Tension test benches for gears, clutches and gear wheels
In order to test whether their mechanical drive systems meet the high requirements for quality, reproducibility and safety, helicopter manufacturers around the world rely on a technology manufactured in Switzerland.
Dhe Rotodiff solution from Viscotherm, as the core of torque test stands, ensures that they can be operated with a fraction of the turbine power. In addition, this allows gearboxes with various ratios to be tested under the prescribed operating conditions. Furthermore, with the Rotodiff, torque test benches can be operated in a mechanically closed circuit, which significantly reduces energy consumption.
In the case of torque test stands in back-to-back arrangement and mechanical closed loop, the input and output shafts of the test specimen are coupled mirror-symmetrically to an identical gearbox. Both are then accelerated up to the nominal speed and held at this level in neutral. A torque is then applied to the drive circuit, causing the gears to be braced against each other. The elastic deformation of the test specimen and the test rig results in a bracing angle between the loaded and unloaded structure. The test is carried out with only a fraction of the nominal power of the gearboxes, as only idle and power loss have to be provided once the nominal speed has been reached. In this way, a helicopter manufacturer saves so much electricity during its annual gearbox tests in the factory that an average city with 30,000 inhabitants can be supplied with it.
Flexible adaptation to all load cases and gear unit types
In order to enable tests with different gearboxes for helicopters, rotodiffs have proven themselves in such test stands for the introduction of torque. In combination with a device for adapting the test rig geometry to the respective input and output shafts of the different gearbox types, a universal test environment is created by compensating the differential speed. For this purpose, the oil volume flow is controlled by a hydraulic unit and thus adapted to the respective over- or reduction ratios. The housing and rotor of the hydraulic motor rotate, the differential speed between the two is always proportional to the volume flow (Fig. 4). Since the test rig and the test specimen are braced against each other, the torque is proportional to the pressure and is regulated hydraulically. This hydraulic solution allows vibrations to be completely eliminated. Depending on the size, the housing can reach speeds of up to 6500 min-1, while the differential speed of the rotor can be between ±0.5 and ±200 min-1 - with a torque of up to 100 000 Nm.
With the Rotodiff it is also possible to adapt the direction of rotation of the housing and rotor to the gear unit under test. The gearbox is protected against overload by a pressure relief valve. The combination of the various variables results in a flexible torque test rig that allows tests of different gearbox variants at high torques and different frequencies with minimal power consumption. In cooperation with Viscotherm, Airbus Helicopters, for example, has developed its test stand technology up to the latest generation of "multipurpose" test stands.
Motor or pump, leading or lagging rotor
To illustrate the function of the Rotodiff, it is divided into eight operating quadrants which can be viewed individually and shown with their respective hydraulic circuitry. In quadrants I, III, VI and VIII, the Rotodiff functions as a hydraulic motor: the hydraulic power fed in by a high-pressure pump is converted into torque and rotary motion. In quadrants II, IV, V and VII, on the other hand, the Rotodiff operates as a pump: the mechanical work supplied to the rotor or housing via a shaft is converted into hydraulic power.
In this case, the rotor turns slower than the housing, i.e. lagging, whereas in quadrants I, IV, VI and VII the rotor runs faster than the housing, i.e. leading. In quadrants IV and VII, the drive torque must be applied to the rotor, while in II and V the rotor or housing can be selected. Thanks to these switching options, back-to-back test stands can be adapted to a wide variety of kinematic conditions of the test specimens on the one hand, and on the other hand the desired load parameters can be conveniently achieved by hydraulic control. With the versatile adjustment possibilities of the kinematics, their robustness and a high degree of efficiency, Rotodiffs are now proving their worth in restraint test benches all over the world. In addition, infrastructure costs can be saved due to the compact and simple design of the units. Another advantage is the energy balance, which is lower than with equivalent electromechanical test benches.
