Grease viscosity

Even though grease viscosity is depending on working temperature, why do viscosity of some grease increase and viscosity of some grease drops while working under high temperature?
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Just like with motor oils, different operating environments require different characteristics of different grease products. For example, some characteristics of the grease to consider are the temperature tolerance and the "shear" qualities. Do tolerances get tighter with operating temperatures? What are the desired "shear" characteristics of the lubricant – essentially the strength of the lubricant before it breaks down? When molybdenum disulfide was all the rage, it was all about preventing metal-to-metal contact when conventional oil broke down. Check out this popular web resource for more discussion on oils and lubricants.

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The base oil in a grease usually has a viscosity of between 20 and 500 mm2/s at 40°C. The application where the grease is to be used governs what base oil viscosity is required.
A lubricant`s viscosity will change with changes in temperature. As lubricants get hot, their viscosity drops; as they get cold, their viscosity increases.
Typically, the viscosity is reported at one of two temperatures, either 40°C (100°F) or 100°C (212°F). For most industrial oils, it is common to measure kinematic viscosity at 40°C because this is the basis for the ISO viscosity grading system (ISO 3448).
The viscosity and density of lubricating oil decrease with the increase of temperature.
The absolute or dynamic viscosity is equal to the kinematic viscosity, multiplied by the density of the fluid. It is usually expressed in centipoise (cP) (1 cP = 0.001 Pa. s). The viscosity of lubricating oil changes with temperature and the rate of change depends on the composition of the oil.
Mobilux EP 2 is a high performance, general-purpose industrial-type grease. This lithium hydroxystearate based grease is formulated to provide extra protection against wear, rusting and water washout. It is NLGI Grade 2 with a base oil viscosity of ISO VG 150.
Increased temperature causes a reduction throughout viscosity even though particles have so much more thermal energy as well as can more easily accomplish the attraction forces that hold them collectively.
Lubricity additives and solid lubricants can enhance the natural viscosity of the oil and component performance. Higher viscosity oils, required components of damping greases, have a higher “Viscosity Index,” indicating there is less change in viscosity over temperature.
Mineral Oil Rotational Viscosity Standard viscosity 55.68 mPa. s (25.00 C) Mineral Oil Rotational Viscosity Standards (viscosity 55.68 mPa. s) are specifically formulated for the calibration and verification of rotational viscometers where operating temperature is controlled precisely.
Grease is usually lithium, with a normal viscosity of 100 centistokes. This typically has a maximum operating temperature of 121 C (250 F), but it is usually limited to a service temperature of 93 C (200 F).
Viscosity generally decreases as the temperature increases. Viscosity generally increases as the temperature decreases. The viscosity of a liquid is related to the ease with which the molecules can move with respect to one another.
Viscosity = (2 x (ball density – liquid density) x g x a^2) ÷ (9 x v), where g = acceleration due to gravity = 9.8 m/s^2, a = radius of ball bearing, and v = velocity of ball bearing through liquid. Plug your measurements into the equation to calculate the viscosity of the liquid.
A common feature of greases is that they possess a high initial viscosity, which upon the application of shear, drops to give the effect of an oil-lubricated bearing of approximately the same viscosity as the base oil used in the grease.
In general, the category of “low-viscosity” lubricants covers systems wherein the fluorinated additive is dissolved or suspended in a liquid lubricant and the liquid is not removed or expected to be removed during its lifetime as a lubricant.
Oil and grease are two types of high viscous substances, and the two compounds differ from each other according to the viscosity level. The key difference between oil and grease is that oil has comparatively a low level of viscosity, whereas grease has a very high initial viscosity.
The recommended operating temperature range is from -20ºC to 130ºC but Mobilgrease EP 2 may be used at higher temperatures if the lubrication frequency is increased accordingly.
Mobil Polyrex EP 2 is a shear-stable polyurea grease with excellent extreme-pressure (EP) and load-carrying characteristics. The proprietary polyurea thickener system exhibits excellent resistance to oxidation and oil separation at operating temperatures as high as 160°C (320°F).
Viscosity is a highly important property of fluids and liquids that determines their industrial or commercial use. Different factors such as temperature, pressure, applied force, and complex multiphase liquid properties can influence the viscosity of a fluid.
A fluid that is highly viscous has a high resistance (like having more friction) and flows slower than a low-viscosity fluid. To think of viscosity in everyday terms, the easier a fluid moves, the lower the viscosity.
Viscosity decreases with increase in temperature for a liquid and increases with increase in temperature for a gas.
Viscosity is governed by the strength of intermolecular forces and especially by the shapes of the molecules of a liquid. Liquids whose molecules are polar or can form hydrogen bonds are usually more viscous than similar nonpolar substances.
Currently, the most utilized method for improving wear prevention in low-viscosity engine oils is the addition of friction modifiers. These cost-effective additives improve friction during boundary and mixed lubrication where low-viscosity oils typically suffer.
Heat collapses oil films, causing accelerated abrasion and scuffing conditions. Hot oil shortens the life of filters and seals and accelerates corrosion. Grease separates faster (oil from thickener) at elevated temperatures`
Grease can also be thickened with non-soap materials. Common non-soap thickeners include polyurea, organophilic clay, fumed silica, fluoropolymers, and others. Polyurea is a generic term that includes include diurea, tetraurea, urea-urethane, and many related chemistries.

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