Couplings in Detroit: Critical components for high performance driveline testing
Since a typical electric motor dynamometer layout has a minimum of mechanical components to begin with (e.g., drive motor, coupling, torque meter, device under test [DUT] / brake motor) the need for superbly precise flexible shaft couplings has grown at the same rate.
In no other place is this more evident than Detroit, Michigan, where German based coupling manufacturer, R+W Antriebselemente GmbH, has been taking on increasing technical challenges from automotive testing engineers, and delivering some of its highest performing couplings yet. Here we’ll profile just one example of automotive testing engineers driving R+W couplings to further optimization through customization of materials, layout, and balance. In this case the need arose for a flexible shaft coupling to transmit up to 750 Nm at 20,000 rpm, with extremely high acceleration rates, and unprecedented requirements for reduced size, weight and rotational inertia.
The right tools for the job
From the start, R+W application engineers were aware that the best chance of meeting these performance goals was with a metal bellows coupling. Unlike other precision flexible couplings, the metal bellows offers such unique features as continuous symmetry, extreme natural balance, and the ability to provide parallel flexibility within a short overall length. Metal bellows couplings can be used from 2 to 10.000 Nm. That are all reasons for the installation in highly dynamic motion applications like machine tools, test stands, packaging machines, printing machines, textile machines, automation equipment etc. A number of special solutions like various materials, tolerances, dimensions and performance ratings almost predestined the couplings for all these different applications. Another advantage of the bellows coupling is the easy adaptation of the end hubs, which can be concentrically mounted directly to the flexible element by bonding or welding, and to the adjacent mating components in the machine, without the need for bulky adapters, which add to misalignment tolerances, rotational inertia and imbalance. Precision bellows couplings are generally assembled on a mandrel, which holds the respective end hubs precisely concentric, without placing any bending or crimping stress on the bellows as it’s attached. This makes it the most direct means of installing a flexible shaft coupling into a machine driveline without corrupting performance. A further advantage of the metal bellows is its lack of wear or moving parts. A bellows coupling is torsionally stiff. When properly applied it can tolerate a theoretically infinite number of bending cycles (rotations) while misaligned. This is especially beneficial for high speed applications as the driveline reaches into the hundreds of millions of rotations within its first 100 hours of service at speed.
Custom tailored for optimal performance
In this case the coupling was to be installed between a drive motor and a flanged torque meter. For attachment to the cylindrical motor shaft R+W applied an external conical clamping ring, which draws over an internal taper to compress the coupling hub concentrically around the shaft. Compared with tapered bushings, where any unevenness in tightening could cause the coupling to tilt on its axis, external clamping rings allow the coupling to remain centered on its shaft bore, with any minor tilting from installation error being isolated to the ring itself, without taking the coupling off center. To help achieve the goals of minimal weight and rotational inertia, the clamping ring was made from titanium grade 5. An array of large through holes was also included around the face of the clamping hub, serving two purposes. These holes made the hub much lighter, and they also allowed for socket head cap screws to pass all the way through the hollow bellows body and adjacent hub, and directly into the threaded holes of the torque meter on the other side. This allowed for the flanged hub of the bellows coupling to be extremely thin. In this case, in the pursuit of optimization, the actual torque meter rotor to be used was shipped to R+W to have the centering bore of the coupling flange polished for a precise match with light press fit. This aided even further in guaranteeing concentricity from the coupling shaft bore, all the way through to the opposite face of the torque meter rotor.
When it comes to flexible coupling balance, some debate exists as to whether ISO grade G1.0 is practically deliverable at such high speeds. The argument against it is that, as a flexible element, the coupling could potentially be balanced to that level of precision, but once it is removed from the tooling, packaged, shipped, and ultimately installed in a machine with slightly different alignment positions, this level of balance quality would be lost. To preserve the balance quality as much as possible, in this case R+W used dedicated balance tooling for each coupling, serving as both the balancing arbor, and a support shaft to prevent the bellows from even very small amounts of bending during transport and handling. This allowed the couplings to be carefully removed from their respective balance / support shafts on site and at the time of installation, for a nearly perfect connection.
A tradition of leadership in test stand coupling engineering
R+W has been involved in designing and manufacturing customized precision flexible couplings and torque limiters for rotary testing applications since its inception in 1990. Over time, the application-dependent coupling configurations have ranged from small bellows and elastomer insert couplings up to larger flexible disc pack couplings, and bellows couplings with capacities up to 150,000 Nm. In 2017 the company produced the worlds largest ball-detent safety coupling, with a disengagement torque of up to 20 million Nm for testing of offshore windmill gearboxes. In a variety of ways R+W is a world technology leader when it comes to engineering couplings for dynamic precision testing applications. As the industry continues to push the limits further, testing engineers and component manufacturers will continue to collaborate in their pursuit of the ultimate coupling.