Gearbox Designers Turn to Hybrid Solutions

As wind turbines continue to grow in capacity, designers must determine a way to improve reliability while decreasing mass and cost using lean principles and advanced technology. Hybrid wind turbine architecture is receiving prime consideration today among design houses, especially for larger capacity machines. This common-sense design combines a simplified medium-speed gearbox with a mid-speed permanent magnet generator. The result is usually a compact power train with a relatively low mass.

It is common for a hybrid design to contain a one- or two-stage planetary gearing system intended to simplify the power train and improve performance and reliability. The theory is that reliability is improved by reducing the number of parts and eliminating the high speed stages which are prone to bearing damage induced by some of the dynamic operating conditions. In the planetary stages of these medium-speed gear boxes, it is customary to support the planetary gears on cylindrical roller bearings in a closed planetary carrier – the pin supporting the planetary bearing is supported on each end by one of the two carrier walls that are connected with webs between two adjacent planetary gears.

Some planetary drive trains are gradually being upgraded to incorporate preloaded tapered roller bearings under the planet gears to add reliability by eliminating smearing and micropitting, which can develop when sliding occurs in bearings operating under no-load with clearance. Such condition often occur in a turbine that is rotating but not producing power. With proper design, the preload that can be incorporated through use of the tapered roller bearing will maintain load and traction forces in each of the roller contacts in this no-load state, thus maintaining pure rolling instead of skidding.

Flexpin Technology

Another option for hybrid wind turbines incorporates flexpin technology in the planetary gearing system. There are several new building blocks related to flexpin technology that have been developed in recent years that make adaptation of flexpin technology a straight forward solution for achieving higher reliability. This technology can make the hybrid design more competitive with a direct drive design claiming higher reliability by eliminating the gearbox altogether.

Originally introduced around 1965 by Raymond Hicks, a British inventor, the typical configuration includes multiple spur gear flexpins arranged around a single walled planetary carrier. Pins are cantilevered from the single walled carrier which eliminates the unwanted gear face misalignment that is created by torsional wind-up in a two-walled, straddle-mounted planetary carrier system. This eliminates the need for lead correction on the face of the gears.

Flexpins are built on the principle of a twin cantilever beam which bends under load in a circumferential direction along the pitch line of the planet gears, all the while maintaining nearly perfect gear alignment. The bending at each pin also has the effect of equalising the loads among multiple adjacent planets. It is common for a flexpin system to contain as many as seven planetary idlers, and with careful engineering nearly equal load distribution can be achieved. Therefore, a flexpin system can allow distribution of input torque more evenly over a higher number of planets and eliminate most of the gear face misalignment from torsional wind-up, reducing gear stress.

Design of the Flexpin Bearing

Flexpins can be constructed in a number of ways, but the Integrated Flexpin Bearing (IFB) has a unique but simple construction comprising very few parts: the pin, sleeve, roller sets, gear, and bolt-on rib ring. Its compact design offers good power density because it is possible to integrate the bearing races with their mounting surfaces, combining the cross-sections to achieve sufficient beam strength and bearing dynamic load rating in a lower-profile cross-section. Likewise, adaptation of a narrow bolt-on rib ring to clamp and establish bearing clearance offers a means of reducing overall width. Reducing radial and axial size while maintaining the dynamic load rating is advantageous when downsizing gearbox designs.

Flexpins are also used in conjunction with spur gearing. Spur gears are desirable because they create an optimum force pattern for the flexpin which is meant to bend without misalignment only in a circumferential direction along the pitch line of the planet gears. The two tangential forces acting on the planetary gear are additive and create fairly equal, side-by-side and symmetrical load zones in the two bearing rows. These separate forces effectively cancel each other out.

New Flexpin Bearing

Another new advancement is the Helical Gear Integrated Flexpin Bearing, which makes it possible to increase the beam strength of the tooth, achieve higher gear contact ratios more easily, and reduce noise and vibration. This development represents a true breakthough since, until now, only spur gearing, usually high contact ratio spur gearing, could be considered.

Helical gears add a disruptive element to the force pattern. The helix angles at both meshes on the planet gear generate two equal and opposite thrust forces that cancel; but they create an overturning moment which skews the load zones in the two rows, and adds new misalignment. The solution is to build a flexpin with normal deflection characteristics in the desired plane of bending, and much stiffer deflection characteristics in the radial plane to help minimise the effect of the turning moment. Through careful engineering, this can be accomplished by unifying the sleeve to the pin only in the radial plane to increase their combined spring rate and reduce the undesirable bending to an acceptable level.

Helical Gear Integrated Flexpin Bearings can now be applied to single-walled (open) carrier planetary gearing systems. However, these systems often contain a high number of planets, perhaps seven or eight, to reduce the bending stress on the cantilevered pin to an acceptable level. Such designs employ a larger diameter sun gear, and smaller diameter planet, thereby reducing the step-up ratio. For some designs a lower step-up ratio is acceptable, but when higher output speed is desired, for example to provide input to a medium-speed permanent magnet generator, a higher step-up ratio would be advantageous in order to decrease generator size.

To address this need, engineers have developed the Flex-drive, which places sufficient planets (either spur or helical) to transfer the torque with acceptable pin stresses on two opposing arrays on a two-walled, conventional-looking carrier. By reducing the number of planets in the arrays, planet gear spacing permits larger diameter planets to be used. This construction allows for a flexpin system with a higher number of flexpins for carrying the torque and achieving a higher step-up ratio. Depending on the variability of the duty cycle, a Flex-drive can offer significant increases in power density compared to conventional straddle-mounted planetary gearing systems. This is possible because misalignment from torsional wind-up is eliminated, and better load distribution is achieved among the planetary gears.

The choice of which solution is best for an application depends greatly on the required outputs (torque and speed), surrounding structure and constraints imposed by other elements. But with careful engineering and support, a proper solution can usually be achieved.

Focus on Serviceability

Looking forward, designers of geared wind turbines continue to advance their products, reducing mass and improving reliability. To complement this effort, there is more focus on the serviceability aspect of wind turbines.

One possible solution is a drive-train configuration that utilises two independent modules comprised of an upwind main shaft bearing module on the left, and a downwind gearbox-generator module on the right.

This drive train architecture creates two benefits: firstly, the access and serviceability characteristics of each module are improved, and secondly, if the correct main-shaft bearings are selected for the upwind main-shaft module, then the main-shaft loading is prevented from entering the downwind gearbox-generator module altogether. This represents a significant step towards improving gearbox reliability because gear meshes are not adversely affected by main-shaft deflection, which is often the case in current wind turbine drive-trains.

Preferred bearing options for the main-shaft module include a 2-TS widespread combination, a large diameter double row bearing called a TDO, or a combination of a fixed TDI taper used with a floating NU cylindrical roller bearing. Each configuration has its merits, and should be evaluated in terms of the unique requirements of each wind turbine design.

One design for the downwind gearbox-generator module applies a high ratio split compound planetary gear case that is supported entirely on the upwind side of the pedestal. Suspended entirely from the downwind side of the pedestal is a permanent magnet generator which is connected to the gear case output via a torque limiting coupling that serves as a mechanical fuse to crop off spikes in the driveline torque created from either end of the turbine; that is, from the rotor during turbulence, from grid faults or other transient dynamic events. This mechanical fuse controls the maximum load, effectively protects the gears and bearings from overloading, and allows the gearbox designer to scale down the gear mass to arrive at a very lean design.

These solutions represent real possibilities for gearbox designers to address many of the failure modes prevalent in the wind turbine gearbox industry today. For the gearbox industry to compete in the wind sector, it will need to embrace new technology that will restore reliability while achieving targets on top head mass reduction technology.

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