FEATURES OF THE SYNTHESIS OF PLANETARY MECHANISMS OF AVIATION GEARBOXES

Abstract

This paper considers methods for calculating gear transmissions of aircraft gearboxes, which can increase their load capacity and reduce mass and dimensions. In power gearboxes of aircraft engines, gears are used, obtained by a cutting tool with a profile angle a = 25° and 28°, or with a complex initial generating contour with a protrusion. Another way to increase the bearing capacity of gears is to use wheels cut with an offset (corrected wheels). Such gears have increased contact strength and bending strength. For a rational choice of displacement coefficients, it is necessary to use locking contours that are built for a pair of wheels being designed. The use of gears whose wheels are cut with an offset in planetary mechanisms requires a special approach to the synthesis (design) of such mechanisms. Optimal design of planetary mechanisms involves three main stages. The first stage is the selection of the optimal kinematic scheme of the gearbox that meets the required technical conditions. Analysis of existing kinematic schemes of aircraft gearboxes of domestic and foreign engines makes it possible to choose the optimal kinematic scheme. Such gearboxes include planetary mechanisms of simple and closed circuits – these are schemes АІ,  та ( A – external, I – internal gearing). The next stage of design is the synthesis of the planetary mechanism itself, i.e. rational selection of the combined number of teeth that meet the kinematic and design requirements. To select the number of teeth that are cut with an offset, you can use "general" equations in which the condition of co-axiality is written with an additional parameter indicating the absence of co-axiality, with the subsequent solution of this dependence. The third stage of the synthesis of the planetary mechanism involves determining the optimal geometry of the gear meshing by selecting rational offset coefficients using blocking contours. Selection of offset coefficients for the scheme АІ and  has its own characteristics and is considered separately for each of them. For the AI scheme, first the displacement coefficients are selected for the pair z₁-z₂ of the external gear, and then for the pair of wheels of the internal gear z₃-z₄ in such a way as to ensure the condition of co-axiality. The peculiarity of the scheme is that the satellite z₂ is simultaneously in external gearing with the gear z₁ and internal gearing with the wheel z₃. To select the displacement coefficients for this scheme of the planetary mechanism, special (spatial) blocking circuits are constructed for all three wheels z₁-z₂-z₃, which connect the three displacement coefficients x₁, x₂, x₃ into a single system.