Guidelines for selecting Gearbox Oils

Ideally, it is best practice to inspect and analyze an equipment on-site before recommending any lubricant. Many manufacturers provide guidelines for lubricant selection but fail to look into some crucial elements. The maximum operational reliability of a Gearbox is not only dependent on operating materials but choosing the right lubricants during all design phases as integral structural elements.

Factors that play an important role in selection:

• Gearbox type: Dimensions, the metal used for teeth, sliding properties and gear ratio, open or closed casing
• Operating conditions: Depending on the load, vibration and shock levels, anti-wear and extreme pressure properties
• Service oil temperature: Very low, moderate (+20°C and +80°C), hot (+80°C) and extreme (+120°C)
• Machine environment and maintenance conditions

Once these factors are accounted for, the following criteria for gear oil selection come into the picture:

• Viscosity – One of the most important properties of lubricating oils
• Additives – This determines the category and affects specific performance properties under operating conditions
• Base Oil – Determines the overall operating conditions, gear type and other factors.

VISCOSITY:

This factor is of utmost importance as it determines the formation of the lubricant film. It determines the film thickness between interacting surfaces at a given speed and load. It is difficult to ascertain the ‘load’ for most viscosity selection methods. Hence the ‘load’ is assumed, and ‘speed’ becomes the determining factor.

The degree to which viscosity changes with temperature depends on the base oil, such as mineral oil, polyalphaolefin, ester, polyglycol, as well as on the VI improver additives contained in the lubricant.

One of the most common methods for determining viscosity is the ANSI (American National Standards Institute) and AGMA (American Gear Manufacturers Association) standard ANSI/AGMA 9005-E02. Load, viscosity index and the pressure-viscosity coefficient of the lubricant are the factors used in this method.

ADDITIVES:

While there are many variations, gear lubricants fall into three general categories: R & O, antiscuff and compounded. Operating conditions determine the choice of additives in gear oils.

R&O stands for Rust and Oxidation. Anti-corrosion properties of gear oils are assessed individually for corrosion protection on steel and copper.

The presence of water due to leakage or condensation can lead to combination with ambient oxygen resulting in rust on inadequately protected steel surfaces. Additives that contain polar rust inhibitors form a compact and protective, water-repelling layer. R&O gear oils generally perform well in the categories of chemical stability, corrosion prevention and foam suppression.

Antiscuff; commonly referred to as extreme pressure (EP) lubricants have some performance capabilities that exceed those for R&O oils. In addition to the properties listed for R&O lubricants, antiscuff lubricants contain special additives that enhance their film strength or load-carrying ability.

Compounded; a compounded lubricant is mixed with synthetic fatty acids to increase its lubricity and film strength. The most common application for these gear lubricants is worm gear applications.

BASE OIL:

The quality of the base oil is a crucial factor in determining the performance of your gear oil. High-quality mineral base oils perform well in most applications. Mineral-based oils typically have higher pressure-viscosity coefficients than common synthetics, allowing for greater film thickness at given operating viscosities. There are, however, situations where synthetic base oils are preferable. Gear lubricants based on these synthetic oils have proven to be most effective: polyalphaolefin (PAO), polyglycol (PG) and ester (E)

Are you wondering what to choose between Synthetic or Mineral base oils? Stay tuned for our next blog where we break down this question.

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