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Friday, 24 January 2014

Selecting A High Temperature Lubricant.

When selecting a high temperature lubricant there are many factors to consider!

Selecting a high temperature lubricant is not merely a case of looking at the viscosity rating (for example the 20W50 rating of a multigrade oil): The type of base stock, that constitutes about 80% of the oil bulk, plays a vital role in determining the temperature that the oil can operate at.

It’s common knowledge that when selecting a high temperature lubricant synthetic oils are more stable at higher temperatures and don’t oxidise as readily.


Synthetic oils were originally developed more than 50 years ago and became widely used in jet engines. Operating at ambient temperatures lower than minus 50ºC, 60000 shaft rpm, and +260ºC exhaust temperatures proved too much for conventional oils. Synthetics were created specifically to withstand these harsh conditions and to date every jet engine in the world uses synthetic oils.
 Petroleum base stock oils on the other hand have molecules arranged in long carbon chains that are sensitive to stress and heat. Additionally, various paraffins that are contained in all petroleum products regardless of how well refined they are, cause lubricants to jell like a syrup at extremely cold temperatures. At the other end of the temperature spectrum, high operating oil temperatures and heavy loads cause these chains to break down and the base stock actually boils off causing a change in viscosity and the formulation of sludge.

This can happen at continuous temperatures as low as 100º C and by 120º C many petroleum oils are suffering significant breakdown, which makes them unsuitable as a high temperature lubricant.

Synthetic oils on the other hand are engineered specifically to provide all the lubricating properties that natural oil possesses, but none of the cold thickening or hot thinning properties of petroleum oil.

Synthetics are made up of uniformly shaped molecules with shorter carbon chains which are much more resistant to heat and stress. Synthetic oils can withstand temperatures of 150ºC all day long and still protect your equipment.

What to look for in a high temperature lubricant.

In order to ensure continued performance of a high temperature lubricant, the major factors are:
  • Viscosity 
  • Thermal degradation 
  • Oxidation.
 In a perfect world, if you focused solely on temperature to drive lubricant selection, you would want a oil that would flow at low temperatures while still providing the protection at high temperatures (very high viscosity index), that would not thermally degrade or “cook” onto hot machine surfaces and leave deposits, and that would not oxidize at elevated temperatures. This means that a hydrocarbon-based lube would not be an option.

Even polyalphaolefin (PAO) synthetics, which are the purest and best-performing of the hydrocarbon base oils, cannot withstand continuous temps of 200 degrees C. Their initial thermal degradation temperatures are higher than this, but they oxidize very rapidly at these elevated temperatures, effectively making their lifespan only a few hours which means they’re not a suitable for use as a high temperature lubricant.

Effective lubrication is only available at temperatures below 170 degrees C. Anything above this temperature will almost always require a compromise to be made. These compromises come in the form of reduced life, lower load-carrying capacity, slower speed, higher levels of friction, difficulties in application, compatibility issues, etc.

The use of additives won’t improve the high temperature performance of the oil, but can be reduced by the presence of less stable compounds found in the oil including additives. The following are thermal decomposition temperatures of some commonly used base oils:

 
Oils that operate above these temperature ranges, are usually solids at normal temperatures, which poses a huge problem for transferability.

Selecting the best high temperature lubricant.

As far as lubricant selection, there’s a wide selection for temperatures below 100 degrees C, which makes the decision more difficult. However, the choice of a high temperature lubricant to operate at temperatures above 400 degrees C is severely restricted.

The only oils with long life at these temperatures are liquid metals, liquid oxides, glasses and a few solid oils like molybdenum disulfide.

To increase reliability and reduce downtime and operating costs it’s highly recommended that you use a quality high temperature lubricant such as those supplied by Habot Synthetic Oils. To talk to one of our team of professionals call us now.

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