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 μm |
Class 1-H |
Class 2-H (APT) |
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 |
| Class | Typical Application |
|---|---|
| 1-H | Hypercritical systems - missile, satellite and critical control systems. |
| 2-H | |
| 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. |
| 6-H | |
| 7-H | General uses such as store handling, hoists, winches, cranes and industrial earthmoving equipment. |
| 8-H | |
| 9-H |
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 (ESSO NUTO A32) 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.
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.
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.
 |
|
