Frequently asked questions

Any excess or shortage of substances and/or energy, that has an adverse effect on performance and reliability of the oil system.

It is a numeric value, defined by one of the standards ISO4406:99, SAE AS 4059, NAS1638, showing the amount of contaminants in the oil. Contamination differs in type and size. The required oil cleanliness levels for machinery are usually specified by the manufacturers.

Solid particle pollution can be measured in different ways, and the most common method used today involves a laser particle counter. Microscopic count and control enables particle type identification and based upon the identified types, it is possible to draw conclusions as to their origin.

It is a standard defining the cleanliness of oil using a three digit code – indicating number of particles per millilitre greater than 4, 6, and 14 μm(c).

ISO 4406:1999 Code Chart
Particles per millilitre
More than Up to/including
24 80000 160000
23 40000 80000
22 20000 40000
21 10000 20000
20 5000 10000
19 2500 5000
18 1300 2500
17 640 1300
16 320 640
15 160 320
14 80 160
13 40 80
12 20 40
11 10 20
10 5 10
9 2,5 5
8 1,3 2,5
7 0,64 1,3
6 0,32 0,64

Solid particles are usually of irregular geometric shape and when a particle of 5 μm is measured, it cannot be clearly defined whether this is its largest, medium, or any other kind of measurement. The surface of the shadow of the particle measured by a laser particle counter is converted into diameter which is defined in units of size – μm(c).

Clean oil means that the whole oil system is clean. This ensures maximum availability of machines and equipment and minimises the costs of spare parts and maintenance.

This can happen in a few ways:

  • at the production and fitting stages of system components
  • during machine operation as a result of friction and wear of contact surfaces
  • ingress of pollutants from the environment

The place of sampling should be taken into consideration when obtaining samples. It is recommended to obtain samples while the system is in service; during the circulation of oil in a turbulent region by means of a hand pump at a depth of about 1/3 of the way up the tank.

If the sample shows no visible residue, it does not mean that the oil is clean. The human eye can only see particles larger than 40 µm. Tolerance in hydraulic or lubrication systems is less than 40 μm, therefore difficulties in operation can be caused by particles invisible to the human eye.

It is recommended to measure the cleanliness level by laser particle counter. The result will provide information about the number of particles larger than 4, 6, 14 µm(c).

The recommended oil cleanliness level is usually specified by the manufacturer of the machine depending on pressure and tolerance within the friction pairs in the machine.

To maintain the recommended oil cleanliness level, it is necessary to establish monitoring and control and to adapt the filter system.

Where the manufacturer of the machine did not provide information on the recommended oil cleanliness level, guidelines in Table 2 can be used.

Tablet 2 – Guidelines for oil cleanliness level

Type of system and application Required cleanliness level ISO 4406:99 Required cleanliness level NAS 1638
Very clean oil.
Operating systems with small tolerance and high sensitivity to pollution.
Clean oil.
High pressure hydraulics, servo systems, proportional valves.
Lightly contaminated oil.
For medium and low pressure industrial hydraulics.
Medium contaminated oil.
Mobile hydraulics, motors, medium-pressure working systems.
20/18/14 8…9
Contaminated oil.
Heavy industrial machines, low-pressure operating systems, mobile hydraulics.
21/19/15 9…11
Highly contaminated oil.
Not recommended for oil systems.
22/20/17 12

The system filters (in line) must meet the requirements of the working process and enable flow of a large amount of media at high pressure, wherein particles larger than the declared filter pores penetrate the oil system through the system filters. In such cases, only additional by-pass filter units can ensure the required oil cleanliness level.

Water in oils can occur in dissolved, emulsified and free states. Water content which does not exceed 60% of the saturation point is considered acceptable at working temperature. Therefore it is recommended to carry out repeated or at least occasional measurements of the water content in oil using a water sensor (instrument WSTM + sensor WSPS 05). If free water is present, it means that the oil is completely saturated with water and its further usage represents danger to the oil and all parts of the operating system.

Precoat filters include a wide range of filters, where a coating is added to the main filtering body.

The coating is a layer of filter substance that has sedimented on the surface of the main filtering body (plate filters, sheet filters, candle filters, drum filters,…) immediately before the start of the main filtration procedure. The coating can also be added in cases of gas filtration and fluid filtration.

Its functions are threefold:

  • protection of the base filter substrate from rapid saturation and fouling with fine sticky dirt particles
  • formation of a loose and porous layer of the filtrate, whose particles would normally cause blocking of the base filter substrate pores
  • reduction of the pore size of the base filter substrate in the first stage of the filtration

The most commonly used substances for the formation of the coating are diatomaceous earth, perlite, and in more recent times cellulose fibres and powders.

Sl. 1: How a precoat filter works

To enhance the throughput of filtrate, an additional substance can be added, until the flow becomes so small that the coating must be washed or replaced.

Pressure precoat filters are commonly used to remove fine particles from diluted suspensions where other filtration procedures are economically unfeasible.