Machinery Lubrication magazine published by Noria Corporation
Issue link: https://www.e-digitaleditions.com/i/1103301
ML the invention of automatic particle counters and the push for ISO 9000 during the '80s and '90s, several other attempts were made to create industry standards for cleanliness levels by the Society of Automotive Engineers (SAE), the Aerospace Industries Association of America (AIA) and others. e improvements in particle filters tipped the scales of natural distribution. Filters became more efficient at removing larger particles, which made the larger size ranges less representative of the particle distribution in the oil. With this change in the natural distribu- tion of particles, ISO decided it was no longer necessary to report the concentration of these larger particle ranges. ISO 4406:87 Recognizing the importance of hydraulic oil cleanliness, the shortcomings of NAS 1638 and the deficiencies of other standards organi- zations, ISO aspired to create a standard that would more accurately reflect the concentra- tion of particles. e organization also sought to make its classification or code easier to understand while expanding the standard to all lubricating fluids so there would be one accepted standard across all industries. e process began by reducing the number of categories from five to two with an optional third. It was also decided to use a scale that would count particles of specific sizes and larger, moving away from the range approach. e representative particle sizes chosen were 2, 5 and 15 microns with the 2-micron cate- gory being optional. Unlike NAS 1638, which categorized all particle counts for the various classifications with a single number, ISO 4406 represented each size individually. An ISO 4406 code is always shown with the micron sizes listed from smallest to largest. e second major modification involved changing the scale altogether. What would later become known as the Renard series table was born. e classifications spanned from 0.9 to 30, with each doubling from the lowest to the highest acceptable value. is approach was intended to make each step more mean- ingful and impactful. It also allowed for a simple method of expressing very small and large particle counts with a single value. For example, a cleanliness code might be something like 18/14. is would indicate that there were somewhere between 1,301 and 2,500 particles larger than 5 microns and 81 to 160 particles larger than 15 microns. ISO later dropped the 0.9 code and started the chart at 1 when it was determined that obtaining this cleanliness level was highly unlikely and thus unreasonable to include it in the chart. ISO 4406:99 In the 1990s, there was a push for industry to become ISO 9000 compliant. During this time, it was discovered that the current method for calibrating automatic particle counters (APCs) did not meet the requirements of the ISO 9000 standard. Previously, all APCs were calibrated according to ISO 4402 using Air Cleaner Fine Test Dust (ACFTD). During the process of becoming ISO 9000 compliant, it was determined that this calibration material was untraceable. e exact quantity and size of the particles in the ACFTD were unknown, resulting in inaccurate calibrations. The company that produced the ACFTD calibra- tion fluid also announced that it would no longer be manufacturing the fluid. is led to ISO 11171, which utilizes ISO Medium Test Dust (ISO MTD) from the National Institute of Standards and Technology (NIST). NIST employs a scanning electron microscope (SEM) to accurately measure the number and size of particles down to 1 micron. In addition to the lack of control over the calibration material, it was also discovered that the particle sizes being reported were not the same. Depending on the type of APC used by the laboratory, there were inconsistencies when measuring the same size particles. What was a 5-micron particle in an optical APC (calibrated to ISO 4402) was a 6-micron particle using an SEM. is led to the need for a new calibration medium and standard, thus the creation of ISO 11171 for calibration and ISO 4406:99 for reporting. ISO was presented with a major decision to make: Should there be two standards based on the type of APC and method of calibration, or should the current standard be adjusted so it would be compat- ible with both calibration methods and APC types? It was decided that two standards would create confusion, so the alterative of developing a compatible standard was accepted. With subsequent accuracy improvements in microscope technologies, ISO realized that the reporting of particles smaller than 5 microns was becoming more reliable. us, a third micron size was added as an option to the cleanliness code. What It All Means ISO concluded that the concentration of smaller particles was of greater concern than the larger ones. e organization determined that 4-, 6- and 14-micron particles provided the best representation of the particles closest www . machinerylubrication.com | March - April 2019 | 53 ISO CODE PARTICLES PER MILLILITER > 10 µm NAS 1638 & SAE AS4059F 26/23 140,000 25/23 85,000 23/20 14,000 21/18 4,500 12 20/18 2,400 20/17 2,300 11 20/16 1,400 19/16 1,200 10 18/15 580 9 17/14 280 8 16/13 140 7 15/12 70 6 14/12 40 14/11 35 5 13/10 14 4 12/9 9 3 11/8 5 2 10/8 3 10/7 2.3 1 10/6 1.4 9/6 1.2 0 8/5 0.6 00 7/5 0.3 6/3 0.14 5/2 0.04 2/0.8 0.01 Cleanliness Level Correlation Table