Protecting high-performance test stands reliably

Intensive research and development and compliance with the highest demands on quality are prerequisites in modern production today, for example during the manufacture of different components of a drive line. As a result, component and functional test stands, which permit the practical simulation of the complete spectrum of loads on a drive line and its components, are becoming increasingly significant. They provide accurate data quickly prior to the implementation of complex practical tests, and therefore help to save costs and reduce development times.

The demands on high-performance test stands are constantly increasing: improvements in performance and efficiency, faster measurement value recording and evaluation, savings in terms of energy resources and reductions in environmental impact are top priorities here. The demands on individual components become greater as the concepts increase in complexity. In many test stand applications, torque limiting clutches are used which not only protect the frequently expensive test pieces but also the sensitive measurement units against overload damage. These torque-limiting clutches must comply with the extreme demands of test stand technology, and also work reliably at very high speeds. With the backlash-free and torsionally-rigid EAS-HSC and EAS-HSE torque-limiting clutches, mayr power transmission provides overload protection which is especially tailored to the requirements of high-speed applications.

Should the torque in a test stand exceed the set limit values of the torque limiting clutch, then the clutch disengages and disconnects the input and output within fractions of a second. Following an overload, the measuring shaft may at worst have to be re-calibrated, but the torque limiting clutch will reliably prevent expensive damage to the drive line or the test piece.

The EAS-HSC and EAS-HSE high-speed torque-limiting clutches transmit the torque backlash-free and with high torsional rigidity. They are compact, have a high performance density and a low mass moment of inertia. As a result, the EAS-HSE clutch with a torque of 1000Nm, for example, weighs significantly less than 10kg. With a very small outer diameter of 125mm, a relatively large bore diameter of up to 60mm is permitted for mounting the shaft.

High balance quality

Moreover, a further important basic prerequisite for the use of a torque-limiting clutch in high-speed applications is the high balance quality of each individual component in order to achieve optimum running smoothness in the drive line. As the speed increases, an imbalance causes significant centrifugal forces and vibration. This places the entire test stand design under strain, leads to irregular running and falsifies the test results of the measurement equipment. Therefore, optimisation of the mass unbalance was the top priority during the development of the EAS-HSC and EAS-HSE clutches. Design measures have ensured that many of the individual components incorporated in the torque limiting clutches do not change their position within the clutch even following an overload. Furthermore, the clutches are balanced in a completely assembled state to a balance quality of G 2.5, at a reference speed of 3000 rpm.

The joint clearance between the respective customer shaft and the clutch bore is minimised so that a completely assembled torque-limiting clutch, balanced and set to the required speed, ensures the necessary balance quality, even following integration into the respective test stand application. The clutch bores are designed with quality 6 in the standard series, and the frictionally locking shaft hub connection ensures backlash-free torque transmission.

In addition, a high-quality clutch bearing absorbs the external reaction forces which act on the output flange of the high-speed clutches. As a result, these forces have no influence on the trigger mechanism, and a precise torque limitation is maintained. Furthermore, the clutch bearing is designed so that it can absorb very high accelerations. These occur, for example, when the drive line is blocked as the torque-limiting clutch suddenly disengages, and the input side of the clutch is strongly accelerated against the output side.

In normal operation, the EAS-HSC transmits the torque via balls which simultaneously engage into recesses of the hub and the output flange. Cup springs pre-tension the ball bearings via a ring. The high spring force is proportional to the transmittable torque. This principle remains backlash-free even when the recesses are worn. mayr power transmission uses spring cups in torque limiting clutches with a falling characteristic curve. Therefore the torque is transmitted in a torsionally rigid manner up to the pre-set trigger value.

In case of overload, the pre-tension force of the spring reduces, and the clutch triggers very accurately. After disengagement, a special overload mechanism ensures that the clutch remains in the disengaged position. The drive line can slow down freely, and as a result absolutely no ratchetting impacts occur during the overtravel time which might negatively influence the drive line, especially the torque measurement equipment in the test stand. The design of the torque transmission unit is such that re-engagement is only possible at the disengagement position. In operation, the individual parts of the clutch therefore always have the same angular position to one another, and the balance quality is maintained. The special overload mechanism enables simple, quick re-engagement without major exertion of force.

High speed - up to 20,000 rpm

On the EAS-HSE, individual overload elements transmit the torque via positive locking. In case of overload, these overload elements disengage and disconnect the input and output almost without residual torque and with a high degree of switch-off and repetitive accuracy. The kinetic energy of the rotating masses stored in the system can slow down freely. The elements do not automatically re-engage, but remain separated until they are re-engaged manually or by means of a device. The position for the synchronised re-engagement for maintenance of the balance quality is clearly marked on the clutch.

The compact EAS-HSC clutches cover a torque range from 5 to 1000Nm; the EAS-HSE clutches, based on one overload element, feature a torque range from 100 to 8400Nm. In the standard design, both clutches can achieve speeds up to 12,000 rpm; in a special design, the EAS-HSE element clutches can even reach up to 20,000 rpm.

Besides torque-limiting clutches, shaft misalignment compensation couplings are a decisively important accessory part in test stands, as they minimise the disturbance variables acting on the measurement flange. These disturbance variables, or so-called parasitic loads, often occur due to alignment errors in the drive line. Misalignments between the input and output side occur in almost all applications. For this purpose, shaft misalignment compensation couplings such as the ROBA-DS disk pack couplings by mayr power transmission are used together with the measurement flange. These couplings transmit the torque backlash-free and with extreme torsional rigidity manner, and compensate for radial, axial and angular shaft misalignments. They therefore not only provide precise measurement results, but also protect the bearings installed in the shaft train against unwanted loads, and consequently from unnecessary downtimes and costs. In addition to high running smoothness and a low mass moment of inertia, the couplings are distinguished by high balance quality, and are particularly robust and resistant.

Depending on the construction size, the standard-design ROBA-DS couplings cover a speed range of 8000 to 18,000 rpm and are balanced to a balance quality of G 2.5 (reference speed 3000 rpm). With a high-speed design in which the individual parts are machined with very high accuracy (quality IT5) and narrowed concentricity and axial run-out tolerance, speeds of up to 30,000 rpm are permissible. Here the balance quality of the coupling is G 2.5, at a reference speed of 5000 rpm.

With the ROBA-DS disk pack coupling, the nominal torques specified in the catalogue can be utilised without any limitations. They transmit the torques backlash-free and with a constantly high torsional rigidity. The specified shaft misalignments can be utilised to 100 per cent without influencing the transmitted torque.

For a combination of shaft coupling and torque limiting clutches, mayr power transmission provides complete systems from a single source. For this purpose, the business can call on comprehensive expertise and many years of experience in development and design. All products are extensively tested on calibrated test stands and accurately adjusted to the required values of the respective application.

Please visit www.mayr.co.uk for further information.