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..
Fluide hydraulique viscosity is the most important of these properties, as it reduces machine wear and improves precision by reducing friction.
Comprendre la viscosité des fluides
Viscosity 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.
Because honey has more resistance to flowing around a 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.
Facteurs influençant la viscosité du fluide
La taille et la structure des molécules, les interactions (force de liaison) et la température affectent la viscosité.
Only the temperature can vary in a hydraulic system because the fluid has already been defined. The hydraulic fluid will thicken in cold temperatures and thin in hotter temperatures. The measure of how much and how quickly these viscosity changes occur is measured by the viscosity index.
L'importance de l'indice de viscosité
While viscosity is one of the most if not the most important aspects of hydraulic fluids, viscosity index (VI) is very important too. Viscosity index is a numerical measurement of how fluid increases in viscosity in cold weather and decreases in viscosity in hot weather. The ISO grade of a AW 32 is 32 cSt at 40C. VI will indicate how the product performs at temperatures less than 40C or greater than 40C.
Normal VI measurements will be 95-105 for paraffinic mineral oils. Synthetic hydraulic oils will have a VI of 140-160. Natural esters (Canola) will have VI measurements of 200 and higher. Some synthetic specialty oils (such as for aviation) may have VI’s over 500. One other note is that there are additives that increase the VI that can be included in the product formulas.
La relation entre les systèmes hydrauliques et la viscosité des fluides
1. Productivité des mécaniciens
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. If the viscosity is too low, then the pump will have to work harder to maintain pressure
2. Efficacité en termes de 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. Lubrification
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 and heat buildup.
4. Cavitation
Cavitation occurs when a liquid’s static pressure is lower than the vapor pressure of the gas dissolved in it, causing small cavities to form in the fluid. 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 wear as well as a reduction in oil lubrication. It can be 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 de la chaleur
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 can overheat unless the heat is being transported away from them on an efficient basis. When the oil 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. Air Release
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. Additionally, there are additives that can be added to the fluid to improve air release.
7. Capacité de filtration
Hydraulic fluid’s capacity to filter out debris and other impurities is called its “filterability.” When fluid flows through a filter element, the filter 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. Hydraulic oils can be filtered down to 3 microns in most cases without removing or filtering out the additives in the fluid.
Summary
Understanding hydraulic fluid viscosity is essential for selecting the right fluid to maximize equipment efficiency, protect critical components, and enhance system performance. At BioBlend, we offer a wide range of lubrifiants respectueux de l'environnement designed to maintain optimal viscosity under various operating conditions. Our premium hydraulic oils provide exceptional performance while reducing environmental impact. Explore our full product lineup, learn how we support various sectors on our industries page, or visit our homepage for more information. Need expert advice? Contact us today for assistance.
FR 
EN