Optimized gearbox design - Wind Power Generation and Wind Turbine Design - page 521

leads to a fi nite life which takes account of the total number of hours at varying
loads. The most heavily loaded are in the high torque low speed primary trains and
in particular the planet spindles which sustain the double tooth loads on the planet
meshes with the sun and annulus. The most successful arrangement has been a
pair of preloaded taper roller bearings which ensure that at light loads there is no
risk of skidding.
To maximise the bearing space available between the bore of the planet and the
spindle especially for low annulus/sun ratios it helps to have fi ne pitch teeth to
increase the root diameter, reduce rim thickness and increase the bore. It also helps
if roller outer races are embodied in the planet bores. Timken have gone further by
also integrating the inner races in the planet spindle and using full complement
preloaded tapered rollers. All planet bearings together with all other lower loaded
higher speed bearings in the secondary trains require a pressurised supply of lubri-
cant. No bearing should be subjected to misalignment and self-aligning bearings
should be avoided. They cannot be effectively preloaded because they have clear-
ances which may lead to skidding on low loads.
In this context, the fl exible planet spindle ensures that however much the torque
may transiently vary, the bearing load always stays in the same place, i.e. the plane
of the face width centres so that it is equally shared when two or more bearings are
required to carry the load.
For smaller gears it is quite possible to have fully fl oating suns and annuli whose
dead weight can, without detriment, be supported on their gear tooth meshes but
generally not for planet carriers. As power increases, the tooth force to component
weight diminishes and there comes a point where annulus rings and even sun
wheels have a signifi cant effect on load sharing and need support.
5 Gear arrangements
As shown in Fig. 9 the most commonly used arrangement employs two plan-
etary step-up gear stages (with fi xed annuli) coupled in series with the second-
ary sun wheel driving a parallel shaft wheel via a double tooth type coupling.
This wheel meshes with a pinion having a parallel offset determined by the
required location of the generator which it drives via a proprietary spacer type
coupling. The primary reason for the offset is to provide a co-axial access to
the turbine rotor from the rear of the gearbox for pitch control purposes, e.g.
electrical slip rings.
Figure 10 shows an arrangement of the epicyclic stages featuring a star/plane-
tary differential with its input torque divided between the annulus of a primary star
stage and the planet carrier of a secondary differential stage whose annulus is
coupled to the primary sun wheel. Thus the primary planet carrier is the sole static
torque reaction member of the combined trains, while the secondary differential
sun wheel is the output coupled to the parallel shaft wheel. The signifi cance of this
is that the torque reaction is no longer transmitted to the gear case via a live gear
such as an annulus. This reduces structure-borne vibrations particularly when
fl exible planet spindles are used.