It’s all about the thickness

The most important factor in lubrication selection is oil film thickness. Gearbox manufacturers do not specify film thickness — they specify viscosity. However, film thickness is a function of viscosity, pressure, temperature, relative velocity, surface roughness, and other factors.

Insufficient film thickness can lead to an increase in abrasion, pitting and adhesion wear. If too high a viscosity is selected, gearbox-churning losses will lead to higher oil temperatures, and consequently shorter oil and additive life.

All else being equal — geometry, velocity, load, and viscosity — some base oils will provide a thinner oil film. Mineral oils will provide the thickest film, followed by PAO, PAG, and esters [3, 5]. For a given viscosity grade (e. g., ISO 320) the synthetic base stock will offer higher viscosity at operating temperature, but film thickness may be thinner than if using a mineral base oil.

If changing from mineral to synthetic, one cannot assume that the same viscosity grade will be acceptable. This in no way implies that synthetic oils always have thinner films or inferior performance.

In practice, for a given temperature, mineral and synthetic oil will not have the same viscosity. The synthetics’ higher viscosity at elevated temperatures will offset some or all of the inherent film reduction. Conversely, mineral oil’s poor viscosity index implies higher viscosity — and thicker oil films — at lower temperatures.

If choosing between a mineral and synthetic, one must weigh synthetics’ cost disadvantage against its superior thermal stability, high viscosity index, and oxidation resistance.

Though synthetic oils may offer extended life, they may not offer extended drain intervals on unfiltered lubrication systems. Independent of a lubricant’s remaining useful life, contaminants and debris accumulation may require the same oil change intervals as mineral oil.

If the user requirements dictate operation over a wide temperature range, or oil changes are impractical, then synthetic offers a clear advantage over mineral.

The next thing to consider is the additive package. Inhibited oils are often used in hydraulic cylinders or other applications where there will always be hydrodynamic lubrication. Hydrodynamic lubrication is not always present in gearboxes during starting, reversing, or when experiencing shock load.

AW (anti-wear) oils have chemically mild additives that will form weak bonds with metal and help separate load bearing surfaces under boundary conditions. AW oils offer minimal corrosion and the additives activate at lower temperatures than EP (extreme pressure) additives. EP additives can provide protection under very heavy and shock loading conditions.

Below their relatively high activation temperature, EP additives are inert. When local temperatures are high enough for the additives to bond to metal, they offer superior protection against scuffing, but at the expense of corrosive wear. Under light loading conditions and lower temperatures, EP oils, due to corrosion, may offer increased wear compared to AW oils.

Gearboxes in applications where there may some incidental contact with food will require National Science Foundation (NSF) H1 class lubricants. The base oils may be mineral or synthetic, but there are significant restrictions to the additives. Many common EP additives are not allowed. Higher viscosity oil may compensate.

This article has been extracted from a white paper, ‘Lubrication Technology’, prepared by Mark Lee Johnson, product engineer, Sumitomo Drive Technologies. The complete white paper provides basic lubrication information for industrial gearbox oils and examples of specific recommendations for Sumitomo products.

Basic properties, terms from specifications sheets, and maintenance information are also included. For more information, visit www.sumitomodrive.com.

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