Role of Lubricating Oil
Lubricating oil plays an important role in the maintenance of machinery and equipment. Oil quality is critical to the reliability, operating cost, and lifespan of equipment. More than half of all machinery failures occur due to poor lubrication.
Lubricating oil has the following main roles:
- Remove heat to cool the system.
- Carry contaminants to the filter.
- In hydraulic systems, it also functions as a seal for control system components such as spool valves.
Lubricating oil must form a stable film between moving parts, preventing metal-to-metal contact. Thus, the oil needs to be viscous. Viscosity is related to its load-bearing capacity. Any reduction in the viscosity during operation will result in a reduction in its load-bearing ability. Additives in oil ensure their usability for different applications. Anti-foaming detergent and extreme pressure are some common additives.
How Does Lube Oil Contaminate?
Oil contamination comes primarily from four sources:
1. Generated in the system due to wear and tear
2. Ingressed due to either leaking components in the system or bad maintenance practices
3. Fresh oil added to the system as a top-up
4. Contaminants that are left over (such as welding slag) after maintenance work
Particulate Matter in Oil
The old school of thought proposed that as long as the size of particulate matter in lubricating oil was considerably smaller than the clearances in moving parts, these particles were harmless and would simply pass through the system without causing much harm.
The oil pyramid shows, typically, the distribution of particles by size in lubricating oil. Over 90% of particulate matter is less than 5 microns, and 70% of particulate matter is less than 1 micron. Online and return line filters in engine and hydraulic circuits are typically 8–12 microns. Centrifugal separators may remove particles up to 5~8 microns. Online filters thus do not trap almost 90% of the particulate matter in oil systems.
These small particles, less than 5 microns in size, are the cause of concern to us. Small in size but with large surface areas, in the presence of heat, moisture, and pressure, they act as catalysts that promote oxidation. Oxidation is the nemesis of oil.
Oxidation of Lube Oil
Fig. explains the process of oil oxidation. Oxidation drastically deteriorates the oil quality, reduces its load-bearing capacity, and accelerates wear and tear. This in turn generates larger quantities of fine particulate matter, and the chain reaction continues.
The brown sludge that we observe at the bottom of hydraulic tanks is varnish and sludge, the products of oxidation. Diesel engine oils also oxidize, but in 4S engines, the oil discolors quickly due to the soot from combustion products, and therefore varnish is not visible. Most control components in hydraulic systems ‘fail’ because of the hard brown-colored varnish deposits on them, which restrict free movement.
Standards for oil cleanliness
The primary function of filters is to remove particulate matter from the system. Having filters in the system by itself is no guarantee that the oil will be clean. What needs to be monitored must be measured. Therefore, regular oil sampling (the better solution is online monitoring, which will be discussed later in this article) and analysis are important tools to monitor oil health. While most companies send out oil samples from their machinery at periodic intervals for routine analysis at laboratories, the ‘particle count’ is seldom requested.
Typically, the industry uses two ‘codes of cleanliness’. NAS used to be the industry standard but is now outdated. The current standard in the industry is ISO 4406. The standard specifies the particle sizes of 4, 6, and 14 microns in a 1 ml sample of oil. For example, ISO 4406 19/17/14 implies the oil has up to 500,000 particles of 4 microns or more, 130,000 particles of 6 microns or more, and 16,000 particles of 14 microns or more. The ISO 4406 table is reproduced below.
The minimum recommended cleanliness levels for hydraulic oil are 19/17/14. Certain applications will require oil to be maintained to higher standards of cleanliness.
Why is clean lube oil important for your equipment?
According to the US Bureau of Standards, oil does not mechanically wear out and, thus, theoretically, can be used indefinitely. Yet, more than half of all machinery failures are attributed to bad oil or poor lubrication conditions. It is perhaps the most ignored commodity in any industry.
You can keep your lube oil clean for a longer period of time, which in turn extends the life of your equipment, with better oil quality management with Neptunus.