Hydraulic sealing materials require good elasticity, stretchability, high pressure resistance, wear resistance, and low friction coefficient. These can be reflected by the mechanical properties of the material and are all related to the mechanical strength of the material.
The determination of mechanical strength is relatively easy and is also the basis for other material performance tests. Therefore, it is used as the most basic material performance indicator. The mechanical properties of synthetic rubber and plastic materials generally consider indicators such as hardness, tensile strength, wear resistance, elasticity, and elongation.
1. Hardness
Hardness refers to the ability of the material surface to resist plastic deformation or rupture; at the same time, hardness and strength have a certain approximate relationship, and materials with low hardness show flexibility under stress deformation. Hardness is an important indicator of sealing materials. The hardness of rubber materials is usually expressed in Shore hardness.
Hydraulic sealing materials must withstand oil pressure. High pressure can cause excessive deformation of the material, and even squeeze out of the sealing gap to destroy the sealing performance. Therefore, the material must have a certain hardness to resist this damage. The higher the hardness, the stronger the pressure resistance.
Rubber material is used as sealing material because it is "softer" than metal, so it is flexible, deforms on the rough sealing surface, conforms to the surface shape, and achieves the purpose of sealing. Therefore, low hardness has a favorable effect on improving sealing.
In dynamic seals, material hardness also has a direct impact on motion performance, and it is more complicated. Different sealing types have different ways of affecting motion performance. In general, low hardness has a low dynamic friction coefficient; but it has the opposite effect on starting friction. Wear resistance is related to hardness, and high hardness has strong wear resistance.
2. Tensile strength and elongation
Hardness and tensile strength reflect the ability of materials to resist deformation, and the sealing of seals is closely related to this. Moreover, tensile strength and elongation directly affect the installation performance of seals.
Tensile strength is expressed as tensile stress at fracture. The tensile stress value of rubber materials is usually taken as the stress value at 100% elongation. This is because the stress-strain curve of rubber materials does not obey Hooke's law, so the value at 100% elongation is used instead of elastic modulus. Plastic materials such as polytetrafluoroethylene have a yield point, so the tensile strength is measured by tensile stress within the yield point.
Tensile strength and elongation have little to do with pressure resistance, but materials with a tensile strength lower than 7MPa are not suitable for dynamic seals. As a life measurement indicator, low tensile strength is prone to stress relaxation and permanent deformation, resulting in seal failure.
Elongation is the reciprocal indicator of material rigidity, expressed as a percentage of the ratio of the material's elongation to its length in the natural state. The allowable elongation of a material refers to the maximum elongation that can be applied without permanent damage or permanent deformation. The allowable elongation affects the installation performance of the seal.
3. Elasticity
The elasticity of the sealing material is extremely important for the sealing of the seal. Since the elasticity causes a rebound force to be generated after the material is compressed, extrusion seals such as O-rings rely on the rebound force of the sealing material to obtain the initial sealing pressure; lip seals such as Y-rings, although they are conducive to the self-sealing of fluid pressure, can theoretically seal under oil pressure even if the compression deformation is zero.
However, if the sealing pair is eccentric, leakage may occur at low pressure. At this time, the elasticity of the material can compensate for the insufficient sealing contact stress caused by the eccentricity. Elasticity can be measured by rebound force. Under the same deformation rate, the greater the elasticity, the greater the rebound force.
Elasticity varies greatly with temperature, and the elasticity of the same material is different at different temperatures. Many rubbers (such as nitrile rubber) have a minimum elasticity at a temperature of -20~20℃, while some rubbers (such as silicone rubber) have little elasticity change in a wide temperature range.
4. Permanent deformation
The seal obtains its sealing ability by deformation recovery force due to its certain compression deformation in the sealing groove. Since the synthetic rubber used for sealing is a viscoelastic material, it will have irreversible permanent deformation when under pressure for a long time.
The initial set same elastic compression force will gradually lose its permanent deformation after long-term use, and eventually leakage will occur. Therefore, the material's resistance to compression permanent deformation is an indicator to measure the life of the seal.
The permanent deformation of rubber and plastic polymer materials is not only related to the magnitude of the force, but also to the deformation amount and deformation time. Long-term deformation is difficult to recover, and the recovery after deformation is completed slowly.
Regardless of the material, its permanent deformation is more or less related to temperature. Generally, the compression permanent deformation is the smallest near room temperature, and the low and high temperature parts increase the permanent deformation.
The mechanism of compression permanent deformation increase at low temperature and high temperature is different. The increase of low-temperature compression permanent deformation is because when compressed at low temperature, the molecules are frozen and move slowly, and the deformation remains for a short time. Once the room temperature is restored, the deformation value at room temperature will be restored. Therefore, the residual deformation at low temperature is a reversible deformation.
In contrast, the compression permanent deformation in the temperature range from room temperature to high temperature is the result of chemical changes accompanied by compression. Therefore, even if it is placed at room temperature for a long time, there will be almost no deformation recovery, which is an irreversible deformation. The compression amount of the material in use is generally not more than 30%; the stretching amount after installation does not exceed 5%. Otherwise, permanent deformation will occur and the seal will fail.
It is relatively simple to measure the compression permanent deformation. A cylinder with a standard thickness such as 12.5mm can be taken as a test piece. In practice, an O-ring with a thickness close to the actual product can also be used as a test piece.
Considering the time effect of compression permanent deformation, test low-temperature compression permanent deformation, compress at the test temperature for a certain time, release the pressure at the original temperature, and measure the thickness of the specimen at the test temperature after 30 minutes.
Test high-temperature compression permanent deformation, keep it in the compressed state and at the test temperature for a certain time, release the pressure and place it at room temperature for 30 minutes, and measure the thickness of the specimen at air temperature. Compression permanent deformation at high temperature can be used as an indicator of the life of rubber materials.
5. Wear resistance
For dynamic seals, wear resistance is also an indicator of material life. The wear resistance of materials is generally examined by wear tests, that is, it is measured by the amount of wear over a certain period of time.
Actual wear is a complex process, which is greatly affected by the use conditions such as lubrication state, roughness of the sealing surface, medium working pressure, load, sliding distance, movement speed and temperature.
From the perspective of the material itself, the wear resistance of the material is closely related to hardness. The harder the material, the more wear-resistant it is. In addition, it is also related to tensile strength.
6. Friction coefficient
When the dynamic seal moves at a low speed, friction resistance is the main reason for the unstable movement, which has a negative impact on the performance of components and systems. Therefore, for dynamic seals, friction performance is one of the important performances, and friction coefficient is an evaluation index of friction performance.
The friction coefficient of synthetic rubber is relatively large, but for synthetic rubber used in hydraulic seals, it is not very meaningful to examine the friction coefficient of the material alone. This is because when the seal is in motion, it is usually in a mixed lubrication state with the participation of working oil or lubricant.
Lubrication conditions have a great influence on the friction coefficient. For example, the dynamic friction coefficient of NBR can vary between 0.5 and 3 depending on the measurement conditions.
The lubrication conditions of pneumatic components are worse during operation. The non-oil-supplied cylinder is only coated with grease during installation, and no additional lubricant is supplied during use. For this type of seal, the friction coefficient of the material needs to be carefully selected.
The hardness of synthetic rubber is related to the friction coefficient. The higher the hardness, the lower the friction coefficient; the friction coefficient of synthetic resin is generally lower than that of rubber; the smallest friction coefficient is polytetrafluoroethylene, and the non-lubricated friction coefficient is 0.04.
In addition, the friction coefficient is also related to many factors such as surface state, contact stress, and movement speed, which is very complicated. It is difficult to directly measure the friction coefficient. The general experimental method is to measure the friction under a certain standard state.
Static friction is affected by the aforementioned factors, and the measurement error is large. The measured value can only be used as a reference; in contrast, dynamic friction can obtain a more stable and repeatable measurement value. In practice, friction mainly affects the minimum starting pressure, so the minimum starting pressure is often used as an indicator of friction characteristics.
7. Bending fatigue strength
Synthetic rubber has strong fatigue resistance, but fatigue damage cannot be completely ignored when using it. For sports seals, especially in places with vibration, the shape of the seals changes repeatedly, and attention should be paid to the fatigue damage of the seals.
In pneumatic seals that are more sensitive to friction, in order to reduce friction resistance, the seals are often made into shapes that are easy to deform. In this way, if the lubrication condition deteriorates, the seals will deform repeatedly and fatigue will occur. Therefore, in this case, it is important to master the bending fatigue strength of the material.
Bending strength can be tested by fracture test. The method is to subject the specimen to repeated bending deformation, record the number of bending times and the fracture propagation speed when fracture occurs, to reflect the bending strength.
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