Estimated reading time: 13 minutes
Point 1: Selection of Tubing Material
In hydraulic systems, commonly used pipe materials include steel pipes, copper pipes, rubber hoses, plastic hoses, and nylon pipes. The main basis for selection is the working pressure, working environment, and the overall layout of the hydraulic device, etc., depending on the specific working conditions and referring to the relevant hydraulic manual to determine.
1. Steel Pipe
Divided into two types of seamless steel pipes and welded steel pipes. The former is generally used in high-pressure systems, and the latter is used in medium and low-pressure systems. The characteristics of steel pipe are strong pressure bearing capacity, low price, high strength, and good rigidity, but it is difficult to assemble and bend. At present, steel pipes are the most widely used in various hydraulic equipment.
2. Copper Pipe
Copper tubes are divided into two types: brass tubes and copper tubes, and copper tubes are mostly used. Copper pipe has the advantages of convenient assembly, easy bending, etc., but also has the disadvantages of low strength, poor vibration resistance, high material price, and easy oxidation of hydraulic oil. It is generally used in places where it is difficult to assemble inside.
hydraulic devices or where the pressure is 0.5-10 MPa. Medium and low voltage system.
3. Nylon Tube
This is a new type of milky white translucent pipe, with two pressure-bearing capacities of 2.5MPa and 8MPa. Nylon tube has the characteristics of low price and convenient bending, but it has a short life. It is mostly used in low-voltage systems to replace copper pipes.
- Plastic hose. Plastic pipes are cheap and easy to install, but they have a low pressure-bearing capacity and are easy to age. They are currently only used for leaking pipes and oil return lines.
- Rubber hose. This kind of tubing has two types: high pressure and low-pressure. The high-pressure pipe is made of oil-resistant rubber with steel wire braided layers. The more steel wire layers, the higher the pressure resistance of the tubing. The woven layer of the low pressure tube is canvas or cotton thread. Rubber hoses are used for the connection of hydraulic parts with relative movement.
- Factors affecting the life of hoses and hose assemblies include ozone, oxygen, heat, sunlight, rain, and other similar environmental factors. During the storage, transfer, shipment, and use of hoses and hose assemblies, the first-in-first-out method should be implemented according to the production date.
- When selecting hoses, the maximum recommended working pressure indicated on the hoses in the manufacturer’s sample should be selected not less than the maximum system pressure, otherwise the service life of the hoses will be reduced and the hoses may even be damaged.
- The choice of the hose is determined according to the highest pressure value of the hydraulic system design. Since the pressure value of the hydraulic system is usually dynamic, shock pressure sometimes occurs, and the peak value of the shock pressure will be much higher than the maximum pressure value of the system. However, the system generally has an overflow valve, so the impact pressure will not affect the fatigue life of the hose. For hydraulic systems with particularly frequent impacts, it is recommended to use hose products that are particularly resistant to impulse pressure.
- The hose should be used within the allowable temperature range of the hose quality specification. If the working environment temperature exceeds this range, it will affect the life of the hose and its pressure-bearing capacity will be greatly reduced. For systems where the working environment temperature is too high or too low for a long time, it is recommended to use a hose sheath. The hose usually touches or rubs with hard objects when in use, it is recommended to add a spring sheath outside the hose. The inner diameter of the hose should be appropriate. Too small a pipe diameter will increase the flow rate of the medium in the pipeline, cause the system to heat up, reduce efficiency, and cause excessive pressure drop, which will affect the performance of the entire system. If hose clamps or hoses pass through spacers such as steel plates, pay attention to the outer diameter of the hose.
- Before installation, the hose must be inspected, including the joint form, size, and length, to ensure that it is correct. It is necessary to ensure that the hoses and joints are compatible with the environmental conditions in which they are located. The environment includes ultraviolet radiation, sunlight, heat, ozone, humidity, water, saltwater, chemicals, air pollutants, etc., which may reduce the performance of the hose or cause early failure the elements of. The cleanliness level of the hose assembly may be different, and it must be ensured that the cleanliness of the selected hose assembly meets the application requirements.
- The radius of curvature of the hose should be more than 10 times larger than the pipe diameter.
- The bending amount of the structure is constantly changing. To prevent the hose from being broken or deflected during use, its radius of curvature should be more than 10 times larger than its pipe diameter, as shown in the figure.
- The hose must work within the specified radius of curvature. If the bending radius is only.
- 1/2 of the specified radius, it cannot be used, otherwise, the service life will be greatly shortened. Avoid sharp turns, and the bending radius R≥(9～10)D (D is the outer diameter of the hose). Do not bend near the root of the joint. The shortest distance from the hose joint to the beginning of the bend is L=6D.
- During installation and work, there should be no twisting. When the hose is installed and connected to the oil circuit, it should be noted that when the hose is moving, the hose is not allowed to be twisted.
- At the connection, the hose should hang freely and avoid bending due to its weight.
- The bending of the hose should be on the same plane as the installation of the hose connector and its movement plane to prevent twisting. But under special circumstances, if the joints at both ends of the hose need to move on two different planes, clamps should be installed at the appropriate positions to divide the hose into two parts so that each part moves on the same plane.
- The hoses should be installed at the shortest distance or along the contour of the equipment and arranged in parallel as much as possible. If the hose is too long or is subjected to sharp vibration, it should be clamped firmly with a clamp. However, for hoses used under high pressure, clamps should be used as little as possible, because the hose is deformed under pressure, friction will occur at the clamps.
- Use a hose to connect the pipeline. The length should not be too tight. A certain amount of length should be left to ensure that the hose has a certain bending force to prevent the hose from being damaged; when the hose is compressed, the length and diameter of the hose must be changed. Change (the length change is generally about ±4%). Therefore, in the case of bending, the end joint cannot be bent immediately; in the case of straight use, do not stretch the end joint and the hose, and consider some allowance in the length to make it slacker.
- Do not contact with other hoses or piping to avoid wear and tear. It can be separated by cardboard or properly considered in the piping design; the hydraulic hose of the actuator should avoid friction with other components. When the hydraulic actuator is moving, the hose should be used. When connecting the oil circuit, avoid contact between the hose and other components during installation. Relative movement friction can damage the hose.
- The hose works at high temperatures and has a very short life. Install the hose as far away as possible from the heat source. If necessary, install a heat shield.
- It must be ensured that the hoses and joints are compatible with the environmental conditions.
Several examples of correct and incorrect installation of hoses are given below, as shown in the figure, and are explained as follows.
- After the two ends of the hose assembly is assembled in the picture, the hose should not be straightened, and there should be some slack. Due to the pressure, the length of the hose will change somewhat, and the range of change is -4% to +2%.
- The minimum bending radius of the hose in the picture must be greater than the minimum allowable radius of the hose to make it in a natural state to avoid reducing the service life of the hose.
- Choose an appropriate hose length, and there should be a certain distance from the bend and the sheath. The reasonable use of elbows in the picture can avoid additional load on the hose.
- Install and fix the hose correctly in the picture to avoid friction and collision between the hose and other objects. When necessary, a protective sheath can be used. If the hose must be installed next to the heating object, a fire-resistant sheath or other protective measures should be used.
- As shown in the figure, when the hose is installed on a moving object, there should be enough free length.
The advantages, disadvantages, and applications of various pipe materials can be seen below, and the requirements for use are as follows:
- Generally, hard pipes should be used as much as possible, because hard pipes have low resistance, safety, and low cost;
- Middle and high-pressure systems mostly use seamless steel pipes, and brass pipes can also withstand higher pressures (p≤25MPa);
- The medium and low-pressure system can be welded steel pipe (p≤0.6MPa) or copper pipe (p<6.5～10MPa);
- Nylon pipes can be used for low-pressure systems, and plastic pipes are only used for oil return pipelines;
- According to the different braided framework materials, the hose can be used for low-pressure or high-pressure systems.
Point 2: The Problem of Pipe Joint Selection Commonly Used Pipe Joint Forms Are as Follows.
- Thin-wall pipe flared pipe joints. As shown in the figure, it is a flared pipe joint, which uses the flaring at the end of the tubing I to seal under the compression of the pipe sleeve. The pipe joint has a simple structure and is suitable for the connection of copper pipes, thin-walled steel pipes, nylon pipes, and plastic pipes. It is suitable for medium and low-pressure piping systems, and can also be used for water and air pressure piping systems. The maximum working pressure depends on the pipe material and pipe diameter and is specified as 3.5-16MPa.
- Welded pipe joints. As shown in the figure, the welded pipe joint is formed by welding the pipe and the inner core 1 of the joint. The spherical surface of the inner core of the joint is closely connected with the taper hole surface of the joint body, which has the advantages of good sealing, simple structure, and strong pressure resistance. The disadvantage is that welding is more troublesome, and it is suitable for the connection of high-pressure thick-walled steel pipes. Applicable to pipeline systems with oil as the medium. The working pressure is ≤32MPa; the working temperature is -25～+80℃. During installation, the welding volume is large, the welding quality is required to be high, and the assembly and disassembly are inconvenient.
- Card sleeve-type pipe joints. The picture shows the ferrule type pipe joint, which uses ferrule 2 with excellent elasticity to clamp the oil pipe 1 and seal it. It is characterized by simple structure, convenient installation, and high requirements for the dimensional accuracy of the outer wall of the tubing. The ferrule-type pipe joint is suitable for the connection of high-pressure cold drawn seamless steel pipes, and the manufacturing precision of the ferrule is high. The bite-type pipe joint is used in the pipeline system with oil and gas as the medium. The applicable pressure range has two levels: the medium pressure level is 16MPa; the high-pressure level is 32MPa.
- Withhold type pipe joints. The figure shows a crimping type pipe joint, which is composed of a joint jacket 1 and a joint core 2. This joint is suitable for hose connection.
- Removable pipe joints. The picture shows the detachable pipe joint. The structure of this joint is that the joint jacket 1 and the joint core 2 are made into a hexagonal shape, which is convenient for frequent removal of the hose. Suitable for high-pressure small-diameter hose connection.
- Quick-change connector. The picture shows a quick-change joint, which is convenient for quick disassembly and assembly of oil pipes. The principle is: when clamp 6 moves to the left, the steel ball 5 withdraws from the ring groove of socket 4, the socket is no longer jammed, and can be quickly withdrawn from socket 1. At this time, pipe plugs 2 and 3 close the two pipe ports under the action of their respective spring forces, so that the oil in the oil pipe will not be lost. This type of pipe joint is suitable for hose connections that require frequent disassembly.
- Expansion pipe joints. The picture shows a telescopic pipe joint. This pipe joint is composed of an inner pipe 1 and an outer pipe 2. The inner pipe can slide freely in the outer pipe and is sealed with a sealing ring. The outer diameter of the inner tube must be precision machined. This type of pipe joint is suitable for the connection of pipes with the relative movement of the connecting pieces.
- Wire braided hose joints. It is suitable for pipeline systems with oil, water, and gas as the medium. Pressure range: 6～40MPa depending on the pipe diameter; Medium temperature: oil -30～＋80℃, water<80℃, air -30～+50℃.
- Three-piece hose connector. It is suitable for pipeline systems with oil, water, and gas as the medium, and the working pressure and medium temperature are determined by the hose.
- Quick connector. Applicable working pressure is ≤32MPa; working temperature is -20～＋80℃; pressure loss (underrated pressure) <0.2MPa.
- Tapered threaded pipe joints. Welded pipe joints with taper thread connections are suitable for piping systems where oil is the medium. Working pressure≤16MPa; working temperature is -25～+80℃. The bite-type pipe joint with taper thread connection is suitable for the pipeline system of oil, gas, and general corrosive medium. The flared pipe joint with taper thread connection is suitable for pipeline systems with oil, water, and gas as the medium. The maximum working pressure depends on the pipe material and pipe diameter and is specified as 3.5～16MPa.
- Square flange. It is used for the connection between pipelines and hydraulic components and between pipelines and pipelines. The applicable working pressure is 20MPa.