Dimensions of solid contamination elements or foreign particles in the pressure fluid. These impurities may be caused by various things:
- debris associated with metallic and non-metallic particles produced by wear such as chips from bearing or pump materials, debris from seals, paint particles and oxidation products,
- foreign particles entering the system via poorly sealed or covered areas,
- contamination already present from the time of manufacture (core sand, welding particles, chips) and not properly removed prior to assembly.
Particle sizes can vary greatly and their distribution – in terms of quantity – is not very even either (Figure P 4). For example, smaller particle sizes tend to be more common. This distribution is divided, in accordance with various processes (ISO 4066, NAS 1638, SAE-ASTM-AIM), into contamination classes or cleanliness grades. It is important to remember that the filter can only ever retain a fraction of the wide range of solid particles present. Most damage is caused by residual hard particles which have found their way into the sliding gap between metal surfaces or seals and cause scoring to occur there. Much depends on how their size compares with the respective gap width:
- Particles which are much smaller than the gap to which they have gravitated will pass through without causing any damage. If they are only slightly smaller, they will tend to clog up the gap at a low flow velocity (Figure P 5 a).
- Particles about the same size as the gap cause the most damage, since they lead to jamming and scoring in the gap. The greater the pressure, the more dangerous the somewhat larger particles will become, since the gap will tend to widen under pressure (b).
- Particles which are significantly larger will remain in front of the gap without causing any damage.
Figure P 4: Average particle distribution in hydraulic systems
Figure P 5: Particle size and gap width a) at various flow velocities, b) depending on pressure