NAS 1638, “Cleanliness Requirements of Parts Used in Hydraulic Systems,” has just be issued as the new Revision 4. The 3 page National Aerospace Standard remains inactive for new design and is available for purchase from Document Center Inc. Replacement documents are AS-4059, “Aerospace Fluid Power – Cleanliness Classification for Hydraulic Fluids,” and NAS-380, “NAS Documents, Preparation And Maintenance, In The Inch System Of Units.”
NAS 1638 was first issued in 1964 to provide a means to control the amount of contamination delivered in aircraft hydraulic components. The standard contains a series of 14 classes covering very clean to very dirty levels, where the interval between each class is double the contamination level. This principle is a feature of many of the classes in more recently developed standards, such as the AS 4059 (adopted as ISO 11218).
The replacement AS 4059 defines cleanliness classes for particulate contamination of hydraulic fluids and includes methods of reporting related data. Conversion from NAS 1638 cleanliness class specifications to AS4059 class specifications is defined and the differences explained. This document is preferred because of the versatility in identifying a maximum class in multiple size ranges, the total number of particles larger than a specific size or designating a class for each size. FYI: NAS 1638 classes based on weight of particles are not applicable to either of these classes and are not included.
There has been a move to the use of Automatic Particle Counters over the years, which is another reason the NAS 1638 is now inactive for new design. ISO 11171, “Hydraulic fluid power — Calibration of automatic particle counters for liquids,” is the primary standard used for this.
However, NAS 1638 can be used for existing systems where correlation with earlier data is necessary.
All NAS standards are available from Document Center Inc. Use our website, www.document-center.com, or contact us by phone (650-591-7600), fax (650-591-7617) or email (info@document-center.com). We can assist you with all your standards and compliance questions and requirements.
