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Hydraulic cylinder vibrates to a 400 Hz sine.

(DYNA-MESS Gmbh) and H?nchen reach new heights of performance

Modern quality standards mean that components are now facing more varied and useful tests than at any time before. DYNA-MESS Testing Systems specialises in testing machines for dynamic component and material testing. With its broad and varied range of standard and special test machines, the Aachen-based company exploits the possibilities of modern fluid technology to the best advantage. Its pièce de resistance is a special testing machine for elastomers which, with the help of standard hydraulic cylinders from H?nchen, realises controlled vibrations of 400 Hz in order, for instance, to test the damping of bearing elements made of rubber-metal composites for the automotive industry.

Hydraulics is cutting-edge technology

According to Dr. Marcus Jarchow, Managing Director of DYNA-MESS, "these test units are exposed to extreme stresses with a nominal force of 10 kN and a frequency of anything from 0 to 400 Hz. It has been extremely difficult to find a suitably high-performance hydraulic cylinder on the hydraulics market", because the test device depends on the perfect stick-slip behaviour of the hydraulic system. Cylinders with hydrostatically supported piston rod guides are normally used for this type of application, but the technical expenditure for this solution is enormous. "We have nevertheless kept our ears to the ground to try and find out who can offer some help with finding a solution" stated the Managing Director of DYNA-MESS. "Everybody hesitated when we mentioned the technical requirements, with the exception of H?nchen Hydraulik. A further argument in their favour was that they had their own test equipment and would be able to base their results on extensive experience. Ultimately, the hydraulic specialists from Ostfildern, near Stuttgart, offered a standard cylinder with patented, floating annular gap seal which is considerably more cost-effective than drives with hydrostatically supported piston rod guides". The PZL assembly series from H?nchen, however, also provides these high-end cylinders.

The PZR floating annular gap seal

The patented, floating annular gap seal from H?nchen permits the use of cost-effective PZR high-end test cylinders, in which a steel sleeve is positioned through a choke gap and thereby creates a contact-free sealing gap of just a few 1/100 mm. This process can, however, only function correctly with a production accuracy of only a few μm, as leaks would otherwise lead to an unacceptable loss of hydraulic fluid. This PZR assembly differs only by the slightly lesser absorption of the lateral forces from a cylinder with hydrostatically supported piston rod guide, but yields a considerable cost advantage. Thanks to their negligible friction, however, PZR cylinders offer maximum positioning and repeat accuracy and are stick-slip free, making them ideally suitable for extremely slow as well as fast movements and minimum travel over ranges of a very few μm. As lateral forces are not experienced in this 400 Hz DYNA-MESS test equipment, the decision was made in favour of these cylinders.  

High-quality peripherals

The cylinder is equipped with a special inductive position measuring system and a force transducer with an extremely low natural frequency and yield measuring strips. Both sensors supply analog output signals which are digitalised with a resolution of 16 Bit and tested with more than 20 kHz. They supply the control with the necessary data, which must be converted practically in real time by a control card with its own high-performance processor. Tried-and-tested DYNA-MESS software modules were adapted to the special conditions to facilitate programming. A PC forms the basis for the Windows user interface. The valves represented yet another technical challenge, as they have to work with a control frequency of 16 kHz. The fluid circuit has been designed for 300 bar and operates at 40 l/min. This high power density enabled a low design size, because a high frequency with a wide frequency spectrum places extremely high demands on the mechanical construction: the frame must not have any natural resonance over the whole of the frequency range.

Commissioning

Commissioning proved to be a time-consuming exercise, particularly due to the need to tune in to the various frequency ranges. The frequency was increased in steps of 10, 20, 30 … up to 400 Hz, with the resonance having to be minimised at every stage. The complete control was also tuned during this time, in that the runtime delays between the reaction of the processor and its execution by the hydraulic system are captured and included in the control. Finally the control quality and curve form were optimised, this operation being followed by fine tuning with the help of a friction-free and loss-free spring, which measures the dynamic without phase displacement and so enables material tests to be carried out under clearly defined conditions. After up to 400 Hz had been reached, Dr. Jarchow’s team carried out further tests up to a frequency of 1,000 Hz. "We were surprised at what outputs were possible, up to double the nominal output at 800 Hz", said the hydraulic specialist. "It was only above this frequency that the limits of what is technically feasible at the moment were clearly reached. The hydraulic cylinder has more than matched the requirements placed on it, and that was with the use of a catalogue cylinder and at a very attractive price”. The finished system was then commissioned during the first quarter of 2002.

Practical application

H?nchen’s PZR hydraulic cylinders are now being used by a Daimler-Chrysler supplier to test vibration dampers for engine and gearbox suspensions for load carrying ability, damping behaviour and service life. These components are made from rubber-metal composite material with embedded passive hydraulic damping. The sealed glycol circuit in this component is used to dampen the vibrations. A new test bench enables the developers to optimise the damping characteristics. Sweep tests with a pre-selected load which corresponds to the engine or gearbox weight are used to determine at which frequency damping is most effective. For this test the frequency is superimposed over the static load with the aim of achieving an optimised damping over the whole frequency range of up to 400 Hz."

Our engineering competence, the experience and evaluation of our suppliers as well as the ability to develop test solutions together with our customers were the deciding factors", explained Dr. Jarchow, whose team performs all calculations, is responsible for design, software, electronic development, control, configuration as well as installation and realises the complete system in co-operation with the suppliers. "Suppliers whose products conform to our own high standards play a critical role. That is why we are working together with H?nchen Hydraulik".