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A Few Discussions on the Cleanliness of Hydraulic Cylinders

2026-03-25 00:00

During the acceptance inspection of hydraulic cylinders, customers often conduct cleanliness tests, with the general requirement being no higher than -/19/16 or NAS 8 grade. Typically, the residual oil remaining in the hydraulic cylinder is sampled for testing and used as the basis for determination.

However, this approach is fraught with several common misconceptions, namely:

1 Can the classification of solid particulate contaminants in residual oil within a hydraulic cylinder be used as a basis for determining the cylinder’s cleanliness?

2 Ultimately, do the hydraulic fluid analysis results reflect the cleanliness of the hydraulic cylinder or the solid particle contamination level of the test fluid?

3 How should the contaminants entrained into the oil tank by the hydraulic fluid prior to draining the residual oil from the hydraulic cylinder be calculated?

4 What is the actual cleanliness level of the hydraulic cylinder?

To put it another way, for example: we first determine the cleanliness of the hydraulic cylinder by detecting the residual oil inside the test cylinder. NAS 8 grade; under these conditions, if we switch to a hydraulic fluid cleanliness level of NAS 5 The hydraulic system of the corresponding grade was used to test this hydraulic cylinder, and the test results were undoubtedly lower than NAS 8 grade, then how should we interpret the actual cleanliness level of the hydraulic cylinder?

Currently, the practice of sampling residual oil from within the hydraulic cylinder is a widespread misconception in hydraulic cylinder cleanliness testing. Specifically, after flushing with high-cleanliness hydraulic fluid and then analyzing the cleanliness of the flushed fluid, the results obtained are more likely to “ Cleanliness of flushed hydraulic oil ” , rather than the inherent contamination level of the hydraulic cylinder itself.

This method can only demonstrate “ After flushing, the cleanliness of the hydraulic oil remaining in the hydraulic cylinder cannot be determined; moreover, it is impossible to specify how much contamination was flushed out. ”， At the same time, a new problem is also being faced: “ If the system’s working fluid is changed, under normal operating conditions, Will the hydraulic cylinder itself continue to shed contaminants? ”

About a dozen years ago, when I was still an intern, my mentor told me that the cleanliness of the hydraulic fluid on the test bench was 8 For such grades, hydraulic cylinders typically machined using honing (at the time, scraping, rolling, and boring were still relatively rare) will see their cleanliness drop to 10 Around the grade, deep-hole boring is employed. + The cleanliness of the large hydraulic cylinder in the rolling process will decrease to 9 around the grade; the difference here reflects the actual cleanliness of the hydraulic cylinder ( (Excluding the pollutants that were removed) 。

The cleanliness of a hydraulic cylinder refers to the total amount of contaminants adhering to its internal surfaces—such as the cylinder bore, piston, piston rod chamber, and seals—that could become dislodged during operation, given that the cylinder is a solid component. The core principle is to measure the total amount of contaminants actively washed off or removed from the hydraulic cylinder, rather than merely looking at the final rinse water. “ Foot-washing water ” How dirty it is.

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