A lubricating grease is a solid or semi-fluid lubricant containing a thickening agent in a liquid lubricant. Grease can be likened to a sponge soaked in lubricant. In cases of a soap based grease the soap can be likened to the sponge soaked in a mineral oil as the lubricant.
They are usually made from mineral oil stock of widely ranging viscosity but similar to those used for liquid lubricants. Synthetic oils may also be used were high temperatures are expected.
Specialised heavy equipment lubricant
More a thick heavy oil, lighter oil blends improve pour point.
Contains additives that improve wear resistance under extreme pressures. A chemical reaction between the metal and the chemical brought about by the heat generated by pressure as the faces come together.
These additives include compounds containing Chlorine, Phosphorous, Sulphur and phosphates.Zinc, lead or even asbestos may be added as a filler to cushion shock loading in gears
Semi solid grease exclusively used to lubricate journal bearings. Typically the grease bar is profiled
This is the largest category and is split up into groups determined by the base oil or thickener
Sodium-base (Sodium Hydroxide) greases are also general-purpose greases. They are
used higher temperature stability is required having a drop point 300 to 350 0F
Sodium-soap greases have a spongy or fibrous texture and are yellow or green in color. They may be used for lubricating rolling element bearings more typically they are used in rough, heavy duty appilcations.Tend to be water soluble
Barium-soap greases are general-purpose types, valued for their ability to work over
a wide temperature range. The drop point is over 3500F although it is not recommended
to use them continuously over 2750F
They are used in many applications although the high soap content makes them unsuitable for high speed or low temperature applications. They have a buttery of fibrous texture and are reddish-yellow or green in color.
Lithium-soap (Lithium Hydroxide) grease is suitable for high and low temperature
applications. It has a drop point of 350 0F and may be used at continuous temperatures
They may be made with oils having a very low viscosity to make them suitable for very low temperature applications
They have very good stability; good water resistance, and are also readily pumpable. They have a buttery texture and a brownish-red color. More expensive than sodium soap
Calcium-soap(calcium Hydroxide) are inexpensive general purpose greases with a relatively
low drop point of 800C due to water content drying out, also called lime-soap greases.
Uses include axle grease, water pump grease and general machinery applications and
have the major advantage of water resistance
Calcium complex greases has a very high heat resistance of over 500oF making it particularly useful to extreme pressure applications although lack of mechanical stability has limited their usage
Calcium-soap greases are yellow or reddish in color, and have a smooth buttery texture.
Aluminum-soap are special purpose greases which are inherently very sticky
Multi-purpose greases combine the properties of two or more specialized greases. This permits the use of a single type of grease for a variety of applications. Most of the multi-purpose greases have a soap base of barium, lithium, or calcium complex. For example, the lithium-soap greases discussed earlier. They are not only water-resistant and corrosion inhibiting, but they have very good mechanical and oxidation stability as well.
Additives to grease
Advantages of grease lubrication
Do not use grease when
note: Spherical roller thrust bearings are almost designed to operate on oil lubrication
The larger the bearing the more frequent the re-lubrication
The higher the speed the more frequent the re-lubrication
Oxidation of mineral oil doubles every 25'C temperatures rise
Fill housing cavities 1/2 to 2/3 full, no more, or churning and overheating will occur
relubrication period(hrs) = K x (14 x 106/ N x D-1/2) - 4D
where K = bearing type factor
N = speed (rpm)
D = bearing bore (mm)The amount of grease in ounces is determined from
G = 0.00018 DB
where D= outside dia of bearing in mm
B= bearing width in mm
alternately amount(ounces)= DxB/6000
The relubrication interval of a bearing should be halved for every 15 to 20 oC increase in operating temperature. Before regreasing the old grease should be flushed out.
Drop point of grease
The drop point of grease is the temperature at which it begins to pass from a solid to a semi solid state, under standard test conditions. It is used as a rough guide to the resistance of a grease to heat. It is not the maximum allowable working temperature which may be several degrees higher. However if the drop point is exceeded the consistency of the grease will have deteriorated due to breakdown of the gel.
Greases using the same thickener are usually compatible, different thickener are usually incompatible. Greases are very good at absorbing water and may take between 40 to 100% by volume without failure. Heavier grease have better water wash resistance.
The base oil oxidises causing darkening of the grease. Acidic oxidant products causes softening of the thickener leading to oil bleeding
This oxidation tends to begin at localised hot points due to the poor heat conductivity of the grease, carbonisation and progressive hardening reduces the effectiveness of the grease
Grease has a maximum temperature at which it can safely be used. Therefore, it follows that it must also have a minimum temperature. This minimum temperature is the point where the grease becomes too hard for the bearing, or other greased component, to be used. Again, the base oil of the grease determines the minimum temperature. Obviously, the base oil of the grease for low-temperature service must be made from oils having a low viscosity at that temperature.
MoS2 is extracted from small veins in granite by crushing and flotation. In its purified state it has a low coefficient of friction, resists heat, oxidation, common solvents , and chemicals and can be used under extreme pressures. It shears readily and bond with ordinary engineering materials and plastics. Due to high cost it is not seen as a replacement lubricant but offers a solution to difficult problems.
Molybdenised lubricants exist with combinations of MoS2 and oils, greases, water, soap, solvents, silicones, and other fluids.
MoS2 is in the form of a laminar structure with a slice of molybdenum atoms being held between to slices of Sulphur atoms. The low coefficient of friction is due to the ease of shear between adjacent Sulphur slices.
MoS2 has a melting point of 1185oC
Care should be taken with the use of this product in relation to tightening torques. Manufacturers may be quote figures under the assumption that lubrication of the nut is by light mineral oil only. Alternately they may give different torques for Light oil, copaslip and molykote.