Hydraulic Cleanliness
Opening words
Approach this article critically, as what I am sharing here reflects my current understanding of hydraulic cleanliness. I am at the beginning of my journey in this field, and there is still much to learn. This is a compilation of my notes on hydraulic cleanliness, and I am addressing the topic at a general level.
? If you notice any mistakes in my article, please send me a message. All feedback to improve this article is welcome!
? I am intentionally leaving out the methods for measuring cleanliness from this article, as it would otherwise become too long. Gravimetric analysis and particle counting are familiar to me, and I use them frequently in my work.
An Introduction to Hydraulic Cleanliness
The cleanliness of a hydraulic system is crucial for its operation. Various sources state that contamination causes 70-80% of hydraulic system failures. Contamination reduces the reliability and productivity of the system and can increase maintenance costs. In the worst case, it can lead to dangerous situations.
Contamination can be visible to the naked eye or so small that the human eye cannot detect it. Visible contamination can be observed as small particles in the hydraulic oil, which may also be detectable by touch or smell. Microscopic contaminants can be detected with special tests and observation devices. In particular, contamination that cannot be seen with the naked eye is the most harmful to the hydraulic system. By the time visible contamination appears, various components of the system may already be damaged. Contamination is not always due to a single factor but can result from multiple factors.
Contamination & Particles
When we talk about contamination, we are referring to impurities present in the system. Contamination is anything that does not belong in the hydraulic system.
Common Types of Contamination:
Common Sources of Contamination:
Description of a Conventional Hydraulic System:
Built-in:
During the manufacturing or sub-assembly phase, impurities can remain inside the part or system. Built-in contamination can include solid greases, chemical pastes, metallic particles, welding splatter, paint particles, fibers, and water.
Ingressed:
Contamination that has "intruded" into the system, for example, through breathers, leaks from seals, or contamination that has entered the system along with the surface of the cylinder rod.
System-generated:
Particles generated by internal mechanical wear and particles created by various damages.
Maintenance-generated:
During maintenance, contamination can inadvertently be introduced into the system. Contamination can come with new oil, spare parts, unplugged connections, or in the form of fibers from wiping with different fiber cloths. Additionally, particles can be released during the tightening of connections, and large particles can be released during the over-tightening of connections.
During Storage:
During storage, oxidation from condensation can form in the system, and seals can compress and create permanent deformations. These deformations can introduce contaminants into the system, while oxidation/rusting caused by condensation can generate particles that wear down the system.
Particle
"A particle is an object or entity that is clearly distinguishable from its background or surroundings."
Contaminants are solid and gel-like particles. When measuring the cleanliness of a system, the number and size of particles are examined. Particles smaller than 20 microns, which are beyond human detection capabilities, are particularly harmful.
What is a Micron?
A micron is a micrometer, and the symbol for a micrometer is "μm". Examining small particles requires a microscope. When investigating hydraulic cleanliness, there is particular interest in particles that reflect light. Light-reflecting particles are most often metallic and are especially harmful in a hydraulic system.
I drew a picture to illustrate micrometers:
Microscopic images of different types of particles:
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Consequences of a Contaminated System
Contamination can cause friction within the system, getting stuck in valve clearances and causing jerky movements in the machine's various functions or even preventing certain movements/features. Particles sized 5-15 μm cause the greatest abrasive wear. Contamination can also degrade the lubricating properties of the oil, which accelerates system wear.
Signs of Contamination in a Machine:
Easily Observable Signs During Maintenance:
Particles in the hydraulic system
How to control cleanliness
A quote from Six Sigma fits well for this heading:
"We do not know what we do not measure."
In cleanliness management, the most important points are understanding the system requirements, the conditions to which the system is exposed, and the state of the system.
System Requirements
This involves understanding the maintenance needs of the system. How often should each maintenance task be performed, what spare parts should be used, what oil should be used, and how should each maintenance operation be carried out?
Conditions and System Usage
What kind of conditions is the system exposed to:
Conditions and system usage can affect the cleanliness of the system. Maintenance intervals may need to be shortened if the machine is exposed to fine dust. It is also important to consider the machine's usage: does short-term use or working between outdoor and indoor environments create moisture in the system? Is the machine stored outdoors or indoors? Does the system always reach its normal operating temperature, or is the use so short that the system does not have time to warm up?
It is advisable to plan the machine's usage and storage so that the system can reach normal operating temperature. The system's moisture should be monitored, as the presence of moisture can cause oxidation and corrosion within the system.
Many sources claim that moisture is the most common type of contamination in the system. Moisture can cause corrosion, releasing corrosion particles into the system. These particles can then cause abrasion in components with small clearances.
I found this picture related to moisture:
System Condition
During maintenance, it is important to visually observe the cleanliness of the system. It is also advisable to regularly take hydraulic oil samples from the system and perform a cleanliness analysis. The information collected from these analyses should be recorded and monitored. With this data, measures can be taken to maintain cleanliness before the values fall outside the required standards. The information can also be used to assess the impact of machine usage or the environment on the system's cleanliness.
Cleanliness Culture
As mentioned at the beginning, cleanliness is one of the most important aspects of hydraulic system operation. The challenge with this topic is that contaminants often cannot be detected without special equipment or tests. Failures caused by contamination are not always correctly attributed to it, thus not giving cleanliness the appreciation it deserves. In manufacturing and maintenance, there might be "inherited practices" that are unsuitable for maintaining cleanliness.
Maintaining Cleanliness
Understanding is one of the most important tools for maintaining cleanliness.
Sources:
You should also familiarize yourself with the following:
Web pages:
Videos: