Hydraulic Fluid Viscosity

Hydraulic Fluid Viscosity

The fluid’s viscosity describes the liquid’s property, which is how it does not readily flow or its thickness. In other words, it defines the extent to which a fluid is subjected to pressure or stress.

The deformation will take place with little ease. Fluid with substantially higher viscosity is thicker and flows sluggishly compared to fluid with low viscosity, which is thinner and flows freely.

Units of Measurements for Fluid Viscosity in Hydraulics

One method is to measure the resistance to the flow of fluid with an applied force. The alternative is to measure the resistive flow of a fluid due to gravitational force.

Given the above definition, we refer to this as an indicator of fluid viscosity—kinematic viscosity.

The standard unit of dynamic viscosity is Pascal seconds. Also, the measurement should be noted as centipoise. The SI unit for dynamic viscosity is one newton-second per square meter (N•s/m2).

Dynamic viscosity is usually measured in centipoise (cP), as the viscosity of water at 20°C amounts to 1.002 cP.

Kinematic viscosity is measured in units of square meters per second (m2/s) or centistokes (cSt). This is the SI unit that is used to measure viscosity, which refers to dynamic viscosity divided by density.

Units of Measurements for Fluid Viscosity in Hydraulics
High Fluid Viscosity vs. Low Fluid Viscosity in Hydraulic Systems

High Fluid Viscosity vs. Low Fluid Viscosity in Hydraulic Systems

High-viscosity fluids have greater resistance to flow and lead to a higher frictional nature in the hydraulic system. It ensures better lubrication and less friction loss due to running components, reducing the effect of high-pressure conditions.

Although it resulted in increased energy consumption, a lack of efficiency, and increasing sluggishness due to increased frictional losses.

Low-viscosity fluids flow better, requiring fewer efforts to be pumped through the hydraulic system circuit.

The benefits of using low-viscosity fluids include increased energy efficiency rates, rapid response times, and minimized resistance across different components.

On the one hand, it helps lower lubrication efficiency and susceptibility to leakage, which may potentially lead to overheating under significant loads.

How Temperature Affects Hydraulic Fluid Viscosity

Temperature plays a major role in hydraulic fluid viscosity level, affecting the behavior and performance of the fluid under such conditions.

Higher temperatures usually lead to reduced viscosity in hydraulic fluids, which consequently become thinner and flow quite freely.

This reduction in viscosity could be helpful as it reduces resistance to flow, therefore making hydraulic components and systems run more smoothly.

High temperatures above the optimum limit can result in the thinning of fluids, which affects the effectiveness of lubricants, leading to additional damage from increased wear and friction. Colder temperatures cause an increase in viscosity for most hydraulic fluids.

When the temperature drops, the liquid becomes much more viscous and increases its resistance to flow.

Low temperatures of operation will increase the viscosity within the hydraulic systems, leading to sluggish operations and low efficiency. The cold may also bring about the thickening of the fluid with elevated flow resistance and the possibility of damage to the system as a result of increased pressure.

How Temperature Affects Hydraulic Fluid Viscosity

Viscosity Index in Hydraulic Fluids

The viscosity index refers to the amount of change the viscosity of a fluid alters with temperature. High VI implies low viscosity with temperature variation, which makes the fluid stable and consistent in performance under different operating conditions.

Hydraulic fluids are classified from viscosity to very high viscosity as indicated in the table below.

Classification of viscosity Viscosity index
Low below 35
Medium 35 to 80
High 80 to 110
Very high above 110

Importance of Knowing Hydraulic Fluid Viscosity

Importance of Knowing Hydraulic Fluid Viscosity

· Efficient System Operation

For a hydraulic system to operate and provide the required results it is necessary to understand the impact of the hydraulic fluid on the system’s efficiency.

Through enhancement of the viscosity, smoothness in movement through components is enhanced as friction and loss of energy are minimized. This productivity translates to better production efficiency and low operational costs.

· Component Protection

The application of viscosity fluid knowledge assists in the choice of the optimal lubrication parameters.

Correct viscosity provides the necessary film thickness between moving parts to avoid metal-to-metal contact and wear and tear on system components. This way, equipment lifetime increases and maintenance expenses are reduced.

· Temperature Management

The viscosity of the hydraulic fluid essentially influences its behavior across a wide range of temperatures.

Knowledge of viscosity characterization allows picking fluids with proper viscosity indices for particular operation conditions. This guarantees stability, does not allow for the thickening or thinning of fluids, and ensures system sustainability under different environmental circumstances.

· Fluid Contamination Control

Viscosity knowledge is important to ensure the maintenance of cleanliness within fluids. Viscosity changes remain as a part of contaminant presence, which helps in the early recognition of possible issues.

Routine monitoring permits timely filtration or fluid replacement, which eliminates the buildup of particles and conserves system efficiency and lifetime.

· Safety Assurance

The correct viscosity of the fluid is conducive to overall safety levels. The viscosity parameters provide the necessary information to ensure system safety and mitigate potential leakage, equipment failure, and hazardous conditions.

This knowledge creates a safe working environment for employees and prevents accidents or damage to equipment.

How Fluid Viscosity Affects Hydraulic System Performance

How Fluid Viscosity Affects Hydraulic System Performance

· Mechanical Efficiency

Mechanical efficiency is the amount of mechanical energy that has been absorbed by hydraulic oil to reduce the moving components’ internal friction. Low mechanical efficiency, in turn, leads to poor flow to lubricated parts and increased friction that takes time to do operations and hence causes mechanical failure.

· Volumetric Effectiveness

Volumetric efficiency is the percentage of power lost due to internal leakage or fluid loss between components such as valves and pistons.

When the viscosity of a fluid becomes too low, it becomes thinner; hence, less energy is wasted. This leads to lower volumetric efficiency and reduced system responsiveness.

· Cavitation

Low oil supply in hydraulic systems results from the effect of pressure called cavitation.

These spaces can easily explode which causes wear, high temperatures, leakages, and corrosion. Viscous fluid is likely to have cavitation which makes it slower to pump oil.

· Heat Dissipation

Hydraulic fluids have the key responsibility of transferring heat that is generated by the system when it is working.

It then dissipates the heat from the components of the hydraulic system like pumps. Therefore, if the fluid loses its viscosity it will not perform this function effectively which means it needs to have a high viscosity index.

· Air Release

Trapped air within a hydraulic system causes ineffectiveness of the system and cavitation. Hydraulic fluids, especially the viscous ones help release air that seems like bubbles. It provides a medium for the air to escape.

Viscosity of Hydraulic Fluid for Hydraulic Pumps

· Vane

In the pump, therefore, there are several slots fitted around a shaft that is rotatable eccentrically relative to a cam ring.

Vanes are subjected to a reduction in wear because of an internal contact between the two contiguous surfaces from rotors inside the ring. Under normal operating conditions, vane pumps typically need a viscosity range of between 14 and 160 centistokes (cSt).

· Piston

A piston pump can produce as much pressure during operation as 6,000 psi compared to a vane pump.

Hydraulic piston pumps represent the middle of the road, a rather obscure type of hydraulic pump. The viscosity range of piston pumps operating at process temperatures is 15 to 160 cSt.

· Internal gear pumps

With internal gear pumps, there are many viscosity options available, of which the maximum is 2,200 cSt. Such a type can produce pressures of 3,000 to 3,500 psi and run silently.

· External Gear Pumps

Despite being inferior in power, secondary gear pumps still present some advantages. They are cheaper to purchase and operate, and they have continuous flow and simplicity of maintenance.

Like the internal gear pump, this type produces pressures between 3000 and 3500 psi, but it can accommodate a viscosity range of only 300 cSt.

Choosing the Right Hydraulic Fluid Viscosity for Your System

· Viscosity

When selecting the oil, ensure that the viscosity of this material is appropriate for the pressure and temperature under which your hydraulic system operates.

The thin protective coating between the moving components is required through the viscosity index; as a result, wear is reduced. If the oil is too viscous, it will overheat the system and drain energy, as well as cause higher resistance.

· Compatibility

The dissimilar hydraulic fluids have poor inter-solubility properties. If you are changing brands or types, ensure that the new oil matches the residual one in the system.

Incompatibility can lead to problems such as seal degradation, foaming, or even system failures.

· Temperature Range

All hydraulic fluids have a working temperature range that is considered the best condition for optimum functioning.

Beyond this boundary, its characteristics can change, leading to underperformance or, worse yet, damage to the system. For instance, you would require an oil that remains liquid and does not become viscous in colder regions.

· Additives

These include additives that are mixed with the base oil to serve as make-up in terms of prolonging its life and functioning.

Anything from stopping corrosion and oxidation to viscosity maintenance in the face of temperature changes is dependent on additives.

The required performance level may require a specific additive package, and hence you would need an oil that can meet the needs of your hydraulic system.

· Environmental Concerns

The impact these hydraulic oils have on the environment is a top concern in today’s world of environmentally friendly living.

Where your operating site is close to fragile ecosystems or bodies of water. On the one hand, eco-friendly or biodegradable oils are better for the environment; on the other hand, such oils may be required by regulations.

· Price

Though it may be alluring to make the least expensive choice, think about the long-term effects. Long-term cost benefits can result from an oil that is somewhat more expensive but provides superior protection, lifespan, and performance by lowering maintenance, downtime, and component replacement expenses.

Types of Hydraulic Fluid and Their Estimated Viscosity

Note: You can expand this table; please research and list all possible hydraulic fluids with their respective viscosities.

Hydraulic Fluid Estimated Viscosity for Hydraulic Fluid
Mineral Oil Based
– ISO VG 32 32cSt
– ISO VG 46 46 cSt
– ISO VG 68 68 cSt
Anti-Wear (AW) Hydraulic Oils 32-150 cSt
R&O (Rust and Oxidation) Hydraulic Oils 32-68 cSt
Synthetic Fluids
Polyalphaolefins (PAOs) 32-150 cSt
Polyglycols (PAGs) 32-150 cSt
Phosphate Esters 46-680 cSt
Water-Based Fluids 32-150 cSt
Get in Touch With Us Now
Upload File
AddressDinghai Road 66#, Zhenhai, Ningbo, Zhejiang, China
Update cookies preferences
Scroll to Top