Shaft Couplings For Rail Vehicle Drives BerhardTenfelde, Engineer KTR Kupplungstechnik GmbH – Rheine, Germany In rail vehicle drive trains, shaft couplings appear to be insignificant components. However, this report will set forth many reasons for the designer and the user of rail vehicles to pay particular attention to this component. It contributes to the easy maintenance and long life of the drive – but the comfort of level of rail vehicle passengers as well. The design of powertrain drive rail vehicles has advanced tremendously in recent years requiring higher performance and higher speeds. There is an increased demand to meet passenger comfort levels through vibration and noise reduction while minimizing maintenance time and service requirements. Features of rigid shaft couplings The need to simplify drive systems has led many design engineers to question the viability of the torsionally elastic coupling. They have integrated rigid couplings or eliminating the coupling entirely on the premise that modern manufacturing is much more accurate, eliminating the need for flexible couplings. This theory has been challenged by examinations made by KTR Kupplungstechnik GmbH, Rheine. Testing and field experience has proven that a rigid drive system is susceptible to more damages as a result of internal wear, electrical short circuits and external drive shocks. The cause for premature fatigue in a drivetrain are: lack of torsionally flexible component (i.e. ROTEX Coupling), radial and axial forces due to rigid shaft connections with multiple in-line bearings, manufacturing machining tolerances, external loads and system heat. The renunciation of flexible shaft couplings in train drives began with concern regarding the drive capability of components originally developed for other types of applications, their service life, load capacity and safety. Fortunately, for the last twenty years, ROTEX shaft couplings have been modified specifically to keep pace with rail drive technology. ROTEX Coupling The ROTEX coupling consists of two hubs with interlocking jaws and an elastomer (spider) between. (Image 1). The crowned jaw design ensures that the flexible spider has sufficient free space to avoid edge pressure, which can result from misalignment of the drive components. (Image 2)

Image 1: Principle presentation of the ROTEX shaft coupling

Image 2: Spider under load In the event of extreme torque, the spider expands to fill the concave jaw design. To keep wear at an absolute minimum, KTR has developed a spider with special leg quadrant geometry to be used in rail applications. This design enables ROTEX couplings to reach performance levels of more than 1 million kilometers. Due to the geometry of the crowned spider leg and hub jaw, ROTEX couplings provide a single cardanic pivoting point, allowing efficient power transmission even with slight misalignments (angular and parallel) of mating shafts. Mating shaft misalignments occur even with the most accurate manufacturing and assembly procedures. Therefore the use of a flexible shaft coupling is extremely important! The concave jaw design ensures an even tooth surface load. The design geometry permits a compact design. The short overall length and diameter of the ROTEX coupling can transmit high torque loads. An important contributing factor to the long service life of the coupling system is the precise manufacturing of the coupling components. The surface of the ROTEX hub presents tight machining tolerances and high surface qualities. Also, high-quality plastic materials are used in the production of the spiders. During the development of the jaw design, calculations utilizing Finite Elements Analysis (FEA) were used (Image 3). Examinations of the spider structure confirmed the long service life and the reliability of the coupling established by the calculations. The damping effect, the permanent elasticity, the remaining deformation and the restoring elasticity of the various hardnesses were also verified.

Image 3: FEM Calculation of the jaws of the ROTEX Coupling

Operating safety The ROTEX coupling's tendency to assume a permanent deformation was also checked and recorded in a load deformation curve. For temperatures up to 80°C, only slight load deformations were noted. Only under a long-term high power load, with higher temperatures does permanent deformation occur. In practice, ideal operation is guaranteed with temperatures from 40°C to + 100°C. Peak temperatures of up to 120°C cannot damage the coupling spider. Furthermore, the ROTEX spider, which is often used in drives for the chemical industry, is resistant to lubricants and other typical working media of train technology. The ROTEX coupling offers a fail-save power transmission design. Since hub jaws are interlocked, the drive will remain operable in the (improbable) event of an overload on the spider. Another tremendous advantage to this coupling is the ability to perform visual maintenance inspection in its assembled condition.

Damping of torsional vibrations The primary task of ROTEX couplings is to reduce vibrations and shocks in train drives. In addition to the mechanical shocks generated by wheels and rail, shock loads from short circuits of the traction motor must be absorbed. The natural vibrations and noise frequencies of the drive must be analyzed and adjusted to acceptable levels. Through computerized simulations using DRESP software (with and without loads) KTR is able to determine and recommend the proper size coupling and spider hardness. By selecting different spider shore hardnesses, the dynamic stiffness of the system can be controlled. Spider hardnesses provide dynamic torsional stiffness ranging between 1.75 x 105 Nm/rad (Polyurethane 95 Sh A) to 1.40 x 105 Nm/rad (Nylon polyamide). A spider made from Nylon polyamide offers only 50% of the damping power of the polyurethane spider. (Image 4)

Image 5: Damping curves of different spider materials Jaw-type coupling with spherical plain bearing In addition to reduced damping of flexible shaft couplings, the couplings also isolate structure-borne noise and contribute significantly to noise reduction, thereby increasing passenger comfort.

Image 6: ROTEX Coupling with spherical plain bearing

Maintenance

The coupling is very compact and easily assembled axially by plugging the hubs into one another with the spider between. Simple assembly and disassembly procedures facilitate the overall maintenance of the drive system as compared to disc couplings, which require numerous adjustments during final installation. Due to their non-lubricated design, ROTEX couplings require no further scheduled maintenance. Rail operators can visually inspect the condition of hubs and spiders during normal scheduled gearbox and motor maintenance checks. Possible assemblies ROTEX shaft couplings are suitable for various train drive configurations including individual axle drive, four wheel longitudinal drive, individual axle cross drive and asynchronous wheel hub motors. The assembly motor/gearbox can be performed with the following methods: slide fit with bolt and washer, shrink fit with hydraulic release or slide fit with tapered bushing. (Photo 5)

1- Slide fit with bolt and washer 2- Interference fit with hydraulic release 3- Slide fit with tapered bushing

Image 5: Assemblies of the ROTEX coupling Integration of components In some areas of Power Transmission, the integration of components has made considerable progress. Couplings designed with overload systems, impellers and disk brakes are commonly used. This reduces both space and costs because the number of components is decreased. By combining the ROTEX coupling with a spherical plain bearing, varied radial loads can be carried with a single motor shaft design. KTR is currently testing this coupling design. (Photo 6) Outlook Some applications for ROTEX shaft couplings in train technology are trams and light rail vehicles. Image 7 shows a ROTEX coupling being utilized between a gearbox and electric motor in a light rail passenger vehicle.

In this report, we draw upon our 20 years of experience in providing maintenance-free and long life (1 million Km) drive couplings to the rail industry.

Image 7: Use of the ROTEX Coupling in single axle drive