Condition Monitoring of Gear through Oil Analysis

Laboratory Analysis

There are many test methods available to provide information about the condition of the lubricant in service of a gear box. The basic oil analysis should include the following tests:

Type of Oil
Industrial Gear Oil	Kinematic Viscosity at 40 degree C.in cSt	Water content in PPM or %age	Acid number mg KOH/g	Particle count and size distribution.as per ISO 4406.No of particle per ml	Oil Additive, Contamination and wear elements in ppm

In all cases, comparison of the results should be made to a sample of the new oil that was actually used in the equipment to be sure that the starting material was within the specified limits stated by the supplier. The baseline values of the new oil also should be based on analysis of the actual sample. Based on the results of the analysis, lubricant can be changed or upgraded to meet the specific operating requirements.

Viscosity

Viscosity is a key property of the gear oil. The oil might lose its ability to lubricate properly if its viscosity changes significantly. Oxidation of oil leads to an increase in viscosity. There are many other possible causes for an increase or decrease in viscosity. Kinematic viscosity can be checked by ISO 3104 / ASTM D445 test method. It is recommended to change the gear oil if its viscosity changes more than ± 10%.

Water Content

Water is a significant factor in lubricant degradation. When combined with iron or copper particles, water becomes even more powerful in attacking lubricant base stocks and additives. The adverse effects of water in oil include: Lubricant breakdown, through oxidation and additive precipitation, changes in viscosity affecting the ability of a lubricant to maintain the film thickness necessary to protect the lubricated surfaces and corrosion.

Water content can be checked by ASTM D6304 test method. It is recommended to change the gear oil when its water content is greater than 0.05% (500 ppm).

Acid Number

The acid number (AN) also referred as total acid number (TAN) test is one of the methods to estimate the amount of additive depletion, acidic contamination and oxidation. It may be noted that AN test does not directly measure the rate of oxidation, it merely measures the by-product of oxidation. It is also beneficial to trend AN to determine the rate of depletion of certain additives.

AN is the measure of acid concentration in petroleum products and lubricants. It is determined by the amount of potassium hydroxide (KOH) required to neutralize the acid in one gram of the sample. The standard unit of measure is mg KOH/g. The AN measurement detects both weak organic acids and strong inorganic acids. However, AN does not represent the absolute acid concentration of the oil sample. AN tests can be carried out by two titration methods: potentiometric and color-indicator. The potentiometric method (ASTM D664 / ISO 6619) uses a potentiometer to detect the acidic constituents and coverts it to an electronic read out. The output is plotted and analyzed to determine the inflection of the test method. The color-indicator method (ASTM D974 / ISO 6618) uses p-naphtholbenzein solution (orange in acid and green-brown in base). Once the acidic constituents have been neutralized by the KOH, the sample change from orange to green, indicating the end point. Acid numbers should not be allowed to increase more than +0.5 AN higher than that of new oil, and if +1 AN is spotted immediate action is required (i.e. if new oil has 0.5 AN, then 1.0 AN is alert and 1.5 AN is alarm value). Acid can be neutralized or removed from oil in different ways. The obvious is to use the alkalinity of the oil to neutralize incoming acid. This is done in gas and diesel engine lube oils. These oils utilize high base numbers (BN or TBN), i.e. new oil is having high BN.

Particle Count and Size Distribution

A good filtering system for the lubricant is very important. The design filtration level may vary, but filtration to a 12 micron or finer nominal particle size is a generally accepted value. Filtration finer than 12 microns is recommended when light turbine lubricants are used, particularly for higher operating temperatures.

Since particle contamination of oil is one of the main reasons for a machine to break down, monitoring the level of hard contaminants is vital. ISO 4406 establishes the relationship between particle counts and cleanliness in hydraulic fluids (common practice has extended the application of this standard to lubricants as well). The ISO 4406 method for coding the level of contamination of solid particles is a classification system that converts a given particle count into an ISO code.

The test methods used most frequently for counting particles are: automatic particle count according to ISO 11500 and manual particle count according to ISO 4407.

In automatic particle count method according to ISO 11500, the contamination level of a liquid sample is determined by automatic particle counting, using the light extinction principle. In this method, the particle concentration is reported at three sizes: ≥4, ≥6 and ≥14 μm

In manual particle counting method according to ISO 4407, particles deposited on the surface of a membrane filter are counted using an optical microscope. It includes particle counting by two manual methods and image analysis, using either transmitted or incident lighting systems. In this method, the particle concentration is required to be reported at three sizes: ≥2, ≥5 and ≥15 μm.

Elemental Analysis for Additives and Wear Metals

Some additives, such as antiwear and extreme pressure additives and rust, oxidation, and corrosion inhibitors, are consumed as they are used. When all of a particular additive has been consumed, the lubricant will be no longer be capable of performing as originally intended. Monitoring the concentrations of wear metals in a lubricant can indicate abnormal wear of the machine components if base line concentration data are available for comparison. Elemental analysis determines concentration of wear metals, contaminants and oil additives in a sample for condition monitoring. ASTM D5185 method is used for the ICP instrument. Generally, ASTM D5185 method is used for elemental analysis.

A total of 22 elements (Al, Ba, B, Ca, Cr, Cu, Fe, Pb, Mg, Mn, Mo, Ni, P, K, Na, Si, Ag, S, Sn, Ti, V, Zn) can be determined by the ASTM D5185 test method - “Standard Test The test methods used most frequently for counting particles are: automatic particle count according to ISO 11500 and manual particle count according to ISO 4407. Method for Multi-element Determination of Used and Unused Lubricating Oils and Base Oils by Inductively Coupled Plasma Atomic Emission Spectrometry (ICP-AES)”.

Typical wear metal elements, contaminants and additive elements are as under:

• Wear metals are: Aluminium (Al), Boron (B), Copper (Cu), Chromium (Cr), Iron (Fe), Lead (Pb), Silicone (Si) and Tin (Sn)

• Contaminants are: Silicone (Si) from dirt, Sodium (Na), Potassium (K) and Boron (B) from coolant

• Additive elements are: Boron (B), Barium (Ba), Calcium (Ca), Magnesium (Mg), Manganese (Mn), Phosphorus (P), Sulphur (S), Silicone (Si) and Zinc (Zn)

It may be noted that some elements are used as both additives and wear metals. Monitoring their content is then crucial because a decreasing or an increasing value will induce different actions. When additives are consumed as they are used, usually lubricant requires replacement. However, in some cases replenishment of the additives is possible. The lubricant manufacturer should be consulted before this is attempted.

Note: It is recommended to consistently use the same lab and test method for a specific lubricant analysis to eliminate reproducibility error. As per ASTM, reproducibility is "the difference between two single independent results obtained by different operators working in different laboratories on identical test material."

Abnormally High Temperature Oil level too high- If the oil level in a gearbox is so high that the gear runs in the oil, then the resulting churning action will heat the oil.

Hot weather- Obviously, a high ambient temperature will cause abnormally high oil temperature. To prevent this, provide adequate ventilation around the drive.

Low oil pressure (in case of a forced lubrication system) - If the oil flow to the bearings and gear mesh is below normal (indicated by below normal oil pressure), the heat created by friction at the mesh and bearings will cause abnormally high temperatures. To correct this situation, check the lubrication system for proper operation. Low Oil Pressure Use of a lubricant which has a viscosity less than that for which the lube system was designed.

Many times several orifices are installed in the lube system. They are sized for lubricants with a particular viscosity. A lubricant with less than this normal viscosity will pass through the orifices without building up pressure. This situation can be prevented by using the lubricant designated on the name plate of the gear unit. Abnormally low viscosity may also result from high lubricant temperatures.

Clogged oil filter- Replacing the filter will allow more oil to flow through it, thus bringing the oil pressure back to normal. Pump cavitation - Should the oil level in the reservoir get so low that the pump suction line sucks both air and oil, then the oil pressure will drop. This problem may be cured by maintaining proper oil level in the reservoir.

Air leak in the suction line to the pump- This situation is similar to pump cavitation in that air gets in the oil and results in low oil pressure. To remedy this problem, check and tighten all pipe fittings in the suction line.

Incorrect relief valve setting- Adjusting the relief valve setting properly will avoid venting the pump discharge line back to the sump.

Unusual or Excessive Noise

Worn parts - One common cause of unusual noise is worn parts. If a part wears enough to cause slack in the system, the slack may be heard as a rattle or noise of some sort. A mechanic's stethoscope may be used to pinpoint the worn part which should be replaced.

Tips for Visual Inspection of Gear box

Visual inspection of gears for wear/damage and tooth contact pattern through an inspection port should be carried out periodically. Detecting a problem in its earliest stages can save time and money in the long run. If the gears or bearings are damaged but still functional, you may decide to continue operation and monitor damage progression. In this case, the gearbox should be continuously monitored. You should also make certain that there is no risk to human life. Many times gearboxes operate in dirty environments. Therefore, areas around inspection ports should be cleaned before they are opened for inspection. Inspectors should take care not to drop anything into the gearbox. Inspection ports should never be left open during breaks and should be closed after the inspection is complete.