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 hy-draulic 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 op-eration. 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 condi-tions. 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 qual-ity 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 op-eration. 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 pres-sure with the same properties. In contrast, normal cylinders in a differential construction can only imitate largely symmetrical proper-ties through complex electronic regulation measures. With the dif-ferential 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 in-spection 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 hy-draulic cylinder is coated with a specialised plastic. An additional pressure-free leak oil connection with a closing leak oil seal en-ables a practically leak-free structure with minimal non-slip inclina-tion, despite these frictionless seal pairings. In recent years, Hänchen has expanded a structural feature to all catalogue cylin-ders: 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 movements 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 quality. With the catalogue cylinders, these are the series, 126 synchro-nous/differential, 166 synchronous, 306 synchronous/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, partially also in synchro-nous 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. However, this process only works with a production precision in the range below μm, as the leakage would otherwise lead to high hydraulic 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 provides a cost benefit of around 30%. Through their negligible friction, cylinders in Servofloat quality offer the highest positioning and repetition preci-sion, 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 scrapers cause a minimum measure of friction. As the hydrostatics in these sealing combinations ensure centring of the piston rod against po-tential 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 In-dustrial 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 previous test dimensions necessary”, summarises Hänchen. “We needed to overcome the challenge with significantly larger structural lengths, lifts, forces and speeds, with the proven precision. With this, the new Airbus has also become a milestone of state-of-the-art hy-draulics 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.
Airbus A380 under continuous Stress(IABG GMBH))
Test bench with Hänchen cylinders pushes the boundaries of what is possible
Turbulence can cause the wing tips of the A 340-600 to move over four metres. Anyone observing the constant up-and-down movement of the wings spanning several metres in trial conditions gets a glimpse of the material stress to which commercial aeroplanes are subject. This Airbus will complete at least 35,000 flights - and won't even leave the hangar directly adjacent to Dresden Airport to do so. The aeroplane is moved by 94 Hänchen hydraulic cylinders which form the core mechanical components of a time accelerator test, which simulates over a period of 18 months the sequences of movements of the entire life-span of an aeroplane lasting 25 - 30 years.
30 types of aeroplane in 40 years
The aeroplanes which have been tested since the 60's with Münchner IABG acting as main contractor range from the Tornado to the Airbus - including the Airbus numbers 300, 310, 320, 330 and 340. The aim of this extended time test is to provide experimental evidence that the airframe has sufficient serviceable life and to eliminate any possible weak points still in evidence. Clients requiring these tests include all the major European aircraft manufacturers. The current test is carried out by the IABG in conjunction with Dresdener IMA Materialforschung und Anwendungstechnik GmbH. It only took less than two years to design and construct the test. The static tests were started in April 2001 and shortly before the first flight in September, the endurance tests were started with a new control technique developed by the IABG to realize the tests in a extremely short timeframe.
Tests with Hänchen cylindersSince 1974 Hänchen hydraulic cylinders have been used for the majority of these dynamic tests for aircraft. The cylinders are supplied with a throughput of 4,400 litres of compressed oil per minute over many kilometres of pipelines with diameters of up to 20 cm on the main pipes. Process computers co-ordinate the sequences of movements in such a way that the loads correspond to the day-to-day flight realities of the Airbus A 340-600. In order to make these movements as realistic as possible, the hydraulic cylinders are driven by proportional valves. This process mainly uses testing machine cylinders with floating annular gap sealing, patented by Hänchen. For special tasks, however, cylinders with hydrostatically mounted piston rod guides are also used.
Simulation not only on the computer
"Discovering damage in this kind of test is part of everyday life", is how Klaus Woithe, graduate engineer and IABG branch manager in charge of the project sums up the experience gained over 40 years. "Even modern computer models with FEM, the finite elements method, still cannot replace the dynamic endurance tests", he maintains. Indeed, computer analysis cannot, as a general rule, reproduce certain effects occurring in reality with the required precision. Since aeroplanes are usually designed nowadays to resist a certain amount of damage, cracks several centimetres long can occur in the fuselage skin without compromising the safety of the aircraft. The demonstration trials in Dresden are carried out on the wing structure with approx. 60 metres wing span and a 33 metre long fuselage segment. Undercarriage and engine pylon dummies serve to introduce the loads coming from these components.Hydraulics as a core technology
"Hydraulics is the core technology for load simulation in the dynamic test for material fatigue on aircraft. This is because control, measuring and fluid technology work hand in hand here", says Woithe. "The computers have to make stipulations in real time which are then activated in the set-actual comparison with the help of PLCs via control circuits with load cells. Apart from control they also serve to prevent overload. High-quality testing cylinders are a basic prerequisite for guaranteeing that this test is realistic. In this respect we have, over 27 years together with Hänchen, pushed the boundaries of what is possible time and time again, yet we have nevertheless achieved a good cost-performance ratio while remaining highly committed to running on schedule. The testing cylinders from Ostfildern were successful on account of their strengths, especially their low friction, optimum tightness, excellent response, low abrasion, extreme piston speed, low initial break-out torque, fatigue strength and long serviceable life.
Two and a half lives in 18 months
For safety reasons experimental evidence is taken over more than two and a half times the expected life of an aircraft. In order to test the material fatigue, all the stages of flight are simulated. This includes take-off and landing as well as all stages of flight in which the Airbus is subject to load alternations, namely vertical and horizontal gusts and flight manoeuvres. In this way even a long-haul transatlantic flight in good weather conditions can be condensed into a simulation program of quarter of an hour or half an hour. In the categories of short, medium and long-haul flights a series of typical flights in each case were defined from the standard flight right through to the difficult flight in extreme conditions. They consist of load data for the airframe mapped on a gradient diagram. This is because the cabin is set at an increased internal pressure by means of a compressor unit and two air chambers depending on the simulated altitude, in order to simulate the difference in pressure between the cabin and the surroundings depending on the altitude of each flight. A flight-by-flight program sequence encompassing more than 1,000 flights is generated from these types of flight. It is repeated as many times as necessary to reach the stipulated total number of flights. Constant monitoring by inspectors, comprehensive, day-long inspections of the entire test structure as well as regular measuring of 3,600 strain gauges and 80 displacement transducers guarantee that damage is detected as soon as it occurs. Since the aircraft is constructed in such a way as to resist a certain amount of damage, the development of cracks is observed from the point at which they arise until they reach a critical length. Then they are repaired or the part is replaced. A cleverly devised monitoring system ensures that the aircraft is not inadvertently exposed to unintentionally excessive loads in particular.
Floating annular GAP sealing
Precision requirements dictate that disruptive forces such as stick-slip effects of the cylinders should be avoided. Very low restoring force occurs on the wingtips, for example, but they must be moved at the same time at up to 670 mm/s. In the process the wings are moved upwards up to 2.9 m from the zero position and downwards up to 1.2 m. Spurious oscillations of the flexible structures can easily occur, however, if the pistons and piston rods of the hydraulic cylinders are not as smooth running as possible.
In this case tolerances of only 3 per cent of the nominal load of the cylinder are accepted; in practice they are under 2 per cent. Spurious oscillations would lead to unwanted load variations and would distort the test results. For this reason, the cylinders with the floating annular gap sealing patented by Hänchen are particularly favoured for use in structure trials on aircraft, since they always have the same friction irrespective of the pressure. A steel bush inside them deforms through a choke gap and thus produces a non-contact packed sealing gap of a few 1/100 mm. The prerequisite for this technology is a production precision in the region of a few µm, since otherwise the leakage would lead to high hydraulic losses. This cylinder series (PZR) has a cost advantage of around 30 % when compared with cylinders with hydrostatically mounted piston rod guides (PLZ). This is because the PZR cylinders, on account of their very low friction, offer the possibility of very high positioning and repeat precision, are stick-slip free and admirably suited to both extremely slow and extremely fast movements alike.
Another crucial factor determining the choice of the Hänchen cylinders, however, was their stability. After all, the trial runs for 24 hours seven days a week.
Dry run for the Airbus:
Dry run for the Airbus:2 - 3 aircraft lives of flights without taking off
Before an aircraft can take off for real it has to prove its prowess on the hydraulic test bed. It must withstand a tough set of tests representing at least 2 - 3 aircraft lives with all conceivable "real situations". For this purpose no less than 90 hydraulic cylinders have to be coordinated with a high degree of control effort. A dry run which pays off for all aircraft in the air. The testing machine cylinders from Hänchen not only ensure a "real" feeling of flight but also permit the sustained and highly precise reproducibility of all movements.
The Airbus 340 already has more than 20,000 flights behind it: taxiing on the runway, operation of the landing gear on uneven ground, take-off, flight and landing in good weather as well as in vertical turbulence with 7.6 m/s gust speed. This scenario takes place on a hydraulic test bed at IABG in Ottobrunn near Munich. "Since 1970, since the first fatigue strength tests on the Airbus A 300, we have been using Hänchen hydraulic cylinders. Together we defined, developed and tested the necessary units. And even on extreme special constructions such as synchronous cylinders with a five metre stroke and six metre length the Swabian cylinder manufacturer, unlike other companies, never failed," reports Dipl.-Ing. Siegfried Schmid, head of the hydraulic loading unit group at IABG. The hydraulic unit for the Airbus tests is a fluid system in the superlative: 90 hydraulic cylinders and two hydraulic motors are supplied by six fluid pumps with up to 2,000 litres of oil per minute at a power consumption of 1.2 MW.
The mega-characteristics of the test stand are reflected in the details.
At a working pressure of 210 bar the cylinder transfers up to 1600 kN of load - depending on the cylinder size - to the structure. This enables conditions to be simulated in which the wings are subjected to a loading of up to 250 tonnes, because the A 340 is a wide-bodied long-haul aeroplane with a wingspan of 60 metres. The entire double wing with the central fuselage element was constructed complete in the hall of the Ottobrunn Technical Inspectorate (TÜV) - in addition to tests on components for the European launch vehicle Ariane V and other aircraft.Despite the giant dimensions involved, the problems for the testing facility reside more in the detail. Coordinating 90 sources of power requires a highly elaborate control system, especially as the movements have to exactly reflect the real situation: Expensive servo-valves and as far as possible low stick-slip cylinders with a friction level distinctly below 1 % at 10 to 100 % nominal loading, which even if mounted at an angle and under lateral forces of up to 10 % do not cant, are just as important as the sophisticated control technology.
The actual testing programme, a cycle of 2,000 flights with different characteristics, which is run-through 20 times, draws its values from the specifications provided by the Airbus producers. As purely static flight phases are irrelevant for the dynamic test each simulated flight consists of five minutes to two hours in which the dynamic sections are combined. The test values are stored in a matrix which defines the current specified force for each cylinder.
High computer power essential
Extensive expert knowledge and high computer capacities are needed in order to create this matrix. Its values are assigned via one microprocessor per cylinder to a dedicated electronic control unit which controls the respective regulator or proportional valve. The piston movement thus triggered in turn exerts an effect via a plunger on a network of pressure points which has the purpose of causing an as far as possible equally distributed impact of forces. Between the plunger and the cylinder rod end is located a load cell with a double sensor. The first sensor closes the control circuit and supplies the respective electronic control unit with the actual value, the second sensor sends its signal to the central test computer. This redundancy permits control through two separate systems which in the event of a fault can bring the testing to a halt independently of each other.
A decisive point in the testing strategy becomes clear here: Through the value of the unit being tested, the duration of the tests, the effort involved in a repeat test and the complexity of the process, the unit being tested must not - unlike for instance in tests conducted in the auto industry - be damaged by factors not envisaged in the test programme.
In the two cylinders for simulating the landing gear loading this led to a special development which was a challenge even for the Hänchen designers: As the landing gear is not available in the laboratory and after take-off positions are also simulated which are below the normal position, the piston in the 'landed' position is not in the final position. A locking system taking up a minimal amount of space therefore had to be found to ensure this position. Given the maximum load of 90 tonnes a clamping unit could not be used. A second, end-position piston was therefore integrated in the cylinder which during operation is recessed in the bottom of the main cylinder. In resting position it is moved out of the bottom and thus provides an end position for the main cylinder.
A laterally fitted auxiliary cylinder then pushes a blocking pad between the end-position piston and the lower cylinder bottom, rendering the system statically stable. Four receivers, duplicated valves and a control block ensure that even if the pressure supply fails the system can be returned to this static condition. This guarantees that if the system fails no untypical loadings occur in which the landing gear for a prolonged period touches down, as it were, 10 centimetres below the surface of the runway.
IABG is a leading companies in Europe for such complex testing tasks, in particular using hydraulic systems. In this environment the Munich-based company sees itself as a provider for complete testing operations. Around 1,500 staff, 50 % of them graduates, offer tailor-made static and dynamic testing operations according to customer specifications as a complete package.
Tailor-made testing operations as a package of services
Experienced development teams, programmers, electronic testing experts, fluid engineers and maintenance specialists support the customer, backed by an extensive testing Infrastructure, computer and measuring system capacities, buildings for testing extremely large assemblies and extensive hydraulic facilities.
With this background of experience units of virtually any kind can be tested with the equipment available. "Particularly interesting for our customers is the fact that they can outsource complete testing projects without, for example, having to make investments in expensive testing systems as a result of new requirements on the quality control side and without having to invest in specialists who perhaps cannot even be fully utilised. In addition to complete testing programmes, we can also handle any partial task, design concept or plant assignment," reports Dipl.-Ing. Wolfgang Mohr, Marketing Manager at IABG.
Projects ranging from dynamic endurance tests on motorcycle forks through to the testing of complex new transport technologies, for example for the Transrapid, show how many and varied the testing tasks are which have to be performed even just as far the hydraulic testing systems are concerned. "Hydraulic testing cylinders represent the core technology," explains hydraulics specialist Schmid. "And in this sector Hänchen offers unbeatable solutions despite the strong competition. Because the Swabian hydraulics company has specialised to such an extent on the actual cylinder construction that unique know-how exists at Ostfildern, especially for test cylinders." The many years of cooperation between the testing company and the cylinder manufacturer has produced numerous detailed solutions. "IABG with its strict requirements prompted us as early as the mid-1970s to develop our own test cylinder programme," reports Hans-Dieter Fabrowsky, Sales Manager at Hänchen. "As a result we were able to occupy major market positions at an early stage and today supply test cylinders for example to all prominent car manufacturers in Germany. The cylinders for the Airbus tests were, however, our biggest challenge."
Perfect technology through relentless testing
To handle these special tasks, tests were also carried out in cooperative partnerships. For instance, Schmid had tests carried out on a cylinder which was tested with 100,000 load cycles including the lateral forces arising under typical testing conditions and which was then dismantled. This enabled precise specifications to be made to the Hänchen design engineers as to where further developments were necessary. "We have been doing business with the cylinder producer for all these years because the design engineers in Ostfildern-Ruit have always managed to meet our most extreme requirements and to respond flexibly to our expectations," said Schmid.
