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Design and selection of hydraulic seals

2019-04-02

Design and selection of hydraulic seals-2


Design and selection of hydraulic seals

About hydraulic cylinder:

The hydraulic cylinder is an important element for the final operation in the hydraulic system. The hydraulic cylinder is composed of cylinder, piston, piston rod, cylinder head, seal, etc. The function of the seal in the cylinder is to prevent leakage of the working medium and intrusion of outside dust and foreign matter. 

 

External leakage will cause the waste of working medium, pollute the machine and the environment, and even cause mechanical operation failure and human injury; Internal leakage will cause the hydraulic system volumetric efficiency to drop sharply, fail to reach the required working pressure or even work normally; 

The dust particles that intrude into the system will cause and aggravate the wear and tear of hydraulic components, which will further lead to leakage. Modern hydraulic systems have extremely high requirements for seals, and their operational reliability and service life are an important indicator for measuring the quality of hydraulic systems.

 

Classification of hydraulic seals

According to the manufacturing materials, installation methods, structural forms, etc. of the seals, the hydraulic seals are further divided into different sub-categories: non-metallic seals (O-rings, rubber gaskets, PTFE guide tapes, etc.), semi-metal seals (combined gaskets) , metal static seal (metal gasket), hydraulic static seal (sealant seal), self-sealing compression seal (O-ring, slide ring compact seal, special-shaped seal, etc.), self-sealing self-tightening seal (U-ring, combined U-ring, Vee packing, composite lip seal, two-way combined lip seal, etc.), piston ring, mechanical seal, oil seal (rotary frame oil seal, high pressure oil seal, etc.) , dust seal (dust ring, skeleton dust ring).

 

About the principle of sealing

In addition to the gap seal, a seal is used to control the gap between two adjacent coupling surfaces below the minimum gap, through which the liquid need to be sealed can pass. This minimum gap is determined by the pressure of the liquid, the surface tension (viscosity), and the molecular weight.   

Self-sealing compression seal has the function that it can generate contact pressure between the seal and the coupling member obtained by the pressing force (pre-compression force and medium working pressure) (it automatically increases with the working pressure of the medium). The sealing ring is run-fit on the sealing surface to block the leakage path to achieve the purpose of sealing; the self-sealing self-tightening sealing (lip sealing) is to utilize the reaction force generated by the deformation of the sealing ring to perform initial sealing under the medium pressure. 

 

Under the action, the sealing lip is opened to make it close to the coupling surface, and the contact pressure is also increased with the increase of the working pressure of the medium to achieve the sealing effect.

About the principle of selection  

1. Basic requirements

A. Under the working pressure, it should have good sealing performance, it can automatically improve its sealing performance with the increase of pressure, and there is no obvious increase in the leakage under high pressure;

B. To make hydraulic seals work in fluid medium for a long time, it should keep the materials stable;

C. Movement. Due to the small static friction resistance of the seal, the friction coefficient should be stable and the movement parts can not be stuck or the movement is uneven;

D. Small wear, long service life;

E. Simple manufacturing, easy to disassemble and install, and low price.

 

2. Factors affecting sealing performance

A. The type of working medium;

B. The oil temperature (based on the temperature of the sealing part);

C. The size and waveform of the pressure;

D. The slip speed of the sealed coupling surface;

E. The size of the extrusion gap;

F. The degree of eccentricity of the seal and the coupling surface;

G. The roughness of the seal coupling surface, the form, structure, size, position, etc. of the seal and the installation.

 

3. Design principle  

The selection of hydraulic seals should first be based on the conditions and requirements of the sealing equipment, such as load conditions, working pressure, speed, changes, the environment, and specific requirements for sealing performance. The sealing structure should match the structure of it; 

Then according to the type of working medium and temperature, the sealing material is reasonably selected; In addition, when using the sealing device in an environment with dust and impurities, it is necessary to select appropriate dust rings according to the pollution situation and the requirements for dust proof.  

 

4. Design criteria  

In the selection and design of hydraulic seals, international standards ISO, Chinese national standard GB, German industrial standard DIN, and Japanese industrial standard JIS should be obeyed.  

About sealing materials

 

1. Requirements for the use of sealing materials  

A. It should have good stability in the working medium which means it has small volume change (not easy to expand and shrink), difficult to dissolve, soften, and harden;

B. Large recovery in the working medium and permanent deformation;

C. Appropriate mechanical strength and hardness. After being affected by the working medium, the change is small;

D. Good heat resistance, cold resistance, and vibration absorption performance;

E. Good wear resistance, small friction coefficient;

F. The materials are dense;

G. It features softness and elasticity to fit the sealing surface;

H. Good ozone resistance and aging resistance, durability;

I. Easy to manufacture and process, cheap price.

  

2. Types and performance of commonly used seals  

In the hydraulic system, the commonly used sealing materials are mainly synthetic rubber and synthetic resin. Among them, synthetic rubber is the main sealing material, which has the excellent elasticity and mechanical strength necessary for sealing materials and has good oil resistance and heat resistance. 

Therefore, its application range is very wide. Under the conditions beyond the temperature specified by the rubber material and chemical abrasion resistance, when synthetic rubber cannot be used, synthetic resin materials such as polytetrafluoroethylene may be used; Under high temperature and high pressure, synthesis rubber and synthetic resin materials are not available, but metal materials can be used. 

When working on site, the requirements for seals are higher than those with one assembly (new machine assembly). In the secondary assembly (maintenance replacement), the accuracy of the machine and the cleanliness of the oil are not as good as before. As the using environment of the seals are getting worse and worse, and the working strength of the replacement seals, therefore, should be highly valued when using and assembling seals!

 

Attention should be paid to the selection and installation:  

1. Select relevant equipment according to the working conditions of the equipment.

2. Try to use the authorised edition

3. Improve the assembly style (such as installing a drain conduit at the position of the rod seal) when conditions permit (there are some design problems on the machines at the beginning)

4. Strict requirements for regular assembly steps to reduce leakage.

 

Cylinder design, processing, and installation precautions

1. Processing accuracy of the cylinder  

Experiments have shown that the surface roughness Ra exceeds 0.8 um on the moving working surface in contact with the oil seal. The leakage and wear values of the oil seal will rise linearly. So it is recommended that the working surface roughness is Ra 0.1-0.8um. In order to ensure the sealing performance of the PU and avoid damage to the seal during the assembly process, the following aspects should be considered in the design and installation of the hydraulic cylinder:  

 

2.Cylinder  

Material: Generally used aluminum alloy, bronze, stainless steel, etc. which should be processed under the low pressure system and good friction conditions.

Inner surface quality and roughness: The inner surface is generally required to be honed, polished or rolled. It is required to achieve a roughness of Ra 0.1-0.8 um and have no vertical and horizontal knives.  

 

3. Piston rod  

Material: Carbon steel and chrome-plated steel;

Surface quality and roughness: The required roughness is Ra 0.2-0.4um. After heat treatment, it will become hard chrome plated.

The piston rod of the hydraulic cylinder for construction machinery may be scratched by sand and gravel, and the surface hardness is required to be above HRC60.

 

4. Oil seal installation groove  

Structural form: Integral groove and split groove

 

Rubber hydraulic seal  

Rubber materials are one of the most commonly used materials for hydraulic seals. It has excellent resistance to mineral oil and water. Different hardness and different shapes can be made according to requirements. The rubber material can be vulcanized, injection molded, and machined. The addition of the fiber and cotton can help expand its range of use.

Rubber seals mainly have three different shapes including U-rings, L-rings, and V-rings. Due to the increasing hydraulic pressure, the required sealing reliability and service life are increasing as well. In order to meet these requirements, great improvements have been made in the manufacturing process, materials, sealing lip design and sealing structure of the rubber seals in the past two decades.


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