With the state of technology, the answer is currently not, but perhaps not for the reasons most assume or believe. It is self-evident that pitch bearings need lubrication to reduce their internal friction and wear, thereby giving predictable and repeatable behaviour. Also the properties of the chosen grease (as the lubricant universally used) need to match the operational and environmental conditions. So, in particular, that is the service temperature range, the corrosion protection of the bearing surfaces, the mitigation of potential false brinelling surface damage, the compatibility with seal type and the essential low wear of the raceways, rolling elements, cages and gear surfaces given the high localised stresses present.
The lubrication regime is one of boundary lubrication, where surfaces are not fully separated, hence wear is more likely. The increasing use of individual blade pitch control (IPC) for maximising turbine output means pitch duties are increased (in terms of pitch amplitude and number of cycles) and whilst this impacts pitch bearing fatigue life, it does tend to reduce the concerns related to false brinelling as bearing movement is more frequent than in prior pitch control methodologies.
The classical approach to avoiding boundary lubrication under elasto-hydrodynamic (EHL) conditions that occurs at gear and rolling contacts is to increase the base oil viscosity. Here oil film thicknesses are governed by entrainment velocity and oil viscosity with lower effect of load. However, simple viscosity increase can result in severe pitch torque penalties at low ambient temperatures and erosion of pitch safety margins within the control system. The growing size of wind turbines and hence the increased diameter of pitch bearings means this issue cannot be avoided, even with the use of fully synthetic base oils. Increased base oil viscosity also tends to reduce the flow of oil (as released from the grease matrix) into the Hertzian contact, thereby increasing the likelihood of false brinelling damage. So this is a compromise and it is interesting to note that most major pitch bearing lubricant suppliers now have remarkably similar base oil viscosities in their formulations.
So assuming we have chosen the ‘right’ grease for the pitch bearing based on evaluations of the above properties by the appropriate (and verified) laboratory screening tests, the question is how the pitch bearing grease degrades in service and age, as that has to determine the re-lubrication strategy (volumes, frequency, even lubrication and collection method). The reality is they do not chemical degrade or oxidise, likewise grease softening is not a major factor, so re-lubrication based on these classical factors does not in practice apply, as grease temperatures are too low. Nor, in the author’s opinion, that much critical attention has to be paid to lubricant storage life in any re-lubrication strategy, because the application is simply not that sensitive. So issues related to grease ageing are less critical.
Pitch bearings continue to pitch with internal fatigue damage far beyond what is considered acceptable for main, generator or gearbox bearings. The key reason for lubricant addition is control of grease contamination. Such contaminants come from internal bearing wear (primarily cage, but also raceways), gear surface wear, water and other fluids present in the hub, in addition to dust and sand from the local environment.
Much can now be done to reduce bearing internal and gear wear by appropriate treatments and alternative materials, but there is delta cost related to such additional processes in manufacture. Seal technology, life and performance have improved, due to increased demands from wind turbine manufacturers, but it is not possible to prevent the daily temperature changes that allow humid air to be present in the bearing internals or the external gear surfaces. So there is dynamic balance between near unavoidable contamination increase and the amount of grease added to dilute this effect. Debris ingress risks surface-initiated fatigue, whilst water increase can reduce rolling contact fatigue life and also promote false brinelling damage. This is primary limitation that prevents lubricated-for-life pitch bearings, or more realistically extended lubrication intervals.
If lubricated-for-life is not currently possible, the final considerations are the re-lubrication strategy, in particular the use or not of automated equipment, the frequency and quantity of grease added and how the new grease is applied and old grease collected. Multi-injection automated systems are the norm, but it must be noted that the total addition per year is typically a small percentage of the overall grease volume present, grease mixing normally occurs within pitch bearings and there is no net grease flow direction simply due to the grease injection method.
So in terms of grease condition or quantity present, the net benefits of such complex systems in improving bearing life are an open question. Multiple inputs and outputs risk localised grease flows that may not lead to optimum grease distribution or uniform collection.