The hydraulic system is composed of a series of hydraulic components (such as hydraulic pump, hydraulic cylinder, control valve and auxiliary components). In order to ensure the normal relative movement between the components of the hydraulic components, there must be a certain fit clearance. When the oil flows through the gap, there will be oil leakage, which is called leakage. Leakage is a common problem of hydraulic equipment, it is almost impossible to make the hydraulic equipment completely without leakage. Therefore, in the hydraulic technology, the leakage is generally reduced as far as possible and limited to a certain range. Generally speaking, the leakage fault refers to the leakage beyond the allowable range.

1、 Classification and harm of leakage fault

Leakage is divided into internal leakage and external leakage. A small amount of liquid inside the hydraulic components leaks from the high pressure chamber to the low pressure chamber, which is called internal leakage. It mainly produces the flow of hydraulic valve and hydraulic pump from the high pressure chamber to the low pressure chamber. A small amount of liquid leakage from the inside of the component is called external leakage, such as oil pipe leakage and hydraulic cylinder leakage.

The harm of leakage fault to hydraulic equipment is mainly shown in the following aspects:

1. Reduce the system pressure.

2. The thrust of the actuator is insufficient, and the speed is slow or unstable, even unable to work.

3. Oil consumption increases, system efficiency decreases and oil temperature increases.

4. Operating costs and other material waste increase.

5. It pollutes the environment, worsens working conditions and hinders health.

2、 Causes of leakage in hydraulic system

Generally, the leakage of hydraulic system occurs after a period of use, which is mostly caused by the failure, damage or aging of seals. There are mainly: oil pollution, deterioration, seal deterioration, etc., resulting in excessive sealing clearance.

1. The leakage of the pipe joint is related to the low machining accuracy of the joint, which makes the pipe joint loose due to the pressure pulsation and is not repaired in time.

2. The leakage caused by the seal is related to the failure or damage of the seal, which is mainly caused by the inconsistency between the material quality and the service conditions, insufficient compression, aging, damage, insufficient geometric accuracy, improper installation of the seal, insufficient surface processing accuracy, and the failure to replace the seal in time after its service life.

3. The leakage of the shell mainly occurs in the defects of castings and weldments, and gradually expands under the pressure pulsation or impact vibration of the hydraulic system.

4. The main reasons for the leakage of the system itself are the rough assembly of the system, the lack of vibration reduction, the failure to timely check and deal with the system according to the regulations when using the system overpressure, and the failure to timely replace the vulnerable parts according to their service life.

3、 Common faults and leakage prevention methods

1. The common leakage faults and prevention methods of hydraulic pump (1) the internal radial and axial clearance caused by machining size, shape and position error, improper assembly and wear is too large, which causes the internal leakage of oil from high pressure area to low pressure area. Troubleshooting: find out the cause, eliminate the symptoms, and remove the unnecessary radial and axial clearance.

(2) Due to the gap caused by uneven joint surface, loose connection, burr and dirt, the internal and external leakage of hydraulic pump is caused. Elimination method: correct and clean the joint plane, ensure that the plane is closely combined, and eliminate the unnecessary gap.

(3) The form, structure, material and size of the seal, the structure and size of the installation position, improper installation and wear cause the seal failure and the internal and external leakage of the hydraulic pump. Troubleshooting: find out the cause of seal failure, and eliminate it to ensure the sealing effect.

(4) The internal and external leakage of hydraulic pump is caused by the sand hole and air hole of pump body and cover. Troubleshooting: replace the parts with air holes and sand holes.

(5) The internal parts are damaged and the sealing of corresponding parts is damaged, causing internal leakage. Troubleshooting: replace the damaged parts.

2. Common leakage faults and prevention methods of control valve

(l) Due to the machining size and shape and position error of the parts, the clearance between the inner hole of the valve body and the valve core is too large, the sealing effect of the sealing ring between the high and low pressure cavities is not good, and the sealing of the cone valve seat is not tight, causing internal leakage. Troubleshooting: find out the existing problems and eliminate them to ensure the normal operation of the valve.

(2) The valve body and valve cover end face is uneven, with burr, dirt, no paper pad or paper pad damage and connection is not tight, resulting in improper clearance, causing internal and external leakage. Elimination method: find out the existing problems, and eliminate the gap between the joint surfaces.

(3) There are sand holes and air holes in the valve body and cover, causing internal and external leakage. Troubleshooting: replace the parts with air holes and sand holes.

(4) The improper selection of the spring in the valve and the inappropriate size of the valve body and spool make the sealing part in the valve unable to be sealed and cause internal leakage. Troubleshooting: replace the spring, repair or replace the valve body and valve core to ensure the sealing of the sealing parts.

(5) Due to the improper thread processing of the oil sealing screw plug on the valve body and valve cover, or the screw is not tightened, the external leakage is caused. Troubleshooting: replace or tighten the plug to eliminate leakage.

3. Common leakage faults and prevention methods of hydraulic cylinder

The leakage of hydraulic system accounts for 20-30, and the leakage of hydraulic cylinder accounts for about 30.

Therefore, the leakage of the hydraulic cylinder has become the main fault of the hydraulic system.

(l) Improper form, size and material of sealing elements, improper structure and size of the installation position of the seal (an example of sealing innovation of a pump), unqualified installation, damage or falling off of the seal, etc. cause the oil leakage from the high-pressure chamber to the low-pressure chamber at the piston. Troubleshooting: find out the cause and eliminate it to ensure good sealing of piston.

(2) The results show that the hole diameter of the hydraulic cylinder is too large, the straightness is too bad, the middle part is waist drum shape, and the coaxiality between the piston and the hydraulic cylinder hole is too poor, resulting in the oil leakage from the high pressure chamber to the low pressure chamber.

Troubleshooting: find out the cause, solve the problem pertinently, and eliminate the factors that affect the sealing effect.

(3) The piston of the hydraulic cylinder is not installed correctly or is blocked by dirt, which causes the piston of the hydraulic cylinder to be pulled and the oil at the piston leaks from the high pressure chamber to the low pressure chamber. Troubleshooting: repair or replace the hydraulic cylinder piston.

(4) For the hydraulic cylinder with clearance seal, the clearance between cylinder and piston is too large, which makes the oil in piston leak from high pressure chamber to low pressure chamber. Troubleshooting: re match the piston to ensure a reasonable fit clearance.

(5) The air hole and sand hole on the piston make the oil in the piston leak from the high pressure chamber to the low pressure chamber.

Troubleshooting: replace the piston.

(6) Due to the poor machining and assembly accuracy of cylinder block, cylinder head, piston and piston rod, when the hydraulic cylinder works, the cylinder block or piston bears unilateral radial force, resulting in the inclination of the axis between the piston rod and the cylinder block, causing unilateral deformation of the seal ring of the cylinder head, causing external leakage at the fitting place of the piston rod and the cylinder head. Troubleshooting: repair or replace the relevant parts to ensure the assembly accuracy; change the structure of the hydraulic cylinder or reduce the unilateral radial force on the cylinder block and piston rod.

(7) The bending deformation of the piston rod causes the unilateral deformation of the seal ring at the cylinder head, which causes the external leakage of the matching part between the piston rod and the cylinder head. Troubleshooting: correct or replace the piston rod.

(8) The sealing ring at the cylinder head is made of hard material, poor elasticity, small section size and improper adjustment, which leads to poor sealing and external leakage at the matching place of piston rod and cylinder head. Troubleshooting: replace or readjust to ensure that the piston rod and cylinder head meet the technical requirements.

(9) There are air holes and sand holes on the cylinder head, which cause the external leakage of the piston rod and cylinder head. Troubleshooting: replace qualified cylinder head.

4. Common leakage faults of oil tank and prevention methods

(l) The welding seam of the oil tank is not tightly welded or there are air holes and sand holes on the casting oil tank integrated with the equipment, causing external leakage. Troubleshooting: repair or replace the oil tank.

(2) The oil mark hole and screw hole on the oil tank are not sealed tightly, causing external leakage. Troubleshooting: modify the structure of the oil tank or strengthen the sealing.

(3) The oil return tank is too high and splashes. Troubleshooting: modify the structure of the oil tank, deepen the oil pool and improve the return oil.

5. Prevention and treatment of common faults, joints and pipeline leakage

(l) There are holes and crevices in the pipeline due to lax welding, and cracks are produced when quenched with water, resulting in external leakage. Troubleshooting: repair the cavity, gap and crack by welding, and eliminate the leakage.

(2) The pure copper tube cracks during cold bending, causing external leakage. Elimination method: replace pure copper tube and use hot bending process.

(3) The angle and length of the bell mouth at the end of the pure copper tube do not meet the requirements, resulting in leakage. Troubleshooting: modify the bell to meet the requirements.

(4) The section size of the sealing element at the joint is small, and the size of the mounting hole is large, which can not guarantee the due compression or leakage of the sealing element, causing external leakage. Troubleshooting: replace the sealing element, modify the size of the seat hole, and re install the sealing element.

(5) During installation, the connection plane is inclined; the sealing element is not installed in the seat hole or screwed, the connection is not tight, causing external leakage. Troubleshooting: eliminate unreasonable installation and ensure the integrity of seals.

(6) During transportation, the equipment vibrates and relaxes, causing external leakage. Troubleshooting: re tighten the connection part.

(7) The joint tightening torque is not enough, causing leakage. Troubleshooting: increase the tightening torque to tighten it.

(8) Before assembly, there are dust, sand, scrap iron and other dirt on the thread, causing leakage. Troubleshooting: carefully clean the thread before assembly.

(9) In the assembly process, the thread was abraded, causing leakage. Troubleshooting: carefully clean the thread before assembly; apply sealant or grease for lubrication during assembly.

(10) During assembly, due to the excessive tightening force, the joint will crack and cause leakage. Troubleshooting: replace the joint to prevent excessive tightening torque.