Because of the special position of the mechanical structure and the hydraulic actuator, a large number of hoses are used in the pipeline design of the hydraulic system. Unlike metal pipes, the hose is flexible, so it is mainly used to allow for relative movement between the parts connected at both ends of the hose, and can simplify pipe layout and installation. Laying a hose around or through a series of obstacles is much easier than bending and installing a rigid pipe. Building a hard tube system is often more expensive and time-consuming than building a hose system.
l Principles that should be noted when using hoses
1) Determine the appropriate bending radius
Although many hose manufacturers are currently able to provide hoses that can be bent to a smaller radius than the published industry standards specify, it is important to avoid the bending curvature of the hose less than the recommended minimum radius of curvature, so as not to shorten the service life. Therefore, the tube must first provide sufficient bending radius. The bending radius of the hose after installation should not be less than 8 to 10 times the outer diameter of the hose, the root of the joint of the two ends of the hose is not allowed to bend, should retain a certain straight line section, its length is not less than 6 times the outer diameter of the hose: in order to avoid sharp bending, can be taken outside the hose around the spring or support steel belt and other methods to bend gently.
2) Determine the hose length correctly
Because the hose is flexible, it is necessary to consider its flexibility when determining the length of the hose. Depending on the type of hose, the hose can be extended by 2% or shortened by 4% when the pressure changes. This change in length can fatigue the wire layer of the hose and cause damage, especially at the hose connection interface. Therefore, taking into account the shrinkage of the hose, it is necessary to leave a little longer when cutting to compensate. If the hose length is not enough, it can be lengthened by connecting two hoses through a transition joint.
3) Avoid hose being twisted
When the hose is bent in the same plane, when there is relative movement between the two components connected, it is necessary to avoid twisting the hose, otherwise it will reduce the bearing capacity of the hose. The test results show that twisting the high-pressure hose by 5° will reduce the service life of 70%, and twisting 70 can reduce the service life of 90%. In order to avoid distortion, a straight line parallel to the center line can be drawn on the hose with chalk before assembly to check whether the hose is twisted after installation.
4) The hose needs to be bent in multiple planes
The layout of the hose is usually done late in the design work, so it is difficult to find the ideal path. Multi-plane bending can often be avoided by reorienting the hose. If this does not work, a hose clamp should be installed between the two bends, providing sufficient length on both sides of the clamp to release tension from the hose wire layer. This length depends on the quality of the hose, the degree of bending and the wire layer of the hose, so it must be designed separately for different objects.
An alternative method is to use a separate section of hose for each bend in the plane, with a hose connector and hose clamp installed between the two bends. However, because this method is not only costly and long installation time, but also increases the potential leakage possibility at the hose connection. So there is less adoption.
5) Provide protection
Hose manufacturers now offer a wide range of products with wear-resistant housings. The analysis found that approximately 80% of hose failures were due to external physical damage, with wear proving to be the primary cause. Wear is mainly caused by repeated friction between hoses and equipment surfaces or between hoses. To avoid wear, hose clamps should be used to secure the hose so that it does not rub against adjacent surfaces. The clamp should be properly tightened to prevent the hose from moving, but if it is too tight, it will compress and damage the hose. The hose on both sides of the clamp should have a suitable margin to compensate for hose expansion. Further protection can be provided by a bushing, a spring-like metal bushing that protects the hose from being squeezed. Flexible bushings protect the hose from wear, and some types of bushings must be mounted by sliding along the hose starting at the unconnected end. Tubing with openings along the length is installed without disconnecting the hose ends. Both types of bushing can tie multiple hoses together. Contact and friction between the hose and the machine or the driving shed should be avoided as far as possible to prevent the wear of the outer rubber layer and the wire braid layer. In areas where mechanical friction may occur with the outside world, canvas tape or rubber hose should be wrapped for protection.
6) Get used to exercise
In addition to torsion and wear, if the hose is not properly adapted to the movement of the working device. That can also damage it very quickly. For example, when connected to a swinging hydraulic cylinder. The length and arrangement of the hose must be appropriate to avoid entanglement or bending beyond the recommended minimum radius. When multiple lengths of hose are arranged close to each other. Some of them will have linear movement, and a hose conveyor should be used to keep the hose neat and avoid entanglement, twisting and friction with each other. Some special types of hose conveyors can also isolate external conditions such as falling objects, friction, chemicals or high temperatures.
2 Issues to consider
Most hoses are wire reinforced, which makes the hose a conductor, since machinery may be used next to power lines or the hose is very close to a flammable solution that can be ignited by the static electricity emitted. Therefore, manufacturers can provide non-conductive hoses. Static electricity may sometimes be discharged through the wall of the tube. The result is positional combustion, which thinens the wall of the tube and even creates pinholes in the wall. In this case, the hose with electrical conductivity needs to be designed to discharge through the fitting of the hose end rather than through the hose.
High temperature as well as torsion can gradually shorten the hose life. An external heat source, such as an exhaust pipe on a construction machine, can quickly soften the hose from the outside of the pipe wall. Therefore, it is important to keep the hose away from external heat sources. If you can't stay away. A protective sleeve should be used to isolate the heat transferred to the hose.
The heat source from the oil itself can also reduce the hose's life. The system oil temperature only exceeds the maximum allowable temperature of the hose by about 10 ° C, which can reduce the life expectancy of the hose by 1/2. This problem is compounded by the fact that machine operators are often unaware that the oil temperature may exceed the hose manufacturer's recommended value, especially when the high temperature occurs intermittently.
The layout of the hose should be as clean as possible, which can not only avoid entanglement, torsion, friction of the hose, but also help the maintenance of the system. Use transition joints with caution, as they increase the number of parts in an assembly, which increases assembly time, expense, and potential leakage points. However, the use of bent transition joints (such as 90° joints) at both ends of the hose can simplify the hose assembly if applied properly. In these assemblies, the joint of the pipe end must pay attention to the installation direction so as not to twist the hose during installation.
For hydraulically operated towed farm tools (or dump trailers), safety joints should be installed at the hose joints between the farm tools and the tractor to prevent hose breakage and oil leakage after the farm tools have been disconnected.
3 Hoses are bent in 2 planes
The hose arrangement in Figure 1 goes against the design recommendation and is bent in 2 planes between the pump port and the fitting. Hose design guidelines recommend that the hose not bend in more than one plane, which will result in an uneven distribution of wire tension in the hose. When pressure pulsation occurs in the hydraulic system, the overloaded steel wire is subjected to additional tensile stress, and after continuous cycling, the overstressed steel wire will fail due to fatigue, and finally cause the hose to break. A hose rupture can be very dangerous if it happens without warning.
Perhaps because of the position of the parts, it is impossible to avoid bending the hose in 2 planes. In this case, a straight hose held in place by a clamp between the bent hose can serve as a transition area allowing the wire layer to relax. The hose in Figure 1 has a straight pipe section between the two bends, but it is clearly not held in place by the clamp or wall. Unless the maximum pressure of the system is much lower than the pressure level of the hose, the hose will eventually rupture (if the system pressure is much lower than the pressure level of the hose, it is unnecessary to spend more money on high pressure hose).
A simple solution is to install a 45° pipe joint at the pump port. The fittings solve the problem of bending in one plane, so that the hose only needs to bend in the other plane, reducing the bending can improve the flow. In addition, the use of 45° fittings can reduce the length of the hose.
4 Avoid confusion in hose layout and positioning
In some places, the layout and positioning of hoses are confused, the layout of hoses is not planned in advance, and some hoses cause friction with each other or with surrounding components. Finally, due to the continuous circulation of the bent part will chafe the hose protective layer, once this happens, leakage begins and increases over time. In order to avoid confusion, the designer should estimate each hose needed before installing the first hose, design a piping plan, and build a clean pipe. If possible, the hoses should be arranged parallel to each other and then clamped in place so that they do not rub against each other.
In fact, good tube design can not only make the system neat and clean, but also improve the reliability of the machine by avoiding wear. It also makes maintenance easier, as technicians don't have to work around a jumble of hoses or have to remove them to service machinery.
5 New trend of hydraulic pipeline development
A thermoplastic tube called a "molded hose" (the hose is heated or cooled to form a special shape) is used in hydraulic systems. The "molded hose" is suitable for temperatures from -54 ° C to more than 121 ° C, and can withstand pressure up to 70MPa, which can be used in harsh environments. Its advantages are:
(1) Zero-waste pipe assembly can be directly assembled, without the need for traditional pipe bending and piping.
(2) Fewer leakage points Pipelines can be multi-dimensional shaped over the entire length of the chassis, pipe joints can be replaced by pipes, and the use of O-rings can reduce leakage.
(3) The problem of traditional rigid tube fatigue point will not occur at the turning point.
(4) Reduce assembly time It can be easily assembled, saving more time than the traditional forming of hard tubes and setting up numerous transition joints.
(5) Reduce the cost of parts When pipe joints and transition joints are usually replaced by pipes, the overall cost will be reduced.
(6) When the design side is damaged, it is easy to be replaced in the spare parts market, and it is easier to distribute tubes than hard tubes.
Contact: China Zhuofan Hydraulic Co., Ltd
Add: Jing County, Hengshui City, Hebei Province, China