In wind power plants, electrical circuit problems are known to cause short-term torque peaks. In this event, slip clutches protect the costly gearbox against overload by slipping at a defined maximum torque in order to temporarily interrupt the drive.

The slip process takes place not on the generator shaft surface, but inside the CENTA Torque Hub, which is mounted on the generator shaft in the coupling’s clamping set.  CENTA’s latest development in slip systems-”CENTA Torque Set”-now positions the slip unit to the middle section of the shaft. By relocating the slip function, manufacturing tolerances at the generator shaft no longer cause variations in the slip torque, resulting in improved accuracy of the slip function.

Another advantage of the new design is that the slip system is made to be pre-mounted as a complete unit. The maximum torque is set on a certified test bench according to the manufacturer’s requirements and documented in a test report, eliminating the need for on-site adjustments.  The low-maintenance slip system is also designed to allow for multiple slip processes (> 200) without having a major impact on the slip torque setting. Depending on the size, a torque range of up to 120 kNm is covered. The new slip system can also be provided as a low-cost system component which does not include the coupling unit.

KTR says its design simplifies assembly in small pods and nacelles because instead of usual large bolts that requiring large tools and high manually generated loads, a Radex-N needs only a conventional-torque sensing screwdriver thanks to special clamping nuts. The necessary prestress on screws comes from a combination of several small screws.

The coupling also provides electric insulation so a current leak cannot get from the generator to the gearbox where it might damage bearings and splines. A positive side effect of the insulating feature is that the total weight is reduced and service simplifies.

Laminates inside the coupling are connected to hub and spacer alternately by means of high-strength shoulder bolts. Apart from the unit’s ability to absorb high misalignments, this combination of engagement and positive locking increases the coupling’s power density.

The company says it developed the steel-laminate packages guided by finite-element analyses. The goal was to find a best design with regard to torque transmission and torsional rigidity, taking into account the necessary displacements.

In addition to couplings, the company offers a frequently needed disk brake that mounts on the gearbox side. The disk, up to 1.5-m dia., includes a sensor for speed monitoring. An overload monitor ensures accurate speed limitation in unfavorable winds.

The Ruflex torque limiter and brake is well suited for its tasks, thanks to friction linings that lets it operate smoothly without stick-slip and with good wear resistance. It is calibrated to a turbine manufacturer’s configuration and built into the coupling spacer. When the equipment senses a max torque, it limits power flow so the turbine is protected against load peaks on the generator side. This also protects the gearbox against high stress and thereby reduces service costs.

KTR also designs combined equipment consisting of coupling, electric separation, brake, sensor disk, and overload devices based on individual components developed for use on wind-power stations. A few recent turbine designs have no gearboxes and so couple the rotor to the generator. These setups make it important to have a powerful overload device to protect against high torque peaks. KTR overload systems provide for the necessary safety on such drives.

To select the right device for an application, the company uses computer-aided torsional vibration analysis, FEM, and tests in company laboratories. The firm can perform tests of service life and load. A climate chamber simulates expected and extreme environmental conditions.

The company says it designed its first coupling for use between a gearbox and generator in 1988. Since then, some 25,000 KTR couplings have been used on windpower stations around the world. The company adds more than 10,000 new applications each year and custom designs are also common requests.

“These couplings withstand extreme misalignment while remaining torsionally stiff and have passed the ‘hell hole’ test at Tehachapi, California Wind Farm,” reports James Motz, member of the Wind Team for Zero-Max.

“The couplings were tested under conditions simulating a 20 year load spectrum of continuous operation. Once fatigue tested, the ‘hell hole’ location was selected for field testing in a wind turbine whereby the coupling would experience wind conditions in excess of 80 mph with continuous wind direction changes. The Zero-Max couplings survived these conditions that put over 50 wind turbines not using our coupling out of commission. The Zero-Max coupling continues to operate uninterrupted at this writing,” Mr. Motz added.

The Zero-Max wind turbine couplings are designed with composite disk packs at both ends of a center spacer. These patented designed disk packs provide the true strength and calculable flexibility of the coupling. The composite disk packs (flex elements) provide a distinct advantage over other coupling component designs by allowing a surplus of parallel and axial misalignment while remaining torsionally stiff through all harmonic ranges of the wind turbine’s oscillating load.

Depending on application, the Zero-Max’s center spacers can be machined out of steel, composite glass fiber or 6061-T6 aluminum. Through the use of Finite Element Analysis (FEA), these center spacers can be engineered to withstand in excess of 70,000 Nm of torque depending on the material selected.

Zero-Max wind turbine couplings have many additional design advantages. The flex elements electrically insulate the turbine’s generator from the gear box, eliminating amperage leak across the coupling thus preventing gear box damage. The coupling protects the generator by transferring lower reaction loads to the generator bearings. Also, the coupling’s composite material withstands all types of environmental elements including temperature extremes from -57 to 121C and from moisture and chemicals native to wind turbines. The table shows a few specs for the coupling.

Zero-Max
www.zero-max.com

Hydraulic pitch control for wind turbine blades is possible with rotary unions from Deubline Inc.

Generating steady shaft speed on a wind turbine means constantly adjusting the pitch of each blade to accommodate wind variations. The blades connect to a huge hub mounted to a shaft that turns a gearbox and a generator. Best turbine efficiency calls for a continuous pitch control on the blades.

Blade pitch is powered by either an electric or hydraulic drive. Electric-pitch control uses slip rings to conduct power to motors rotating in the hub. Hydraulic systems, on the other hand, use a rotary union to deliver hydraulic power to the drive motor. The industry is split about 45% electric and 55% for hydraulic controls. The advantage of the hydraulic control is that its power density is higher than electrical equipment and it needs fewer components, making for a simpler system. There are other pluses.

Rotating unions, such as those from Deublin Co., are precision mechanical devices for transferring fluid from stationary sources to rotating machinery. Ball bearings in a typical union support the rotating components (attached to the machinery) against the stationary component (attached to the fluid supply) and a mechanic seal prevents leaks. While often found on wind turbines, rotating unions device work well in other applications such as air clutches, gearboxes, machine tool spindles, and more.

The Ixilflex link coupling absorbs misalignment of power transmission shafts through bushes linked to alternate flanges. The bushes are made by vulcanizing rubber to metal parts under high compression. The coupling can be run in either direction without reversing the coupling. Other features include high misalignment and torque capability, torsional damping, low restoring forces, silent operation, easy replacement of elastic elements, easy visual check of the conditions of the rubber bushes, and electrical isolation is available. Couplings are rated from 600 kW to 5 MW wind turbines.

The composite-link coupling, developed by Spain-based Jaure, combines the best features of steel-disc and elastomeric couplings. A high-misalignment capacity, high-torsonal stiffness, and long service life with free maintenance makes the Complink well suited for wind turbines, especially the megawatt and offshore units. A few additional features include high fatigue resistance for long service life, high flexibility for increased misalignment, low reaction forces, and excellent corrosion resistance for low maintenance. Other features include electrical insulation capability, low weight for easier installation and service, and insignificant heat generation. What’s more, the coupings have zero backlash, low inertia, and they are torsionally stiff.