INTERNATIONAL DESIGN CONFERENCE - DESIGN 2000 Dubrovnik, May 23-26, 2000. TWINTORS - DIAPHRAGM COUPLINGS FOR TURBO MACHINES H. Birkholz, P. Dietz, E. Dehner and M. Garzke Keywords: diaphragm coupling, machine elements, design, numerical and experimental investigations Abstract: Diaphragm couplings are ideally suited for the torsionally rigid transfer of very high torque and rotational speeds while at the same time compensating for the axial, radial and angular offset of the shaft ends that are to be connected. This paper describes the construction and properties of this design of couplings. 1. Introduction Couplings are essential elements of drive systems, and can be subdivided into shifting and nonshifting couplings. In addition to their primary task of transmitting torque, they must sometimes perform other functions, e.g.: reducing intermittent loads or vibrations (flexible coupling) safeguarding against overload (overload coupling) linking or separating the shaft ends (shifting coupling, centrifugal coupling) compensating for shaft offsets (compensation coupling) Numerous different designs have been developed specifically for compensating shaft offsets, with different designs aimed at compensating axial, radial and/or right-angled offsets. The primary causes of shaft offsets are imprecisions in alignment or assembly, expansion due to heat, elastic deformation, displacement of the base, or design factors /1/. The following are all examples of torsionally rigid compensating couplings: claw coupling, parallel-crank coupling cross-recessed coupling propeller shaft (universal joint) denture coupling spring-shackle coupling metal-bellows coupling diaphragm coupling The fundamental distinguishing feature of these couplings is the implementation of the compensation function by means of guides and joints or through flexible metal components (so-called metal springs) /2/. The many components that make up a coupling lead to problems of wear associated with the positive transfer of the torque (e.g. denture coupling, cross-recessed coupling), and it is essential that the contact points should be lubricated. Usage is thus limited by a maximum permissible temperature. The same limitation applies to the maintenance-free, dry-running tooth type coupling, in which the 605
internally-toothed sleeve is made from plastic (polyamide); this coupling can be used up to a maximum temperature of around 80 C. Diaphragm couplings, on the other hand, transmit torque without any clearance, and as a result the dynamic characteristics of the drive line are significantly improved, particularly in reverse operation or when counterbalancing wheels. Furthermore, there is no need to use any lubricant and the coupling can be operated at temperatures above 250 C. Other important characteristics of diaphragm couplings are discussed below. 2. Design layout of diaphragm couplings From a technical point of view, diaphragms refer to thin plates with a low bending strength. These diaphragms can be produced either as a one-piece disk or with a segmented structure, whereby the two types demonstrate different characteristics. All diaphragm couplings consist of two hubs, the diaphragm components and the linking elements. Possible configurations are illustrated in Figure 1 /3/. With the single arrangement of the compensating element "diaphragm" the angles of deflection are relatively small, and the version arranged in tandem (double arrangement) is more functional. Figure 1. Arrangement of diaphragm couplings /3/ The following features represent special advantages of diaphragm couplings: economical to construct due to the symmetry of the surfaces and uniformity of components simple, spring-actuated linking of hubs and diaphragm components 3. TWINTORS diaphragm couplings The company BHS-Cincinnati Getriebetechnik Sonthofen (Germany) has worked with the Institut für Maschinenwesen (IMW) of Technical University of Clausthal (Germany) on the well-planned further development of existing diaphragm coupling designs and the skilled combination of the advantages described above, and the result of this collaboration is the TWINTORS diaphragm coupling with optimised features /4/, as shown in Figures 2 and 3: Integration of the connecting components (middle pieces) with the one-piece diaphragm disks. Transmission of high levels of torque at extremely high rotational speeds, depending on the overall dimension T up to 1500 knm, n up to 80000 rpm Extremely precise operation, small masses and mass moments of inertia. When the coupling is designed for fatigue strength, this results in a limitless service life as wear-free operation is guaranteed. Replacing the screw connection between diaphragm membrane and centre piece with a welded connection reduces windage losses, and the smooth outer surfaces of the diaphragm disks also have the effect of reducing the production of noise and heat. These excellent characteristics predestine these couplings for use, amongst other situations, in gas and steam turbine plants and in high speed gears. 606
Figure 2. Two TWINTORS diaphragm couplings connected by a spacer (BHS Cincinnati Getriebetechnik Sonthofen) Figure 3. Design of TWINTORS diaphragm coupling 4. Response of the diaphragm couplings under load To guarantee the application of the coupling in accordance with the safe-life principle, numerous numeric and experimental tests have been conducted on the stress response and deformation response under load of torque, bending moment and axial force. Figure 4 illustrates the deformation characteristics of the TWINTORS diaphragm coupling under bending moment, gained by FEM calculation. The point of highest stress under load of bending moment can be seen clearly at the radial outer position of the diaphragm labelled Point 1, and the small distance between the lines for equal radial tensions, suggests a high stress gradient. The position of maximum shearing stress under the action of torque is located in the vicinity of the transition region to the mating flange, and is shown as Point 2. The spring characteristics of the diaphragm couplings under load of torque and bending moment are shown in Figures 5. The stiffness characteristics of the coupling when subject to bending moment only have a progressive spring character when the bending angles are very large and exceed the size 607
declared as permissible in BHS-Cincinnati's brochure. In the safe range where the coupling should be operated, the design features a linear stiffness behaviour which allows the dynamic control of the vibration-resistant "drive line" to be performed more effectively, especially at high rotational speeds. In Figure 5 (right) a linear spring characteristic is shown for comparison. Figure 4. Diaphragm coupling under load of bending moment; lines of equal radial stresses Figure 5. Diaphragm couplings under load of torque (left) and bending moment (right) 5. Experimental tests In order to establish the stress resistance limits of the TWINTORS diaphragm couplings, extensive testing was carried out at various test stations in IMW /5, 6/: Static torsion and alternating bending up to twice the safe bend angle Alternating torsion torque Static torsion to determine the shearing moment. 608
The tests on static torsion with an overriding bend angle were carried out on an eccentric test station. This test station attains a rotational speed of up to 760 rpm with a torque load of up to 7500 Nm (Figure 6). Tests using an increasing torsion moment were carried out on a dedicated torsion test station, which enabled static, increasing or alternating torque to be exerted on the specimen. Figure 6. Eccentric test station (left), Bracing box (right) Along with life expectancy studies, another commonly set aim of testing is to determine the maximum static torsion moment that can be transmitted or to establish the torquing angle for a specified load. The bracing box shown in Figure 6 is used for the tests to determine shearing moment. Figure 7 shows, by way of example, a coupling broken in a single-stage test with an impermissibly wide bend deflection angle. The location of the fracture is entirely consistent with the highly-stressed areas of the component as described in Section 4. The area in which the damaged layers lie with a clear separation between them under torsion, bending and axial load makes it easy to identify the cause of the failure in the event of claims for damages. Figure 7. Fatigue fracture of a diaphragm coupling in consequence of to much rotaring bending moment (test load over allowed value) Some results of the experimental works are shown in the component stress-number-curve (figure 8). The test angle of bend in relation to the safe life angle of bend. Furthermore, surface treatments by shot peening process has been carried out to investigate the increasement of safe life loads. Figure 8 demonstrates that the fatigue limit of shot peened diaphragm couplings can be increased at minimum of 10%. 609
Figure 8. Stress-number-curve of tested diaphragm couplings 6. Perspective The test results clearly confirm that the excellently rigid properties of the TWINTORS diaphragm couplings make this a balanced and high-performance design. However, both BHS-Cincinnati Getriebetechnik and the IMW are of the opinion that the potential for optimising the design has not yet to be fully exploited. Further tests are currently in progress in order to further increase the loadcarrying capacity of these diaphragm couplings. References Dubbel Taschenbuch für den Maschinenbau, Springer-Verlag, 19. Auflage, 1997 Peeken, H.; Troeder, C.: Elastische Kupplungen, Konstruktionsbücher Band 33, Springer-Verlag, 1986 Ehrlenspiel, K.; Henkel, G.: Membrankupplungen als drehstarre, biegenachgiebige Ganzmetallkupplungen, VDI-Berichte 299, VDI-Verlag, 1977 Rohbeck, N.: Doppel-Membrankupplung Neuentwicklung mit verbesserten Eigenschaften, Antriebstechnik 30 (1991), Nr.2 Garzke, M.; Henschel, J.; Schäfer, G.: Prüfstände zur Bauteiluntersuchung am IMW (Teil 1), Institusmitteilung Nr. 23, 1998 Birkholz, H.; Heider, G.: Prüfstände zur Bauteiluntersuchung am IMW (Teil 2), Institutsmitteilung Nr. 23, 1998 Dipl.-Ing. Hagen Birkholz Fritz-Süchting-Institut für Maschinenwesen Techische Universität Clausthal Robert-Koch-Str. 32, 38678 Clausthal-Zellerfeld, Germany Phone: +49 5323 722270; Fax: +49 5323 723501 e-mail: Hagen.Birkholz@tu-clausthal.de 610