Oil Cleanliness Standards


Dirt comes in all sizes but the main concern is dirt between 1 and 100μm. To put these sizes in perspective a grain of salt is about 100μm and the smallest particle that can be normally seen by the naked eye is 40μm.

Systems are classified according to the dirt they contain. Typical figures are shown for the number of particles in each size range.

Particle Size
Class 1-H
Class 2-H
Class 3-H
Class 4-H
Class 5-H
Class 6-H
(New Oil)
Class 7-H
Class 8-H
Class 9-H
1 – 5 30882 420000 860606 2840000 8568000 14280000 17136000 22848000 27720000
5 – 10 1617 22320 50050 165168 450864 714240 857088 1142784 1428480
10 – 15 701 1810 3677 12137 29143 42181 50442 67240 84051
15 – 25 290 370 529 1744 3480 5220 6090 7830 9570
25 – 50 94 104 133 439 829 1220 1415 1805 2196
50 – 100 15 15 15 51 89 128 147 185 224
100 + 7 7 7 22 32 43 47 57 68
Particle size range and counts (per 100ml).


Class Typical Application
1-H Hypercritical systems - missile, satellite and critical control systems.
3-H Subminiature Servo-valves and certain aircraft hydraulic fluids as new.
4-H Miniature Servo-valves.
5-H High pressure industrial and Hydrostatic transmission systems.
7-H General uses such as store handling, hoists, winches, cranes and industrial earthmoving equipment.
Classifications with examples of typical applications.

It will be noted that the APT tilt hydraulic system lies in category Class 2-H, this compares with new oil which is in Class 6-H and is therefore considered insufficiently clean for use without processing prior to application on APT.

New oil from the barrel is much too dirty to be used in Class 2-H systems.

If comparisons are made with dirt and typical clearances within components it can be seen that cleanliness is very important.

  1. Servo valve spool to bush clearance 1.5 μm
  2. Main pump piston/bore clearance 15–30 μm

To satisfy these cleanliness standards certain precautions must be taken. Perhaps the most important of these is to flush dirt out of the system before starting to run. Secondly oil must be precleaned before use and finally, clean systems must not be disturbed without cleaning on reassembly.

Atmospheric dirt in the working area tends to enter the system and so it is undesirable to allow grinding or filing in the vicinity.

Flushing Trolley

A flushing trolley has been designed in order to simplify the above procedures. The trolley accepts oil from a drum and recirculates it internally until it is cleaned to Class 2-H. This oil can then be used to flush out the train hydraulic system until the returning oil reaches Class 2-H.

An additional feature is the ability to pressure test the hydraulic system to confirm correct assembly.

Oil cleanliness class ratings can be assessed by passing a fixed quantity of oil through a fine filter, one of mean pore size about 1μm, and looking at the contamination collected on the surface through a microscope. This is compared with a pair of 'go / no go' slides to determine the dirt classification. If the sample is too dirty further flushing must take place.

The flushing trolley contains all the oil sampling equipment and microscope to enable these checks to take place. The two kinds of tilt pack are equipped with special test points for oil sampling and pipework is checked by samples from return oil flow to the flushing trolley.

APT Tilt System Flushing trolley

Extract from APT3 Technical Training Note.

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© R G Latham 2023.