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.