Hydraulic fluid acts as the medium for power transfer in heavy machinery, presses, and lifting equipment, converting mechanical energy into pressurized force. This specialized fluid must possess specific physical characteristics to perform efficiently under high pressure and varying conditions. Among these properties, viscosity, which describes a fluid’s inherent resistance to flow, is the most important factor governing the fluid’s performance.
Defining Viscosity and Measurement Standards
Viscosity is precisely quantified through two related measurements: dynamic and kinematic. Dynamic viscosity measures a fluid’s internal resistance to flow when an external shearing force is applied, and it is commonly expressed in centipoise (cP). Kinematic viscosity is the standard measure for hydraulic fluids, derived by dividing the dynamic viscosity by the fluid’s density. This measurement, typically expressed in centistokes (cSt), reflects the fluid’s resistance to flow solely under the force of gravity.
Manufacturers classify industrial hydraulic fluids using the International Organization for Standardization Viscosity Grade (ISO VG) system. This system categorizes fluids based on their kinematic viscosity measured at a standardized temperature of \(40^\circ\text{C}\) (\(104^\circ\text{F}\)). The ISO VG number corresponds to the midpoint of the allowable kinematic viscosity range in centistokes; for example, an ISO VG 46 fluid has a midpoint viscosity of \(46\text{ cSt}\) at the reference temperature. A higher ISO VG number indicates a thicker, more resistant fluid, which is selected for systems operating under higher loads or temperatures.
The Dynamic Influence of Temperature
The viscosity of any hydraulic fluid is not constant but is inversely affected by temperature, meaning the fluid becomes thinner as it heats up and thicker as it cools down. This thermal thinning and thickening is quantified by the Viscosity Index (VI), a dimensionless number that describes how much a fluid’s viscosity changes across a given temperature range. The VI is typically calculated by measuring the kinematic viscosity at both \(40^\circ\text{C}\) and \(100^\circ\text{C}\).
A fluid with a high Viscosity Index maintains a more stable viscosity across wide temperature fluctuations. This characteristic is desirable for outdoor machinery, such as construction or agricultural equipment, which must operate reliably in both freezing cold and extreme heat. Manufacturers often use polymer additives, known as Viscosity Index improvers, to formulate high-VI fluids that resist excessive thinning at high operating temperatures.
The Essential Functions of Correct Viscosity
The correct viscosity enables three primary functions essential to the longevity and efficiency of a hydraulic system. The first is lubrication, where the fluid forms a separating hydrodynamic film between moving metal components, such as in a pump or cylinder. An adequately viscous fluid ensures this film is thick enough to prevent damaging metal-to-metal contact, reducing friction and wear. However, if the viscosity is too high, it increases the fluid’s internal friction, which generates excessive heat and reduces mechanical efficiency.
The second function is sealing, where the fluid acts as a dynamic sealant within the tight clearances of internal components like pistons and valves. A proper viscosity minimizes internal leakage, often called slippage, which is essential for maintaining system pressure and maximizing volumetric efficiency. If the fluid is too thin, it bypasses these clearances too easily, causing a loss of power and slower operation. The third function is heat transfer, where the fluid absorbs heat generated by friction and pressure from internal components. The fluid then transports this heat to the reservoir and coolers, helping to keep the system temperature within an optimal operating range.
Signs of Viscosity Failure
When the fluid’s effective viscosity deviates from the manufacturer’s specified range, observable symptoms and component damage follow. If the fluid is operating at a viscosity that is too low, it leads to a failure of the hydrodynamic lubricating film, resulting in accelerated wear and scoring on pump and motor internals. This thin fluid also increases internal leakage past seals and clearances, causing a loss of holding power and sluggishness in actuator movement. Furthermore, excessive thinning can lead to overheating because the fluid’s reduced film strength generates more friction, which accelerates the fluid’s degradation.
Conversely, if the fluid is too thick, the system will exhibit sluggish operation, particularly during cold starts. The pump must work harder to move the fluid, which increases energy consumption and reduces overall mechanical efficiency. A consequence of high viscosity is cavitation, a phenomenon where the pump inlet cannot draw enough fluid, causing vapor bubbles to form and violently implode near the pump’s internal surfaces. This implosion generates a loud, gravelly noise and causes severe pitting damage to the pump.