection7. At the opposite, upper end, as depicted inFIG. 1, the main part2is provided with an increased wall thickness9. Welded to the free end of the upper end is a metallic housing end part13, along the weld seam11. The housing end part13has the form of a domed cup, which is closed apart from a centrally located filling opening15, which is closed by a weld nugget17.

The metallic bellows3that is located inside the main part2is welded at its open bellows end, shown at the top inFIG. 1, with its last bellows fold19to a metallic retaining ring21. Retaining ring21is provided with a thickened circumferential edge23with which it is secured to the accumulator housing1in such a manner that the thickened circumferential edge23overlaps the weld seam11between main part2and end part13. The welding method used is a full penetration welding process using electron-beam welding (laser welding) so that the circumferential edge23of the retaining ring21is also welded on. The opposite, lower end of the bellows3is closed by a metallic, plate-shaped bottom part25to which the lowest bellows fold19is welded.

FIG. 1depicts the operational state in which the bellows3is fully extended and the oil end27is empty. The associated fluid connection7is closed with a protective screw cap29. When used in aeronautical engineering systems, the bellows3forms the moveable separation element between a viscous hydraulic fluid on the oil side27, such as a phosphoric acid ester oil (HFD-R), which, as a low-flammability liquid, is permitted for aeronautical applications, and a process medium, such as a process gas, such as nitrogen gas, which is present on the gas side28of the accumulator housing1under a pre-filing pressure. The gas side28is formed by the inner space of the bellows3as well as the housing part located above the retaining ring21.

To facilitate the guidance of the axial movements of the bottom part25, which changes the volume of the bellows, the bottom part25is guided by a guide ring33disposed on its circumferential edge31. Guide ring33is made from a synthetic material with good sliding properties, for example tetrafluoroethylene. In order to facilitate the passage of fluid from the oil side27that is adjacent to the fluid connection7to the outside of the bellows3despite the axial guide provided by the guide ring33, so that also the area of the gap37between the outside of the bellows and the inner wall of the housing35is part of the oil side27, the guide ring33has the shape of a flat ring that is formed in such a manner as is shown for a guide ring that fulfils a comparable function that is depicted inFIGS. 2 and 3of the document WO 2011/079890 A1 and referenced with the number47, cited above as prior art. In accordance with that the guide ring33is provided with circumferentially spaced, radially protruding guide sections that are in contact with the inner housing wall35and between which are radially recessed gaps that allow the fluid to pass through.

As is most clearly shown inFIG. 2, a gap37is provided between the inner wall35of the main part2of the accumulator housing1and the outer diameter of bellows3that is established by the tips of the bellows folds19. The width of this gap is indicated inFIG. 2by the arrows39. In conjunction with the interspaces41, which are formed between its folds19by an at least not fully compressed bellows3, the volumes of which change accordingly with the movement of the bellows in operation, damping spaces are formed between the outside of the bellows and the inner housing wall35as part of the oil side27. As is apparent fromFIG. 2, damping throttles are formed between the tips of the folds19and the inner housing wall35for the fluid flow that results from the volume changes of the fold interspaces41caused by the bellows movement in operation. The chosen gap size determines the throttle cross-section. In addition to the pulsation damping effect due to the movement of the bellows against the pressure cushion of the process gas, a hydraulic damping effect is achieved on the outside of the bellows. For reasons of clarity the folds19and the interspaces41between the folds19are not all marked as such inFIGS. 1 and 2.

The size of the gap may be selected depending on the desired damping effect; for example a gap of 2.0 mm for a relatively low damping effect or of 0.25 mm for a high damping effect. With such small gap sizes and a highly viscous medium on the oil side27, such as phosphoric acid ester oil, a lubricating film is formed between the inner housing wall35and the outside of the bellows3. The lubricating film acts as axial guidance for the bellows folds19inside the housing1and provides protection for the folds19with respect to vibrational loads. The bellows accumulator according to the invention is then particularly suitable as a damping device for fluids with high-frequency pressure spikes. The invention is also suitable for use in high-frequency pulsation applications because it is not necessary to attach an additional oscillating mass to bellows3for the guidance of the bellows folds19, as it is necessary in the above-described, known solution that provides a sleeve on the bellows that surrounds the folds.

FIG. 3depicts a modified or second exemplary embodiment in which the accumulator housing1is closed at the upper end of the circular-cylindrical main part2through a domed cover part45that is connected to main part2. The lower end of main part2is closed along the weld seam47by a flat housing bottom49, which corresponds to the bottom5of the first exemplary embodiment and which, in likewise manner, is provided with a central filling connection7for the hydraulic fluid. The metallic bellows3in this exemplary embodiment is open towards the bottom49so that the inside of the bellows3forms the oil side27. The retaining ring21, which is welded to the last bellows fold19at the open end of the bellows3, is formed in this exemplary embodiment by a flat ring, which is fixed along the weld seam47to the lower end of the main housing part2.

As in the first exemplary embodiment, the bottom part25, which forms the closed end of the bellows3and which is welded to the last bellows fold19that faces it, is axially moveable. Again, as in the first exemplary embodiment, the bottom part25is fitted at the circumferential edge31with the guide ring33that is provided with passages so that the gap37, which is formed, like in the first exemplary embodiment, between the inner housing wall35. The outside of the bellows3, inFIG. 2is indicated by the gap dimension39, forms together with the interspaces41between the folds19damping spaces. Damping throttles formed between the tips of the folds19and the inner wall35for the flow of process gas when the bellows3moves, causing volume changes of the interspaces41. As in the first exemplary embodiment, this causes on the outside of the bellows3an additional damping effect. As inFIGS. 1 and 2, the folds19and the interspaces41between the folds19shown inFIG. 3are not all marked as such for reasons of clarity.