Selection of hydraulic oil for construction machinery and simple judgment of its quality status

Selection of hydraulic oil for construction machinery and simple judgment of its quality status

Most faults in the operation of hydraulic systems are caused by contamination of hydraulic oil and improper selection. Among them, improper selection and careless use of hydraulic oil are an important aspect. Therefore, the correct and reasonable selection of hydraulic oil is of great significance for enhancing the reliability and economy of hydraulic equipment operation, prolonging the service life of systems and components, ensuring the safe operation of equipment, and preventing accidents.

The usage requirements for hydraulic oil in construction machinery:

Appropriate viscosity

Viscosity is one of the main factors that need to be considered in the hydraulic system of construction machinery. Viscosity is an indicator of the flow performance of oil, indicating the magnitude of the frictional resistance between molecules when the oil flows. Excessive viscosity will increase the transportation resistance in the pipeline, increase energy loss during operation, increase the no-load loss of the main unit, raise the temperature, and "cavitation" phenomena may occur at the suction end of the main pump. If the viscosity is too low, it cannot ensure good lubrication conditions for the mechanical parts, accelerate the wear of components, increase system leakage, and cause a decrease in the volumetric efficiency of the pump.

(2) Good viscosity-temperature characteristics

Viscosity-temperature property refers to the degree to which the viscosity of an oil changes with temperature, and is usually expressed by the viscosity-temperature index. The larger the viscosity-temperature index is, the less the viscosity of the oil decreases with the increase of temperature during operation, and thus the internal leakage of the system will not be too large. The working conditions of construction machinery are generally harsh. During operation, the oil temperature of the system varies with the load and ambient temperature. Therefore, the viscosity-temperature index must not be lower than 90.

(3) Good antioxidant and hydrolytic stability

The working temperature of general hydraulic oil is best between 30℃ and 80℃, as the service life of hydraulic oil is closely related to the working temperature. When the oil temperature exceeds 60℃, for every 8℃ increase, the service life of the oil is halved. This means that at 90℃, the lifespan of the oil is about 10% of that at 60℃, and the reason for this is oxidation. At atmospheric pressure, each liter of oil contains slightly less than 0.1 liter of air. In fact, oxygen is always present. It reacts with the hydrocarbons in the oil, causing the oil to oxidize gradually. The color of the oil turns black and its viscosity increases. Finally, the oxide no longer dissolves in the oil but deposits as a brown viscous layer somewhere in the system. This will cause valve adhesion, increased friction between the ball bearing valve core and the hydraulic pump piston, etc., and at the same time, oxidation generates corrosive acid liquid that damages various hydraulic components. The oxidation process begins to proceed slowly. At a certain stage, the oxidation rate suddenly increases and the viscosity suddenly rises. The result is that the working oil temperature rises, and the oxidation process accelerates. When the oxidation precipitate, high viscosity and the accumulated acid solution accumulate to a certain limit, it will cause fatal damage to the entire system. This will cause fatal damage to the entire machine.

(4) Flame resistance and shear stability

To improve the viscosity index of the oil, high-molecular polymers such as polymethacrylate and polyisobutylene are often added to the oil. These substances have long molecular chains. When flowing through the slits of hydraulic components, they are subjected to significant shearing effects, which often cause molecular chain breakage and a decline in the viscosity-temperature characteristics of the oil. When construction machinery is in operation, the frequent reversing of the directional control valve and the throttling of the throttling element all require the hydraulic oil to have strong shear resistance.

(5) Compatibility with sealing materials and the environment

Hydraulic oil can cause the sealing elements in contact with it to swell, soften, harden, etc., causing the sealing material to lose its sealing function. Due to leakage, seal failure and other reasons, hydraulic oil may flow out of the hydraulic system. If the hydraulic oil is incompatible with the environment, it will cause pollution to the environment.

(6) Good anti-wear and lubricating properties

The purpose is to reduce mechanical friction and ensure the service life of the main unit.

(7) Good flame resistance

Hydraulic oil should have a relatively high flash point, ignition point and autoignition point.

2. Simple on-site judgment of the quality status of hydraulic oil:

(1) Simple judgment of water content in oil

A. Burst test: Heat a thin metal sheet to above 110℃ and drop a drop of hydraulic oil. If the oil bursts, it indicates that the hydraulic oil contains water. This method can detect a water content of more than 0.2% in the oil.

B. Test tube sound test: Pour 2 to 3 milliliters of hydraulic oil into a dry test tube and let it stand for a few minutes until the oil bubbles disappear. Then heat the oil (for example, using a lighter), and at the same time listen to the small "puff" sound of the oil (located at the top of the test tube opening). This sound is caused by the steam produced when water particles in the oil collide and boil.

C. Cotton ball test: Take a clean cotton ball or cotton paper, dip it in a little of the hydraulic oil to be tested, and then light it. If you hear a "crackling" sound and a flashing phenomenon, it indicates that the oil contains water.

(2) Judgment of appearance color

A. If the hydraulic oil appears milky white and turbid, it indicates that water has entered the hydraulic oil.

B. If the hydraulic oil appears blackish-brown, it indicates that the hydraulic oil has undergone high-temperature oxidation

(3) Judgment of smell

A pungent smell indicates that the hydraulic oil has been oxidized and deteriorated at high temperatures. If there is a smell of diesel or gasoline, it is possible that fuel has been mistakenly added.

(4) Simple judgment of viscosity

A. "Hand pinch" method: The viscosity is judged by hand. Due to the change in viscosity with temperature and personal perception, there is often a large human error. However, it is feasible to compare the changes in viscosity of the same oil product before and after use by this method.

B. Glass tilt observation method: Take a drop of each of the two different types of hydraulic oil and place it on a clean tilted piece of glass. Observe which one flows faster and has a lower viscosity.

(5) Oil droplet spot test for oil contamination:

Take a drop of the tested hydraulic oil on the filter paper and observe the change of the spot. If the hydraulic oil spreads rapidly and there is no sediment in the middle, it indicates that the oil quality is normal. If the hydraulic oil diffuses slowly and sediment appears in the middle, it indicates that the oil has deteriorated.

There are many ways to determine the quality of hydraulic oil, such as using chemical methods to check if water has entered the oil. The above are just several simple methods considered from the perspective of fieldwork for construction machinery under harsh conditions.