The degradation of oil is inevitable, regardless of the machine in which it is used. This is because, over a period of time, contaminants will enter the oil, causing damage to internal components, hydraulic seals, O-rings and the like. When lubricant degrades, an entire system and hydraulic pressure can be affected, leading to reduced reliability and performance of equipment.
The degradation of oil can be prevented with lubricant additives. These additives protect the base oil by degrading before it does, which also protects the base oil’s molecular properties. Once the additives have been consumed, however, base oil is once again vulnerable to degradation, the first sign of which is a change in colour. Additives exist in many forms, including emulsifiers, detergents, rust inhibitors and anti-wear agents.
Oxidation of Oil
Another type of additive is the antioxidant. This product prevents the oxidation of oil, but is unfortunately depleted as it protects the base oil. It only takes 70% of an antioxidant additive to deplete in new oil before the lubricant begins to exhibit physical changes. Following this, further issues can occur in terms of varnish and sludge formation, total acid number and viscosity. When the antioxidant content is being monitored, however, oxidation can be prevented, along with thickening and varnish and acid formation.
What Accelerates Additive Depletion and Oxidation?
Additive depletion and the resulting oxidation of oil can be accelerated when operating temperature is high, and when particles and moisture have contaminated the oil.
Heat in a hydraulic system exists in the form of thermal and mechanical energy. Although the heat generated by equipment operation is unavoidable, too much heat will increase the rate at which antioxidants additives are consumed. Every 10°C rise in operating temperature over 100°C will cause lubricant to oxidise at double the rate.
Particles can enter oil in many forms and from many sources. Any particle contamination left unchecked will lead to erosion, fatigue and abrasive wear. Contamination can originate in the air, as particles are light enough to float. If there are exposed reservoirs, particles can easily enter as oil levels decrease, In cases of new oil contamination, large amounts of air and dirt contamination due to reservoirs with frequently-changing oil levels are to blame.
When contaminated by particles, oil increases friction as it loses its lubricating properties. As a result, heat is generated, which does its own damage as mentioned above.
Moisture is the catalyst for oxidation, and when it enters oil, can cause the rate of additive depletion to increase rapidly. This is because several additives negatively react with water, chemically fragmenting them and rendering them ineffective. Moisture is also the catalyst for corrosion and rust, which can both lead to filter plugging. There are many causes for moisture in a hydraulic system, including leaky hydraulic seals, temperature drops which free dissolved water and heat exchangers.
Polymerisation, or the generation of sludge, occurs during the oxidation process, causing molecular structures that are sticky, polymerisation produces a dark substance not unlike resin, which can adhere to any surface in the system. Because sludge molecules are microscopic, traditional filter systems will not remove them and as a result they build, causing the system to lose efficiency and reliability.
The best way to prevent the oxidation of lubricants is to monitor the degradation of additives, as well as levels of particulate and moisture contamination, renewing lubricant when necessary. Doing this will ensure that additives never deplete to the point of oil degradation, which can cause permanent damage to the system and cost money in lubricant replacement and equipment repair.