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Analysis of common faults of hydraulic hoses

Hydraulic hose is an important part in construction machinery such as concrete conveying pumps, excavators, loaders, etc. However, it is often not valued. When leakage, crack, rupture, loosening and other failures occur during use, it is often not analyzed. The cause of the failure was simply replacing the hose, and the same failure was repeated shortly after use. The loosening or rupture of the hydraulic hose not only wastes oil and pollutes the environment, but also affects work efficiency and even accidents, endangering human and machine safety.

Reasons for failure analysis of hydraulic hoses: improper use, unreasonable system design and unqualified hose manufacturing.

In the following, I will discuss it in detail in combination with my experience in actual work.

First, the characteristics of the pipe are analyzed. The hose is divided into rubber tube and plastic tube. Plastic tube is cheap and easy to install, but it has poor bearing capacity and is easy to age. It is only suitable for oil return pipe or drain pipe. Rubber tube is divided into two types: high pressure hose and low pressure hose. The outer rubber layer, steel wire reinforcement layer, middle rubber layer and inner rubber layer are composed of four parts. The inner rubber layer is in direct contact with the hydraulic oil. One is made of synthetic rubber. The load-bearing capacity of the hose depends on the reinforcement layer, which is the skeleton of the hose and is usually woven or wound with steel wire. Most medium and high voltage steel wire braids are used as the skeleton, and high voltage and ultra high pressure are mostly steel wire wound skeletons. The outer rubber layer is generally made of abrasion-resistant and corrosion-resistant rubber to ensure that the reinforcing layer is not subject to mechanical damage, chemical corrosion, moisture and rust. Low-pressure rubber hoses use braided cotton and twine instead of braided steel wires. pipeline.

Hose failures are generally manifested in the following four aspects: failure of the outer rubber layer, failure of the inner rubber layer, failure of the reinforcement layer, and failure at the rupture. The failure causes and countermeasures of hydraulic hoses are as follows:

1. Failure of the outer rubber layer

(1) Cracks on the appearance of the hose

The main cause of cracks in the appearance of the hose is that the hose is bent in a cold environment. If there is a crack on the hose surface, pay attention to whether there is a crack in the hose rubber and decide whether to replace the hose immediately. Therefore, do not move the hose or dismantle the hydraulic system at will in cold environments, and perform it indoors if necessary. If you need to work in colder environments for a long time, you should use cold-resistant hoses.

(2) Bubbles appear on the outer surface of the hose. The reason for the bubbles appearing on the outer surface of the hose is that the production quality of the hose is unqualified or it is used improperly at work. If the bubbling occurs in the middle of the hose, it is mostly a quality problem of the hose production, and the qualified hose should be replaced in time; if the bubbling occurs at the joint of the hose, it may be caused by improper installation of the joint. There are two types of hose joints: detachable and crimping. The detachable pipe joint is made into a hexagon on the outer sleeve and the joint core, which is convenient for frequently disassembling the hose. The crimping pipe joint is composed of the joint outer sleeve and the joint core. The outer rubber layer must be peeled off during assembly, and then buckled on the special equipment. , So that the hose gets a certain amount of compression.

(3) The hose is not broken but there is a large amount of oil leakage. The hose has a lot of oil leakage but no fracture is found. The reason is that when the high-pressure liquid flow passes through the hose, the inner rubber is eroded and abraded until a large area leaks out of the steel wire layer, causing a large Oil oozing. This fault usually occurs at the bend of the pipeline. Check and replace the hose, and ensure that the bending radius of the hose in use meets the technical requirements.

(4) The outer rubber layer of the hose is seriously deteriorated, and micro cracks appear on the surface. This is a manifestation of the natural aging of the hose. Due to aging and deterioration, the outer layer is continuously oxidized so that its surface is covered with a layer of ozone, which thickens with time. As long as the hose is slightly bent during use, micro cracks will occur. In this case, the hose should be replaced.

2. Failure of the inner rubber layer

(1) The inner rubber layer of the hose is hard and has cracks: the main reason is that plasticizers are added to rubber products to make the hoses flexible and plastic. However, the hose is overheated and will cause plasticizer to overflow. In addition, when the superheated oil passes through the cylinder, valve or other components in the system, if a large pressure drop occurs, the oil will decompose, causing the rubber layer in the hose to oxidize and harden. In this case, you should first check whether the operating temperature of the system is normal, whether the valve throttling point, the pump suction channel is unobstructed, etc., remove all factors that cause the high oil temperature and decompose the oil, and then replace the hose. If the inner rubber just below the rupture of the hose is hard, and the upper is well maintained, this is because the excessive pressure of the hose rupture, excessive bending or twisting will cause the oil to heat up and accelerate oxidation, resulting in inner rubber hair. hard.

(2) The inner rubber layer of the hose is severely deteriorated and swelled significantly: The inner rubber layer of the hose is severely deteriorated. The reason for the obvious swelling is that the rubber material of the hose is incompatible with the oil of the hydraulic system, and the hose is deteriorated by chemical action. . If this occurs, the fuel tank should be inspected, as broken rubber pieces may be found at the oil return port. In this regard, check the compatibility of the system oil with the material of the hose rubber and whether the working temperature meets the standards.

3. Failures manifested in the reinforcement layer

(1) The hose is broken, and the rusted braided steel wire near the break is broken. The outer rubber layer is peeled off, and it is found that the braided steel wire near the break is rusted. This is mainly due to the effect of this layer on damp or corrosive substances, which weakens the strength of the hose and causes it to rupture under high pressure. When this happens, the outer rubber layer is usually accompanied by cracks, abrasions, or severe deterioration, causing the outer layer to lose its protective effect on the reinforcing layer. In this regard, the mechanical damage, chemical corrosion and high temperature baking of the outer rubber layer must be checked and eliminated before replacing the hose. However, there is also a case where the outer rubber layer is maintained well and the reinforcement layer rusts and breaks. The rupture is usually within 200mm from the hose joint number. The reason is mostly that the joint is unqualified, and moisture enters the inner edge of the jacket. The reinforcing layer absorbs moisture, which leads to rust and reduces the pressure resistance of the hose and breaks.

(2) The reinforcing layer of the hose is not rusted, but the broken wire appears irregularly in the reinforcing layer. The hose was cracked, and the reinforcement layer was not rusted when the outer rubber layer was peeled off. However, irregular broken wires appeared in the length direction of the reinforcement layer. The main reason was that the hose was subjected to high-frequency impact. There are many intersections between the steel wire and the steel wire of the braided reinforcing layer. When the pressure in the pipe changes greatly, these intersections also shift with the change of the pipe diameter, and the steel wires rub against each other. If the hose is subjected to high-frequency impact pressure, frequent friction occurs at the intersection, causing the wire to break. For hoses that are frequently subject to high frequency shocks, a steel wire wound skeleton should be used as the reinforcing layer.

4. Malfunctions at the rupture

(1) One or more of the hoses are broken, the cracks are neat, and the other parts are kept well. The main reason for this phenomenon is that the system pressure is too high and exceeds the pressure resistance of the hose. If the system pressure meets the requirements, check whether the actual pressure resistance of the hoses used meets the design requirements.

(2) Torsion occurs at the rupture of the hose. This phenomenon occurs due to excessive twisting of the hose during installation or use. After the hose is twisted, the structure of the reinforcement layer is changed, and the gap between the braided steel wires is increased, which reduces the pressure resistance of the hose, and the hose is easily broken under high pressure. Therefore, during the use or storage of the hose, do not subject the hose to torsional torque. When installing the hose, try to keep the axis of the two joint numbers on the plane of movement to prevent the hose from being twisted during movement.

(3) The steel wire at the break of the hose is curled, and there is obvious necking phenomenon near the break. The phenomenon of loosening of the reinforcing layer is that the hose is subjected to excessive tensile deformation and the layers are separated, which reduces the compressive strength. Under the action of high pressure, the hose will contract or extend in the length direction, and the normal expansion and contraction amount is + 2% to 4% under normal conditions. If the hose is selected too short during installation, it will be greatly stretched during work, and serious failures such as cracking or loosening will occur; in addition, if the hose span is too large, the hose's own weight and oil weight will It will give a large tensile force to the hose, and the above-mentioned failure will also occur in severe cases. Solution: If the installation is too tight, replace the lengthened new pipe; if the span is too large, add an intermediate support clip.

In summary, through the above analysis, the following matters should be paid attention to when using hydraulic hoses:

① Hose layout should avoid heat source as far as possible, and stay away from the performance of engine exhaust pipe. If necessary, a device such as a sleeve or a protective screen can be used to prevent the hose from being deteriorated by heat.

② Where hoses must cross or where friction may occur with mechanical surfaces during work, protective devices such as hose clamps or springs should be used to prevent damage to the outer layer of the hose.

③ When the hose must be bent, the bending radius should not be too small, it should be greater than 9 times the outer diameter. There should be a straight line at the joint between the hose and the joint that is greater than 2 times the outer diameter of the pipe.

④ When installing the hose, avoid being in a tight state. Even if there is no relative movement of the two ends of the hose, keep the hose loose. The tensioned hose will expand under pressure and the strength will decrease.

⑤ Do not twist the hose during installation. A slight twist of the hose may reduce its strength and loosen the joint. The joint should be tightened on the hose instead of the joint during assembly.

⑥ If the hose is configured on key parts, it is recommended to regularly check or replace it.