Optimized gearbox design - Wind Power Generation and Wind Turbine Design - page 518

primary fatigue condition. However, when torque is variable, there is clearly an
associated secondary unidirectional fatigue condition in the planet spindles as well
as the gear teeth. In any case, even with constant torque, sun and annulus gear teeth
are subject to unidirectional fatigue as they rotate, in and out of mesh with the
planet wheel whose teeth are subject to full fatigue load reversals as they alter-
nately mesh on opposite fl anks with the sun and annulus. Therefore, all gears
whether epicyclic or otherwise, have to be designed to accept primary fatigue
loads as well as the secondary effects of torque fl uctuation.
As stated previously, conventional planet carriers cannot be made completely
rigid so that inevitably, the webs joining the two fl anges which support either end
of the planet spindle are subject to shear and bending defl ections that create a tor-
sional defl ection of one fl ange with respect to the other to misalign the planet
wheel. While it is feasible to calculate this defl ection and compensate for it either
by boring the carrier skewed or by helix corrections on the mating gears this only
helps at one nominal torque. It is therefore, usual to crown the face widths of the
gear teeth to avoid edge contacts on either end which would otherwise occur at
different loads. This reduces the contact area and increases the local stresses. Fur-
thermore, the planet bearing load is no longer on the centre of its spindle which
can also be a source of bearing problems.
Figure 8 shows the principles of the compound cantilever fl exible planet spindle
comprising a fl exible inner member and a comparatively rigid co-axial outer sleeve.
Central tooth loads at the planets sun and annulus mesh points create equal and
opposite moments at either end of the inner pin with a point of infl ection at the cen-
troid of load, where the bending moment is zero. The spindle is very soft in an angu-
lar sense to such an effect that it cannot sustain any unequal loading across either
gear face, e.g. if a planet wheel has a helix error which could lead to heavier loads
at opposite ends of its respective face contacts with the sun and annulus, then the
Tooth load
Spindle
Planet carrier
Flexible pin
Planet
Point of inflexion
Figure 8: Flexible planet spindle.