Factors affecting the quality of hydraulic hose fitting assemblies

Factors affecting the quality of hydraulic hose fitting assemblies

I. Composition and Classification of Hydraulic Hose Fitting Assemblies

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

1. Classify by working pressure range

1) The low-pressure working pressure is below 3 mpa, mainly hydraulic hoses woven from cotton thread (fiber). It is mainly used to control oil circuits, automotive brake lines and some hydraulic machine tools.

2) Medium pressure working pressure is between 3 and 10MPa, mainly steel wire braided type I and Type II large diameter (?) Hydraulic rubber hoses above 25. It is mainly used in medium and low pressure oil circuits and return oil circuits

3) The high-pressure working pressure is between 10 and 31.5 MPa, mainly steel wire braiding? Type I, II, III and steel wire wound pipes with a diameter of less than 25. It is mainly used in high-voltage systems.

4) The ultra-high pressure working pressure is above 31.5 MPa, mainly steel wire braiding? Steel wire wound pipes with a diameter of 31.5 or less. With the development of ultra-high pressure and high-power hydraulic machinery, the demand for it is increasing.

2 According to the connection method between the rubber hose and the joint, they can be classified into detachable type and crimped type

The crimping type rubber hose joint assembly is made by pre-matching the rubber hose with the joint and then forcing the outer sleeve of the joint to contract inward by a certain size in the cold state with external force, ensuring a reliable connection between the rubber hose and the joint.

2) Detachable assembly, whose joint and rubber hose are connected by a core with an outer cone compressing the inner rubber layer of the rubber hose, making it closely adhere to the inner cone of the joint sleeve. It is connected by the inverted conical gap formed between the core and the joint sleeve, which simultaneously presses the inner and outer rubber layers of the rubber hose. However, the connection quality is unstable. Therefore, domestic professional manufacturers generally adopt the crimping type.

Ii. The Relationship between the Structure and Performance of Hydraulic Hose Joint Assemblies.

1. Rubber hose

The hydraulic rubber hose is composed of an inner rubber layer, a reinforcing layer and an outer rubber layer (as shown in Figure 7). The inner rubber layer is in direct contact with the oil, so it is required not to be corroded by the fluid under long-term working conditions and to be leak-proof. It can withstand certain pressure under the action of the reinforcing layer. Therefore, nitrile rubber is recommended. Besides the rubber compound, the main factors affecting performance also include the hardness, thickness and permanent deformation of the inner rubber layer. Hardness and permanent deformation have a significant impact on sealing performance. Generally, the higher the hardness, the smaller the permanent deformation after compression, and the better the sealing performance. Generally, a Shore hardness of 70 to 85 and a compression set of 50% are considered the best. The thickness of the inner rubber layer is preferably 1.5 to 2.5 mm. If it is too thick, it will increase the flow volume during crimping, causing the excess rubber to accumulate on the contact end face between the joint core sleeve and the rubber hose, reducing the flow cross-section. If it is too thin, it will be cracked when crimped. At the same time, the uniformity of the inner adhesive layer wall thickness is also very important. If the thickness is uneven, it will cause one side to crack and the other side to accumulate glue after compression. The pockmarks that appear on the surface of the inner adhesive layer are also an important factor affecting performance and quality.

2) Hydraulic hoses mainly rely on the reinforcing layer to withstand pressure. The braided rubber hose is firmly bonded to the inner and outer rubber layers by the rubber paste. Due to the mutual contact between the steel wires within the same braided layer, when subjected to dynamic pressure, they will expand and contract in different ways, causing friction between the steel wires and affecting their durability. The wound pipe is composed of two layers wound in different directions to form a working layer. There is an intermediate adhesive between the two layers, so there is no intersection point between the two layers of steel wire in the same working layer. Therefore, it will not form stress concentration or frictional wear due to the cross-bending between steel wires when subjected to dynamic pressure. Therefore, it has good durability and can withstand high pressure.

3) The outer rubber layer of the hydraulic hose adheres to the reinforcing layer to provide protection, and chloroprene rubber is generally used. Attention should be paid to preventing its aging and cracking, which may affect the lifespan of the entire rubber hose.

2 Connector

The crimping type rubber hose joint is composed of a joint core, a nut and an outer sleeve.

The main factors affecting the performance of the joint core are the length of the core rod, structural shape, wall thickness and material. From the perspective of sealing and preventing disengagement, the core rod should be as long as possible, but if it is too long, it will waste materials and increase manufacturing costs. In addition, to prevent rubber accumulation at the end of the core during crimping, the general design requirement is that its length be slightly longer than that of the outer jacket.

2) There are various structural shapes for the core rod part, among which the main ones are the R groove (Figure 1) and the sawtooth groove (Figure 2), both of which can increase the friction surface and provide a rubber-holding groove for rubber flow. At present, R grooves are used for steel wire braided pipes, while serrated grooves are used for wound pipes with higher pressure.

3) Material: The core material of the joint is generally 20 #, 35 #, and 45 # carbon structural steel. To prevent the inner hole of the core from deforming during crimping and increasing the liquid resistance, the wall thickness of the core should be carefully selected, usually 1.5 to 3.5 mm. The larger the diameter, the larger the value. The outer circle of the core and the inner hole of the rubber hose should not have too much interference, as it will damage the inner rubber and make the cavity matching difficult. If the inner hole of the core is too small, it will increase liquid resistance and cause pressure drop loss. Due to the above-mentioned restrictions on the outer diameter of the core. However, the size of the rubber hose itself varies greatly. To meet the requirement of the minimum pass for the performance of the joint assembly, it is necessary to solve the problem by optimizing the crimping amount.

3 Joint cover. Its shape is shown in Figure 3. The inner hole size should be appropriately larger than that of the reinforcing layer; otherwise, it cannot be installed and may even cause the reinforcing layer to completely disintegrate. The general gap is 1 to 1.5 mm. The groove shape inside the outer jacket will directly affect the quality of the joint. At present, there are straight hole slotless type, sawtooth groove type, and a combination of ring and sawtooth groove type. (Figure 3

Cotton thread (fiber) braided rubber hoses all adopt a slotless structure due to their low operating pressure and small pull-out force. When steel wire braided rubber hoses are crimped, their reinforcing layers are prone to bending deformation, so serrated shapes are mostly adopted. Steel wire wound pipes usually have a large number of layers. The maximum single-sided thickness of the reinforcing layer can reach 3.6mm. When crimping, to make the reinforcing layer form a wavy deformation and be embedded in the groove, a combination of annular and serrated grooves must be adopted. This groove shape is 2/3 wider than the serrated groove and has a stronger anti-pull-out force.

The crimping process is the most crucial factor affecting quality.

The most significant technological factor affecting the quality of the crimping type rubber hose joint assembly is the crimping process. At present, there are mainly two crimping methods. One is the axial push type (Figure 4), which can be achieved on a general-purpose press with a special mold. The amount of the crimping is not easy to adjust, and the outer surface of the jacket is prone to damage during the crimping process. In order to remove the crimped parts. The mold must be made in a split type. This results in a seam, which makes the appearance of the finished product not good. Another type is the radial crimping type (Figure 5)

Its molds are classified into double-sided, three-piece, six-piece and eight-piece molds. Suitable for mass production, six-petal and eight-petal molds are generally used. The basis for ensuring the crimping quality is to accurately control the compression amount of the inner rubber of the rubber hose and the crimping amount of the outer sleeve. Based on the test of production practice. It is relatively simple to calculate the compression amount of the inner rubber by empirical formulas (refer to Figures 6 and 7).

t = (B-e+f) ×x

t- Compression of inner rubber (or crimping of outer rubber, on one side) mm, B- thickness of inner rubber on one side, mm; (B can be measured directly or calculated according to the following formula.)

B = (d2-d) / 2-nt1

d- Outer diameter of the steel wire layer of the rubber hose, mm; D - Inner diameter of the rubber hose, mm

n- Number of middle rubber layers (two layers of steel wire, one layer of middle rubber; three layers of steel wire, two layers of middle rubber)

t- The thickness of the middle adhesive layer, with a value of 0.3mm

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

d1- Outer diameter of the core, mml

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

d3- Inner diameter of the joint sleeve, mm

x - The percentage of compression of the inner rubber layer (40 to 50% for steel wire braided rubber hoses, 42% for one layer of steel wire; 44% for two layers of steel wire; 48% for three layers of steel wire; 55 to 65% for steel wire wound hoses; about 20% for cotton fiber braided hoses.)

For fiber braided tubes without the outer rubber removed, B = (outer diameter of the rubber tube - inner diameter of the rubber tube) / 2. The outer diameter of the crimped sleeve D = D-2t.