Perhaps you've felt like a child in a candy store when wanting to choose synthetic gear oil out of a glitzy pamphlet? How frequently would you have paid for someone to teach you how to choose the best type of synthetic gear oil? Continue reading... In this 3 part series we’ll look at the subject in depth.

Just like synthetic lubricants in any application, synthetic gear oil is typically used when crude based gear oil has reached the performance limit and can no longer satisfy the application demands; for instance, at very low or high temperatures, higher than normal loads, extraordinary ambient conditions, or if they fail to meet special conditions such as flammability.

Although additives can enhance several attributes of crude based oils,
it is extremely hard to modify all their qualities. This applies especially to physical attributes including following:
• evaporation losses
• thermal resistance
• flash point
• decreased temperature qualities (fluidity, pour point)
Synthetic gear oil ensures several benefits. Nevertheless, it doesn't necessarily out-perform mineral oil in all aspects and might even result in several drawbacks in spite of the positive aspects.

The advantages of synthetic gear oil (depending on the base oil) include:

• lower evaporation loss
• improved lubricity (in some cases)
• enhanced thermal and oxidation stability
• bettered viscosity-temperature performance, high viscosity index (frequently)
• enhanced low temperature properties
• diminished flammability (in some instances)
• lower propensity to form residues
• increased resistance to ambient media

Possible disadvantages of choosing synthetic gear oil include things like:

• restricted miscibility with petroleum based oils
• greater price
• reactions with water (hydrolysis, corrosion)
• material compatibility problems (paints, elastomers, specific metals)

Application-associated benefits frequently prevail, increasing the usage of synthetic lubricants as gear lubricants, particularly under critical operating conditions. The most typical synthetic varieties being used include synthetic hydrocarbon oils (SHC), polyglycols (PAG) and ester oils (E).
So, what are the various types of synthetic gear oil offered.

Lubricating Oils Based On Synthetic Hydrocarbon Oils
Synthetic hydrocarbons are similar to petroleum based hydrocarbons in their chemical composition. They have nearly identical attributes pertaining to their compatibility with sealing materials, disposal, reprocessing and miscibility with petroleum based oils. The primary benefit in a synthetic gear oil is the exceptional low temperature behavior.

Lubricating Oils Based On Polyglycols
These lubricants ensure especially low friction coefficients, which makes them appropriate for gears with a higher sliding percentage (worm and hypoid gears). With the correct additives, they present superb antiwear protection in steel/bronze worm gears, and have a very good extreme pressure performance. In gear systems, higher polarity polyglycols permit more interaction on the metal gear surface. This provides polyglycols mild extreme pressure performance even without additives.

Polyglycol oils may have a severe effect on sealing materials and may possibly dissolve some paints. At operating temperatures above 212°F (100°C), only seals made of fluorinated rubber or PTFE are resistant. Before making use of PAG oils in production applications, it might be useful to test compatibility with paints, seals and sight glass materials.

Miscibility with petroleum based oils is limited; mixtures must therefore be avoided. Polyglycols are neutral toward ferrous metals and almost all nonferrous metals. If the application has a loaded contact with one element consisting of aluminum or aluminum alloys (rolling bearing cages containing aluminum), there could be greater wear under dynamic load (sliding movement and substantial load). In such circumstances, compatibility tests must be carried out prior to moving over to this type of synthetic gear oil. If a worm gear is made of an aluminum bronze alloy, polyglycols should not be used because the reaction in the load zone may lead to greater wear.

Lubricating Oils Based On Ester Oils
Ester oils are the result of a reaction of acids and alcohols with water cracking off. There are many different kinds of esters, all of them having an affect on the chemical and physical attributes of synthetic gear oil. Previously, these kinds of lubricating oils were primarily used in aviation technology for the lubrication of aircraft engines and gas turbines in addition to gear systems in pumps, starters, etc.

Ester oils have a high thermal resistance and extremely good low temperature performance. In industrial applications, quickly biodegradable ester oils will gain importance merely because it seems possible to achieve the same results as with polyglycol oils by selecting an appropriate ester base oil.

Some ester oils may exhibit reduced hydrolytic stability. Hydrolysis is the cleavage of the ester into an alcohol and an acid in the presence of water. Ester lubes should be hydrolytically stable because they are often exposed to moisture in use. In practice, hydrolysis might be a less serious problem than often reported. The hydrolytic stability of an ester would depend on:
• how the ester was processed
• the sort of ester being used
• the sort of additives being used
• the application

Now that we understand a bit more about the candy in the store, next week we’ll look at the advantages of selecting a synthetic gear oil.