Wednesday, 20 June 2012

Grease

Grease
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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.
  • Grease can be divided into 6 general categories or types;
    • MINERAL OILS MIXED WITH SOLIDS
      Specialised heavy equipment lubricant
    • HEAVY ASPHALTIC-TYPE OILS BLENDED WITH LIGHTER OILS
      More a thick heavy oil, lighter oil blends improve pour point.
    • EXTREME-PRESSURE GREASES
      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 aditives 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
    • ROLL NECK GREASES
      Semi solid grease exclusively used to lubrictate journal bearings. Typically the grease bar is profiled
    • SOAP THICKED MINERAL OILS
      This is the largest category and is split up into groups stermined 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 luricating 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 continously 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 coninuous tempeatures over 3000F
      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 GREASE
      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.
    • Bentonite( clay base)- operational up to 200'c
    • Silicone- operational up to 260'C
  • Bentonite( clay base)- operational up to 200'c
  • Silicone- operational up to 260'C
    Additives to grease
    • Extreme Pressure additives
    • Oxygen and corrosion inhibitors
    • Pour point depressants
    • Water repellents
    • Pigments
    Advantages of grease lubrication
    • Convenient to apply and replenish
    • Easily retained in the housing and does not require complex seals
    • Protects bearing surfaces from corrosion particularly when machinery is shutdown
    • Serves as a seal preventing the ingress of dirt and moisture into the bearing.
    Do not use grease when
    • Temp' are high and the lube must function as a coolant for the bearing
    • When a bearing is lubricated in conjunction with other components in a single lubrication system e.g. gearbox
    • Where resistance to bearing rotation must be kept to a minimum e.g. light precision instruments
    • When speeds are beyond the limit of grease lubrication
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

Grease relubrication

relubrication period(hrs) = K x (14 x 106/ N x D-1/2) - 4D
The amount of grease in ounces is determined from
G = 0.00018 DB
where D= outside dia of bearing in mm
B= beairng 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.

Other

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.

Service Deterioration

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.

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