Grease: Selection matters

 

Choosing the right grease can play a key role in helping to protect equipment and ensuring a trouble-free operation. However, when making that selection, it is vital to not only consider the quality and performance of a product, but also the requirements and challenges of the specific application. 

 

Operations that function at extreme temperatures and under intense pressure pose a number of lubrication challenges which can only be met by technology that is specifically designed to meet such requirements. Factors such as base oil type, viscosity, thickener type, stability of the composition and operating temperature must all be considered. 

 

Grease formulation

It is important for maintenance professionals to understand how greases are formulated in order to ensure that they select a grease which will deliver the performance required to protect their equipment in the long term. 

A simple way to think of grease is to consider it like a sponge soaked with lubricating oil. Upon application of external stresses, such as heavy loads or high temperatures, the thickener (sponge) releases the oil to lubricate the mechanical parts. When the stress is removed, the thickener re-absorbs a portion of the released oil for later use.

Typically, greases are applied to mechanisms in which a lubricant cannot stay in position, or re-lubrication is infrequent, difficult, or simply not economical. A few examples include - drive shaft couplings, universal joints and fan shaft bearings.

Selection

In extreme temperature applications, there is a risk that a grease will perform poorly due to degradation resulting from thickener and base oil oxidation, or due to the loss of base oil from grease bleed and evaporation. One of the driving factors that can limit the ability of a grease to provide lubrication at higher temperatures is proper viscosity and oxidative resistance. Oxidation is a chemical reaction that occurs between oxygen and the in-service lubricant, and is accelerated in high temperatures.  

A grease’s oxidation rate generally doubles with every 15°C rise in temperature above 60°C. This rule-of-thumb varies, based on the type of thickener used in the grease, and for soap thickeners, the amount of metal contained in the structure (lithium, calcium, aluminum, etc). 

Typically, greases with synthetic base oils can provide a wider operating temperature range than conventional, mineral-based greases. For example, the Mobil SHC Polyrex Series is a range of synthetic bearing greases that uses advanced polyurea thickener technology in order to achieve excellent high temperature performance up to 170°C.

Even at extreme temperatures, the polyurea thickener technology resists oxidation and loss of structural stability allowing re-lubrication intervals to be extended while maintaining equipment protection when compared to competitor mineral oils. This carefully balanced combination of thickener, base oils, and additives yields a grease with efficient load carrying capabilities and rust protection.

Boosting productivity

An aluminum plant in Germany is one of the companies to benefit from the use of advanced greases in a high temperature environment. To protect wheel bearings, the plant had been using a grease with a base oil viscosity of 100cSt and had been experiencing routine bearing failures and significant mechanical wear and tear due to lubricant degradation as a result of excessive heat. The plant was also incurring high lubricant costs.

ExxonMobil was asked by the plant’s maintenance team to help provide an analysis of the plant’s operations and equipment maintenance practices to help address the issues. Following the audit, the ExxonMobil team made specific recommendations to convert the lubricant protecting the wheel bearings to Mobil SHC Polyrex 462, in addition to implementing a best practice programme to help increase re-greasing intervals.  

The higher base oil viscosity offered by Mobil SHC Polyrex 462 provided the machinery with improved levels of protection, resulting in a noticeable reduction in component damage and an annual saving of €39,000*.

With ongoing developments in machinery to increase output, and the high ambient temperatures of many industrial sectors, lubricants that can operate effectively in extreme conditions will help companies build a competitive advantage into their operations by reducing downtime and increasing productivity.

 

* This proof of performance is based on  the experience of a single customer. Results can vary depending on the type of equipment used and its maintenance, operating conditions and environment, and any prior lubricant used.