Why Its Crucial to Understand Hydraulic Fluid Viscosity

In order to have a working hydraulic system, the manufacturer or designer must consider different hydraulic fluid properties such as viscosity, compressibility, demulsibility, fire retardancy, etc! 

hydraulic fluid in a functioning machine

Hydraulic fluid viscosity is the most important of these properties, as it reduces machine wear and improves precision by reducing friction!


Understanding Fluid Viscosity

It is the most crucial topic to grasp when trying to better understand hydraulic fluids!

Scientifically, viscosity is a phrase that characterizes a fluid’s resistance to flow. It’s common to refer to this as the fluid’s thickness, although that’s not the precise terminology. The viscosity of honey, for example, is substantially higher than that of water.

Here’s a visual comparison between honey and water! Because honey has more excellent resistance to flowing around the spoon than water, it takes more force to move a spoon through a jar of honey. A fluid’s resistance to an object’s movement and the pressure needed to force it through a tube or pipe is determined by the amount of friction produced by the fluid’s molecules.


Factors Influencing Fluid Viscosity

The size and structure of molecules, interactions (bond strength), and temperature affect viscosity.

Only the temperature can vary in a hydraulic system because the fluid has already been defined. The hydraulic fluid must not thicken or thin significantly across an extensive temperature range. The viscosity index of a fluid increases in direct proportion to temperature change.


The Significance of  Viscosity Index

The ISO provides a specification in which several oil grades are mentioned. ISO 3348 is a standard for determining the viscosity or flowability of a substance. Hydraulic fluid thickness or thinness can be determined by the viscosity index, which measures the friction between fluid layers.


The Relationship of Hydraulic Systems and Fluid Viscosity

1. Mechanics’ productivity

The hydraulic fluid’s ability to lower the mechanical friction of the interior moving parts is considered mechanical efficiency. It is more challenging to lubricate moving parts when the fluid viscosity is too high, which results in reduced mechanical efficiency.

2. Efficacy in terms of volume

Power loss due to internal leakage is measured by a metric called “volumetric efficiency.” It is simpler for fluid to flow through these tiny spaces between moving parts if the viscosity is too low and fluid is “thinner,” which means more energy is lost. As a result, the system’s responsiveness decreases, and its volumetric efficiency drops. It also reduces component life by increasing heat, wear, and damage.

3. Lubrication

The capacity of hydraulic fluid to lubricate moving parts by forming a thin film of oil between two surfaces is known as hydrodynamic lubrication, or “fluid film.” It’s a lot like when a car aquaplanes on a slick road. The lubricant’s viscosity must be precisely calibrated in this application. The lower the viscosity, the more likely two surfaces will come into contact. Viscosity raises the hydrodynamic film’s thickness, but it also raises the film’s friction coefficient. Viscosity decreases as a result of increased friction.

4. Cavitation

While cavitation can occur in any system, it’s most common in those with low pressure and insufficient oil supply. It causes metal erosion and wears as well as a reduction in oil lubrication. It is due to the heavier nature of the oil, which means it takes longer to pump. Thick oil is more susceptible to cavitation because of this.

5. Dissipation of Heat

Heat transport is one of the most important functions of hydraulic fluids. Hydraulic fluid, in the process of doing its job, generates heat. As a result, essential components such as pumps and actuators are now overheating since the heat is being transported away from them. It means, however, that when it is heated to operating temperature, it will lose some of its critical properties, such as viscosity; the fluid’s viscosity index must be high enough to absorb as much heat as necessary without losing too much viscosity.

6. Streaming

The hydraulic fluid’s capacity to release trapped air is known as “air release.” Cavitation and low system efficiency can result from trapped air. Higher viscosity fluids, like honey, can hold on to air bubbles longer than thinner oils because of their viscosity, but water bubbles burst quickly. System designers use reservoirs of varying capacities when high viscosity oils are necessary to get around this problem.

7. Capability of filtration

Hydraulic fluid’s capacity to filter out debris and other impurities are called its “filterability.” When fluid flows through a filter element, it functions as a limitation. To determine how much pressure is needed to drive fluid through a filter, filter makers use the term “Pressure drop.” A higher pressure before the element and a lower pressure after the element are the outcomes of a flow restriction (a pressure drop). Oils with a higher viscosity drop in pressure because they are more difficult to filter.