Wikov Flexible-pin Gearboxes for Industrial Applications By Jan Vosatka, Wikov Industry a.s. and Vilem Rosko, Orbital2 Ltd. Introduction Various industrial driven machines are demanding continuous powertrain development and optimization. When gearbox is used, either speed reducer or increaser, such has to cope with unpredictable loading with possible overloads, and additional loads into the structure very often. At the same time, high durability and long life, as well as high efficiency and low maintenance cost together with good NVH (Noise, Vibrations and Harshness) are required. Economic factors, such us low CAPEX and OPEX play a key role in decision making too. There are efforts to eliminate the gearbox from the powertrain completely. When higher speed reduction ratio is needed, direct drive is usually not cost effective solution though. Various arrangements of parallel or planetary gearboxes, sometimes in combination with bevel or worm gears, are typically used for industrial applications. Those are being constantly improved by using the latest CAE tools, the newest design standards, highstrength materials as well as manufacturing methods. The planetary gearboxes represent a very robust solution with an excellent power to weight and size ratio. Gearbox durability is maintained at the same level or even increased while the overall size and weight are significantly reduced over parallel shaft gearboxes. There is a clearly visible cross-industrial trend for a shift from parallel shaft gearboxes to planetary ones where possible, as those can offer reduction in terms of size, weight and cost with no compromise in the performance. Planetary gearboxes have been around for decades, one can say centuries, and still most of the arrangements rely on a traditional design approach with 3 planets mounted in a rigid carrier. Such solution offers improvements over parallel shaft gearbox arrangement as described in previous paragraph. Further development of such concept needs to be considered though, as there are solutions for further significant improvement in terms of all key aspects. Wikov Flexible Pin Achieving higher power/torque density by using more than 3 satellites in one planetary stage is well known approach. By adding more planets, the power/torque is split into more paths (Fig. 1), and while using the same size of gearing and keeping overall dimensions, an increase of the power/torque capacity can be achieved. Pagina1
Annulus gear Sun gear Planets Fig. 1 Power-split in planetary stage with 3 and 5 planets Theoretical comparison of single stage capacity based on number of planets and gear ratio is shown in fig. 2. It can be seen the 5 planets gear stage has 166% capacity of the 3 planets one. Fig. 2 Single planetary stage capacity vs. number of planets However, if conventional rigid carrier is used, problems caused by key component deformations from both, internal as well as external loading, manufacturing tolerances and mainly behaviour under various operation loads may occur, as the load may be unevenly distributed between planets, and the life of overloaded components, e.g. gears and bearings, can be reduced significantly. There are limitations in such approach though. The traditional 3 planet arrangement, where one member is kept free (floating, e.g. the sun gear), is statically determinant. Pagina2
That means equal power split between planets is always assured. When more planets are added, the system becomes in-determinant and very sensitive to manufacturing tolerances as well as deformations in the system. Manufacturing tolerances can be compensated in some ways by tighter tolerances as well as careful part selection during assembly. Both of which means additional on-cost though. The traditional approach to cope deformation is to make key components stiffer, and less prone to deformation. Increased size, weight as well as cost are the penalty though. The approach successfully applied by Wikov is different. Rather than fight with deformation, we use it for better functionality and durability of the system. Such approach allows us to achieve even smaller, lighter and cheaper gearboxes with improved performance over both, parallel shaft as well as traditional planetary gearboxes. The key technical feature in Wikov design, which has been well proven in wind, tidal, mining and off-shore, is now being implemented into other industrial applications. It is so called Wikov flexible pin technology. This technology, which has been patented by Wikov, offers significant advantages over a traditional approach to a planetary gearboxes design. The key benefits are: Better safety factors and increased lifetime of gears and bearings Shock load resistance The possibility of using Multi-satellite arrangement (up to 8 planets) Compact dimensions High Power Density Simple open carrier design Optional overload stop The Wikov flexible pin allows to add controlled flexibility into the system which compensates for the structural deflections and manufacturing errors and high power density can be achieved in a very safe way by using multi-satellite arrangement (Fig. 3). Fig. 3 Multi-satellite planetary stage Unlike similar systems using flexible annulus (ring) gear or carrier, both of which is not an ideal solution in terms of fatigue life, Wikov solution is introducing the flexibility into the system through the specially designed flexible pin. Pagina3
Benefits of a flexible pin over a conventional design are described on a simple example. In case of conventional planetary stage (Fig.4), the stiffness, as well as gear micro geometry, can be well optimized for a nominal torque under such, both, gear contact pattern as well as planet bearing load sharing are good. However, in case of unexpected overload in terms of torque or external force applied on the gearbox, deformation in the structure leads to an uneven gear contact pattern as well as planet bearing loading. One side will be loaded more than the other one. Both, gear and bearing life is compromised as any unbalance in the system leads to local overloading and potential risk of failure. Nominal load Overload Fig. 4 Conventional planetary stage If Wikov flexible pin planetary stage is used (Fig.5), extra flexibility in the specially designed pin allows planet to float under load in a limited manner. By a nature of the pin design (double-cantilever arrangement), planet stays parallel to both, sun and annulus gear. Equal planet bearing load sharing, as well as even gear contact pattern is achieved for various loads including overload. Both, gear and bearing life, are significantly improved as the load is always equally spread, and the risk of failure is significantly reduced. Nominal load Fig. 5 Flexible pin planetary stage Overload Pagina4
If the flexible-pin is used in a fairly harsh application with uncontrolled shock loads and significant overloads, there may be a risk of exceeding allowable stress in the flexible pin which can lead to a potential failure. Wikov patented overload stop prevents such state through a controlled reduction of movement of the flexible pin under extreme conditions while still keeping perfect load sharing Wikov Flexible Pin gearboxes for industrial applications All industries are under constant pressure for performance and productivity increase. And so there is an understandable pressure on reduction of unexpected shut-downs and repairs for all equipment. Condition monitoring systems for various pieces of equipment, for example gearboxes, are slowly becoming a standard. But surely there is a pressure to make gearboxes more reliable, too. Such represents quite a challenge for all gearbox manufacturers as, at the same time, there is a constant pressure on the equipment cost reduction. Wikov has got a long tradition in supplying gears for all kind of other market segments. High level of diversification brings a possibility to share an experience gained in one sector into the other ones, and thus accelerate the development in various gearbox applications. Wikov have supplied more than 700 hundred flexible pin gearboxes to the wind turbine industry over the last decade, while experienced zero failure rate for planetary stages. Such experience triggered the effort to introduce the flexible pin technology into general industrial market sectors also. Since 2015, Orbi-fleX catalogue planetary gearboxes have been supplied into various industries including mineral processing, shredding, mining, rubber, food processing (primarily sugar), lifting and also hydro power. Summary The flexible pin planetary gearboxes offers many advantages over traditional designs while being cost effective. They can contribute to further equipment optimization through improved power density and reliability increase at the same time, and thus significantly reduce CAPEX and OPEX. The Authors J.Vosatka graduated in Vehicle and Driveline Engineering at the Czech Technical University in Prague, and worked for several international engineering and consultancy companies before joining Wikov in 2013. He has been involved in design studies and complete delivery projects for a number of gearbox projects for automotive (both mass production and niche market) as well as general industrial segments. He is currently working as the Group Technical Director at Wikov responsible for engineering and manufacturing, new product development and R&D activities, across the Wikov Group. V. Rosko graduated in Automotive, Combustion Engine and Railway Engineering from the Czech Technical University in Prague, and worked for the Czech manufacturing company and design house before joining the Orbital2 in 2006. He has been involved in conceptual design studies and complete designs for a number of gearboxes for renewables as well as various industrial market segments. He is currently working as the Engineering manager at Orbital2 responsible for engineering teams in two offices delivering state-of-the-art innovative planetary gearbox and driveline designs, development and manufacturing support to global customers. Pagina5