High loads on the shaft assembly
From a mechanical point of view, the drive train represents the core component of hydraulic plants. In order to reliably transmit the power produced by the turbine runner, the shaft-hub connection was previously realized either with a large pitch circle for a high number of bolts - with a cost-intensive shaft flange - or using form fitting elements with high costs and equally high assembly effort. "The alternative use of friction shims in the joint means a considerable efficiency and cost advantage, while at the same time ensuring function over the entire service life of the component," explains DI (FH) Klaus Eichlberger, Head of Mechanical Engineering at Global Hydro Energy GmbH. The company has been working intensively on this solution for several months and sees its expectations fully confirmed by its own test series.
Full-scale test series with 3M™ Friction Shims
The evaluation and calibration of a new measuring system using a full-size test specimen provided a welcome opportunity to also test 3M™ Friction Shims in full size. The new CoreIn measuring system from the start-up company core sensing GmbH was used for the tests. Here, a strain gauge-based deformation body was configured specifically for the component and installed directly in the shaft to be tested, turning the complete component itself into a sensor, so to speak, whose measured values could be transmitted directly via Bluetooth without cabling.
Among other things, the tests served to determine the load capacity of the shaft-hub connection when using a friction shim. In order to sound out the limits of the connection, a minimum stress condition was first created and then forces and torques were introduced via hydraulic cylinders.
A total of three test series were carried out:
- Loading of the joint with the theoretically determined limit load according to VDI: The one-time slow increase of the torsional load introduced via the hydraulic system towards the theoretical limit value of 186 kNm (the actually measured torsional load was approx. 163 kNm) did not result in any twisting (slipping movement) in the joint.
- Cyclic repetition of the theoretical limit load: 10 repetitions with 186 kNm (the actual measured load was 170 kNm to 180 kNm) as a pulsating load. Again, no relative movement was detected in the joint.
- Cyclic slippage of the connection: Increasing the cylinder pressure caused the joint to slip at 249 kNm. This was indicated by an abrupt drop in cylinder pressure. The maximum load for initial joint slippage was thus found. Subsequent torque increases delivered an almost unchanging slippage torque of approx. 211 kNm.
Expectations met and even exceeded
In order to take advantage of the friction-enhancing shims, the manufacturer 3M specifies, among other things, a maximum roughness in the friction joint (flange components) and a minimum contact pressure of 50 MPa. After the tests, Global Hydro experts state, "Despite the fact that this requirement is met only locally at 4 positions of the flange pattern, this test provides very promising results and also suggests a minimum friction coefficient of 0.7 before and 0.6 after the first slippage of the joint, even for different pretensioning conditions (variation of the tightening factor)." The conclusion of calculation engineer DI Markus Peer: "The promising results have verified the previous calculations, which means that the new connection method with friction shims will be released for customer projects."
Advantages of friction-enhancing shims
At the same time, cost advantages, their durability as a service life component, and the comparative ease of assembly speak in favor of the future use of friction-enhancing shims in hydropower plant technology. The surfaces of such large components are often quite rough because fine machining is very cost-intensive for shafts of 400 to 500 mm diameter. The manufacturer compensates for this by selecting the appropriate friction shims - in this case, 3M™ Friction Shims Type 35. The friction-enhancing shims consist of a steel substrate with an electroless nickel coating and diamonds embedded in it. These create a micro scale form fit between the friction partners. This opens up a wide range of possibilities for lighter and more compact designs without compromising force and torque transmission.
Innovations in hydropower plant technology
At Global Hydro, the experts expect to make 3M™ Friction Shims a standard solution in their applications in the future, for example to make it easier to realize flange connections between shaft and turbine wheel.
The company, headquartered in Hofkirchen im Mühlkreis (Austria), together with its 12 international subsidiaries, employs around 300 people worldwide and is known in the market for its innovative strength. Global Hydro's customers include international customers in the small- and medium-hydro sector. They expect robust, durable and reliable technology, which, among other things, can be realized thanks to friction shims.
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