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Haenchen hydraulic test the super bird

Test cylinders for the aircraft industry -
testing the operational stability of the Airbus A380

User report - Test cylinder Ratio-Test®

Differing seal combinations are the recipe for success for the largest order in the company's history

Stick-slip-freedom is a basic prerequisite when 163 hydraulic cylinders work in precise rhythm under computer control and test the largest passenger aircraft of all times. In order to test the operational stability of the new Airbus A380 in flight operation, hydraulic cylinders simulate the changing forces occurring on wings, engine suspensions, tail units, flaps, fuselage and landing gear during flight operation. Through a complex control system, the forces are applied that have an impact during take-off, landing and flight under widely ranging normal and extreme conditions. Thus, the possible risks, such as material fatigue and damage tolerance properties can be determined prior to the A380 taking off with passengers for the first time.

"The optimal seal between the fastening and rod is the key to quality for Hänchen", summarises the managing partner, Hartmut Hänchen. "Our technological advantage in this field is the basis for our having been involved in the testing of all Airbus models for the past 35 years." What quality means is shown by the damage that can be caused by a single cylinder: It destroys the expensive test device and the result of many months, or even years, of testing. The computer-controlled common mode must be perfect in order to simulate how the aircraft performs under years of continuous operation. Common mode cylinders are a mechanically optimal drive for such sensitive testing and inspection tasks: After all, only in this way do hydraulics react in both lift directions on the applied pressure with the same properties. In contrast, normal cylinders in a differential construction can only imitate largely symmetrical properties through complex electronic regulation measures. With the differential cylinder, the effective annular ring surface of the piston on the piston rod side is significantly smaller than the annular surface of the on the fastening side. In order – particularly for dynamic inspection and testing tasks – to apply pressure to the same surface in both lift directions, with the synchronous cylinder, hydraulics use a fluid technology trick: the piston receives a piston rod in each lift direction, even if the second rod only has the function of ensuring the necessary symmetry of the drive.
However the basis for such high-quality testing cylinders are the seal combinations, for the constant development of which Hänchen carries out a high level of research and development.
Already in its basic version, the piston rod guide of a Hänchen hydraulic cylinder is coated with a specialised plastic. An additional pressure-free leak oil connection with a closing leak oil seal enables a practically leak-free structure with minimal non-slip inclination, despite these frictionless seal pairings. In recent years, Hänchen has expanded a structural feature to all catalogue cylinders: with most of its catalogue cylinders, a plastic guide is sprayed on and reworked. With this, a certain absorbance capabil-ity for transverse forces is given. This fitting the fastening on the piston side is now the basic quality at Hänchen, under the name of Servoslide®.
Under the protected brand name of Servocop®, Hänchen offers a seal combination that lies in the mid-price segment, despite its very high performance. Through an additional Teflon ring on the piston side, lower guide play and extremely stick-slip-free move-ments are also achieved with very low and very high piston speeds: even speeds below 0.02 m/s take place completely evenly, a stick-slip-type effect only occurs with even lower speeds. The Servocop® quality covers a broad spectrum of inspection tasks, so that all test cylinder types can be equipped with this seal qual-ity. With the catalogue cylinders, these are the series, 126 syn-chronous/differential, 166 synchronous, 306 synchro-nous/differential and 327 synchronous. The latter type series mentioned emerged in the environment of earlier Airbus tests and is specially reinforced. The Servocop® quality enables speeds of up to 4 m/s and is also frequently used for testing the super bird, par-tially also in synchronous cylinders with a shortened design.

Where even higher standards exist with this simulation, the pat-ented floating annular gap seal of the Servofloat® model is used. Here, a steel bushing deforms through a throttling gap, thereby creating a non-tangential sealing gap of a few 1/100 mm. How-ever, this process only works with a production precision in the range below µm, as the leakage would otherwise lead to high hy-draulic losses. This type series 328 only distinguishes itself through its somewhat higher sensitivity to lateral forces from a cylinder with a hydrostatically supported piston rod guide, but pro-vides a cost benefit of around 30%. Through their negligible fric-tion, cylinders in Servofloat® quality offer the highest positioning and repetition precision, are stick-slip-free and suited to extremely slow and fast movements.

For the highest standards, cylinders with hydrostatically supported piston rod guides are an optimal structure. With these, the piston rod is clamped hydraulically, floating in the oil flow, through the application of hydraulic pressure in four pockets. Only the scrap-ers cause a minimum measure of friction. As the hydrostatics in these sealing combinations ensure centring of the piston rod against potential transverse forces, drive solutions can be realised in the threshold range of what is feasible. However, Hänchen also uses this technology for other aircraft tests, as well as in different Industrial application with a mechanically challenging environment.

"The challenge with the current Airbus order was in the size of the test device, which partially made double the dimensions of previ-ous test dimensions necessary", summarises Hänchen. "We needed to overcome the challenge with significantly larger struc-tural lengths, lifts, forces and speeds, with the proven precision. With this, the new Airbus has also become a milestone of state-of-the-art hydraulics on the technical side." With this, the hydraulics specialists from Ostfildern, near Stuttgart, have shown what state-of-the-art sealing combinations can achieve. This experience, of course, flows into all of the company's series and special products.

Jörg Beyer

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