Greases and oils perform the same basic function — lubrication.  So how are they different? Simply put, grease is an oil with a thickener added to it. Lubricant selection has conventionally been driven by base oil viscosity, in the belief that a higher viscosity enhances film thickness and performance.

This logic is understandable, since the lubricant film-forming behaviour of grease approaches that of its corresponding base oil at high speeds. But what about the film formation at low speeds — a question that is highly relevant to lubrication of wind-turbine bearings?

This article will highlight a study that suggests another basis for choice of wind-turbine lubricant by giving a different perspective of the lubricant film-formation behaviour of greases and how it differs from that of oils, especially at low speeds.

Much is known about the film-forming behaviour of oils — knowledge dating back to film-thickness experiments conducted in the 1960s. As is widely acknowledged, oil film thickness increases with speed, following the classic EHL (elastohydrodynamic lubrication) theory with a log-log film thickness-speed exponent of 0.67.

However, film formation with greases can differ significantly from that of oils in low-speed applications, and the behaviour is influenced by many factors, including:

- thickener content and type

- base-oil viscosity

- replenishment properties

- rheological properties

- test conditions

- application (such as bearing type, bearing dynamics).

A common belief concerning grease lubrication is that the film thickness of the grease approaches that of its corresponding base oil at high speeds due to similar rheological properties. However, at low speeds, the grease film thickness can be significantly higher by a factor of "d" as shown in Figure 1. This indicates that conventional EHL oil theory could be applicable to grease lubrication at high speeds, but may not be so for low speeds.

-2017 China(Guangzhou) Int’l Bearing and Equipment Exhibition