Hydraulic hose is an important part of construction machinery such as concrete delivery pump, excavator, loader, etc. However, it is often ignored by people. When leakage, crack, rupture, loosening and other faults occur during use, it is often The cause of the failure and simply replacing the hose, repeats the same failure shortly after use. The loosening or rupture of hydraulic hoses not only wastes oil, pollutes the environment, but also affects work efficiency, and even causes accidents and endangers 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 based on my experience in actual work.

First, the characteristics of the pipe are analyzed. Hose is divided into rubber tube and plastic tube. Plastic tube is cheap and easy to install, but has poor bearing capacity and is easy to age. It is only suitable for oil return pipe or oil drain pipe; rubber tube is divided into high-pressure hose and low-pressure hose. It consists of four parts: outer rubber layer, steel wire reinforcement layer, middle rubber layer and inner rubber layer. The inner rubber layer is in direct contact with the hydraulic oil and is made of synthetic rubber. The load-carrying capacity of the hose depends on the reinforcement layer, which is the skeleton of the hose and is usually braided or wound with steel wire. Most of the medium and high pressure use steel wire braid as the skeleton, and the high pressure and ultra-high pressure mostly use steel wire winding skeleton. The outer rubber layer is generally made of rubber with good wear resistance and corrosion resistance to ensure that the reinforcement layer is not damaged by mechanical damage, chemical corrosion, moisture and rust. The low-pressure hose is made of braided cotton and twine instead of braided steel wire, which is generally used for low-pressure oil return. pipeline.

The failure of the hose is generally manifested in the following four aspects: the failure of the outer rubber layer, the failure of the inner rubber layer, the failure of the reinforcement layer, and the failure of the rupture port. The reasons and countermeasures for the failure of hydraulic hoses are as follows:

1. Failure of the outer rubber layer

(1) Cracks appear on the surface of the hose

The main reason for cracks on the surface of the hose is that the hose is bent in a cold environment. If there are cracks on the surface of the hose, pay attention to observe whether the inner rubber of the hose has cracks, and decide whether to replace the hose immediately. Therefore, in a cold environment, do not move the hose or dismantle the hydraulic system at will. If necessary, it should be done indoors. If you need to work in a cold environment for a long time, you should use a cold-resistant hose.

(2) Bubbling on the outer surface of the hose The reason for the bubbling on the outer surface of the hose is that the quality of the hose is not up to standard, or it is used improperly during work. If bubbling occurs in the middle of the hose, it is mostly a quality problem in the production of the hose, and the qualified hose should be replaced in time; if the bubbling appears at the joint of the hose, it is likely due to improper installation of the joint. There are two types of hose joints: detachable and crimped. The detachable pipe joint is made into a hexagonal shape on the jacket and the joint core, which is convenient for frequent disassembly and assembly of the hose; the crimped pipe joint is composed of the joint jacket and the joint core. When assembling, the outer rubber layer must be peeled off, and then crimped on special equipment. , so that the hose gets a certain amount of compression.

(3) The hose is not broken, but a large amount of oil seeps. The hose sees a lot of oil but no rupture is found. The reason is that when the high-pressure liquid flows through the hose, the inner rubber is eroded and scratched until a large area leaks out of the steel wire layer, resulting in a large amount of leakage. Oil seepage. This fault generally occurs at the bend of the pipeline, and the replacement hose should be checked and ensured that the bending radius of the hose meets the technical requirements in use.

(4) The outer rubber layer of the hose is seriously deteriorated, and there are micro-cracks on the surface, which is the manifestation of the natural aging of the hose. Due to aging and deterioration, the outer layer is continuously oxidized to cover the surface with a layer of ozone, which thickens over time. As long as the hose is slightly bent during use, tiny 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 the addition of plasticizers in rubber products makes the hose flexible and plastic. But the hose is overheating and the plasticizer will spill out. In addition, when the superheated oil passes through the cylinders, valves or other components in the system, if a large pressure drop occurs, the oil will decompose, causing the inner rubber layer of the hose to oxidize and harden. In this case, you should first check whether the working temperature of the system is normal, whether the throttling of the valve, whether the oil suction channel of the pump is smooth, etc., and replace the hose after eliminating all factors that cause the oil temperature to be too high and the oil to decompose. If the inner rubber just below the rupture opening of the hose is hard, but the upper part is kept well, this is because the phenomenon of excessive flattening, too fast bending or twisting at the rupture of the hose will cause the oil to heat up and accelerate oxidation, causing the inner rubber to break. hard.

(2) The inner rubber layer of the hose is seriously deteriorated and obviously swollen: the inner rubber layer of the hose is seriously deteriorated, and the reason for the obvious swelling is that the material of the inner rubber of the hose is incompatible with the oil used in the hydraulic system, and the hose is deteriorated by chemical action . If this happens, the fuel tank should be checked, as it is possible to find pieces of rubber at the return port. In this regard, the compatibility of the system oil with the material of the inner rubber of the hose and the operating temperature should be checked.

3. The failure of the reinforcement layer

(1) The hose is broken, and the braided steel wire is rusted near the break. The hose is broken, and the outer rubber layer is peeled off for inspection. It is found that the braided steel wire near the break is rusted. This is mainly due to the layer being exposed to moisture or corrosive substances, weakening the hose and causing it to rupture at high pressure. When this happens, it is generally accompanied by the rupture, abrasion or serious deterioration of the outer rubber layer, which makes the outer layer lose its protective effect on the reinforcement layer. In this regard, all unfavorable factors such as mechanical damage, chemical corrosion, high temperature baking, etc. of the outer rubber layer must be checked and eliminated, and then the hose is replaced. However, there are also cases where the reinforcement layer is rusted and ruptured even though the outer rubber layer is well maintained, and the rupture opening is usually within 200mm of the hose joint. The reinforcement layer absorbs moisture, causing rust to reduce the compressive strength of the hose and rupture.

(2) The reinforcement layer of the hose is not rusted, but the reinforcement layer has irregular broken wires. The hose was broken, and the outer rubber layer was peeled off, and no rust was found on the reinforcement layer, but 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 force. There are many intersections between the steel wires of the braided reinforcement layer. When the pressure in the pipe changes greatly, these intersections also move with the change of the pipe diameter, and the steel wires rub against each other. If the hose is subjected to high-frequency shock pressure, frequent friction occurs at the intersection, causing the wire to break. For hoses that are often subjected to high-frequency shocks, steel wire wound skeletons should be used as reinforcement layers.

4. The failure manifested in the rupture

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

(2) Torsion occurs at the rupture of the hose. The reason for this phenomenon is that the hose is excessively twisted during installation or use. After the hose is twisted, the structure of the reinforcement layer changes, and the gap between the braided steel wires increases, which reduces the compressive strength of the hose, and the hose is easy to rupture under the action of high pressure. Therefore, in the process of using or storing the hose, do not subject the hose to torsional moment. When installing the hose, try to make the axis of the two joint numbers on the moving plane to avoid the hose being twisted during movement.

(3) The steel wire at the rupture opening of the hose is curled, and there is an obvious necking phenomenon near the rupture. The reason for the phenomenon that the reinforcing layer is loose is that the hose is excessively stretched and deformed, and the layers are separated, which reduces the compressive strength. Under the action of high pressure, the hose will shrink or elongate in the length direction, and the general expansion and contraction amount is +2% to 4% of the normal state. If the hose is too short during installation, it will be greatly stretched during work, and in severe cases, failures such as rupture or loosening will occur; in addition, if the span of the hose is too large, the weight of the hose and the weight of the oil will also be affected. It will give the hose a large tensile force, and the above failure will also occur in severe cases. Solution: If the installation is too tight, a new extended pipe should be replaced; if the span is too large, an intermediate support clip should be added.

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

①The hose arrangement should try to avoid the heat source and keep away from the performance of the engine exhaust pipe. If necessary, a casing or a protective screen can be used to prevent the hose from being heated and deteriorated.

②Where the hose must cross or may rub against the mechanical surface during operation, use protective devices such as hose clamps or springs to avoid damage to the outer layer of the hose.

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

④When installing the hose, it should be avoided to be in a tensioned state. Even if there is no relative movement at both ends of the hose, the hose should be kept loose. The tensioned hose will expand under the action of pressure and the strength will be reduced.

⑤ Do not twist the hose during installation. A slight twist on the hose can reduce its strength and loosen the fitting. Assembly should be done with the fitting tightened on the hose, not the hose on the fitting.

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