lET'S START WITH AN OVERVIEW OF fluid main functions in hydraulic systems
There’s a long list of functions for oil in hydraulic tipping systems.
First of all it must be recognized as an ENERGY TRANSFER MEDIUM.
Mainly because of its incompressibility or stiffness, hydraulic oil is used for providing consistent transmission of power throughout a hydraulic system, which allows cylinders to complete the work.
The advantage of incompressibility ensures a hydraulic machine fast reaction working:
This is achieved by a hydraulic oil that does not easily trap gasses. On the other hand, trapped gas (as well as foaming formation) would bring a higher level of compressibility to the fluid and, as a consequence, to a less efficiency and reliability of the system as a whole.
This is the reason why in general air contamination must be kept under control and minimized as much as possible! Obviously not to mention even more detrimental effects it can cause, such as cavitation, diesel effects,… (see also AIR CONTAMINATION)
| ENERGY TRANSMISSION |
| LUBRICATION |
| ANTI WEAR |
| ANTI CORROSION/OXIDATION |
| HEAT DISSIPATION |
| CONTAMINATION REMOVAL |
| SEALING |
In addition, hydraulic oil creates a boundary layer on surfaces, reducing friction between sliding (or rotating) mechanical components that are in contact with each other and therefore preventing from wear.
The oily film on components also protects the metal surfaces from chemical rections caused by air and water (oxidation and corrosion).
Preventing any metal-to-metal contact, it also reduces over-exposure to heat. Furthermore hydraulic fluids help to cool a system by dispersing heat throughout the whole system, particularly by carrying it away from devices that create heat (like pump) to the oil tank.
This function is is very simple, as the fluid simply carries any particle and water away from critical components to the filters and oil tank where they are captured or settled.
Also, common additives help with a number of problems that can occur, such as anticorrosion agents, rust and oxidation inhibitors, fire-resistance, anti-foaming agents and more.
For major clearances between parts, seals and O-rings are appointed to this job, but hydraulic oil also I also involved and it can help with those components featuring tight clearances, such as valves, pumps and motors.
The thin film of oil helps to reduce leakage over these clearances and reduce friction on sliding and rotating surfaces in a system.
Critical to this sealing function is one of the main properties of the fluid: VISCOSITY.
The hydraulic fluid can be considered as the main component in hydraulic systems. It is actually a crucial one, to be carefully selected, becaue it affects performances, efficiency and lifetime of the whole system.
To select th suitable hydraulic fluid it is importamt to consider:
- any requirementsof the system/machine
- operating conditions
- safety and environmental conditions
- regulations
An important concept to introduce is VISCOSITY: although various hydraulic fluid characteristics have an impact on different performance functions, viscosity is a key element, when selecting the suitable fluid for a hydraulic system.
In fact, VISCOSITY influences almost all the other hydraulic fluid main properties and functions.
Viscosity rating is a measure of the oil’s resistance to flow (or thickness), which can vary in relation with temperature variations.
In other terms, all oils, including hydraulic oils, have variable viscosities at different temperatures. In addition it must be considered that viscosity characteristics variation, in relation to the temperature change, is very nonlinear.
It is therefore very important to know what temperature is usually reached within the hydraulic system, Usually it is related to the climate zone, the type of application and operating conditions of the machine. This is important for better choosing the suitable oil, able to work at such temperatures, not reaching too high or too low viscosity.
Generally higher temperature will lead to lower viscosity (thinner oil) and vice-versa lower temperature will lead to higher viscosity (thicker oil).
Low viscosity fluids will flow more easily and faster, with consequent less risk of overheating and pressure drops, but increased risk of leakages and less efficiency (mainly due to the fact that if the oil is too thin not only it is hard to pump, but slippage will accrue due to leakage past clearance spaces, between gears, piston seals, spools,…)
If the fluid viscoity is too low, the oil film on surfaces will be thinner, increasing the risk of metal to metal contact, which leads to excessive wear on components.
On the other hand, high viscosity fluids will flow more slowly. If you use oil that is too thick, in general the system will suffer from a sluggish movement/reaction, resulting in a overall reduced mechanical efficiency, generating energy losses and unnecessary heat. As a specific consequence, the pump may not be able to draw the fluid efficiently from the reservoir and cavitation may occur.
OIL RECOMMENDATIONS
To be considered as general indications and guidelines, based on practical experience (from Binotto User Manual)
The viscosity of the fluid is influnced by climate, environment, operating temperature and system design.
A correct selection of the fluid with the proper viscosity is key for efficiency and performance of the hydraulic system.
In the table representing Binotto oil recommendations, there's a selection of the ISO VG table (ISO VG stand for "International Standards Organization Viscosity Grade") .
ISO viscosity classification uses mm 2 /s (cSt) units and relates to viscosity at 40° C.
The complete classification consists of a series of 18 viscosity brackets between 1.98 mm 2/s and 1650 mm 2/s, each of which is defined by a number, ranging from 2 all the way up to 1500.
It is important to distinguish between the 2 concepts. In fact, VISCOSITY is the hydraulic fluid property describing the fluid's resistance to flow at a certain temperature, meaning that temperature of the fluid affects on viscosity.
Viscosity Index (VI) is the value that indicates the effect of temperature on the viscosity; high VI fluids maintain their viscosity relatively stable with temperature changes.
TIPS FOR OPERATORS
Especially when operating in a cold environment, it is important to monitor start-up and operating temperatures, allowing enough time for the machine to warm up before putting it into high speed or heavy operation.
Other important phenomena connected with air in relation with hydraulic oil, like turbulences and vortex or foaming are connected with oil viscosity and deserve a special dedicated section.
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