The time when the sample was taken can affect certain values. It is not without reason that we advise as follows in our sampling guidelines: „During operation or just after standstill as dirt and wear particles are suspended.“ If, however, you take oil, for example, in the morning from a machine that was not in operation over the weekend or hours after a wind power station was stopped operating, the sample may not contain all information. However, the time factor not only plays an important role when taking samples. It also has a significant role in our laboratory. In order to be able to optimally carry out the visual check, the sample containers must rest upside-down for approximately 15 minutes at 40°C. This allows the wear particles from the highly viscous oils to settle on the white cover seal.
The sealed sample containers are also placed upside down on the particle quantifier to determine the PQ index‘. It detects magnetisable iron wear particles in the sample using its magnetic coils. While the value given in the lab report for „iron“ in mg/kg only relates to particles smaller than 5 µm, the PQ index provides information on all magnetisable iron particles regardless of particle size.
In order to obtain a representative sample, they should always be taken as close as to when the machine is in operation as possible. However, some values can only be determined after a rest time. The velocity at which particles in the oil move and deposit through gravitation depends on:
- the size of the particle and its density (its weight, too)
- the oil viscosity, which is also temperature dependent.
The velocity at which a spherical particle sinks or sediments in a liquid can be calculated using „Stoke‘s Law“. For non-spherical bodies half of its equivalent diameter is used rather than the particle radius. We have used this „Stoke‘s Law“ to obtain comparable values for the sinking velocities of metal particles in a lubricating oil.
During these calculations we assumed that the metal particles are almost spherical in shape. A magnetisable chrome steel, which is typically used for gear teeth in gearboxes, was taken as the material for the particles. At approx. 8 kg/dm³, it is almost nine times as dense as the oil which is approx. 0.9 kg/dm³. The sinking velocity of particles with a diameter of 5 µm (> 4 µm smallest particle size during particle count in accordance with ISO 4406), 50 µm (just visible with the naked eye) and 500 µm (particle from a damaged aggregate) was calculated. A conventional CLP 320 gearbox oil was taken as the lubricant. Depositing was calculated for temperatures of 0°C, 20°C and 60°C, as can occur when taking typical samples.