Hydraulic systems rely on fluids to transfer energy from one point to another. Known as hydraulic fluids, these substances serve a wide range of industries, from farming and construction to forestry, mining, steel, and aviation.
Beyond their role as a power transmission medium, these fluids also provide sealing, lubrication, and cleaning functions. Our earlier articles cover hydraulic oil types, chemical structures, and roles in full detail.
This brief guide outlines the most important characteristics that define a high-quality hydraulic oil.
Viscosity describes how strongly a fluid resists flowing. A hydraulic oil must maintain optimal viscosity across both cold and hot operating conditions to ensure effective power transmission.
Hydraulic oil that is too low in viscosity is essentially too thin — it fails to deliver adequate protection and sealing, which raises the risk of wear and tear. Conversely, oil with excessively high viscosity becomes harder to pump through the system, dragging down overall working efficiency.
It is worth noting that viscosity requirements differ based on the pump type used within the hydraulic system. The three pump categories are vane, piston, and gear.
Pressure oscillations and working temperature both influence the volume of hydraulic fluids. As temperatures rise, oil density drops and hydraulic oil volume expands. Hydraulic oil compressibility refers to the degree of volume change brought about by pressure fluctuations.
In high-pressure fluid systems, elevated compressibility can trigger serious performance issues — among them servo failure, cavitation and metal fractures, and power loss. For this reason, selecting hydraulic oils with low compressibility is strongly advisable.
Oxidation stability describes the chemical reaction that occurs when oxygen comes into contact with hydraulic fluid.
The oxidation rate accelerates over time and is further hastened by high temperatures, water, acids, and catalysts. This process shortens a lubricant's service life by causing oil deterioration and sludge formation.
The oxidation rate is influenced by several variables, including base oil type, oil quality, and the additive package employed. Compared to mineral-based alternatives, synthetic fluids generally offer superior oxidation stability and tolerate higher operating temperatures. Antioxidants are incorporated into the additive package to enhance oxidation stability.
Thermal stability refers to a hydraulic fluid's capacity to resist degradation when subjected to elevated temperatures. It plays a critical role in the performance and efficiency of all mechanical systems.
Antiwear additives have a finite service life and break down gradually over time. This decomposition can generate sludge that clogs filters, diminishes oil flow, and increases downtime. At extremely high temperatures, antiwear additives deteriorate further, producing acids that attack and damage the metal components within hydraulic systems.
Hydraulic oils with strong thermal stability help minimize these performance issues while extending the service life of hydraulic system components.
When water comes into contact with hydraulic fluid, it disrupts the additive system and promotes acid formation, which then attacks metal surfaces. Fluids with poor hydrolytic stability also undergo hydrolysis, generating insoluble salts that can block filters and valves, interrupting consistent oil flow. Insufficient lubrication and power transmission resulting from this can lead to severe hydraulic system failure.
This is why hydraulic oils with high hydrolytic stability — meaning they resist chemical degradation when water is present — are the preferred choice. They enhance both the overall performance and the lifespan of hydraulic machinery and equipment.
Air entering the hydraulic oil through the reservoir or via air leaks is the primary cause of foaming. Under high pressure, the fluid can carry large quantities of dispersed air bubbles, which expand and produce foam once pressure is released.
Foam ranks among the most damaging threats to hydraulic systems, compromising operational efficiency and lubrication alike. The straightforward remedy is selecting hydraulic oils formulated with anti-foaming additives.
Water introduced into hydraulic fluid accelerates corrosion of system components. It also speeds up the breakdown of additives within the oil formula and can generate chemicals that harm yellow metals in the hydraulic system.
For this reason, quality hydraulic oils are formulated with anti-rust and anti-corrosion inhibitors that shield metal surfaces from attack by water and other reactive chemical components.
Using poor-quality or unsuitable hydraulic fluids can significantly reduce the service life of heavy-duty industrial machinery. The consequences include frequent breakdowns, expensive repairs, and extended downtime — all strong reasons to select high-quality lubricants matched to your hydraulic system's specific requirements.
Valvoline's carefully selected product range includes both mineral and synthetic hydraulic oils. Every oil in our lineup has undergone rigorous testing and delivers exceptional hydrolytic, thermal, and oxidation stability. Available across a variety of viscosity ranges, these products protect against corrosion, foaming, sludge formation, and dust.
Unsure which hydraulic fluid from Valvoline's range suits your needs? Reach out to a Valvoline distributor in your area or consult our helpful Lubricant Advisor to identify the ideal hydraulic oil.
