Common faults and cause analysis of hydraulic hoses

Hydraulic hoses are important components in construction machinery such as concrete pumps, excavators, loaders, etc. However, they are often not given enough attention. When faults such as leakage, cracking, rupture, and loosening occur during use, the cause of the fault is often not analyzed and the hoses are simply replaced. Shortly after use, the same faults may occur again. The loosening or rupture of hydraulic hoses not only wastes oil and pollutes the environment, but also affects work efficiency and even causes accidents, endangering human and machine safety.

Reasons for hydraulic hose failure analysis: 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 pipe material. Flexible hoses are divided into rubber hoses and plastic hoses. Plastic hoses are cheap and easy to install, but have poor load-bearing capacity and are prone to aging. They are only suitable for return or drain pipes; Rubber hoses are divided into two types: high-pressure hoses and low-pressure hoses. High pressure hoses generally consist 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 in direct 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 wrapped 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 wrapped skeletons. The outer rubber layer is generally made of rubber with good wear resistance and corrosion resistance to ensure that the reinforcement layer is not mechanically damaged, chemically corroded, or damp rusted. Low pressure rubber hoses are replaced with woven cotton and hemp thread instead of woven steel wire, and are generally used for low-pressure return oil pipelines.

Hose failures generally manifest in the following four aspects: failure of the outer adhesive layer, failure of the inner adhesive layer, failure of the reinforcement layer, and failure at the rupture port. 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 whether there are cracks in the rubber inside the hose, and decide whether to replace the hose immediately. Therefore, do not move hoses or disassemble hydraulic systems in cold environments. If necessary, they should be carried out indoors. If you need to work in a colder environment for a long time, you should replace it with a cold resistant hose.

(2) The reason for the appearance of bubbles on the outer surface of the hose is due to poor production quality or improper use during operation. If bubbles appear in the middle section of the hose, it is often a quality issue with the hose production, 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 connector numbers: detachable and crimping. The detachable pipe joint is made into a hexagonal shape on the outer cover and joint core, which facilitates frequent disassembly and assembly of the hose; The crimping type pipe joint consists of a joint jacket and a joint core. During assembly, the outer adhesive layer must be peeled off and then crimped on specialized equipment to achieve a certain compression of the hose.

(3) The hose is not broken but has a large amount of oil leakage. The hose has a large amount of oil leakage but no rupture was found. The reason is that when high-pressure fluid flows through the hose, the inner rubber is eroded and scratched, until a large area of steel wire layer leaks out, causing 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 is continuously oxidized to cover 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. Malfunction of the inner adhesive layer

(1) The rubber layer inside the hose is hard and has cracks: the main reason is that plasticizers are added to the rubber products, making the hose flexible and pliable. But if the hose overheats, it will 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 rubber layer inside the hose. In this situation, it is necessary to first check whether the system's operating temperature is normal, whether the valve throttle and the pump's suction passage are unobstructed, and eliminate all factors that cause high oil temperature and oil decomposition before replacing the hose. If only the inner rubber below the hose rupture becomes hard, while the upper part remains good, it is because the hose rupture is excessively flattened, bent too quickly, or twisted, causing the oil to heat up and accelerate oxidation, resulting in the inner rubber becoming 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 because the rubber material inside the hose is incompatible with the hydraulic system oil, causing the hose to deteriorate due to chemical reactions. If this phenomenon occurs, the fuel tank should be checked as broken rubber pieces may be found 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. Malfunctions manifested in the reinforcement layer

(1) The hose is broken, and the woven steel wire near the break is rusted. The hose is broken, and the outer rubber layer is peeled off for inspection. It is found that the woven steel wire near the break is rusted. This is mainly due to the influence of moisture or corrosive substances on the layer, which weakens the strength of the hose and leads to rupture under high pressure. When this situation occurs, it is usually accompanied by phenomena such as breakage, abrasion, or severe deterioration of the outer layer, causing the outer layer to lose its protective effect on the reinforcement layer. Before replacing the hose, it is necessary to first inspect and eliminate all adverse factors such as mechanical damage, chemical corrosion, and high-temperature baking on the outer adhesive layer. However, there are also cases where the outer adhesive layer remains in good condition and the reinforcement layer rusts and ruptures, and the rupture is usually within 200mm of the hose joint number. The reason for this is mostly due to the unqualified joint, where moisture enters the inner edge of the jacket, causing the reinforcement layer to absorb moisture and rust, resulting in a decrease in the pressure resistance strength of the hose and rupture.

(2) The reinforcement layer of the hose is not rusted, but there is irregular wire breakage in the reinforcement layer. The hose ruptured, 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 in the woven reinforcement layer. When the pressure inside the pipe changes significantly, these intersections also move with the change of 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 often subjected to high-frequency impacts, a steel wire wrapped skeleton should be selected as the reinforcement layer.

4. Malfunctions manifested at the rupture site

(1) The hose has one or more ruptures, 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 ensure that it meets the design requirements.

(2) Twisting occurs at the site of hose rupture, which is caused by excessive twisting of the hose during installation or use. After the hose is twisted, the reinforcement layer structure changes, and the gap between the woven steel wires increases, reducing the pressure resistance strength 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 twisting torque. When installing hoses, try to keep the axis of the two connection heads on the plane of motion as much as possible to avoid twisting the hoses during movement.

(3) The steel wire at the rupture port of the hose is curled, and there is a noticeable necking phenomenon near the rupture port. The reason for this phenomenon is that the hose is subjected to excessive tensile deformation, causing the layers to separate and reducing the compressive strength. The hose will shrink or elongate in the length direction under high pressure, usually by+2% to 4% of the normal expansion and contraction. If the hose is installed too short, it will experience significant stretching during operation, and in severe cases, it may break or loosen; 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 large tensile force, and in severe cases, the above-mentioned faults may occur. Solution: If the installation is pulled too tightly, a new extended pipe should be replaced; If the span is too large, intermediate support clamps should be added.

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

① The arrangement of hoses should try to avoid heat sources and stay away from the engine exhaust pipe. If necessary, sleeves or protective screens can be used to prevent the hose from deteriorating due to heat.

② 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.

③ 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 hose and the joint that is greater than twice the outer diameter of the pipe.

④ When installing a hose, it should be avoided to be in a taut state. Even if there is no relative movement between the two ends of the hose, the hose should be kept loose. A tensioned hose will expand under pressure, reducing its strength.

⑤ Do not twist the hose during installation. Slight twisting of the hose may reduce its strength and loosen the joint. When assembling, the joint should be tightened on the hose instead of the hose.

⑥ If the hose is installed on critical components, it is recommended to conduct regular inspections or replacements.