16. 08. 2023.

We lubricate your aircraft: Greases 

Greases are lubricants consisting of oils mixed with other ingredients to give a gel like consistency. When they are used as lubricants, greases behave like oils in many ways. They reduce friction by providing a film which separates moving surface. In fact, this lubrication is actually carried by oil rather than the grease itself. Greases are specially designed to have a very high viscosity making greases effective an effective seal against moisture and solid contaminants. 

so how is a grease made? 

All greases consist basically of a base oil, a thickener (carrying the structural stability) and performance improving additives. Varying these three basic ingredients leads to various greases suitable for a numerous different application. However, in broad terms, a typical grease would consist of about 80% – 90% base oil, 10% – 15% thickener and about 5% – 10% additives. 

1. Base Oils 

Choosing a base oil for a grease is essentially determined by the type of application. For example, with mineral-oil-based greases, if the application involves heavy load conditions, a grease with a heavy viscosity base oil will be the right choice. However, high viscosity mineral oils do not perform well at low temperature as they become too thick, and so for more general use a lighter base oil will be used. 

Greases which are required to operate over a very wide temperature range are often base on synthetic oils, often derived from esters rather than mineral oils. Keep in mind that these oils are more aggressive to seal materials, use them only when specified for your aircraft. These greases are used commonly on commercial aircraft where there is frequent exposure to extremely low temperatures at altitude.

2 Thickening Agents

Thickening agent has the biggest effect on its consistency. Depending what king of thickening agent is used, you will get a softer or stiffer type grease. A very simplified way to describe the synergy of thickener and oil is to imagine the thickener like a sponge that holds the oil in a lattice. When pressure or stress is applied, oil is released which then lubricates the mechanism and, when the stress is released, the thickener and oil return to a semi-solid state. 

The consistency of greases can vary from very soft, semi-fluid cream like consistencies through to hard wax-like solids. The consistency is measured by allowing a standard metal cone to sink into the grease and measuring its penetration. This is often called the “unworked penetration” value of the grease. 

In environments with vibration, the “working stability” of a grease is a crucial indicator since most greases tend to soften slightly when heavily worked. The working resistance of a grease is measured by the so-called “Working Penetration”. With this method, the grease is worked with perforated mechanical plunger and the penetration value is measured afterwards. The difference between the worked and unworked penetration values gives an indication of the working stability of the grease. The majority of aviation greases have unworked penetration values of around 260-320. 

The common types of thickeners are generally “soap” based or “clay” based. 

• Calcium soaps have a smooth texture and have good water resistance, but have very low operating temperatures normally not more than 60 °C, although some chemical improvement in the temperature tolerance is possible 

• Sodium soaps have a higher melting point than plain Calcium soaps, have a fibrous texture and have good adhesive properties. However, they have relatively poor water resistance and risk suffering water washout in very wet conditions. 

• Lithium soaps have a butter like texture, relatively high temperature (up to about 135 °C) and have good water resistance. 

However, all of the soap thickeners are limited by their high temperature performance as the thickeners tend to melt and the grease softens markedly. 

To address this issue, AeroShell use f.e. a patented thickener known as Microgel. This is a specially treated clay-based thickener which has: no upper temperature limit (the upper temperature is limited by the base oil), very little change in consistency with variation in temperature, extremely good water resistance due to the treatment of the clay and low oil separation, or “bleeding” of the grease. 

Non-soap type thickeners can also be used in high speed / high temperature applications with an upper temperature limit not achievable with soap type thickeners.[/expand] 

  

3. Additives 

The additives used in greases have similar characteristics to oil additives. These are typically antioxidants, corrosion inhibitors, load carrying additives, although some greases are enriched with solid film lubricants. These are particularly suited to heavily loaded, sliding applications such as splines, landing gear bogie pivot pins and the like. 

 mixing greases. 

Again, grease performance depends on the three main ingredients shown before: the thickener, the base oil, and the additive. Greases with same thickener type (e.g., clay, Li-complex) are not necessarily compatible. Additives may act in synergy, compete or act antagonistically. Mineral and some synthetic oils do not mix well. 

Be aware that by mixing incompatible greases you can compromise the grease’s lubrication properties and the ability of the grease to perform as designed in application. Such incompatibility can result in change in consistency, shear stability, oil separation and/or oxidation stability issues. The most common effects include: 

• Softening, which can lead to grease leakage followed by lubrication starvation 

• Stiffening, which can result in reduced grease flow or prevention of oil release 

• Frictional drag, which increases operating temperature 

• Separation of the thickener and oil phases, which can lead to a premature lubrication failure or distribution system plugging 

• Reduction in dropping point, which can cause leakage and eventually lubrication starvation at peak operating temperatures 

What factors should be considered when converting from one grease to another? 

If you are using a new grease, always validate compatibility. If compatibility is borderline, apply one extra regreasing at half the normal interval. In case the two greases do not mix well at all regrease at half the normal regrease interval for several cycles. 

For a better result take the extra time to clean the parts before you apply the new grease. Sometimes cleaning is not an option, or not recommended. In that case purge out the old grease until old grease is no longer visible. Again, increasing the relubrication interval for a period of time is a good idea to make sure you minimize the duration of grease mixture.[/expand] 

  

best practices. 

Storage 

Keep the grease indoors in dry environments. Make sure that storage temperatures range from 0 °C to maximal 40 °C. In the event that the stored grease is briefly exposed to severe temperatures, consult us or the manufacturer with concerns. Depending on the grease, even quick exposure can compromise the grease. 

[expand title=”More storage tips.” swaptitle=”Show less” swapalt=”Show less!”]Do not take the shelf life for granted once a container is opened. Since the rate of grease degradation can be impacted by exposure to contamination and/or storage and handling conditions, the listed shelf life is no longer applicable. Keep the grease pure like you do with the jam at the breakfast table: always use clean and dry transfer/dispensing equipment to avoid contamination. 

As an inventory management best practice, we recommend the “first-in, first-out rule. By rotating older stock to the front or to a designated area, you will reduce the potential for use of product beyond its expiration date. This practice will save you money by preventing disposal of aged product. 

While we cannot recommend storing lubricants outdoor, sometimes it cannot be avoided. In this case find a waterproof place at least 20-30 cm above the ground to prevent moisture damage.[/expand] 

  

Expiration 

You might ask if the expiration date should be really viewed so strictly. While the degradation of a particular product depends on storage and handling – as shown above –, you will most certainly risk quality issues by using expired grease. Such an application holds similar risks associated with using an unapproved product. 

[expand title=”More about expiration.” swaptitle=”Show less” swapalt=”Show less!”]As the product exceeds its shelf life, a decrease of performance additives that provide wear protection, oxidation stability, and grease structural stability may be observed. 

However, you can apply recertified grease light-heartedly. An aviation authority like the FAA would not reapprove or recertify a grease if there was an issue with the product.[/expand] 

  

Application 

Oil separation to a greater or lesser extent occurs with all greases. Unless the separation is excessive, you can use after you stirred it. Always use protective gloves. This is not just for your own protection but also to prevent oil from your fingers from effecting the lubrication. 

When re-greasing a bearing with a grease gun, always apply enough grease for fresh, clean grease to be seen coming out of the bearing. This ensures that the bearing purged of the old, degraded grease which can the be wiped off and discarded. 

interesting developments. 

AeroShell has been working on an impressive project to develop a new genuine multi-purpose, corrosion inhibiting grease in conjunction with Beoing. Boeing wanted to rationalise the multitude of greases used on their airline fleet and in collaboration with Shell drew up a very demanding set of criteria. 

To meet this demand Shell has developed AeroShell Grease 33 which is now used on all new manufacture commercial Boeing aircraft and Shell is the only manufacturer to have a grade currently capable of meeting the specification. AeroShell Grease 33 can do the job previously covered by 4 different greases and of over 359 grease application points on a Boeing 737, all but 9 are now lubricated by AeroShell Grease 33. 

  

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