Jan Vosátka, Technical Director, Wikov Group
A flexible-pin gearbox that improves load sharing between the various gear elements makes it valuable for high peak load applications, such as wind and tidal-power turbines. Furthermore, it enables the use of more than three planets – in some cases up to eight – in one planetary stage, thus allowing for a significant increase in power density.
This should be good news for the wind industry, which has been struggling to extend the life of its gearboxes. Results suggest that the flexing-pin design has a role to play.
Traditional gearboxes use solid planet bearing mount in carriers. In many cases, it is not difficult to optimize these designs for one load, but it can be quite difficult to do so for applications with variable loads, such as wind power.
Renewable energy applications must commonly withstand variable and high-load peaks. These harsh conditions may lead to deformation in the system, such as radial and angular misalignments that can result in non-symmetrical planet-bearing load patterns in gearing, which reduce gearbox life.
Most gearbox manufacturers address this concern by increasing the stiffness of the system to avoid deformations, usually at the price of weight penalty. However, our philosophy is different. We work with flexibility rather than fight it. We have a design that places the planet gears on a flexible pin on the planet carrier. A hollow sleeve called a spindle, carries the planet bearing and gear, and mounts on a pin, which is then mounted into the carrier. The pin is allowed to bend in a controlled manner, introducing flexibility to the system. And, thanks to the geometry of the assembly, this design allows for appropriate planet movement.
As a result, when a load is applied to the gears, the flexible pin lets the planet float mostly parallel to the axis, minimizing gear tilt and improving load sharing among planets. This alternative design allows for increased life of gears and bearings, greater resistance to shock load, and a 30 to 40% gearbox weight reduction in comparison to conventional gearboxes of similar power.
Furthermore, a patented overload stop is usually used to reduce planet movement under critical conditions. We also have a version available for helical gears in which planet tilt in the gearing must be handled due to thrust forces.
This technology has been proven in the field in systems ranging from hundreds of kilowatts to seven MW in size. In a 350-kW wind turbine that has been in operation for more than 10 years, the turbine’s gears and bearings show no significant sign of wear or damage.
What’s more, we used this technology in a tidal-power project where we supplied a twin 650 kW main drive for a tidal current turbine. Those gearboxes have been in trouble-free operation since 2008.
These are significant applications because wind and tidal power turbines are often in remote locations where maintenance is costly. Tidal power, in particular, is an extreme application in which the gearbox may be fitted in a closed nacelle on the seabed, making it impossible to access for several years, unless you take the whole turbine out of the water.
As a result, we had to ensure that the gearboxes could operate maintenance-free for many years to help ensure customer profitability. In order to address these issues, we looked at lubricants as well. Specifically, we partnered with ExxonMobil, which had recently launched a new synthetic lubricant – Mobil SHC Gear 320 WT – specifically formulated for renewable and wind-energy applications.
After running in-house tests on a 3-MW planetary wind gear, the lubricant displayed the right properties, including high-viscosity stability, strong anti-foam properties, and better fluidity at cold temperatures. Mobil SHC Gear 320 WT is now formally approved for our flexible-pin design in wind applications.
Following the outstanding performance of Mobil SHC Gear 320 WT in wind turbines, we tested it for tidal power applications, and found it to be an ideal lubricant for this application as well. It offers a higher viscosity index, easier and faster startup in cold environments due to lower viscosity and thus limited need of oil pre-heating, and higher protection against wear at high temperatures –an important feature when you consider that temperatures can rise to 100ºC in the load area.
As for tidal power, our gearboxes have a long track record of success in some of Europe’s largest projects. Moving forward, we expect the market for planetary gearboxes to grow across all industrial applications, and we believe that our flexible pin design will offer customers a compelling alternative to traditional designs.
About the company
Founded in 1878 and with more than 130 years of mechanical engineering expertise, Wikov Group is one of Central and Eastern Europe’s leading manufacturers of gears and mechanical gearboxes. From its three production facilities in the Czech Republic, the company manufactures products for customers across the globe, including in Europe, the United States, Russia, India and South Korea. Its gearboxes are used in a wide range of industries, including oil and gas, rail, chemicals, and mining.
The company focuses on delivering advanced technology that meet specific equipment needs. Wind turbines are one such area. One of the company’s most exciting designs is flexible pin technology, which is ideally suited for high demand industries and renewable energy applications such as wind and tidal power turbines.