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PortuguêsTips for good maintenance
When we are faced with making the choice for a hydraulic fluid for starting up, or for maintaining, a pressurised hydraulics system for a machine, we generally come across the information that is provided by the manufacturer of the equipment, which can often be scarce and even, on several occasions, limited to the naming of a small list of commercial product names, without any more detail being given regarding the characteristics, specifications, or properties, or any other factors that need to be taken into account when properly choosing which hydraulic oil is to be used, as well as the equipment’s operating temperatures during routine operation.
For this reason, this article is intended to reveal the basic concepts that needs to be taken into account before any decision is made.
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REQUIREMENTS
Hydraulic pumps, control valves, and hydraulic cylinders operate under pressure, often at high speeds, and are built with a wide range of materials (metals, synthetic materials, plastics, elastomers, etc.).
Taking these facts into account, as well as the range of operating temperatures and the environmental conditions that a hydraulic system must endure during operation, the hydraulic fluid must at least satisfy the following requirements:
• Maintain a suitable viscosity within the operating range of temperatures (start-up temperature, operating temperature, maximum temperature).
• Offer great resistance against oxidation and degradation at high temperatures, thus avoiding the formation of lacquers and varnishes inside the system and the appearance of residue in the valves, actuators, tank, and filters, thus increasing the useful life of the equipment and that of the fluid itself.
• Tested anti-corrosive capacity for various metals (mainly steel and copper and their alloys).
• Compatibility with the various materials, especially with plastics, the paint in the tanks, and the elastomers in the joints, seals, and hoses.
• Anti-foaming capacity, avoiding the fluid compressing when subjected to high pressures, as well as avoiding cavitation in the pump.
• Easily filtered within the range of filterability required according to the operating pressures and the other components in the circuit.
• Good deemulsifying properties (rapid separation from water).
• Deliver good lubricating and anti-wear capacity, even when water or moisture are present in the system.
• According to the application, other properties may be required, such as: resistance to fire, biodegradability, atoxicity, etc.
ISO/DIN CLASSIFICATION
The provider of the hydraulic fluid must state its classification according to ISO 6743-4:2015, ISO 11158, ISO 15380, DIN 51502, and/or DIN 51524, which gives information about its properties and principal applications.
SPECIFICATIONS FROM MANUFACTURERS
As well as the ISO and DIN classification for a hydraulic fluid, the manufacturer of the equipment can require the use of a fluid with a specific level of quality associated with a sample or a manufacturer-specific regulation that must appear on the technical data sheet of the product that has been selected. Some examples: Bosch Rexroth, CAT, Cincinnati Milacron (CM Pxx), Eaton, JCMAS, Komatsu, New Holland, Parker, Poclain, Sperry Vickers, etc.
VISCOSITY ACCORDING TO ISO 3448
The viscosity of industrial lubricants, including hydraulic fluids, is measured according to kinematic viscosity V [mm2/s] at 40 ºC, measured by dividing the absolute viscosity by the density. It is typically given in units of millimetres squared per second [mm2/s] or centistokes [cSt] and their classification is given as ISO VG xx, where ‘xx’ is the average viscosity within the limits allowed for each grade. For example, an ISO VG 46 hydraulic could have a viscosity measured between 41.4 and 50.6 [mm2/s] at 40 ºC, which would give it an average or standard viscosity of 46.0 [mm2/s] at 40 ºC.
VISCOSITY INDEX
The viscosity of lubricants varies with temperature, meaning that when selecting the most appropriate viscosity, it is essential that we account for the range of temperatures that is going to be experienced by our system at work, since the viscosity determines the thickness and the resistance of the lubricating film that will affect the wearing of the moving parts (pump, valves, shafts, cylinders), as well as having an effect on the flow and pressure that the pump can transmit (as the viscosity increases, pressure rises and flow decreases and vice versa, at lower viscosity, flow increases and pressure falls).
Having said this, when selecting a hydraulic fluid, it is essential to take the average operating temperature into account, as well as the range of maximum and minimum temperatures at which the hydraulic oil will be operating. In order to determine the variation of the viscosity of an oil according to the temperature, its Viscosity Index (VI) is used as a reference. The VI is an empirical and adimensional value and we must be aware that the ‘VI’ of a conventional mineral hydraulic oil is between 95 and 105, below which values we must not work and that ‘as VI increases, the variation of the viscosity with a change in temperature is reduced’. Therefore, hydraulic oils with an improved VI have an IV > 130, and in the majority of applications, hydraulic oils with a VI > 150 are recommended, which will be classified as HV or HVI hydraulic oils according to ISO 6743, commonly known as multi-grade hydraulic oils.
OIL CLEANLINESS GRADES ACCORDING TO ISO 4406
The cleanliness of a hydraulic fluid is ‘the big unknown’ and the factor that will hugely affect the useful life of our equipment, meaning that we must pay close attention to this parameter.
The tables below will help us to understand the importance of the cleanliness of the hydraulic fluid in service and the impact of the cleanliness on the operating life of the machine, taking as a reference the most widespread regulation at a global level (ISO 4406).
The ISO 4406 classification or code of a fluid is defined by three separated numbers, for example: 20/17/13. The first number refers to the quantity of particles measured in the fluid larger than four microns, the second to the quantity of particles larger than six microns, and the third to particles larger than fourteen microns. These numbers can vary from six to twenty-four and will give us the particulate content according to Table 1*.
*Sometimes, manufacturers refer solely to the latter two numbers due to the fact that the larger particles are the ones that cause more damage. We can thus see that an oil classified as 17/13 or as */17/13, ignoring the quantity of particles smaller than or equal to four microns.
In Tables 2 and 3 we can see an example of the particles measured in a conventional hydraulic oil and what its ISO classification or code would be (in this case, 20/17/13).
In Table 4, we can see the cleanliness grade that we should set as the desired in a hydraulic fluid in accordance with the parts that make up the system.
We must see that selecting the correct hydraulic fluid on the basis of its viscosity (suitable for the equipment’s operating temperatures), its viscosity index (to reduce variations in the viscosity caused by changes in temperature), and its cleanliness grade will guarantee a longer operating life for the equipment and a longer useful life for the oil, will reduce energy consumption, and increase the equipment’s reliability and productivity, with the resulting savings in costs.
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ADVICE FOR GOOD MAINTENANCE
• Remember to install the appropriate hydraulic filters and set working pressures for the proper functioning of the system.
• Require your supplier of hydraulic fluids to certify the cleanliness grade with which new oil is supplied.
• Install filters to capture dirt and moisture in the vents of the tanks.
• Keep the containers of oil properly closed and sealed against dust and water.
• Use appropriate transfer equipment, clean and not contaminated.
• Perform regular analyses of the oil in service in a certified laboratory to determine the degree of degradation and cleanliness of the oil in service.
More information can be found at www.olipes.com
and by directing any enquiries to the email address: olipesresponde@olipes.com