Wear metals in lubricants

Wear metals occur as corrosive wear due to water or acids in the oil or as abrasive wear due to contact or local welding of roughness points. Based on their composition, the abrasion particles analyzed in the oil can be assigned to specific machine elements and components that come into contact with the lubricant. OELCHECK lubricant analyses identify any signs of wear and their causes at an early stage. OELCHECK's assessment of wear metals is based on limit values resulting from the outcomes of over 4 million samples tested.

Metal analysis is the core of oil analysis. It detects the often microscopically small wear particles, defines them and, in conjunction with other values, enables conclusions to be drawn about their origin.

This feeds into a diagnosis based on limit values:

  • The type of engine, machine or plant and its components
  • The operating hours of the machine, gearbox or motor
  • The operating time of the oil filling and significant oil refill quantities
  • The oil grade or type used

Test methods

OELCHECK uses four analytical methods to detect wear metals: atomic emission spectroscopy (AES), PQ index, "DoubleCheck" and ferrography.

Atomic emission spectroscopy is part of every OELCHECK analysis. It detects over 20 abrasion, contamination or oil metals simultaneously in just one measurement. The result is given in mg / kg (mg of metals per kg of oil filling) or in ppm (parts per million). By the metal or metal combination itself and by its quantity, the wear of a component made of the metals found is concluded. However, the AES can only detect particles down to a size of less than 5µ.
Detailed information about the AES

The PQ index (Particle Quantifier Index) specializes in the detection of all magnetizable iron particles. It provides information about all iron particles present in the sample, regardless of particle size (Caution: Rust is not magnetic). OELCHECK determines the PQ index (Ferro index) for each oil and grease analysis.

Ferrography shows the size and structure of iron particles on a specially prepared slide under a camera-assisted microscope. By displaying them according to particle size, it is thus possible to make an accurate assessment of the wear condition and the cause of the particular way in which the wear particles are formed. This examination is carried out only with a separate order.

Limit values for wear metals

A limit value is a value intended to indicate whether the amount of wear particles found in the oil is still tolerable or whether an oil change or other measure must be carried out. Limit values for wear are given in mg wear metal per kg oil or in ppm.

A limit value for dust, water or other contaminants in the oil is relatively easy to define for engines, machines and plants. However, there are no generally applicable limit values for the permissible proportion of wear metals in oil.

Too many factors influence these limit values for wear. Even the engine and machine manufacturers provide information on limit values only extremely rarely. Only a complex consideration of all influencing factors and the entire spectrum of all detected different wear metals allows a specification and the evaluation on the basis of limit values.

OELCHECK uses a device-specific matrix when evaluating an oil sample. In it, the respective machine element is defined as precisely as possible. The operating time of the oil, the operating hours of the machine, a possible oil change or a major refill are taken into account as well as the oil type.

This is compared with limit values from an OELCHECK database, which draws on the findings of over 4 million OELCHECK analyses carried out internationally. From this interaction, the experienced OELCHECK tribologists obtain the basic information for the final personal assessment of the oil sample. With OELCHECK, experts - not a computer - comment on the wear metals detected.

Like a blood sample in medicine, each oil sample should be considered on its own or as part of a trend analysis.

Basically, when estimating the values, the following must be taken into account:

  • Everything is o.k. if the determined value is lower than the lower table value.
  • Everything is critical if the determined value is above the highest table value.
  • All values must be seen in interaction if one of the values is in the intermediate range.

It should also be noted that:

  • the larger the oil quantity
  • the shorter the operating time
  • for motors: the lower the rpm
  • for hydraulics: the higher the operating pressure
  • for gears: the higher the circumferential speed

the smaller the limit values for wear metals!