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"We have a problem with some of our wind turbines. Their yaw drive systems are very noisy. Currently, we use a diurea grease. Can you advise a way to reduce the noise?"
An unusual noise is only a sign of an issue, not necessarily an issue itself. Therefore, the first thing that should be done is a thorough investigation into the origin of the noise in hopes of finding the root cause, as opposed to making an assumption that a lubricant will solve the issue.
If the noise is due to a mechanical problem such as severe bearing wear or misalignment, then an adjustment in grease type or usage will not resolve the issue.
Another thing to consider is the way the grease is delivered to the yaw bearings. It is possible that the bearings are not receiving a sufficient amount of grease due to the low speed and heavy load application or the method in which it is being greased, including the regreasing location, the amount of grease and the regreasing intervals.
If all other root causes have been exhausted and improper lubrication appears to be the issue, then the use of the diurea (thickener) grease should be addressed. The diurea grease refers to the thickener type, not the base oil with which it is combined. The base oil and its additives are what provide the lubrication to the bearing system, and characteristics such as the base oil’s viscosity must be determined to better evaluate its suitability. In general, a diurea grease can perform well under extreme pressure and temperatures and offers good wear protection.
Without knowing the exact grease composition currently in use, a specific lubricant-related solution cannot be recommended. There are several grease products on the market that should meet the requirements of a system like this, but they must be chosen based on the parameters of the particular wind turbine design, the environment and loading conditions.
However, if the lubricant in use is specified by the equipment manufacturer and adjustments are made based on the wind turbine's parameters, then a mechanical issue is probable. It is important to investigate the cause of the noise, as it may be an indication of possibly a bigger issue. The noise could be stemming from a high frictional area within the yaw system, but that does not necessarily suggest an improper lubricant is in use.