The ability of lubricating grease to withstand the addition of water to the lubricant system without adverse effects. Water Resistance is generally considered to be made up of four components as listed below:
Water Washout Resistance – The ability of a lubricating grease to resist being removed from a bearing when operated with exposure to water. Generally measured by ASTM D1264 or ISO 11009.
Water Absorption – The characteristics of a lubricating grease when water is added to the lubricating system. Water Absorption Characteristics may be measured by any of several suitable tests in which the lubricating grease may react in any of three ways, described as follows:
Water Soluble – The lubricating grease absorbs the water, and then de-gels to semi-fluid consistency.
Water Absorbent – The lubricating grease absorbs relatively large quantities of water with little or no change in consistency and without a separate phase of free water.
Water Resistant – The lubricating grease does not absorb more than small amounts of water, does not change appreciably in consistency, and most of the added water is a separate second phase.
Water Corrosion Resistance – The ability of a lubricating grease to prevent corrosion of metal surfaces in the presence of water. May be measured either statically by any of a number of standard tests, or dynamically by actual operation of bearings with water added to the lubricant reservoir, as in ASTM D1743, D5969, D6138 and ISO 11007.
Water Spray Resistance – The ability of a grease to resist displacement from a surface by the impact of water spray. The method of test used to evaluate this characteristic for lubricating greases is given in ASTM D4049.
Lubricating greases for various types of service may not need any of the several types of water resistance characteristics described above. They are not measures of quality except for specific situations where water resistance is required.
Damage that involves the cumulative and gradual removal of material from surfaces. Three types of wear are described below:
Abrasive Wear – Damage that occurs when surfaces are in contact and undergo relative motion, and cutting or abrasion by hard particles (usually a contaminant) remove material from the surfaces.
Adhesive Wear – Damage that occurs when two surfaces are in contact and undergo relative motion, and high loads and/or temperatures cause asperities on these two surfaces to weld together and then immediately separate, removing material from one or both surfaces. Adhesive wear can be mild (frosting), moderate (scuffing), or severe (galling, scoring, seizing).
Corrosive Wear – Damage that occurs when chemical reactions at a surface result in the removal of material. Corrosion can be localized (e.g., pitting) or general (not local).
Electrical Discharge Wear – Removal of material from solid surfaced due to sparks or high-amperage electrical discharge.
Erosive Wear – Progressive loss of material from the surface or edge of a solid component when a liquid or a large number of solid particles impinge upon it..
Fatigue Wear – Wear of a solid surface that is caused by fatigue, i.e., repeated heavy loading that causes dents, cracks, fracture and removal of fragments from a metal surface.
Fretting Wear – Wear that occurs as a result of fretting, i.e., small amplitude oscillations or vibrations between two solid surfaces that are in contact.
Polishing Wear – Occurs when very fine hard particles in a lubricant remove material from a solid surface, producing a brightly polished surface.
Bearing failure characterized by the appearance of white cracks on the surface of the bearing. White etch cracks are typically found in high-stress applications such as wind turbine gearbox bearings. In-situ hydrogen generation by corrosion or other causes and stray electrical currents have been implicated as factors leading to white etch cracking.
The subjection of lubricating grease to any form of agitation or shearing action beyond simple transfer to any test apparatus, package, or application.