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The Benefits of Changing from Clay Based to Lithium Complex Based Greases

As we have already discussed in Greases Explained, a grease is made up of a mineral or synthetic base oil, a thickener, and an additive package. 

Clay-based thickeners, such as Microgel® in AeroShell Grease 7 and 22, have been used in aircraft since the 1960s owing to their good water-resistance and high-temperature properties. They have served the aviation industry well and they will continue to serve, where high temperature performance is required. 

However, over time the requirements for greases became more and more complex requiring the use of a wider range of sophisticated additives. At the same time, the customer expectation regarding durability was also rising.

To address all these needs, the industry shifted toward Lithium-complex thickeners, as those have a good compatibility with a wider range of high-performance additives, which means greases like AeroShell Grease 33 and AeroShell Grease 58 offers superior:

  • oxidation and corrosion control;
  •  wear protection;
  • extreme pressure load carrying capacity.

Lithium-complex greases have excellent mechanical stability and water resistance compared to Clay-thickened greases. This helps the grease stay where it’s supposed to, instead of softening and potentially moving out of the grease application.

The AeroShell Grease Family

Lithium Complex Greases

Microgel (Clay) Greases

Calcium Soap Greases

AeroShell Grease 7 and AeroShell Grease 33

AeroShell Grease 7 and AeroShell Grease 33 are both multi-purpose airframe greases approved to MIL-PRF-23827. Grease 7 is a clay based grease approved to Type II while Grease 33 is a lithium-complex soap based product approved to Type I.

As a lithium complex based grease, AeroShell Grease 33 exhibits superior performance characteristics compared to AeroShell Grease 7 in both washout resistance and load carrying capacity.

Chart displaying Aeroshell grease properties

AeroShell Grease 22 and AeroShell Grease 58

AeroShell Grease 22 and AeroShell Grease 58 are both designed to protect aircraft wheel bearings operating under high loads and speeds and wide temperature ranges and as such are approved by major operators and OEMs.

As a lithium complex based grease, AeroShell Grease 58 exhibits superior performance characteristics compared to AeroShell Grease 22, as well as MIL-PRF-81322 and SAE-AMS 3058 competitor alternatives in load wear protection.

Chart demonstrating Aeroshell Grease 58 performance properties

Corrosion can cause excessive or abnormal wear in bearings. AeroShell Grease 58 exceeds SAE AMS 3058 corrosion requirements.

Not all the popular, lithium-complex wheel-bearing greases meet these specifications or protect against the effects of the latest runway de-icing fluids.

Chart demonstrating Aeroshell Grease 58 performance compared to competitors with images of corrosion comparison

Grease Changeover

Airframe and grease manufacturers do not recommend mixing different grease types as they are not always compatible, but changeover is straightforward: 

  • Remove all the old grease from the bearing surfaces and internal cavities of the lubricated mechanism before applying the new grease. 
  • If this is not practicable, then purge the system by injecting the new grease until it has displaced the old product and only new grease is returned. 

Please consult your aircraft manufacturer’s maintenance manual for its recommended purging or changeover procedure.

The final note to remember is to check that the alternative grease is approved for your application. Although used in similar applications, different products carry different approvals. It is important to check that any alternatives are approved by the equipment manufacturer, and details can be confirmed by the Silmid team where necessary.