Influence of hydraulic oil seals _ repair and maintenance

Hydraulic oil is used as a working medium in the hydraulic transmission system, which plays the role of energy transfer, conversion and control, lubrication, corrosion prevention, rust prevention, and cooling of various components in the hydraulic system.

The seals in the hydraulic system prevent fluid leakage from the joint surface, maintain pressure, and maintain energy transfer or conversion.

At present, most of the sealing materials used at home and abroad are polymer elastomers, and some special conditions also use plastics and all kinds of metal.

But no matter which kind of material it belongs to, it should have the following properties:

1, has specific mechanical and physical properties: such as tensile strength, fixed elongation strength, elongation;

2, have a certain elasticity, hardness is appropriate, and permanent compression deformation is small;

3, adapt to the working medium, not easy to produce swelling, decomposition, hardening;

4, wear resistance, have a specific tear resistance;

5, with high temperature, low temperature aging performance.

However, no sealing material includes all of the above performance, according to the working environment, such as temperature, pressure, medium, and mode of movement to choose the appropriate sealing material and formulate materials to meet specific requirements.

Or the use of more than two kinds of composite materials or a combination of structures in the form of their respective strengths to achieve a more comprehensive effect.

The formation of sealing effect: dynamic sealing is divided into non-contact sealing and contact sealing.

The non-contact seal is mainly a variety of mechanical seals, such as graphite packing ring, floating ring seal, etc.;

Rubber-plastic composite seals and rubber-plastic composite seals are:

Hydraulic system seals for static seal (end seal), reciprocating dynamic seal (piston, piston rod seal) and rotary seal.

Factors affecting the sealing effect are the choice of filling structure and oil film formation, pressure, temperature, compatibility of materials, material, hardness, geometry, and surface finish of the working surface touched by dynamic sealing.

Several sealing materials with excellent dielectric resistance and their characteristics;

1, nitrile rubber:

2, silicone rubber:

3, fluorine rubber:

4, EPDM rubber:

5, PTFE:

6. Polyurethane

7, propylene ester rubber:

Two, sealing material and hydraulic oil (liquid) compatibility

One of the sources of particle pollution of hydraulic oil is the “debris” or “debris” produced by the sealing material that does not adapt to the hydraulic oil.

The seals are damaged by the “debris” or the “extracted” inorganic materials and filling reinforcement materials. The seals are damaged and invalid, the oil is polluted, and the hydraulic oil deteriorates and fails.

Vane pump is widely used in the hydraulic system. When its working pressure is more excellent than 6.9MPa, the wear problem becomes prominent, so the antiwear agent is used in hydraulic oil.

To adapt to the high-temperature heat source and open fire near the hydraulic system, flame-resistant phosphate ester, water-glycol hydraulic fluid, water-in-water, and oil-in-water emulsion.

And should be “used” and the antioxidant, rust, and other types of composite additives configuration for different purposes of hydraulic oil (liquid) variety.

The antiwear hydraulic oil compound type of zinc-free (ash-free) antiwear hydraulic oil compound uses hydrocarbon sulfide, phosphate ester, phosphate ester, and s-P-N extreme pressure antiwear agent containing sulfur and phosphorus and nitrogen three elements.

In extreme pressure industrial gear oil, p-S type radical pressure agent is given priority.

Seals produce “dissolution” or “extraction” is the hydraulic oil additives contain a variety of chemical elements and their concentration, according to the principle of “similar phase dissolution,” have different effects on different sealing materials, but also the medium resistance of sealing materials.

For example, Shell Omala 320 gear oil and Shell Omala 460 gear oil shows polar phosphorus (P) element concentration in about 300ppm, so nitrile butadiene rubber because of containing propylene nitrile group has a polarity, has excellent oil resistance, but is not suitable for the type of oil medium conditions.

With the continuous development of hydraulic oil varieties, the impact of oil, various antiwear additives, extreme pressure additives, metal deactivators, demulsifiers, and anti-foam additives is, improved sealing materials need to be verified through experiments.

Three, sealing material oil (liquid) performance test and evaluation.

Rubber seal oil performance generally adopts standard test oil, according to the test temperature conditions prescribed by the standard and test the steeping time, using immersion test values before and after contrast (such as material hardness, tensile strength, change rate, breaking elongation rate of change, volume change rate, permanent compression deformation, etc.), and evaluate its performance.

No1 standard oils are “low volume additive oils,” mixtures of mineral oils made from solvent extracted, chemically treated, dewaxed paraffin residues, and neutral oils;

No3 standard oil is a “high volume additive oil” consisting of two lubricating oil components obtained by vacuum distillation from selected naphthenic crude products.

The evaluation of the index value of materials is different for different purposes.

Four, the influence of hydraulic oil (liquid) on the performance of seals

Sealing theory considers a complete lubrication film between a dynamic, flexible seal and its mating surface.

Under normal conditions, it is with the help of this layer of lubrication film to achieve sealing and prolong the life of seals.

In the hydraulic system, hydraulic oil (liquid) affect the performance of the packing seal in addition to the medium/intermiscibility of sealing material, oil viscosity, viscosity changes with temperature, speed of movement of the reciprocating (rotation), or the fabric of the piston rod, cylinder pressure and shaft, hardness, geometric shapes, surface finish, and the structure of seal, factors influencing the characteristics of lubrication film formation and film.

All affect the sealing effect.

In general, the formation of continuous lubrication film can achieve the desired sealing effect.

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It is considered that the sealing mechanism of oil seal (dynamic rotary seal) consists of lubrication characteristics and sealing principle.

Lubrication characteristics: the friction characteristics of the oil seal are controlled by the viscosity and sliding speed of the fluid. The relative sliding surface of the oil seal and the shaft moves under the lubrication state of oil film separation to keep the friction resistance and wear small.

Sealing principle: the flow of oil on the sliding contact surface of the oil seal is a cycle from the atmospheric side to the oil side and from the oil side to the atmospheric side.

The sliding surface is well lubricated to prevent wear and tear, so there is no leakage.

When the system’s speed is too high, the formation of continuous lubrication film will be affected, increasing friction heat, and the sealing parts will be damaged if the temperature range of the sealing material is exceeded.

When the pressure is too enormous and affects the formation of oil film, it will also produce a “squeeze gap” effect on rubber and plastic seals, which can be improved by adding a “retaining ring.”

Or choose small friction coefficient, good self-lubrication PTFE seal structure will help improve the lubrication between the friction pair, suitable for high and low-speed reciprocating motion and high-pressure system cylinder piston, piston rod seal.

The key to obtaining practical sealing effect, reliability, and service life are to select seals and sealing materials reasonably according to system working conditions and medium environment and install, use and maintain seals correctly.

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