Aircraft Lubricants : Grease Vs Oil

EASA MODULE 7
SUB MODULE 7.3
TOOLS

Lubrication Methods:

 

Solid surfaces are never perfectly smooth, as the actual rubbing contact (the friction), between two moving surfaces, is concentrated on a number of high spots on the respective surfaces. Any load between the two surfaces tends to wear away these high spots until the area of contact is large enough to support the load without further deformation.

 

During the wearing action fragments of the surface are broken away and high local temperatures are generated. The effects are potentially dangerous, in that the fragments can cause serious damage to the surfaces while excess heat leads to expansion of the materials.
 

The expansion is likely to cause higher frictional forces, leading to further damage (due to more particles breaking from the surface) and can possibly result in eventual ‘seizure’ and stoppage of movement. Seizure could have disastrous results in any moving mechanism.
 

Lubrication is a process for reducing friction and wear, through the introduction of an unguent between two moving surfaces. The materials, commonly used for these purposes, are greases and oils.

A large number of different greases and oils are in use in aircraft maintenance. All lubricants should be kept clean and covered to keep out foreign objects. The correct quantity of the respective lubricant must, always, be used, as over-lubrication may cause all manner of problems, from contamination of electrical and mechanical equipment, to dirt and dust collecting on the lubricant residue.
 

Care must also be taken that lubricants do not remain in prolonged contact with unprotected skin (and particularly eyes) as many of them are severe irritants and present serious hazards to health. Hot oil also constitutes a danger to personnel.

Lubrication methods:

Grease is the preferred lubricant, in certain circumstances, for the following reasons:

Where conditions are dirty, dusty or wet, grease will provide a sealing medium, which will tend to prevent the entry of foreign matter into moving parts

• Grease will stay in vertical bearings, whereas oil will drain away
• Grease, packed into a bearing or housing, will provide sufficient lubrication for prolonged periods of time
• Grease lubrication systems use much cheaper fittings, and less complicated designs than those required for oil systems.

Grease, however, has limitations in certain situations, in that grease:

• Cannot replace oil when heat needs to be carried away
• Cannot be used where contamination is likely to occur
• Cannot be used where debris and contaminants have to be washed away.
  

Greases (as with oils), are produced to a range of specifications that depend upon the applications for which they are used. Some greases have to be water- resistant, while all have to possess good storage characteristics and be of a specific viscosity.

A grease, generally, is required to:

•  Remain chemically stable when it is heated, and especially if that grease is to be used in bearings that are running at high temperature
• Be thin enough to flow into applicators and spaces, whilst having the ability to remain in the desired location
• Resist the tendency to harden at the low temperatures met at high altitudes
• Retain the limits, set on its alkalinity or acidity, to prevent the onset of corrosion.

Oils are also required to remain chemically stable and to possess (and retain) a suitable viscosity during their useful life.
 

In general, every oil must:

• Adequately wet the surface needing lubrication
• Not evaporate too much in service
• Not produce gum, sludge or carbon
• Not damage the material with which it normally comes into contact.
  

In addition to protecting surfaces from corrosion, other properties of oils include the previously mentioned ability to wash away small particles of debris and to remove heat from system components. The heat can be as a result of the friction of motion, or from other sources such as the heat of combustion within a gas turbine or piston-type aero engine.

Oil provides lubrication in two distinct phases, which are referred to as:

• ‘Boundary’ lubrication
• ‘Fluid’ lubrication.

Boundary lubrication occurs in a stationary engine, when the oil tends to drain away from surfaces, leaving only an extremely thin film of oil, clinging to the microscopically ‘rough’ surfaces of
the metals.

Boundary lubrication will assist in the initial movement of one surface against another but, if it is not renewed, the surfaces will slide until the film disappears and seizure follows. Fluid lubrication is a thin, but continuous, film of oil, separating the moving surfaces, and so preventing metal to metal contact.

The oil also acts as a cushion between the surfaces. If, however, the oil is driven from the space between the surfaces, possibly due to excess loading, the fluid film is reduced to almost nothing and again, the surfaces will slide until eventual seizure occurs.

Generally, lubricating oils do not deteriorate during storage, but low-temperature greases can suffer and must be stored in a cool place. If water gets into either type of lubricant, the result can be very serious. Water contamination can cause:

• Breakdown of normal properties under bearing loads
• Oil additives rendered ineffective, giving a tendency to ‘sludge’
• Failure of normal properties due to oil emulsification
• Frothing of engine oil, which can cause excessive loss of oil through the system vents.

Many contaminants, such as rust and dust in suspension, may lead to blocking of oil passages or damage to moving parts. In grease, these solids produce a sort of grinding paste, which wears moving parts very quickly.
When oils and greases are in storage, a number of precautions will prevent subsequent problems. Good lubricant storage and usage demands that: 

• All containers have their lids firmly secured at all times when not in use
• The majority of lubricants need to be stored in a cool plac
• Different types or groups of lubricants must be kept apart, to prevent a risk of cross contamination
• All equipment must be kept totally clean.

Grease Versus Oil:

Grease lubrication versus oil lubrication for bearings both have their own specific advantages:

Grease

• Does not require a lubrication circulating system; pump, filter, sump, piping
• Better for leakage control
• Provides better seals against contaminants
• Can remain in equipment longer
• Lowers the risk of a dry start
• Worn seals and connectors can retain grease better, which lowers the risk of lubricant starvation and leakage
• Many grease lubricated bearings can operate for years without the need to repack the bearings.
• A better choice where a continuous supply of oil cannot be maintained
• Less consumption of the lubricant over time

Oil

• Easier to drain and replace oil lubricants
• Oiled bearing life can last longer than grease lubricated bearings.
• Easier to control the amount of lubricant to use
• Cleaner than grease due to capacity to carry away contaminants
• Better cooling properties
• Larger volume available allows for more available additive performance
• Avoids the problems associated with proper regreasing at proper frequencies
• Less energy consumption during operation
• Better cold start properties
  

EASA PART 66 MODULE 7 ESSAY QUESTION:

What type of lubricant will you use for the lubrication of following aircraft parts

1. Aircraft flight control cables
2. Landing gears and Wheel bearing
3. Engine Bearings

Also explain why you have selected a specific lubricant. 

 

(Note: write your answer in the comment section below)