2 Common Hydraulic Hose Assembly failure and Fault Analysis

Way Liu
3 Companies Founder: 11 Years Hydraulic Hose Assembly & International Trade Operate(B2B) & . Also an Honest Partner For Your China Sourcing.
Abstract: the problems of oil and hose failure often occur in the use of hydraulic hose assembly. through long-term follow-up research, it is concluded that the common failure modes are mainly two types of hose assembly buckle and hose raw material failure. this paper makes a systematic analysis of these two kinds of failure problems in the construction machinery industry.
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Keywords: failure cause analysis; hose assembly; crimping process; leakage failure mode analysis

Introduction

Hydraulic hose assembly is the basic component of the hydraulic transmission system, which is mainly used in construction machinery, mining machinery, coal mining machinery, and automobile and other equipment. The serviceability of the hydraulic hose assembly has an important influence on the stability and reliability of the whole hydraulic system. Only by analyzing the causes of its failure from the raw materials of the hose itself and the working procedures in the production process of the assembly, can the failures that often occur in the use of the hose assembly be reduced or eliminated.

1. Failure analysis of rubber hose.

The hydraulic hose is usually composed of three parts: the inner rubber layer, the outer rubber layer, and the reinforcement layer. The specific structure is shown in figure 1. The quality of the hose directly affects the service performance and service life of the hose assembly, which ensures the reliability of the hydraulic hose assembly.

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Hydraulic Hose Assembly
Hydraulic Hose Assembly 3
hydraulic hose assembly

The inner rubber layer is in the innermost position of the hydraulic hose, which is usually made of durable synthetic rubber, which ensures the sealing of the hose, and its main function is to protect the reinforced layer from erosion and the conveying medium to bear pressure.

The reinforcement layer is the insulation separation layer of the hose assembly and acts as a central skeleton, which is located between the inner rubber layer and the outer rubber layer to protect the service pressure of the hose and the stability of the hose strength. According to the difference in the processing process, its structure is usually divided into two types: steel wire braiding and winding reinforcement: as shown in figures 2 and 3, respectively.

On the other hand, the outer glue layer plays the role of protecting the reinforced layer so that the reinforced layer will not be damaged.

Through the long-term use practice and the physical and chemical test of the damaged rubber hose, it is found that the common failure causes of the rubber hose are mainly as follows:

  • Both the inner and outer layers of the hose are broken, but the synthetic rubber is soft at room temperature: the reason analysis is that this failure is caused by the bending of the hose at a very low ambient temperature, as shown in figure 4.
the cracked hose
cracked hose 4
  • The lateral blasting of the bending section of the rubber hose has an oval cross-section: when this occurs in the intake pipe of the pump, the pump will make noise or heat. The discharge pipe of the pump becomes hard and brittle: the reason is that the installation bending radius of the hose is smaller than its allowable bending radius, which is often the cause of this malfunction. Check the minimum bending radius of the hose assembly and ensure that the hose is used correctly. In the oil intake pipeline of the pump, the local bending and flattening of the rubber hose will make the pump generate gas holes, increase the noise and increase the temperature rise. Is a very serious situation, if not corrected, it will lead to serious damage to the pump.
  • When the hose is flattened or twisted in one or two places, it will break and twist in some positions: the reason is that the torsion of the hydraulic hose will loosen the gold wire interlayer, thus increasing the distance between the braided wire strands and causing the hose to break. It is necessary to ensure that there is no twisting force on the hydraulic hose at any time in order to avoid this phenomenon, as shown in figure 5.

Hose bad 1
Hose bad 5
  • The outer skin of the hose blisters and the oil can be found after puncturing the bubbles: the analysis of the reason is that the tiny needle hole in the inner layer of the hose will let the high-pressure oil infiltrate between it and the outer skin, and finally form bubbles in the weak bond between the outer skin and a mezzanine. For detachable pipe fittings, due to insufficient lubrication between the hose and the joint, the dry pipe wall may be torn by sticking to the rotating screw, which is enough to form leakage and cause the above situation. This can also happen to other damage to the hose.
  • Serious damage due to the expansion of the inner layer of the hose, as shown in figure 6: the reason analysis is that the material of the hose is not compatible with the liquid transported, and the compatibility of the hose should be explained to the supplier, or the sample of the working liquid should be analyzed in advance. In addition, make sure that the working temperature inside and outside the hose does not exceed the recommended value.
hose inner
Hose expansion 6
  • The rubber hose is broken, the outer skin is seriously damaged, and the rubber surface is cracked: the original analysis is the common phenomenon of rubber aging, and the cracking phenomenon is the result of natural aging and oxidation that exceeds the lifetime of the rubber hose.
  • The winding sandwich hose is burst and completely split, and the wire is tangled together: the main reason is that the length of the hose is too short to adapt to the length change under the working pressure of the hydraulic system.
  • The hose is broken, but there is no sign that the wire has broken many times on the whole length of the hose. The hose may also break in many places: the main reason is that the pressure of the system may have exceeded the minimum burst strength of the hose, so it is necessary to choose a hose with higher strength. On the other hand, it is also necessary to eliminate the overpressure fault of the hydraulic circuit.
  • After the outer rubber layer is seriously worn, the hose blisters cause oil leakage, so appropriate protection should be taken for the hose assembly in this part.

2. failure analysis of buckling place.

The most important technological factor affecting the quality of buckle hose assembly is the buckling process. In actual production, we often encounter leakage, pull-out failure, and bursting at the buckle place of the rubber hose jacket, according to the daily follow-up analysis.

The leakage or pull-out of the hose assembly is generally related to the quality of the raw materials of the hose itself, and whether the amount of buckle in the buckling process is reasonable or not is the key point that directly affects the overall performance of the hose assembly. It is proved by practice and experiments that the failure form of leakage or pullout at the buckle place of the high-pressure hose assembly accounts for 20% of the total failure form, so the accurate control of the buckle pressure is the main measure to control this failure form.

First of all, the factors affecting the buckling amount are closely related to the hardness and permanent deformation of the internal adhesive material, such as high hardness, small permanent deformation after compression, and the better the sealing performance after buckling, which has been proved by practice.

At 70-85 Shaw hardness, the compression permanent deformation of 35-50% is the best, and the thickness of the inner glue layer is 1.5-2.5. Too thick will increase its flow during buckling, resulting in the accumulation of excess glue in the contact end face of the joint core, jacket, and hose, reducing the cross-sectional area of the fluid, and fracturing when it is too thin.

Secondly, the uniformity of the thickness of the inner glue layer is also very important. if the thickness is not uniform, it is easy to cause one side fracture on the surface of the inner glue layer after compression, and the leakage phenomenon will be caused by stacking glue on the other side.

Once again, the appearance of pockmarks on the surface of the inner glue layer is also an important factor affecting the performance and quality, so the rubber hose should sort out the questions such as the thickness and uniformity of the inner glue layer and whether there are pockmarks or not before assembly.

Analyzing the thickness and uniformity of the inner glue layer from the above phenomena, the buckling quality of the whole hose assembly is very important. in addition, it is necessary to correctly control the process of clasping the jacket, in general, the leakage and pull-out failure of the hose assembly and the bursting at the tail of the buckle sleeve are mainly due to the following reasons:

  • First, if the amount of buckle is not enough, there will be leakage in the front buckle groove and the tail of the sleeve in contact with the rubber hose, and serious pullout will occur.
  • Second, the outer glue layer of the high-pressure oil pipe whose reinforcement layer is wound steel wire is thick during buckling, and when the structure design of the sleeve and the joint core is a tooth structure of stripping glue, the outer glue layer needs to be stripped, and the tool is not allowed to damage the steel wire layer. After stripping, the steel wire layer can be allowed to have a uniform glue layer of W0.2 mm, so that the strengthening layer is in direct contact with the sawtooth groove of the sleeve, and the sleeve shrinks under the action of external force. It will firmly buckle the steel wire of the reinforcement layer and prevent it from being pulled out.
  • Third, especially the reinforced layer hose assembly for non-stripping and buckling steel wire braiding or winding, in addition to the special tooth structure design of the sleeve and joint core, we should also pay attention to checking whether the outer rubber thickness of the rubber hose exceeds the prescribed range:
  1. Braided pipes generally stipulate that the maximum unilateral thickness of the outer rubber layer of this kind of rubber hose is W1.2 mm. If it exceeds it, it will be regarded as unqualified.
  2. the winding pipe assembly with non-stripping buckling or partial stripping is shown in figure 7. When the rubber hose is selected in the production process, there will also be a certain requirement for the thickness of the external rubber layer. in general, it is necessary to summarize the corresponding allowable maximum thickness of the outer rubber layer after the durability test of the winding tube assemblies with different sizes from small to large. If the thickness of the outer rubber layer is too thick, after buckling, the grip on the outer rubber layer of the sleeve will be difficult to transfer to the steel wire of the reinforcement layer, which can cause the pull-out phenomenon. According to the regular production process, the ideal external glue thickness of some winding pipes is summarized, as shown in Table 1.
Non skiving hose assembly construction 2
Non-skiving hose assembly construction 7
Hose ID φ13 φ16 φ19 φ25 φ32 φ38
Outer rubber thickness 1~1.6 1~1.6 1~1.6 1.4~2 1.4~2 1.4~2
Table 1

As mentioned earlier, the buckling process is the most critical factor affecting the performance of the hose assembly. There are also many factors that affect the buckling amount, which mainly lies in the reasonable formulation of the buckle size of the sleeve according to the manufacturing parameters of the hose itself, that is, the dimensional tolerance of the hose, the plasticity of the rubber and the hardness of the reinforced layer.

3. Concluding remarks.

Hydraulic hose assembly is widely used in all kinds of hydraulic systems, almost all the main engine products can not do without it, and its quality and reliability directly affect the performance of the whole host product. This paper analyzes the common faults of hydraulic hoses in use and discusses how to improve the reliability of hydraulic hoses.

It is concluded that in order to improve the reliability of the hose assembly, it is necessary to strengthen the inspection of the ex-factory quality of the hose, that is, to reduce the defects and faults of the hose; the second is to accurately grasp the degree of buckling at the interface between the hose and the joint in the process of production. and in the use of a hose to pay attention to the correct use and maintenance of these four aspects to optimize.

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