Formation of a void due to reduced pressure in lubricating grease or oil. Cavitation of a lubricating grease in a dispensing system can prevent the grease from flowing.
A standard that certifies that an individual possesses a defined level of expertise in the field of lubricating grease. Certification indicates that NLGI recognizes the individual as a grease expert. Certification is awarded after an individual passes a two-hour exam that consists of 120 questions about lubrication fundamentals and grease types, selection, manufacturing, applications, maintenance, testing, etc.
1) The tendency (usually desirable) of a channel to form when grease is worked in a bearing, leaving shoulders of unworked grease that serve as a seal and a reservoir of oil.
2) The tendency of liquid lubricants and flow-type lubricating greases at low temperatures to form a plastic structure sufficiently strong to resist flow under gravitational forces. Similar to, but not identical to the pour point of liquid lubricants, it is measured with empirical tests such as Method 3456.2 in Federal Test Method Standard No. 791 (D).
Consider two relatively flat surfaces that are in contact, such as a block on a tabletop. When there is no relative motion between these two surfaces, the coefficient of initial friction is the ratio of the force applied to initiate relative sliding motion (parallel to the surfaces) to the force due to the load (perpendicular to the surfaces). The coefficient of kinetic friction is the ratio of the applied force to the load in order to maintain sliding motion. The term coefficient of friction usually refers to the coefficient of kinetic friction.
Attractive forces between molecules in a substance. For example, cohesion between molecules in grease contribute to its resistance to flow.
A substance that consists of microscopically dispersed insoluble particles that are suspended throughout another substance, typically in a fluid. Lubricating grease is a colloidal system of thickener particles or fibers in oil. (see also Thickener).
(of lubricating grease) The shade and intensity observed when lubricating grease is viewed under conditions that eliminate Bloom. For example, lubricating grease in an opaque container such as a metal package can be observed under reflected light from a position approximately perpendicular to the surface. Grease color can also be observed with transmitted light by placing the sample on a transparent plate. It is important to indicate the method used to determine the color of grease. Colors of lubricating greases are best described in terms of the predominant hue such as amber, brown (or perhaps green, red, or blue for dyed grease) with a qualifying adjective describing intensity in terms of light, medium, or dark.
(of lubricating oil) â€“ That shade shown when viewed under transmitted light only. Usually lubricating oil colors are obtained by viewing under specified conditions in test equipment. Several such methods are available, the most widely used being ASTM D1500, which describes the colors in terms of numbers.
A soap thickener wherein the soap crystal or fiber is formed by co-crystallization of two or more compounds:
1) The primary soap (such as metallic stearate or oleate).
2) Complexing agents such as metallic salts of short chain organic acids or inorganic salts. The complexing agent modifies grease characteristics and usually increases the dropping point.
A device described in ASTM D217 and D1403 that measures the consistency of lubricating grease. The device consists of a freely moving cone and shaft assembly, a pedestal for placing the grease sample below the cone, a locking device to stop the motion of the cone and shaft, and a scale for measuring the depth to which the cone penetrates the grease.
The degree to which lubricating grease resists deformation under the application of force. Consistency characterizes the plasticity of a solid in much the same way that viscosity characterizes a fluid. Grease consistency is usually measured by cone penetration according to ASTM D217 (IP 50) or ASTM D1403.
The gradual destruction and/or pitting of a metal surface due to chemical reactions with the environment. The most common form of corrosion is caused by oxidation of metal or electrochemical reactions of metal with oxygen or aggressive ions. (See also Fretting).
An additive that is used to enhance the corrosion prevention properties of the grease to which it is added. Test methods for corrosion prevention properties of grease include D1743, D5969, and D6138 for ferrous metals and D4048 for copper. Different types of chemistries are used for the different types of metals to be protected.