As modern machinery becomes more complex, production and maintenance engineers are having to think harder about what lubrication management processes to adopt, including having to make crucial decisions on whether to choose synthetic or mineral oils. Both types of lubricants have their strengths and weaknesses, and so it is important to get it right.
Mineral oil, obtained from the separation of crude oil, can be unreliable due to its natural and unpredictable structure, the fact that irregularities in its molecules can actually generate friction, and its tendency to form sludge at high temperatures. They can also enable oxidisation.
However, mineral oils are not without benefits. Generally cheaper than synthetic oils, mineral oils are also typically more straightforward to dispose of or reuse, with an existing infrastructure in place. As a traditional lubricant, they are compatible with seals, metals and coatings which may be damaged by synthetic alternatives, and can also be further chemically manipulated after distillation to produce ‘semi-synthetics’, mineral-based oils with enhanced operational benefits and a more stable structure.
Synthetic oils of various types, meanwhile, are generally specified for machinery operating at very high temperatures; to cope efficiently with low start-up temperatures; for their low volatility and flammability properties; and for the reduced risk of residue build-up and evaporation loss. However, mineral oils generally outperform synthetics in terms of their solvency, hydrophilic properties and compatibility with components including seals and surrounding materials. Additionally, on less advanced machinery working at standard temperatures, for example, synthetic oils may be an unnecessary expense with traditional mineral oils fulfilling requirements.
The decision to adopt synthetic oils should not be taken therefore on a plant-wide basis, but after detailed evaluation of each individual piece of equipment and the components and materials within which are also affected by lubricant choice. An efficient lubrication management system is tailored to each machine based on its own operating conditions and so, in some cases, a mineral oil will still be the most appropriate option.
If, however, the application demands a lubricant that exceeds the performance properties offered by mineral oils, there are various synthetic options offering particular benefits for different purposes.
Synthetic oils are, as the name suggests, based on chemicals rather than refined crude oil, and offer more predictable behaviour due to their highly engineered nature. More common, however, are partial synthetic and semi-synthetic oils which combine the desirable properties of true synthetic and mineral oils to create customised lubrication solutions for individual pieces of equipment. The blending of synthetic and mineral oils creates partial synthetics, while semi-synthetics are distilled from crude oil, then given further chemical manipulation to remove unwanted elements and compounds. Within these sub-groups lie several different classifications of synthetic oils, based on the chemicals used in their formulation, and each offers different characteristics and performance.
Ideal for gear lubrication, compressors, gas turbines, automotive engines and various aviation applications, synthetic lubricants based on polyalphaolefins offer stability at high temperatures, a high viscosity index, low temperature fluidity and low volatility. They are also one of the few synthetics to resist reaction with water, thanks to their pure hydrocarbon structure which repels water. The high natural viscosity index offered by this class of synthetic lubricant offers a high resistance to shearing, and if treated with a small amount of antioxidant material, polyalphaolefins also become more stable than minerals at comparable temperatures – making them less likely to form potentially problematic deposits.
Ester-based fluids, including organic and phosphate esters, are a blend of acids and alcohols with a chemical structure which attracts them to metallic surfaces – resulting in highly effective boundary lubrication. Phosphate esters also offer excellent fire resistance and the ability to perform under extreme pressures. In fact, oils in the ester-class offer the lowest volatility of all the synthetics making them a safe option for hazardous applications. Organic esters, meanwhile, have a high thermal stability which contributes to engine cleanliness and reduced deposits. Throughout the class, esters are derived from such a wide variety of raw chemicals that truly bespoke lubrication solutions can be created by varying the compounds used. Phosphate esters in the past have caused concern over toxicity and potential disposal hazards, however they are biodegradable and with organic esters in particular, it is possible to engineer a higher degree of biodegradability if required.
Originally designed as a replacement for castor oil as a brake fluid, polyalkylglycols are still used for this purpose, but have also found favour as circulating or compressor oils and as hydraulic fluid in fire-resistant applications. With a very low friction coefficient, lubricants in this class are also ideal for gears with a high sliding percentage. Stable at extremes of both low and high temperature, they are ideal for aviation applications, but are also widely used in steel production and mining thanks to their fire resistant properties. As a gear lubricant they are also used in an entirely different production environment – as a water-soluble oil, they are easily washed off fabric and are therefore favoured by the textile industry. However, this also means that polyalkylglycols must be protected from moisture contamination when in operation, or their antiwear characteristics will be compromised. Polyalkylglycols also reduce residue left on machine surfaces when compared with mineral oils, due to the fact that they burn clean and actively dissolve deposits.
A range of additives can produce polyalkylglycol-based synthetic lubricants with a variety of properties. For example, thermal stability can be improved up to 250ºC while antioxidative additives can provide oxidative stability. Another application of this class of synthetic oil is in water treatment plants, dredgers or other applications where the lubricant may unavoidably enter water courses, due to their low toxicity. However, polyalkylglycols may cause damages to seals especially at high temperatures, and can dissolve paints and other coatings, meaning that compatibility testing is vital.
Generally, synthetic lubricants are an ideal choice for all types of gears, including worm, hyphoid, spur and helical gears, due to the fact that synthetics have a lower friction coefficient than mineral oils. This reduces tooth-related friction, increasing the efficiency of the gear and reducing the operating temperature, reducing replacement and repair costs.
A further benefit of all classifications of synthetic oil is the extended working life span – something which, in many cases, offsets the higher initial cost of the lubricant. The oil change intervals of synthetic products may be three to five times longer than for mineral oils in an equivalent application, offering reduced maintenance costs. Additionally, evaporative loss is vastly reduced when using synthetics over minerals which can justify the often increased disposal costs associated with synthetics.
So there’s a lot to consider. As part of an overall maintenance and machine reliability programme, regularly assessing the lubrication requirements of production equipment can contribute to a more efficient facility. Detailed analysis of the working conditions of each piece of equipment, including force-speed factors and the viscosity-temperature behaviour of different oils, will enable careful selection of the most appropriate formulation for each application.
Mark Needham is an asset integrity specialist at AV Technology