Gearbox Oil Analysis

Oil analysis is a highly advantageous tool for gear systems used in both industrial and mobile equipment. Despite being designed to be highly reliable, they bring about disruption and incurred costs when they wear or break due to contamination or poor operation. Oil analysis helps in detecting the developing failure conditions, and fundamentally, a majority of the gear manufacturers recommend condition monitoring, including oil analysis.

Gear wear can be intensified by poor lubrication, misalignment and contamination. When the gears are worn exceeding their operational tolerances, jamming, gear slippage, tooth breakage, or grinding can take place, requiring a replacement or overhaul, leading to considerable downtime.

In general, gear drives can be categorized into two classes from a lubrication methodology— pressure-lubricated and splash-lubricated. In splash- or bath-lubricated gearboxes, application of lubricant is done by permitting the gear to run partially submerged in the oil. In pressure-lubricated systems, oil taken from the gear case is pumped through a filter, heat exchanger and pressure relief valve, and delivered back to the unit under pressure. Spray nozzles in a manifold are used for applying oil to the system. Oil analysis enables both types of lubricated systems to be monitored; however, the tests may slightly vary.

Recommended oil analysis test packages for gear drives

Viscosity

Viscosity is the resistance of a fluid to flow and is the most significant lubricant physical property for gear drives. It is necessary for lubricants to have appropriate flow properties to ensure that a sufficient supply of oil reaches lubricated parts at different operating temperatures. The viscosity of lubricants differs based on their grade or classification, and also the degree of oxidation and contamination during usage. If the lubricant’s viscosity varies from nominal grade by over 10%, the lubricant supplier usually recommends for an oil change. It is anticipated that oil viscosity increases over time and use; a drop in viscosity is regarded to be more serious compared to an increase. Hence, a working alarm range is +20% to −10%, that is, not less than 10% or not more than 20% of the nominal grade.

Ferrous Density

Ferrous density is a measure of the total amount of ferrous magnetic debris that exists in ppm and is measured using a magnetometer. Ferrous debris that ranges in size from sub-micron to visible will initiate a change in the electrical current proportional to the amount of metal that exists. Trending the amount of total ferrous debris is an important indicator for any gearbox and should be included on all screening test packages. Trending of the actual value in ppm is performed. A 10% increase in the wear rate suggests an abnormal change.

Water

The presence of water in oil is generally undesirable and can be visually detected in the presence of gross contamination (giving a cloudy appearance). Excessive water in a system destroys the potential of a lubricant to separate opposing moving parts, enabling severe wear to occur with ensuing high frictional heat. For a majority of the gear drives, water contamination should not be more than 0.25%; however, certain pressurized systems have lower limits.

Oxidation by Infrared Analysis

Infrared spectroscopy is a proven method for the detection of water, organic contaminants, and oil degradation products in a used oil sample. Over the lifetime of a lubricant, oxidation products pile up, resulting in degradation of the oil and leading to slightly acidic oil in a majority of the instances. Severe oxidation leads to corrosion of the critical gear surfaces by the lubricant. The “oxidation number” is directly proportional to the oxidation level. In systems with oxidation problems, conditions such as sticky rings, varnishing, lacquering, sludge deposits, and filter plugging take place. Infrared spectroscopy also suggests contamination as a result of free water and glycol antifreeze. Manufacturers have provided various guidelines for oxidation numbers and liquid contaminants, but this is typically a trending tool.

Total Acid Number

Total acid number (TAN) is a titration technique developed to detect the relative acidity in a lubricant. It is used as a guide to follow the oxidative degeneration of an oil in use. Oil changes are usually indicated when the TAN value attains a preset level for a given lubricant and application. A sudden drop in TAN would be suggestive of abnormal operating conditions (for example, overheating) that must be investigated. A majority of the lubricant suppliers provide TAN condemnation limits in the bulletins. In general, an increase of 0.5 over the starting value is a cause for concern. It is always advisable to know the starting new oil value—it can be higher than anticipated for certain gear oils due to the presence of the additive packages.

Particle Count

Particle count is a technique used for counting and categorizing particulate in a fluid according to accepted size ranges, often based on ISO 4406 and SAE 4059. This test is very useful for improving the reliability as the reduction in particulates in the oil will increase the service life of the gearbox. It is recommended for pressure-lubricated gears and is regarded to be optional for bath-lubricated gear systems as it is not prudent to measure particle count in the absence of filters, or if there is no plan to filter the oil (using a cart, etc.).

Ferrous Particle Count

Ferrous particle count is a method for quantifying the existing ferrous debris based on quantity and size, and not based on its concentration. Intrinsic magnetometers are usually used at present to measure events when oil flows through the magnetometer. Direct-read ferrography systems are also employed for monitoring and evaluating ferrous wear. These systems are advantageous in gaining insights into the size and quantity of ferrous debris particles and they enhance the spectroscopy and ferrous density methods used for measuring the ratio of large and small ferrous particles in the sample. This data can be used to calculate the severity indexes and the wear particle concentration and to set alarms when these limits reach a specific level.

Elemental Spectroscopy

Elemental spectroscopy is a method for detecting and quantifying metallic elements in used oil that occur due to contamination, wear, and additives. Energization of the oil sample is performed to make each element emit or absorb a quantifiable amount of energy, which denotes the concentration of the element in the oil. The results indicate the concentration of all dissolved metals (from additive packages) and particulates. This test is fundamental for all on-site and off-site oil analysis tools since it offers information on contamination, machine, and wear condition in a comparatively accurate and rapid manner. Its significant drawback is that it has a poor particle detection efficiency for particles with a size of 5 μm or more—the reason for the ferrous density to be measured first.

Wear Debris Analysis/Analytical Ferrography

Wear debris analysis (WDA) is defined either as an analytical or patch technique that isolates magnetic wear particles from the oil and deposits them on a glass slide called ferrogram. Microscopic investigation of the patch or slide enables the characterization of the wear mode and plausible sources of wear in the machine. This method is called analytical ferrography. It is an exceptional indicator of abnormal non-ferrous and ferrous wear. The major disadvantage of this method is that it usually requires a trained analyst.

This information has been sourced, reviewed and adapted from materials provided by AMETEK Spectro Scientific.

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