Have you ever wondered what the quality standards of the lubricating oil you use in your machines actually mean?
You’re probably aware that we have used NAS 1638 since the mid-1960s to discuss cleanliness requirements and particle content in oils for maritime and industrial use. Back then, quality control consisted of placing oil samples under a microscope to count particles of different sizes in a given amount of oil.
The table below shows all 14 different cleanliness classes on the far left, detailing how many particles of five different categories (sizes measured in micrometers or µm) were accepted per 100 ml of oil. Observe that the scale is logarithmic instead of linear – meaning that each step of the scale allows twice as many particles as the previous step:
| Maximum allowance for particles per 100 ml. of stated size (in micrometer/µm) | |||||
| Class | 5-15 | 15-25 | 25-50 | 50-100 | >100 |
| 00 | 125 | 22 | 4 | 1 | 0 |
| 0 | 250 | 44 | 8 | 2 | 0 |
| 1 | 500 | 89 | 16 | 3 | 1 |
| 2 | 1,000 | 178 | 32 | 6 | 1 |
| 3 | 2,000 | 356 | 63 | 11 | 2 |
| 4 | 4,000 | 712 | 126 | 22 | 4 |
| 5 | 8,000 | 1,425 | 253 | 45 | 8 |
| 6 | 16,000 | 2,850 | 506 | 90 | 16 |
| 7 | 32,000 | 5,700 | 1,012 | 180 | 32 |
| 8 | 64,000 | 11,400 | 2,025 | 360 | 64 |
| 9 | 128,000 | 22,800 | 4,050 | 720 | 128 |
| 10 | 256,000 | 45,600 | 8,100 | 1,440 | 256 |
| 11 | 512,000 | 91,200 | 16,200 | 2,880 | 512 |
| 12 | 1,024,000 | 182,400 | 32,400 | 5,760 | 1,024 |
This standard was introduced back in the early 1960s to measure and describe the number of particles in hydraulic oils for the aviation industry. Up until that point, they had almost exclusively been concerned with viscosity and additives in the oil, but NAS 1638 also made it possible to discuss and analyse how microscopic particles affect the lifespan and longevity of machines. To put it in perspective: An improvement of just one NAS class has the potential to double the lifespan of sensitive components!
Cleaner oils are naturally more expensive to produce and purchase, so it’s important to find the right balance between price and purity to fit use case. For industrial hydraulics in Norway, there is rarely a need for anything cleaner than NAS 1638 class 6 – but MHService can of course provide cleaner oils through Texaco and Castrol, as well as other suppliers if needed.
For land use, we at MHService sell a lot of hydraulic oil of the Texaco HDZ type, which is fine-filtered to cleaner than NAS 1638 class 6. This is an oil with added elements that counteract wear, corrosion, foaming, and oxidation – HDZ stands for “Heavy Duty, Zinc-based” – and its favorable viscosity properties makes it very well suited for outdoor use.
However, shortly after the NAS 1638 standard was introduced, the price of very fine-meshed filters started to come down. At the same time, they became more advanced and their quality increased – which made their use increasingly widespread. Modern filtration systems can now achieve cleanliness levels down to NAS 3, so it is no longer very useful to sort oils by five different categories of particle sizes.
In fact, NAS 1638 was formally discontinued in 2001. But since many industries over several decades had grown accustomed to using it, it still prevails – at least in everyday speech. For more advanced use cases, it is far more common to adhere to standards like AS4059E or ISO4406, but that’s a discussion for some other time!
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