Abstract: The quality and cleanliness of hydraulic oil are the primary conditions to ensure the normal operation of hydraulic system, and the pollution of hydraulic system is a major cause of hydraulic failure. In order to ensure the safe and reliable operation of the hydraulic system of drilling machinery, this paper analyzes the hazards and causes of hydraulic oil pollution in the hydraulic system, and puts forward the methods and measures to detect and control the hydraulic oil pollution, so as to realize the pollution control in the working process, including measures such as controlling the oil temperature and filtering regularly.

Paper Keywords: hydraulic oil, pollution, control

In modern transmission mode, hydraulic transmission can realize remote control and automatic control well, and it has been widely used because of its light weight, compact structure and small inertia. However, drilling equipment is mostly outdoors, which is greatly influenced by the external environment. In the actual use process, it is easy to be polluted by hydraulic oil, which seriously affects the transmission of power. Therefore, the control of hydraulic oil pollution is particularly important.

I. Overview of Hydraulic Oil

Hydraulic oil, as an intermediate medium in hydraulic transmission system, plays the role of transferring and converting energy, and also plays the role of lubrication, anticorrosion, cooling and cleaning between parts in hydraulic system. Its main features are:

1. Proper viscosity and good viscosity-temperature characteristic viscosity are the first factors to consider when selecting hydraulic oil. Under the same working pressure, if the viscosity is too high, the motion resistance of hydraulic components will increase, the self-priming ability of hydraulic pump will decrease with the acceleration of temperature rise, and the pressure drop and power loss in the pipeline will also increase; If the viscosity is too low, it will increase the volume loss of the hydraulic pump, increase the internal leakage of the parts, make the oil film of the sliding parts thinner and reduce the bearing capacity.

2. Good lubricity (anti-wear) There are a lot of moving parts in the hydraulic system that need to be lubricated to prevent the wear of the relatively moving surfaces. Especially in the high-pressure system, the requirements for the anti-wear performance of hydraulic oil are much higher.

3. Hydraulic oil with good oxidation resistance will also be oxidized during use. Acidic substances produced by hydraulic oil oxidation will increase the corrosiveness to metals, and sludge deposition will block filters and tiny gaps, making the hydraulic system work abnormally. Therefore, good oxidation resistance is required.

4. Good shear stability. Because hydraulic oil will be subjected to severe shearing when it passes through the throttle and gap of pump and valve, some macromolecules in the oil, such as tackifier, will break and become small molecules, which will reduce the viscosity. When the viscosity is reduced to a certain extent, the oil can't be used, so good shear performance is required.

5. Good anti-rust and anti-corrosion hydraulic oil will inevitably come into contact with moisture and air during use, and the acidic substances produced after oxidation will rust and corrode the metal, which will affect the normal work of the hydraulic system.

6. Good emulsification resistance and hydrolysis stability. Water and condensate mixed by hydraulic oil in different ways are affected by hydraulic pump and other components.

7. Good foam resistance and air release in the hydraulic oil tank. Because the bubbles mixed in the oil circulate with the oil, it will not only reduce the pressure of the system, worsen the lubrication conditions, but also produce abnormal noise and vibration. In addition, bubbles also increase the contact area between oil and air and accelerate the oxidation of oil, so hydraulic oil is required to have good foam resistance and air release.

8. Adaptability to sealing materials Because the adaptability of hydraulic oil and sealing materials is not good, sealing materials will expand, soften or harden, and lose sealing performance, so hydraulic oil and sealing materials are required to adapt to each other.

Second, the pollution and harm of hydraulic oil

According to different pollutants, the pollution of hydraulic oil can be divided into impurities (dust, metal particles, cotton yarn, oxides, etc. ), moisture, air, microbial and chemical pollution. In the hydraulic system of drilling and production machinery, there are mainly impurities (mostly metal particles), moisture and air pollution.

Oil is mixed with water.

(1) the way water enters the oil.

1) Water in the air condenses into water droplets due to alternating cold and hot, and falls into human oil.

2) The seal of the cooler or heat exchanger is damaged or the cooling pipe is broken, so that water leaks into human oil.

3) The wet air entering the system through the piston rod of the hydraulic cylinder is not tightly sealed and condenses into water drops.

4) When using oil, people's moisture and water absorbed by oil exposed to humid environment are brought.

(2) Harm of oil and water mixing

1) When a certain amount of water is mixed in the oil, the hydraulic oil will be emulsified into a turbid state. If the anti-emulsification ability of hydraulic oil itself is poor, after a period of standing, water cannot be separated from the oil, so that the oil is always in a turbid state. When this white emulsified oil enters the hydraulic system, it will not only rust the hydraulic components, but also reduce their lubrication performance, aggravate the wear of parts and reduce the efficiency of the system.

2) After the ferrous metal in the hydraulic system rusts, the peeling rust will flow and spread in the pipelines and hydraulic components of the hydraulic system, which will lead to the internal rust of the whole system and produce more peeling rust and oxides.

3) Water will react with some additives in engine oil to produce pollutants such as precipitation and colloid, which will accelerate the deterioration of engine oil.

4) Water reacts with sulfur and chlorine in oil to produce sulfuric acid and hydrochloric acid, which aggravates the wear of parts, accelerates the oxidation and deterioration of oil, and even produces a lot of sludge.

5) These water pollutants and oxidation products immediately become catalysts for further oxidation, which eventually lead to the blockage or seizure of hydraulic components, resulting in a series of failures, such as hydraulic system failure, oil distribution pipe blockage, cooler efficiency reduction, oil filter blockage and so on.

6) In addition, at low temperature, water condenses into tiny ice particles, which is also easy to block the gap and dead mouth of the control element.

2. Oil invades the air

The air in the oil mainly comes from loose pipe joint, loose joint surface of components, oil pipe exposed to oil surface and seal failure, and the oil exposed to the atmosphere will also dissolve the air. In addition, when the amount of oil in the fuel tank is small, the circulation of hydraulic oil is accelerated, and it is difficult to remove bubbles. At the same time, the depth of the oil suction pipe of the oil pump is not enough, which also makes air easy to enter.

The air mixed into the hydraulic system is usually suspended in the hydraulic oil in the form of bubbles with a diameter of 0.05 ~ 0.50 mm, which has a serious impact on the bulk elastic modulus and viscosity of the hydraulic oil in the hydraulic system. With the increase of pressure in hydraulic system, some air is mixed into hydraulic oil, and the rest still exists in gas phase. When the amount of mixed air increases, the volume elastic coefficient of hydraulic oil drops sharply, the propagation speed of pressure wave in hydraulic oil slows down, and the dynamic viscosity of hydraulic oil increases linearly. Air suspended in oil combines with hydraulic oil to form mixed liquid. The stability of this kind of oil depends on the size of bubbles, which have great influence on the hydraulic system, such as vibration, noise, pressure fluctuation, unstable hydraulic components, crawling of moving parts, inaccurate positioning or disorder. At the same time, it also increases the power consumption, accelerates the oxidation of oil and reduces the lubricating performance of oil. Solid pollutants in oil mainly exist in the form of particles. Some of these impurities remain in the process of component processing and assembly, some are produced in the working process of hydraulic components, and some come from the invasion of external impurities. Their hazards are:

(1) Various particulate impurities in oil will do harm to pumps and motors. When foreign particles enter the gear end face of a gear pump or gear motor, between two end cover side plates, between the tooth tip and the shell, between the blade and the blade groove of a vane pump or vane motor, between the rotor end face and the oil distribution plate, between the stator and the rotor (the top of the blade), between the plunger of a plunger pump or plunger motor and the plunger cylinder hole, between the rotor and the oil distribution plate, between the slider and the inclined plate, and between the sliding pair of a variable mechanism, it is possible to cause a stuck fault. Even if it does not cause a stuck fault, it will aggravate the wear. Impurity particles may also block the oil inlet filter in front of the pump, causing cavitation of the pump or causing various concurrent failures.

(2) Various particulate impurities in the oil will do harm to the hydraulic cylinder. Particulate impurities will deform and wear pistons and cylinders, piston rods and cylinder head holes, and seals, thus increasing oil leakage and reducing volumetric efficiency and effective thrust (pulling force). If particulate impurities block the piston or piston rod, the cylinder will not move.

(3) Contaminated particles in oil will do harm to various valve parts. Contaminated particles may block the slide valve or throttle valve, resulting in valve failure. Even if there is no jamming or blocking fault, contaminated particles will make the motion pair of the valve assembly wear prematurely, increase the fit clearance and deteriorate the performance.

(4) Pollutants breed bacteria, aggravate the aging of oil products, make oil products black and smelly, and further produce pollution. This vicious circle may have the following consequences:

1) pollutants block the oil filter, causing the oil pump to suck air, resulting in vibration and noise.

2) Contaminants increase the friction of oil cylinder or motor, leading to crawling.

3) Dirt will completely make the servo valve and other components with poor anti-pollution ability fail at all, at least the work will be unstable, and the lag will increase. Dirt will block the same small hole in front of the pressure gauge, so that the pressure cannot be transmitted and reflected correctly.

4) Contaminants block the channel of the pressure gauge, so that the pressure cannot be transmitted and reacted correctly.

Three, the main measures to control the pollution of hydraulic oil

In order to ensure the normal and reliable operation of hydraulic system, reduce faults and prolong life, effective measures must be taken to control the pollution of hydraulic oil.

1, control the oil temperature

Excessive oil temperature will often bring the following adverse effects to the hydraulic system:

The viscosity of (1) oil decreases, which destroys the oil film in the moving parts and increases the friction resistance, resulting in the system heating and the actuator (such as hydraulic cylinder) crawling. The decrease of oil viscosity will lead to the increase of leakage and the working efficiency of the system will be significantly reduced.

(2) After the viscosity of oil drops, its characteristics will change when it passes through the throttle, making the movement speed of the piston unstable.

(3) The high oil temperature causes the thermal expansion of the machine parts, which changes the gap between the motion pairs, causing the action to be invalid or stuck, reducing its working performance and accuracy.

(4) When the oil temperature exceeds 55 degrees Celsius, the oxidation of engine oil is intensified and the service life is shortened. According to the data, when the oil temperature exceeds 55 degrees Celsius, the service life of the oil will be shortened by half for every 9 degrees Celsius increase in temperature. Therefore, for machines with different uses and different working conditions. There should be different allowable working oil temperatures. The allowable normal working oil temperature of the hydraulic system of construction machinery is 35-55 degrees Celsius, and the highest is 70 degrees Celsius.

2. Control the filtration accuracy

In order to control the degree of oil pollution, oil filters should be installed at the oil suction port, pressure pipeline and oil inlet of servo speed regulating valve according to the different requirements of the system and components and the required filtering accuracy, so as to control the particulate pollutants in the oil and make the hydraulic system reliable in performance and stable in operation. The filtering accuracy of oil filter is generally selected according to the most sensitive components in the system.

3. Strengthen on-site maintenance management

Strengthening field maintenance management is an effective measure to prevent external pollutants from invading the system and filter out pollutants in the system.

(1) Check the cleanliness of the oil.

When checking the cleanliness of equipment, the equipment management department should also check the cleanliness of system oil, oil tank and oil filter, and establish a three-level scoring system for the cleanliness of hydraulic equipment. Check the hydraulic system of key equipment.

(2) Establish the first-level maintenance system of hydraulic system.

The equipment management department should increase the specific maintenance content of hydraulic devices when formulating the first-level maintenance content of equipment.

(3) Regular oil sampling and testing.

Oil samples should be taken regularly and quantitatively, and the size, quantity or weight of impurity particles in unit volume oil samples should be checked, and qualitative and quantitative analysis should be made to determine whether the oil needs to be changed.

A. Sampling time: For hydraulic equipment with specified oil change cycle, the oil can be sampled and tested one week before oil change; The newly changed oil shall be sampled and tested after continuous operation 1000h; Oil for large-scale precision hydraulic equipment in enterprises should be sampled and tested after 600 hours of use.

B, when taking oil samples, oil containers should be cleaned first, and dirty containers are not allowed to be used to ensure accurate data. The specific methods for taking oil samples are as follows:

When the hydraulic system is not working (that is, in a static state), the same amount of oil samples can be taken from the upper, middle and lower parts of the oil tank, and then the test can be carried out after stirring; When the hydraulic system is working, the oil sample can be taken out from the main oil return pipe of the system; The number of oil samples required for testing is generally 300-500mL/ time; Carry out the test according to the oil test regulations, fill in the test results on the oil test list and store them in the equipment file.

4, regular cleaning

Another effective way to control oil pollution is to regularly remove the dirt inside the filter screen, filter element, oil tank, oil pipe and parts. Attention should also be paid to cleaning when disassembling components and oil pipes, and all oil ports should be sealed with plugs or plastic sheets to prevent dirt from invading the system.

5. Filter the oil regularly and control its service life.

The service life or replacement period of oil depends on many factors, including environmental conditions and equipment maintenance, oil filtration accuracy in hydraulic system and allowable pollution level. Because the engine oil is used for a long time, oil, water, dust, metal wear and so on. It will turn oil into a mixed liquid containing many pollutants. If it is not replaced in time, it will affect the normal work of the system and lead to accidents. Whether to change the oil depends on the degree of oil pollution. At present, there are three methods to determine the oil change period:

(1) Visual oil change method. It is based on the experience of maintenance personnel, according to some conventional state changes of oil (such as black oil, smelly oil, ivory oil, etc.). ), to decide whether to change the oil.

(2) Regular oil change method. According to the environmental conditions and working conditions of the site where the equipment is located, and the oil change cycle of the used engine oil, it is due to be replaced. This method is very suitable for enterprises with more hydraulic equipment.

(3) Sampling inspection method. Oil is sampled and tested regularly to determine the necessary items (such as viscosity, acid value, moisture, particle size and content, corrosion, etc.). ) and indicators, and compare the measured oil quality with the specified oil quality deterioration standard to determine whether the oil should be replaced. Sampling time: the hydraulic system of general construction machinery should be carried out one week before the oil change cycle, and the hydraulic system of key equipment should be sampled and tested every 500 hours, and the test results should be filled in the equipment technical files. Sampling inspection method is suitable for key equipment and large hydraulic equipment.

Pay attention to cleaning when changing oil, prevent stolen goods from invading the hydraulic system, and don't mix them and change them by mistake. The main requirements are as follows:

(1) The new oil to be replaced or added must be the oil specified in this system, and can only be added after laboratory tests confirm that its oil quality reaches the specified performance index.

(2) In order to keep the new oil clean, when changing the oil, the old oil in the oil tank and main pipeline should be discharged, and the oil tank, filter screen and hose should be cleaned. Oil must be filtered when refueling, and the filter screen that has been damaged by fatigue should be replaced.

(3) The amount of oil added should reach the oil mark position of the fuel tank. The refueling method is as follows: firstly, refuel to the highest oil mark of the fuel tank, start the oil pump motor, supply oil to all pipelines of the system, refuel to the oil mark of the fuel tank, then start the motor, and repeat this for many times until the oil quantity is kept within the oil mark.

Therefore, in the process of daily use and equipment maintenance, we should pay great attention to observe the quality of oil to avoid damage to equipment caused by oil pollution.

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