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مُساهمةموضوع: كتب علمية خاصة بالبترول   الخميس سبتمبر 30, 2010 11:03 am

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مُساهمةموضوع: رد: كتب علمية خاصة بالبترول   الجمعة فبراير 24, 2012 11:20 pm

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مُساهمةموضوع: رد: كتب علمية خاصة بالبترول   السبت فبراير 25, 2012 10:42 am

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Lubricating Greases

• Grease is essentially a solid or semi-solid lubricant consisting of a gelling or thickening agent in a liquid lubricant.
• Other ingredients such as fillers and additives may also he incorporated to impart special properties.
• Originally greases were manufactured from mineral lubricating oils with soaps as gelling agent,
• But now synthetic lubricating fluids and non-soap gelling agents are also finding increasing use.








Description
 Grease is a semifluid to solid mixture of a fluid lubricant, a thickener, and additives. The fluid lubricant that performs the actual lubrication can be petroleum (mineral) oil, synthetic oil, or vegetable oil.
 The thickener gives grease its characteristic consistency and is sometimes thought of as a “three-dimensional fibrous network” or “sponge” that holds the oil in place. Common thickeners are soaps and organic or inorganic nonsoap thickeners. The majority of greases on the market are composed of mineral oil blended with a soap thickener.

 Additives enhance performance and protect the grease and lubricated surfaces.
 Grease has been described as a temperature-regulated feeding device: when the lubricant film between wearing surfaces thins, the resulting heat softens the adjacent grease, which expands and releases oil to restore film thickness
Function
 “The function of grease is to remain in contact with and lubricate moving surfaces without leaking out under gravity or centrifugal action, or be squeezed out under pressure. Its major practical requirement is that it retain its properties under shear at all temperatures that it is subjected to during use.
 Greases must be able to flow into the bearing through grease guns and from spot to spot in the lubricated machinery as needed, but must not add significantly to the power required to operate the machine, particularly at startup.”



Grease is generally used for:
 Machinery that runs intermittently or is in storage for an extended period of time. Because grease remains in place, a lubricating film can instantly form.
 Greases are used in sealed-for-life applications such as some electrical motors and gearboxes.
 Machinery operating under extreme conditions such as high temperatures and pressures, shock loads, or slow speed under heavy load. Under these circumstances, grease provides thicker film cushions that are required to protect and adequately lubricate, whereas oil films can be too thin and can rupture.
 Worn components. Grease maintains thicker films in clearances enlarged by wear and can extend the life of worn parts that were previously oil lubricated. Thicker grease films also provide noise insulation.




Grease function

 Functions as a sealant to minimize leakage and to keep out contaminants. Because of its consistency, grease acts as a sealant to prevent lubricant leakage and also to prevent entrance of corrosive contaminants and foreign materials. It also acts to keep deteriorated seals effective (whereas an oil would simply seep away).
 Holds solid lubricants in suspension. Finely ground solid lubricants, such as molybdenum disulfide (moly) and graphite, are mixed with grease in high temperature service or in extreme high-pressure applications. Grease holds solids in suspension while solids will settle out of oils.
 Fluid level does not have to be controlled and monitored.

Advantages of grease lubrication over oil lubrication are as follows
 Grease lubrication gives better protection under extreme operating conditions such as extreme pressures, low speeds, shock loads and bearings operating intermittently or reversing.
 Grease lubrication provides more effective sealing against contaminants such as water and dirt.
 Oil lubrication requires complicated design of housings and bearing enclosures. Grease lubrication permits simplified design of housings and enclosures because grease clings to metal surface more easily and more firmly.
 Grease reduces possible damage to the process or product from leakage. In oil lubrication there is danger of damage from oil spilling or leakage.



Grease lubrication has many disadvantages

 Grease is not suitable for high speed applications.. Oil because of its fluidity offers better lubrication at high speeds and low torque applications.
 Grease, though more effective in sealing, cannot flush away contaminants such as dirt and dust accumulated in the system.

 Poor cooling. Due to its consistency, grease cannot dissipate heat by convection like a circulating oil.

 Grease cannot be circulated while in case of oil lubrication centralized systems can be used.

 Resistance to motion. Grease has more resistance to motion at start-up than oil, so it is not appropriate for low torque/high speed operation
 More difficult to handle than oil for dispensing, draining, and refilling. Also, exact amounts of lubricant cannot be as easily metered.

Common ASTM tests for the grease characteristics listed as following:
 Apparent viscosity.
At start-up, grease has a resistance to motion, implying a high viscosity.
However, as grease is sheared between wearing surfaces and moves faster, its resistance to flow reduces.
To distinguish between the viscosity of oil and grease, the viscosity of a grease is referred to as “apparent viscosity.” Apparent viscosity is the viscosity of a grease that holds only for the shear rate and temperature



 Bleeding, migration, syneresis. Bleeding is a condition when the liquid lubricant separates from the thickener. It is induced by high temperatures and also occurs during long storage periods. Migration is a form of bleeding that occurs when oil in a grease migrates out of the thickener network under certain circumstances.

 Syneresis is a special form of bleeding caused by shrinking or rearrangement of the structure due to physical or chemical changes in the thickener.


 Consistency, penetration, and National Lubricating Grease Institute (NLGI) numbers.
• The most important feature of grease is o consistency. Its resistance to deformation by an applied force.

• Grease consistency depends on
• Type of thickener
• Amount of thickener
• viscosity of the fluid
• The measure of consistency is called penetration. Penetration depends on whether the consistency has been altered by handling or working. ASTM D 217 and D 1403 methods measure penetration of unworked and worked greases.

• To measure penetration, a cone of given weight is allowed to sink into a grease for 5 seconds at a standard temperature of 25 EC (77 EF). The depth, in tenths of a millimeter, to which the cone sinks into the grease is the penetration.
 The NLGI grease classifications along with a description of the consistency of each classification.
NLGI Number ASTM Worked Penetration Consistency
000 445- 475 Semifluid
00 400- 430 Semifluid
0 355- 385 Very soft
1 310- 340 Soft
2 265- 295 Common grease
3 220- 250 Semihard
4 175- 205 Hard
5 130- 160 Very hard













 Contaminants. Greases tend to hold solid contaminants on their outer surfaces and protect lubricated surfaces from wear.

 Corrosion- and rust-resistance. This denotes the ability of grease to protect metal parts from chemical attack. The natural resistance of a grease depends upon the thickener type. Corrosion-resistance can be enhanced by corrosion and rust inhibitors.

 Dropping point. Dropping point is an indicator of the heat resistance of grease. As grease temperature rises, penetration increases until the grease liquefies and the desired consistency is lost.
Dropping point is the temperature at which a grease becomes fluid enough to drip. The dropping point indicates the upper temperature limit at which a grease retains its structure, not the maximum temperature at which a grease may be used. A few greases have the ability to regain their original structure after cooling down from the dropping point.


 Evaporation. The mineral oil in a grease evaporates at temperatures above 177 EC (350 EF).
Excessive oil evaporation causes grease to harden due to increased thickener concentration. Therefore, higher evaporation rates require more frequent relubrication.
 Oxidation stability. This is the ability of a grease to resist a chemical union with oxygen. The reaction of grease with oxygen produces insoluble gum, sludges, and lacquer-like deposits that cause sluggish operation, increased wear, and reduction of clearances. Prolonged high-temperature exposure accelerates oxidation in greases.


 Pumpability and slumpability. Pumpability is the ability of a grease to be pumped or pushed through a system. More practically, pumpability is the ease with which a pressurized grease can flow through lines, nozzles, and fittings of grease-dispensing systems.
• Slumpability, or feedability, is its ability to be drawn into (sucked into) a pump. Fibrous greases tend to have good feedability but poor pumpability. Buttery-textured greases tend to have good pumpability but poor feedability.
 Shear stability. Grease consistency may change as it is mechanically worked or sheared between wearing surfaces. A grease’s ability to maintain its consistency when worked is its shear stability or mechanical stability. A grease that softens as it is worked is called thixotropic. Greases that harden when worked are called rheopectic.





 High-temperature effects. High temperatures harm greases more than they harm oils. Grease, by its nature, cannot dissipate heat by convection like a circulating oil. Consequently, without the ability to transfer away heat, excessive temperatures result in accelerated oxidation or even carbonization where grease hardens or forms a crust. Effective grease lubrication depends on the grease's consistency. High temperatures induce softening and bleeding, causing grease to flow away from needed areas. The mineral oil in grease can flash, burn, or evaporate at temperatures above 177 oC.


 Low-temperature effects.
If the temperature of a grease is lowered enough, it will become so viscous that it can be classified as a hard grease. Pumpability suffers and machinery operation may become impossible due to torque limitations and power requirements. The temperature at which this occurs depends on the shape of the lubricated part and the power being supplied to it. As a guideline, the base oil’s pour point is considered the low-temperature limit of a grease.
 Texture.
Texture is observed when a small sample of grease is pressed between thumb and index finger and slowly drawn apart. Texture can be described as:
*Brittle: the grease ruptures or crumbles when compressed.
*Buttery: the grease separates in short peaks with no visible fibers.
*Long fiber: the grease stretches or strings out into a single bundle of fibers.
*Short fiber: the grease shows short break-off with evidence of fibers.


 Water resistance. This is the ability of a grease to withstand the effects of water with no change in its ability to lubricate. A soap/water lather may suspend the oil in the grease, forming an emulsion that can wash away or, to a lesser extent, reduce lubricity by diluting and changing grease consistency and texture.







Egyptian Classification
Greases are classified into two general groups (ES, 792-1996). Those designated with an "L" and "G" prefix. These groups are further subdivided into categories with intended applications as follows:
 LA: Service typical of chassis and universal joints in passenger cars, trucks, and other vehicles under mild duty only. Mild duty will be encountered in vehicles operated with frequent relubrication in noncritical applications.
 LB: Service typical of chassis and universal joints in passenger cars, trucks, and other vehicles under mild to severe duty. Severe duty will be encountered in vehicles operated under conditions which may include prolonged relubrication intervals, or high laid, severe vibration exposure to water or other contaminates, etc.




 GA: Service typical of wheel bearings operating in passenger cares trucks, and other vehicles under mild duty. Mild duty will be encountered in vehicles operated with frequent relubrication in noncritical applications.
 GB: service typical of wheel bearings operating in passenger cars, trucks, and other vehicles under mild to moderate duty. Moderate duty will be encountered in most vehicles operated under normal urban, highway, and off- high service.
 GC: service typical wheel bearings operating in passenger cars, trucks, and other vehicles under mild to severe duty. Service duty will be encountered in most vehicles operated under conditions resulting in high bearing temperatures






E G Y P T




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