High efficiency, low backlash planetary speed reducer gearbox optimisation by means of a multidisciplinary numerical approach

Franco Concli

Bonfiglioli Mechatronic Research


In the field of automation, extremely precise gearboxes are mandatory in order to ensure the adequate precision required by the automatic process. For this kind of applications, planetary gearboxes represent one of the most attractive solutions. This type of gearing, thanks to its specific design, ensures at the same time high power density and reduction ratios. The low backlash required in the field of automation is obtained by changing the chordal thickness of the teeth. This has the main drawback to promote the power losses generation reducing the global efficiency of the system.
In order to overcome this problem, new designs have been studied by means of numerical approaches by Bonfiglioli Mechatronic Research. The aim was to optimize the actual design of the low backlash planetary gearboxes both in terms of mechanical and fluid-dynamics performances. The efficiency increase is based on a reduction of the module of the gears. This fact, together with other topological modification of the teeth shape (pressure angle, profile shift etc.), allows to reduce the relative sliding between the teeth flanks and, therefore, the power dissipation. CFD simulation performed using a suitable code, have shown that the mechanical optimized design has a positive impact also on the churning power losses. The global winning in terms of power losses reduction is quantifiable in about 50%, depending on the reduction ratio. This implies, besides an energy saving, a reduction of the operating temperatures and, consequently, an increase in the reliability of the system or the possibility to stress the system with more severe duty cycles without reaching dangerous temperatures.
Numerical structural analysis have been also carried out on the efficiency optimized geometry in order to characterize the torsional stiffness of the gears and to choose the best design for shafts and planet-carrier. The torsional stiffness, in fact, is the other important characteristic of this kind of gearboxes. In precise applications, in fact, high stiffness is required in order to ensure good positioning accuracy. All the CAE based calculations have been validated by means of experimental tests performed in the internal lab of BMR. The results have fully validated both the CAE approach and the new design.