Engineers have traditionally considered vibration analysis to be the primary method for condition monitoring, whereas, oil and wear debris analysis is considered to be the sole province of the chemist. Lubricants should, however, be considered as much an integral part of a machine as any of its mechanical components and it must perform many vital functions over extended periods of operation.
By testing a sample of lubricant from the machine it is possible to measure the lubricant’s ability to continue to perform its original function and also to obtain information on the operation and condition of the machine. There are now many references, which cite the efficiency of oil and wear debris analysis to detect equipment deterioration and damage prior to its detection by vibration analysis.
Many different tests are available to measure oil condition and oil contamination. Simple sensory tests, appearance, colour and odour may often be adequate. If however, these are unsatisfactory, supplementary tests must be made to determine whether the equipment or the lubricant is in a condition suitable for further service. Most of these tests use standard analytical procedures providing the data for the engineer, who is familiar with the equipment being monitored, to be able to resolve their significance.
The lubricant must be considered as an integral part of a machine. It will lubricate by forming a fluid film between loaded surfaces, act as a coolant in removing heat, carry away contaminants, act as a hydraulic medium, protect again rust and corrosion, protect against the accumulation of sludge, varnish and similar deposits and resist aeration and foaming. Deterioration of the lubricant’s properties caused by degradation or build up of contaminants due to either ingress or wear of the machine surfaces, will lead to deterioration of machine performance and eventually machine failure.
Testing of lubricants will ensure the machine is clean, the correct lubricant is in use, and that maintenance practises can be progressively improved.
Sampling should commence from the initial commissioning of the equipment wherever possible. Power generation, steel mills, paper mills often have circulating systems which may have an initial charge of may thousands of litres of lubricant or hydraulic fluid which can last for five, ten or more years. It is therefore important that the system is clean and that the fluid is used properly from the time of installation.
Frequency of sampling is dependent on the operating critically of the machine. In the first months of operation the monitoring interval should be short so that a database of information be created for each machine component. A typical sampling frequency for general industrial equipment should be every month for the first six months, then reviewed and adjusted. A sampling schedule for mobile plant equipment would be every 250 hours of operation, whereas transport, such as buses and lorries would be sampled at 2500Km/s intervals with adjustments made to suit the duty cycles.
An efficient condition monitoring programme requires a sample that is representative of the system from which it was taken. The sample must also be uniquely identified by machine, component and sample interval. Details of oil changes, service repairs and operational environment should also be recorded to the sample.
Ideally, sampling valves are designed into the system so that a representative sample may be drawn off through a valve without risk of external contamination. Where this is not possible, a vacuum pump or syringe with flexible tubing attached is an acceptable alternative. The tube should be carefully inserted into the reservoir or sump to sample from just below the fluid surface. When particle counting is a requirement a glass bottle is the preferred container. Clean sample bottles are necessary due to the sensitivity of particle counting to background effects.
Simple sensory tests such as appearance, colour and smell will provide an indication of the lubricant condition. A comparison should be made using either a glass bottle or test tube between the used oil and a sample of the original unused oil.
Water or refrigerant contamination will cause the oil to appear hazy. Water content in excess of 0.1% will bubble or crackle when a few drops of the oil placed on a hot plate heated to 120°C.
Darkening of light coloured oils may signify oxidation or contamination with dark coloured products. Used oils from gasoline and diesel engines may show a variety of colours ranging from light brown through red-brown to black. Colour assessment in these cases is not particularly reliable, although an intense brown colour may be indicative of lacquer and a clear black colour indicative of soot rich oils.
An acrid smell with dark colouring and thickening of the lubricant would indicate serious oxidation.
A further simple check of oil degradation is the use of the blotter spot check. This test which is particularly useful for testing engine crankcase lubricants requires a drop of oil deposited on a piece of blotting paper which will show a distinct ring when dry.
Examination of the system filter is another simple test procedure that is overlooked. The debris trapped by the filter is a historical accumulation of the debris generated by the system. Debris on the filter may be washed with petroleum spirit and examined using a low powered magnifying glass. Identification of the debris will then provide valuable information concerning the operation of that equipment.
The particles of metal found may include one or more of the following: steel, tin, aluminium, silver, copper, bronze, chromium, cadmium or nickel. A visual examination as to colour and hardness will often suffice to identify the metal particles. Positive identification of the metals present may be determined by a few simple tests e.g. a permanent magnet, soldering iron and caustic soda (oven cleaner) – safety standards should be observed.
Many different tests are available to measure oil condition and contamination. Simple site tests should be the first to be considered. If the results from the tests are normal then further testing may not be required. If, on the other hand, these preliminary results indicate a change in condition of the oil, supplementary tests may then be made to determine whether the oil is fit for further service.
|General Properties||Manual Inspection|
|Water Content||Karl Fisher|
|ISO 4406 Rating (Hydraulic)||Hiac Particle Counter|
|Spectrographic Analysis (20 element by Rotrode)||Braid Emission Spectrometer|
|Total Base Number (TBN)||Automatic Titration|
|Neutralisation Value or Total Acid Number (TAN)||Manual or Automatic Titration|
|Particle Count||Hiac Particle Counter|
|Particle Quantifier||Analex PQ90|
|Viscosity at 40 and 100 deg centigrade||Houllion Automatic Viscometer|
|Infra-Red Spectroscopy||Nicolet FTIR system|
The viscosity of an oil may, in simple terms, be considered as a measurement of the fluid’s resistance to flow. Industrial oils are classified in terms of their ISO VG rating i.e. the kinematic viscosity (Centistokes) measured at 40 degrees centigrade. Automotive lubricants however, have various international specifications, which may require the additional measurement of kinematic viscosity at -30, -25, -15, -10, -5 and 100 degrees centigrade (SAE rating e.g. SAE 10W/40).
The changes in viscosity found in engine oils are more complex than changes, which occur in industrial oils and depend on a number of effects, which separately or in combination can either increase or decrease the oil’s viscosity.
In industrial and automotive oils an increase in viscosity may indicate oxidation, contamination with dirt or water or an addition of a higher viscosity oil to the system. Insoluble levels in excess of 5% weight in diesel engines may cause the oil to become extremely viscous and cause difficulty in starting, blocked filters and oil starvation leading to mechanical failure.
It is seldom that the viscosity of an industrial oil decreases in use. If this occurs it suggests contamination with a solvent or lower viscosity oil. Fuel dilution in engine oils will also cause a noticeable decrease of viscosity. Degradation of the viscosity index improver may also result in a viscosity decrease.
Acid and base number determination are carried out in non-aqueous solutions and are therefore not directly related to absolute levels of acidity or alkalinity.
Total Acid Number (TAN or Neutralisation Value):
TAN is a measure of the total amount of both weak and strong organic acidity present in the oil. Compounds formed in the early stage of oil oxidation are not in themselves harmful. Further oxidation will however convert initial oxidation products into acids, which attack and corrode metals. Hydraulic oils have a residual total acid number, which, with service, will increase. Twice the original value is usually an actionable level.
Total Base Number (TBN):
TBN is a measurement of the reserves of alkalinity present in the oil. Crankcase oils are continuously monitored for TBN particularly in marine, residual and natural gas engine applications when the quality of the fuel is suspect. Higher sulphur fuels quickly deplete the reserve alkalinity of the crankcase oil, which is then unable to neutralise the harmful acids formed as a result of the combustion process.
The advantage of site testing is that the results are instantly available for action. Laboratory testing, although more comprehensive, has the disadvantage of a delay associated with first taking a sample then dispatching and awaiting the results and the laboratory analysis usually includes lubricant and wear debris analysis in conjunction with the above tests. A major problem of site testing is the ‘management’ of such a programme. Operators in the present economic climate are often transient and programmes initiated with good intent soon fall into disuse with a change of personnel and loss of experience.
The combination of site testing and laboratory analysis should however be considered the optimum when the results are combined into a fully integrated maintenance management programme.
Techniques for this type of analysis include elemental measurement, oil property and chemical measurement and particulate contamination measurement.
The most common wear metal measurement techniques include atomic absorption, inductively coupled plasma (ICP), atomic emission and X-Ray fluorescence spectrometers. These spectrometers measure the elemental constituents of the ample of lubricant. Knowledge of the metallurgy of the machine from which a sample of lubricant was extracted then enables an accurate diagnostic report to be prepared.
A limitation, which however has to be considered when interpreting the results, is the inability of the spectrometer to analysis particles larger than five to ten microns. (5 to 10um) – this does not apply to X-Ray fluorescence.
The following list contains the most common sources of each element analysed by the spectrometer.
Wear Debris Analysis
As stated on the previous page, spectrometers are only capable of measuring the size of particles less than 5 to 10um. Various analysers such as Ferrography and the Particle Quantifier have been developed to overcome this particle size shortcoming. One example is the Particle Quantifier, which measures the total amount of ferrous debris in a sample of oil or grease with results quoted as the PQ Index.
It may be either a portable or a laboratory based instrument. Analysis may be either directly from the sample bottle or from samples dispensed into small plastic containers. Linear and Rotary Ferrography provide additional information by magnetically separating particles from the sample into a substrate.
Examination of the particles for shape, size, colour and morphology enables a classification to be made of the severity of wear occurring in the system from which the sample was taken. This type of examination, which is laboratory based, is usually only requested for abnormal values of the PQ Index.
Magnetic Drain Plugs (MDP) are often used for the collection of wear debris from a system. MDPs are frequently used in gas turbines and helicopter transmissions and in their industrialised version may be fitted to industrial gearboxes. These Plugs are periodically removed and the debris deposited on the magnet face viewed through a low magnification microscope.
Abnormal shapes and sizes are clear evidence of gear and bearing damage. The limitation of particle size collection is in the range 100 to 1000um. The Quantitative Debris Monitor (QDM) is available as an on-line instrument utilising magnetic plug technology.
Fluid power systems, where contamination has to be maintained at al low level, require particle counters which may be either on-line or off line instruments. Counts are taken for particle size ranges from 1 to 100um. The standard ISO cleanliness code however quotes counts at 2, 5 and 15um particle sizes (these size ranges are due to change in the latest ISO standards).
Counters may use the light obscuration or filter blockage principle. When either water or air entrapment is present in the system light obscuration methods may distort the particle count. As stated previously, sampling procedures for particle counting is of prime importance for measurement repeatability.
Oil properties and chemical measurement: Degradation of a lubricant requires the evaluation of chemical structure, the presence of chemical contaminants and the process of additive depletion – Fourier Transform Infrared Spectroscopy (FT – IR) enables these measurements to be carried out. Compounds commonly detected include water, blow by products, ethylene glycol, unburnt fuel and refrigerant gases.
Degradation of the lubricant through oxidation, nitration and sulphation may be measured directly. The degree of depletion of the additives providing the oil’s detergency, dispersancy, alkalinity and antiwear characteristics may also be measured.