Basics of Characterizing Limiting Friction of Greases

Lubricating greases find their use in various applications such as gears, bearings, chains, guides, etc. Selection of grease for a particular application depends upon various factors ranging from the contact pressure, lifetime, temperature, etc. With ever increasing demand for improvement in the efficiency of systems, selection criteria have also become quite stern. Under such conditions, having knowledge about the characteristics of the grease beforehand is highly beneficial in the selection process. This however is only possible for its inherent properties such as thixotropy, density, oxidation stability, rust protection, etc., and some specific tribological properties like extreme pressure properties, loadability, etc. There are of course various other parameters such as its resistance towards corrosion, long term stability, friction behavior, etc., which need to be characterized individually for each given tribological system.

Lubricating greases find their use in various applications such as gears, bearings, chains, guides, etc. Selection of grease for a particular application depends upon various factors ranging from the contact pressure, lifetime, temperature, etc. With ever increasing demand for improvement in the efficiency of systems, selection criteria have also become quite stern. Under such conditions, having knowledge about the characteristics of the grease beforehand is highly beneficial in the selection process. This however is only possible for its inherent properties such as thixotropy, density, oxidation stability, rust protection, etc., and some specific tribological properties like extreme pressure properties, loadability, etc. There are of course various other parameters such as its resistance towards corrosion, long term stability, friction behavior, etc., which need to be characterized individually for each given tribological system. The current article deals with one such property - the break-away force of greases in a particular tribological system. It is the force required to overcome static frictional resistance of the tribological system and set it into macroscopic motion. The value of coefficient of friction right before the onset of macroscopic motion is known as the limiting friction. Typical applications where this parameter could be of great significance are seat regulators, sliding guides, doors, locks, fishing gear, etc. While in most cases a low breakaway force is wished for, it must also be noted that a certain amount of resistance is still required to inhibit involuntary movements.

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