Hydraulic transmission is mainly the use of liquid pressure energy to transfer energy, and the transfer of pressure energy is inseparable from a variety of rubber hoses, rubber hoses according to the standard, pressure, temperature, media is divided into many types, its selection and design to understand the design needs of the system, but also need to understand the hose and joint manufacturers of product information, more complex and prone to problems, This paper mainly studies the design and selection method of hydraulic hose, the common hydraulic pipeline failures and their causes.1. Selection and design of hydraulic hoseAfter the performance of the hydraulic system, the service life of the equipment and components, and the cost budget are clear, the hydraulic hose assembly that needs to be installed in the design of the hydraulic system generally includes five elements: size, temperature, application, medium and pressure.1.1 DimensionsHydraulic systems transfer energy by transferring fluids with different pressures and flows. In order to minimize pressure loss and avoid system damage caused by excessive heating, the size selection of hoses and joints is very important. The inner diameter d of the pipeline is generally selected according to the flow rate.Where, qv is liquid flow rate m3/s; v indicates the flow rate m/s.Recommended flow rate: suction pipe mouth 1~2m/s(generally less than 1m/s); Pressure tubing take 3~6m/s(high pressure, short pipeline or small hydraulic oil viscosity can take a large value, conversely, small value, local or special cases can be v≤10m/s); The return pipe takes 1.5~2.5m/s.1.2 TemperatureHose selection should consider the temperature of the medium transported inside the hose and the temperature of the external working environment of the hose. Select hoses in the temperature range according to the parameters. The external temperature of the hose working environment affects the service life of the hose assembly. In this condition, the user needs to increase the frequency of pipe inspection. If the rubber layer of the hose is found to be brittle or cracked, the hose or hose assembly must be replaced.Under the comprehensive condition of high temperature and high pressure, the life of the hose will be greatly shortened. At this time, the tube with a high temperature resistant outer rubber layer should be selected. Choose a Pike hose with HT(HighTemperature) mark or hose code end 6, such as 661HT, 881HT, 436, 836 and other series of hose.Under low temperature conditions, low temperature will reduce the softness of rubber products, in cold extreme climate, deep cold and other conditions, the selection of hose on the one hand need to do a good job of pipeline insulation and cold prevention measures, on the other hand, choose low-temperature hose, such as choosing the hose with LT(LowTemperature) logo 371LT, 772LT and so on.1.3 ApplicationsThe application parts are intuitive and easy to choose, such as oil absorption, pressure and return lines. In the oil absorption circuit, it is necessary to consider the selection of hose resistant to negative pressure and vacuum conditions. The pipes of air conditioning and refrigeration systems need to consider the characteristics of medium temperature, medium corrosion and other hose products. In metallurgy and some high temperature open flame conditions, choose high temperature resistant, fireproof outer adhesive layer or add fire jacket outside the pipeline. The pipeline in CNC machine tools and processing equipment is installed in the drag chain, and the work is constantly friction and contact with the surrounding, and the outer rubber layer with wear-resistant rubber layer or ultra-wear-resistant rubber layer hose products are selected. Comprehensive consideration of the various comprehensive factors involved in product selection helps to select the right product, ensure the use of safety and product service life and optimize the overall cost of product use.1.4 MediaFluid media compatibility, the designer according to the fluid medium of the system to choose compatible hose products, such as the medium is phosphate ester hydraulic oil, choose Parker hose series of the last number is 4 hose, the inner rubber layer is generally EPDM material, such as 804, 304, 424, 774 and other series of hoses.1.5 PressureHose and joint selection must ensure that the maximum working pressure of the hose assembly is greater than the maximum pressure of the system. The pulse or peak pressure of the system must be less than the maximum working pressure of the hose. The designer often ignores the scope of pressure application of the hose joint in the selection design, resulting in system failure, because the pressure range of the hose assembly is determined by the lowest working pressure of the components of the hose assembly, so the hose assembly selection, the working pressure of the joint and the hose pressure must be considered comprehensively.The above describes the 5 aspects of hydraulic hose design that need to be considered comprehensively, and other factors are not the same according to different design projects and needs. For example, select the standards that the product needs to meet, product certification, thread sealing and storage.2 Common faults and causesThe correct design and selection of hydraulic hose affect the reliability, safety and normal operation of the whole system. The following is an analysis of the frequent failures of hydraulic pipelines, the main causes of the failures and the measures that should be taken.2.1 Service Life(1) Phenomenon: the hose bursts, the steel wire reinforcement layer is damaged and exposed to the outside, and the steel wire in the area where the rubber layer is lost is slightly inclined.(2) Cause: May be repeated cycle of high voltage pulse or repeated winding in the same place in the hose.(3) Measures: Replace the hose assembly according to the working condition and the actual demand on site.2.2 Hose extraction(1) Phenomenon: joint withdrawal and hose separation.(2) Cause: The hose insertion depth does not reach the recommended value. All teeth of the jacket must be fully fastened to the hose, and the clamping force of the last tooth of the outer sheath accounts for 25% of the total clamping force.(3) Measures: Replace the hose assembly. Mark the tube according to the insertion depth recommended by the hose manufacturer, then insert the connector into the mark.2.3 Failure of inner adhesive layer during assembly(1) Symptoms: there are cracks (pinholes or leaks) or bubbles in the hose near the joint connection, and there is corrosion of the steel wire at the burst point but no obvious damage to the surface.(2) Cause: The inner rubber layer of the hose may be cut or damaged due to the lack of necessary lubrication during assembly, and the incorrect assembly leads to water entering the interior resulting in corrosion of the steel wire reinforcement.(3) Measures: check the correct assembly method and replace the hose assembly.2.4 Hose Distortion(1) Symptoms: The outer layer of the hose is damaged in the twisted area, and the steel wire is damaged.(2) Cause: Due to the lack of necessary lubrication, the hose is distorted when assembling the joint, and incorrect installation may cause the hose to twist.(3) Measures: properly assemble, replace the hose assembly, and use pipe clamps if necessary to ensure that the hose is installed in the same plane.2.5 Hose Wear(1) Symptoms: the outer layer is torn or damaged, and the reinforcing layer has signs of friction or corrosion.(2) Cause: excessive friction between the hose and other objects, or sharp corners or brackets may damage the hose.(3) Measures: Replace the hose assembly, use a protective sleeve or a flexible tube sheath, rearrange the pipe if necessary or use a pipe clamp to avoid direct contact between the hose and other objects.2.6 Incompatible media(1) Symptoms: the inner rubber layer is seriously damaged, swollen or stratified, and even falls off.(2) Cause: the fluid is not matched and compatible with the inner rubber layer material.2.7 Rubber aging(1) Symptoms: cracks appear in the inner rubber layer but it still remains elastic, and no obvious cracks are seen in the joint joint.(2) Reason: Because the air inside the hose is too dry, the inner rubber layer is aging.(3) Measures: Replace the hose assembly and use a hose assembly that is resistant to dry air.2.8 Overtemperature(1) Symptoms: The hose becomes hard, brittle and cracked at ambient temperature. The outer layer shows signs of drying and zooming.(2) The reason: the ambient temperature exceeds the temperature limit of the hose and gradually loses its elasticity, or the oxidation reaction occurs when carbon dioxide enters the inside of the hose, causing the hose to harden.(3) Measures: According to the hose application manual provided by the manufacturer, replace the high-temperature hose or install a heat-resistant protective sleeve.2.9 The temperature is too low(1) Symptoms: At the temperature of use, the inner or outer adhesive layer cracks but remains soft and flexible.(2) Cause: The ambient temperature is lower than the minimum operating temperature of the hose.(3) Measures: According to the manufacturer's product manual, increase the ambient temperature or replace the low-temperature resistant hose.2.10 High Pressure(1) Symptoms: hose burst, reduce the life, burst suddenly, steel wire and rubber layer without wear marks.(2) Reason: the actual working pressure exceeds the rated working pressure of the hose.(3) Measures: Adjust the working pressure of the system or replace the hose with a higher pressure level.2.11 The flow rate is too High(1) Symptoms: hose leakage, inner tube similar to chisel marks.(2) Reason: due to the excessive flow rate, the perforation leaks after the inner rubber layer is worn, and it may also be that the fluid medium contains small particles and makes the inner rubber layer wear.(3) Measures: Replace the hose according to the flow rate and medium.2.12 The bending radius of the hose is too small(1) Symptoms: the hose is broken at the outer rubber layer and the joint connection.(2) The reason: the hose moves back and forth on the joint body, excessive movement or the hose assembly is too short, causing the hose to break, another reason may be the hose clamping error.(3) Measures: when distributing pipes, try to ensure that the assembly is in a relaxed state, and the straight section is at least 2 times the outer diameter of the hose, and the pipe clamp is installed to avoid excessive movement of the hose. Select the right clamping tool and hose clamping machine for easy on-site maintenance.This paper introduces 5 key factors that need to be considered in hydraulic hose design and 12 kinds of faults caused by unreasonable design and hose layout or improper use, and deepens the understanding of various indicators in hydraulic hose selection through fault analysis. It can provide reference for hydraulic engineers and pipe installation, and make the selection and use of hydraulic hose products more scientific.