Synthetics range in composition types, which includes hydrocarbon based, polyglycols, organic and phosphate esters, and other materials. The common trait is that the molecules that comprise the base lubricant are chemically synthesized for targeted properties and a controlled molecular composition. For example, fire resistance requirements in aviation hydraulic systems will favor phosphate ester fluids. Lack of solubility of hydrocarbon fluids and gases in polyalkyleneglycol fluids prevents thinning of the lubricant in natural gas applications. For our short discussion here, the focus will be on synthetic hydrocarbon based fluids.
The decision to use more expensive synthetic fluids depends one whether performance requirements outweighs the added cost. Systems with severe service duty, extreme temperatures or extended drain intervals due to design or necessity tend to be the most common criteria for accepting the higher cost of synthetic hydrocarbon-based products.
Synthetic lubricants are stable in high and low temperatures. Operating environments that experience temperature extremes are particularly suited for these types of products. There is better low temperature fluidity as well as greater oxidation stability at high temperatures. Products formulated for arctic conditions are generally synthetic based. Gearboxes or bearings exposed to excessive heat due to service duty or environmental conditions benefit from synthetic lubricants by providing extend equipment life and reduced rain intervals. The combination of high heat and exposure to oxygen are environments where synthetics perform well. Synthetic lubricants do not breakdown as fast in equipment that is exposed to high temperature and air such as compressors and turbo chargers.
Besides better performance under temperature extremes, certain reservoirs and operating systems do not allow for convenient access or shutdown schedules for drain intervals. The lubricant in some equipment is expected to stay in service for periods that would be detrimental to conventional mineral oil based fluids.
Under certain conditions synthetic lubricants can provide reduced energy consumption, reduced maintenance and downtime, and increased power output. Synthetic lubricants would not necessarily provide a cost benefit or perform any better than conventional mineral oil based lubricants in environments where there is continuous exposure to contaminants or operating conditions that do not stress lubricant service life.
Before deciding to use a synthetic lubricant it should be determined if the performance improvement out weighs the higher cost. Factors to consider can involve the following:
- Severe cold temperatures
- Severe hot temperatures
- Extremes going from hot to cold during the duty cycle
Extended drain interval due to:
- Access and convenience
- Shutdown cost
Improved power consumption due to enhanced fluid film and lubricity
Excessive exposure to air/oxygen combined with high heat
As previously indicated, synthetic lubricants may not provide additional cost benefits over modern conventional lubricants in systems which are not operating under extreme or special service requirements, or have continuous exposure to contaminants.
David Doyle, CLS, OMA I, OMA II