Factors Affecting the Quality of Hydraulic Hose Assembly

1. Composition and Classification of Hydraulic Hose Joint Assembly

The hydraulic hose joint assembly consists of two parts: a rubber hose and a metal joint. It is mainly classified according to the working pressure range and the connection form of the hose and the joint.

Classification by working pressure range

Low pressure: the working pressure is below 3MPa, mainly the hydraulic hose woven with cotton thread (fiber). Mainly used in control oil circuit, automobile brake pipeline and some hydraulic machine tools.

Medium pressure: The working pressure is between 3 and 10MPa, mainly steel wire braided I, II large diameter (above h25) hydraulic hoses. Mainly used for medium and low pressure oil circuit and return oil circuit.

High pressure: The working pressure is between 10 and 31.5MPa, mainly the type I, II, III and steel wire winding pipes with steel wire braided below h25. Mainly used in high pressure systems.

Ultra-high: The working pressure of high pressure is above 31.5MP a, mainly steel wire winding pipes with diameters below h31.5. With the development of ultra-high pressure and high-power hydraulic machinery, the demand for it is increasing.

Classification According to the Connection Method of the Hose and the Joint

The crimped hose joint assembly uses external force to force the joint jacket to shrink inward to a certain size in a cold state, so that the hose and the joint can be connected reliably.

For the detachable assembly, the joint and the hose are compressed through the core with an outer cone to compress the inner rubber layer of the hose to make it close to the inner cone of the joint sleeve.

That is, it is connected by the inverted conical gap formed between the core and the joint sleeve, while pressing the inner and outer rubber layers of the hose. But the connection quality is not stable. Therefore, professional manufacturers in China generally use the withholding type.

2. The Relationship Between Structure and Performance of Hydraulic Hose Joint Assembly

1. Hose

The hydraulic hose is composed of an inner rubber layer, a reinforcing layer, and an outer rubber layer. The inner rubber layer is directly in contact with the oil, so it is required that it should not be corroded by the fluid under long-term working conditions and can prevent leakage. Under the action of the reinforcement layer, it can withstand a certain pressure.

Therefore, nitrile rubber should be used. In addition to the rubber compound, the main factors affecting the performance are the hardness, thickness, and permanent deformation of the inner rubber layer. Hardness and permanent deformation have a great influence on sealing performance. Generally, the hardness is high, the permanent deformation after compression is small, and the sealing performance is better. Generally, it is best when the hardness is 70-85 Shore and the compression set is 50%.

The thickness of the inner rubber layer is preferably 1.5 to 2.5 mm. Too thick will increase its flow during withholding, causing excess glue to accumulate in the contact end face of the joint core sleeve and the rubber tube, reducing the flow cross section; Fractured during withholding.

At the same time, the wall thickness uniformity of the inner rubber layer is also very important. If the thickness is not uniform, it will cause one side to crack and one side to pile up after compression. The pitting on the surface of the inner rubber layer is also an important factor affecting the performance quality.

Hydraulic hoses mainly rely on reinforcement layers to withstand pressure. The braided hose is glued to the inner and outer layers by glue.

Due to the mutual contact between the steel wires in the same braided layer, when subjected to dynamic pressure, the friction between the steel wires will affect their durability due to their different expansion and contraction.

The winding pipe is composed of two layers with different winding directions to form a working layer, and there is an intermediate glue between the two layers, so there is no intersection between the two layers of steel wires in the same working layer.

Therefore, it will not cause stress concentration or friction and wear due to the cross bending between the wires when subjected to dynamic pressure. Therefore, it has good durability and can withstand high pressure.

The outer rubber layer of the hydraulic hose sticks to the reinforcement layer for protection, generally using neoprene. Care should be taken to prevent it from aging and cracking and affecting the life of the entire hose.

2. Joint

The crimped hose joint consists of a joint core, a nut, and a jacket.

The main factors affecting the performance of the joint core are the length of the core rod, the structural shape, the wall thickness and the material. From the point of view of sealing and preventing pulling out, the longer the mandrel is required, the better, but too long will waste materials and increase the manufacturing cost.

In addition, in order to prevent the end of the core from being piled up when it is crimped, the general design requires that its length is slightly longer than that of the jacket.

There are many structural shapes of the core rod, among which there are mainly R grooves and zigzag grooves, which can increase the friction surface and provide a groove for rubber flow. At present, steel wire braided pipes use R grooves, and winding pipes with higher pressure use zigzag grooves.

Material: The material of the joint core is generally 20#, 35#, 45# carbon structural steel. In order to prevent the inner hole of the core from being deformed and increasing the liquid resistance during the withholding, the wall thickness of the core should be selected carefully, generally 1.5~3 .5 mm, the larger the diameter, the larger the value.

Excessive interference is not allowed between the outer circle of the core and the inner hole of the hose, which will damage the inner rubber and make it difficult to assemble the cavity. If the inner hole of the core is too small, it will increase the liquid resistance and cause the loss of pressure drop.

Because the outer diameter of the core is limited by the above. However, the size of the hose itself varies greatly. In order to meet the requirements of the minimum throughput of the joint assembly performance, it must be solved by optimizing the amount of withholding.

Joint jacket: the size of the inner hole should be appropriately larger than the size of the reinforcement layer, otherwise it cannot be installed, and even the reinforcement layer will be completely scattered.

The general gap is 1 ~ 1.5mm. The groove shape inside the jacket directly affects the quality of the joint. At present, there are straight hole without groove type, serrated groove type and combined annular and serrated groove type.

3. The Crimping Process is the Most Important Factor Affecting the Quality

The most important technological factor affecting the quality of the crimped hose joint assembly is the crimping process. At present, there are two main ways of withholding, one is the axial push-in type, which can be realized by using a special mold on a general-purpose press. The withholding amount is not easy to adjust, and the outer surface of the jacket is easily damaged during the withholding process. To remove the clip.

The mold must be made into a split type. This creates a seam that makes the finished product look bad. The other is radial withholding type, and its molds are divided into split, three-lobe, six-lobe and eight-lobe molds.

It is suitable for mass production of six-lobed and eight-lobed molds. The basis for ensuring the quality of crimping is to correctly grasp the amount of compression of the inner rubber of the hose and the amount of crimping of the outer jacket.

According to the inspection of production practice. It is relatively simple to use an empirical formula to calculate the compression amount of the inner rubber.

t=(B-e+f)×x

t-inner rubber compression amount (or jacket withholding amount, unilateral) mm, B-inner rubber unilateral thickness, mm; (B can be measured directly, or calculated by the following formula)

B=(d2-d)/2-nt1

d- the outer diameter of the steel wire layer of the hose, mm d- the inner diameter of the hose, mm

n - the number of layers of medium rubber (two layers of steel wire, one layer of medium rubber; three layers of steel wire and two layers of medium rubber)

t- the thickness of the rubber layer, its value is 0.3mm

e- the expansion of the joint core to the inner hole of the hose mm, e=(d2-d)/2

d1 - core outer diameter, mm

f- the gap between the joint sleeve and the outer diameter of the steel wire layer of the hose, mm f=(d3-d2)/2

d3 - inner diameter of joint sleeve, mm

x—the percentage of inner rubber layer compression (40-50% for steel wire braided hose, 42% for one layer of steel wire; 44% for two-layer steel wire; 48% for three-layer steel wire; 55-65% for steel wire winding tube %; cotton fiber braided tube takes about 20%.)

For the fiber braided tube without peeling the outer rubber, B=(the outer diameter of the rubber tube - the inner diameter of the rubber tube)/2. The outer diameter of the sleeve after withholding is D=D-2t.