1, requirements for hydraulic oil used in construction machinery:
(1) Appropriate viscosity
Viscosity is one of the main factors to be considered in the hydraulic system of construction machinery. Viscosity is an index of oil flow performance, which indicates the size of intermolecular friction resistance when oil flows. If the viscosity is too high, it will increase the conveying resistance in the pipeline, increase the energy loss in the working process, increase the no-load loss of the main engine, and raise the temperature, which may lead to "cavitation" at the oil suction end of the main pump. If the viscosity is too small, it will not ensure the good lubrication state of mechanical parts, aggravate the wear of parts, increase the leakage of the system, and lead to the decline of the volumetric efficiency of the pump.
(2) Good viscosity-temperature characteristics
Viscosity temperature refers to the degree to which the viscosity of oil changes with the rise and fall of temperature, which is usually expressed by viscosity-temperature index. The greater the viscosity-temperature index, the smaller the decrease of oil viscosity with the increase of temperature, so that the internal leakage of the system will not be too great. The working conditions of construction machinery are generally bad. During operation, the system oil temperature changes with the load and ambient temperature, so the viscosity-temperature index should not be lower than 90.
(3) Good oxidation resistance and hydrolytic stability
Generally, the working temperature of hydraulic oil is preferably between 30℃ and 80℃, because the life of hydraulic oil is closely related to the working temperature. When the oil temperature exceeds 60℃, the service life of the oil will be halved for every 8℃ increase. That is to say, the service life of engine oil at 90℃ is about 10% of that at 60℃ because of oxidation. At atmospheric pressure, every liter of oil contains slightly less than 0. 1 liter. In fact, oxygen always exists. It reacts with hydrocarbons in oil, which makes the oil slowly oxidized, the color of the oil turns black and the viscosity rises. Finally, the oxide is no longer dissolved in the oil, but deposited as a brown slime layer somewhere in the system. This will cause the valve to jam, and the friction between the ball bearing spool and the piston of the hydraulic pump will increase. At the same time, oxidation will produce corrosive acid and destroy all hydraulic components. The oxidation process started slowly, and then the oxidation speed and viscosity suddenly increased after a certain stage. Therefore, the working oil temperature rises and the oxidation process is accelerated. When oxidation precipitation, high viscosity and acid accumulation reach a certain limit, it will cause fatal damage to the whole system. Thereby causing fatal damage to the whole machine.
(4) Fire resistance and shear stability
In order to improve the viscosity index of oil, high molecular polymers such as polymethacrylate and polyisobutylene are usually added to oil. The molecular chain length of these substances, when flowing through the slit of hydraulic components, is subjected to great shear action, which often breaks the molecular chain and reduces the viscosity-temperature characteristics of oil. During the operation of construction machinery, the frequent reversing of reversing valves and throttling of throttling elements require hydraulic oil to have strong shear resistance.
(5) Compatibility with sealing materials and environment
Hydraulic oil will expand, soften and harden the seal in contact with it, thus making the sealing material lose its sealing function. Due to leakage, seal failure and other reasons, hydraulic oil flows out of the hydraulic system. If hydraulic oil is incompatible with the environment, it will pollute the environment.
(6) Good wear resistance and lubricity
The purpose is to reduce mechanical friction and ensure the service life of the main engine.
(7) Good flame retardancy
Hydraulic oil should have high flash point, ignition point and self-ignition point.
2. Simple judgment of hydraulic oil quality on site:
Simple judgment of water content in (1) oil
A. Burst test: heat the thin metal sheet to above 1 10℃ and drop a drop of hydraulic oil. If the oil bursts, it proves that the hydraulic oil contains water. This method can test that the water content of the oil is above 0.2%.
B, test tube sound test: put 2 ~ 3 ml of hydraulic oil into a dry test tube and let it stand for a few minutes to make the oil bubbles disappear. Then heat the oil (for example, with a lighter) and listen to the small "Pumbaa" sound of the oil (at the top of the test tube mouth). This sound is caused by water vapor generated when water particles in oil collide and boil.
C cotton ball test: take a clean cotton ball or cotton paper, dip it in a little hydraulic oil to be tested, and then ignite it. If you hear crackle and flicker, it proves that the oil contains water.
(2) Judgment of appearance color
A, the hydraulic oil is milky and turbid, indicating that the hydraulic oil is flooded;
B, the hydraulic oil is dark brown, indicating that the hydraulic oil is oxidized at high temperature.
(3) the sense of smell
A pungent smell appears, indicating that the hydraulic oil has been oxidized and deteriorated at high temperature. If there is a smell of diesel or gasoline, it may be that the fuel was added by mistake.
(4) Simple judgment of viscosity
A, "hand twisting" method: judging viscosity by hand, because viscosity changes with temperature and personal feeling, there is often a large human error. But it is feasible to compare the viscosity changes of the same oil before and after use by this method.
B, glass tilt observation method: Take two different kinds of hydraulic oil drops on the tilted clean glass to see which one flows faster and has lower viscosity.
(5) Oil drop spot test of oil pollution:
Drop a drop of measured hydraulic oil on the filter paper and observe the change of spots. The hydraulic oil spreads quickly and there is no precipitation in the middle, indicating that the oil is normal. The slow diffusion of hydraulic oil and precipitation in the middle indicate that the oil has deteriorated.
There are many ways to judge the quality of hydraulic oil, such as judging whether water has entered the oil by chemical method. The above are just a few simple methods to consider from the aspects of site operation and harsh conditions of construction machinery.