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Common faults and cause analysis of hydraulic hoses

Hydraulic hoses are important components in construction machinery such as concrete delivery pumps, excavators, loaders, etc. However, they are often not taken seriously. When faults such as leakage, cracks, rupture, and loosening occur during use, the cause of the fault is often not analyzed, and the hoses are simply replaced. Soon after use, the same fault will be repeated. The loosening or rupture of hydraulic hoses not only wastes oil and pollutes the environment, but also affects work efficiency and even leads to accidents, endangering human-machine safety.

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

Below, I will provide a detailed discussion based on my practical work experience.

Firstly, analyze the characteristics of the pipes. Hoses are divided into rubber pipes and plastic pipes. Plastic pipes are inexpensive and easy to install, but have poor load-bearing capacity and are prone to aging. They are only suitable for return or discharge pipes; Rubber hoses are divided into two types: high-pressure hoses and low-pressure hoses. High pressure hoses are generally composed of four parts: an outer rubber layer, a steel wire reinforcement layer, a middle rubber layer, and an inner rubber layer. The inner rubber layer is directly in contact with hydraulic oil and 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 pressure systems use steel wire braided structures as the skeleton, while high and ultra-high pressure systems often use steel wire wound structures. The outer rubber layer is generally made of wear-resistant and corrosion-resistant rubber to ensure that the reinforcement layer is not subjected to mechanical damage, chemical corrosion, moisture, and rust. Low pressure rubber hoses use woven cotton and hemp thread instead of woven steel wire, and are generally used for low-pressure oil return pipelines.

The hose failure generally manifests in the following four aspects: failure of the outer rubber layer, failure of the inner rubber layer, failure of the reinforcing layer, and failure at the rupture point. The causes and countermeasures of hydraulic hose failures are as follows:

1. Failure of the outer adhesive layer
(1) Cracks appear on the surface of the hose
The main reason for the appearance of cracks on the hose is that the hose is bent in cold environments. If cracks are found on the surface of the hose, pay attention to observing whether there are cracks in the adhesive inside the hose and decide whether to replace it immediately. Therefore, in cold environments, do not move hoses or disassemble hydraulic systems at will. If necessary, it should be done indoors. If working in colder environments for a long time, cold resistant hoses should be used instead.
(2) The reason for the appearance of bubbles on the outer surface of the hose is due to the unqualified production quality of the hose or improper use during operation. If bubbles appear in the middle section of the hose, it is mostly a production quality issue of the hose, and qualified hoses should be replaced in a timely manner; If bubbles appear at the joint of the hose, it is likely due to improper installation of the joint. There are often two types of hose joint numbers: detachable and crimped. The detachable pipe joint is made into a hexagonal shape on the outer sleeve and joint core, making it easy to disassemble and assemble the hose frequently; The buckle type pipe joint is composed of a joint outer sleeve and a joint core. During assembly, the outer adhesive layer must be peeled off, and then the hose can be compressed to a certain extent using specialized equipment.
(3) The hose is not broken but there is a large amount of oil leakage. The reason for this is that when high-pressure liquid flows through the hose, the inner glue is eroded and scratched, until a large area of steel wire layer leaks out, resulting in a large amount of oil leakage. This fault usually occurs at the bend of the pipeline, and the hose should be checked and replaced to ensure that the bending radius of the hose meets the technical requirements during use.
(4) The outer rubber layer of the hose has severely deteriorated, and there are slight cracks on the surface, which is a manifestation of natural aging of the hose. Due to aging and deterioration, the outer layer continuously oxidizes and covers its surface with a layer of ozone, which thickens over time. As long as the hose is slightly bent during use, it will produce small cracks. In this situation, the hose should be replaced.

2. Failure of the inner adhesive layer

(1) The rubber layer inside the hose is hard and has cracks: the main reason is that the addition of plasticizers in rubber products makes the hose flexible and malleable. But if the hose overheats, it can cause the plasticizer to overflow. In addition, when overheated oil passes through cylinders, valves, or other components in the system, a significant pressure drop can cause the oil to decompose, leading to oxidation and hardening of the adhesive layer inside the hose. In this situation, it is necessary to first check whether the operating temperature of the system is normal, whether the valve throttle and the pump suction passage are unobstructed, and eliminate all factors that cause high oil temperature and oil decomposition before replacing the hose. If only the inner glue below the hose rupture becomes hard while the upper part remains in good condition, it is because the hose rupture is excessively flattened, bent too quickly, or twisted, which leads to oil heating and accelerated oxidation, causing the inner glue to become hard.
(2) The rubber layer inside the hose is severely deteriorated and visibly swollen: The rubber layer inside the hose is severely deteriorated and visibly swollen due to the incompatibility between the rubber material inside the hose and the oil used in the hydraulic system, resulting in the deterioration of the hose due to chemical reactions. If this phenomenon occurs, the fuel tank should be inspected as it is possible to find broken rubber pieces at the return port. It is necessary to check the compatibility between the system oil and the rubber material inside the hose, as well as whether the working temperature meets the standards.

3. Fault manifested in the reinforcement layer

(1) The hose is broken, and the braided steel wire near the break is rusted. After peeling off the outer adhesive layer for inspection, it was found that the braided steel wire near the break is rusted. This is mainly due to the effect of moisture or corrosive substances on the layer, which weakens the strength of the hose and causes it to rupture under high pressure. When this situation occurs, it is generally accompanied by phenomena such as breakage, abrasion, or severe deterioration of the outer adhesive layer, causing the outer layer to lose its protective effect on the strengthening layer. It is necessary to first inspect and eliminate all adverse factors such as mechanical damage, chemical corrosion, and high-temperature baking of the external adhesive layer before replacing the hose. However, there are also cases where the outer adhesive layer is well maintained and the reinforcing layer rusts and ruptures, and the rupture is usually within a range of 200mm from the hose joint number. The reason for this is mostly due to the unqualified joint, where moisture enters the inner edge of the outer jacket, causing the reinforcing layer to absorb moisture, resulting in rust and a decrease in the hose's pressure resistance strength, leading to its rupture.
(2) The reinforcement layer of the hose is not rusted, but there is irregular wire breakage in the reinforcement layer. The hose broke, and no rust was found on the reinforcement layer after peeling off the outer adhesive layer. However, irregular wire breakage occurred in the length direction of the reinforcement layer, mainly due to the high-frequency impact force on the hose. There are many intersections between the steel wires of the woven reinforcement layer, and when there is a significant change in pressure inside the pipe, these intersections also move with the change in pipe diameter, causing friction between the steel wires. If the hose is subjected to high-frequency impact pressure, frequent friction occurs at the intersection, causing the steel wire to break. For hoses that are frequently subjected to high-frequency impacts, a steel wire wrapped skeleton should be selected as the reinforcement layer.

4. Faults manifested at the rupture site

(1) One or more parts of the hose are broken, with neat cracks, while other parts remain in good condition. The main reason for this phenomenon is that the system pressure is too high, exceeding the pressure resistance of the hose. If the system pressure meets the requirements, the actual pressure resistance of the hose used should be checked to see if it meets the design requirements.
(2) Twisting occurs at the point of hose rupture, which is caused by excessive twisting of the hose during installation or use. After the hose is twisted, the reinforced layer structure changes, and the gap between the woven steel wires increases, reducing the pressure resistance of the hose. Under high pressure, the hose is prone to rupture. Therefore, during the use or storage of hoses, do not subject them to torsional torque. When installing hoses, try to keep the axes of the two connectors on the moving plane to avoid twisting during movement.
(3) The steel wire at the rupture point of the hose is curled, and there is a significant necking phenomenon near the rupture. The reason for this phenomenon is that the hose is subjected to excessive tensile deformation, and the layers are separated, reducing the compressive strength. The hose undergoes contraction or elongation in the length direction under high pressure, with a general expansion and contraction amount of+2% to 4% of normal. If the hose is selected too short during installation, it will be subjected to significant stretching during operation, and in severe cases, faults such as breakage or loosening may 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 give the hose a significant tensile force, and in severe cases, the above faults can also occur. Solution: If the installation is too tight, replace the extended new pipe; If the span is too large, an intermediate support clip should be added.

In summary, based on the above analysis, the following precautions should be taken when using hydraulic hoses in the future:

The arrangement of hoses should avoid heat sources as much as possible and be away from the engine exhaust pipe. If necessary, devices such as sleeves or protective screens can be used to prevent the hose from deteriorating due to heat.
In areas where hoses must cross or may rub against mechanical surfaces during operation, protective devices such as hose clamps or springs should be used to prevent damage to the outer layer of the hose.
When 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 section at the connection between the rubber hose and the joint that is greater than twice the outer diameter of the pipe.
When installing hoses, it is important to avoid being in a tight state. Even if there is no relative movement at both ends of the hose, it should be kept loose. Tight hoses will expand under pressure and their strength will decrease.
Do not twist the hose during installation. Slight twisting of the hose may reduce its strength and loosen the joint. During assembly, the joint should be tightened on the hose instead of the joint.
If the hose is installed on critical components, it is recommended to regularly inspect or replace it.

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