7 important performance factors to consider when selecting hydraulic seals
Hydraulic sealing materials require good elasticity, stretchability, high pressure resistance, wear resistance, and low friction coefficient, all of which can be reflected by the mechanical properties of the material and are related to its mechanical strength.
The determination of mechanical strength is relatively easy and serves as the basis for other material performance tests. So it is regarded as the most basic material performance indicator. The mechanical properties of synthetic rubber and plastic materials generally need to consider indicators such as hardness, tensile strength, wear resistance, elasticity, and elongation.
1. Hardness
Hardness refers to the ability of a material's surface to resist plastic deformation or fracture; At the same time, there is an approximate relationship between hardness and strength, and materials with low hardness exhibit flexibility in deformation under stress. Hardness is an important indicator of sealing materials, and the hardness of rubber materials is usually expressed in terms of Shore hardness.
Hydraulic sealing materials must withstand oil pressure, and high pressure can cause excessive deformation of the material, even squeezing out from the sealing gap and damaging the sealing performance. Therefore, the material needs to have a certain hardness to resist this damage, and the higher the hardness, the stronger the pressure resistance.
Rubber material is used as a sealing material because it is softer than metal and therefore has flexibility. It can deform on rough sealing surfaces, conform to surface shapes, and achieve sealing purposes. Therefore, low hardness has a beneficial effect on improving sealing performance.
In dynamic sealing, material hardness also has a direct impact on the motion performance, and it is relatively complex. Different sealing types have different ways in which hardness affects the motion performance. Generally speaking, low hardness results in a low coefficient of dynamic friction; But it has the opposite effect on starting friction. Wear resistance is related to hardness, with high hardness and strong wear resistance.
2. Tensile strength and elongation
Hardness and tensile strength reflect the ability of materials to resist deformation, and the sealing performance of seals is closely related to this. Moreover, the tensile strength and elongation directly affect the installation performance of the seal.
Tensile strength is expressed as the tensile stress at fracture, and 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 follow Hooke's law, so the value at 100% elongation is used instead of the elastic modulus. The plastic materials of polytetrafluoroethylene have a yield point, so the tensile strength is determined by the tensile stress within the yield point.
The tensile strength and elongation have little relationship with the pressure resistance, but materials with tensile strength below 7MPa are not suitable for dynamic sealing. As a measure of lifespan, low tensile strength can easily lead to stress relaxation and permanent deformation, resulting in seal failure.
Elongation is the reciprocal index of material rigidity, expressed as a percentage of the ratio of the material's tensile strength to its natural length. The allowable elongation of a material refers to the maximum elongation that can be applied without permanent damage or deformation, and the allowable elongation affects the installation performance of the seal.
3. Elasticity
The elasticity of sealing materials is extremely important for the sealing performance of seals. Due to elasticity, the material generates a rebound force after being compressed. Squeezing type seals such as O-rings rely on the rebound force of the sealing material to obtain the initial sealing compression force; Lipstick seals such as Y-shaped rings, although beneficial for self sealing under fluid pressure, theoretically can seal even with zero compression deformation under oil pressure.
But if the sealing component has eccentricity, leakage may occur under low pressure. In this case, the elasticity of the material can compensate for the insufficient sealing contact stress caused by this eccentricity. Elasticity can be measured by rebound force, and at 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 varies at different temperatures. Many rubbers (such as nitrile rubber) exhibit their minimum elasticity at temperatures of -20~20 ℃, while some rubbers (such as silicone rubber) show little change in elasticity over a wide temperature range.
4. Permanent deformation
Seals have a certain degree of compression deformation in the sealing groove and obtain sealing ability through deformation recovery force. Due to the fact that the synthetic rubber used for sealing is a viscoelastic material, prolonged compression can result in irreversible permanent deformation.
After prolonged use, the initially set compression force will gradually lose its permanent deformation and eventually leak. Therefore, the compressive permanent deformation resistance of the material is an indicator for measuring the sealing life.
The permanent deformation of rubber and plastic polymer materials is not only related to the magnitude of the applied force, but also to the amount and time of deformation. Long term deformation is difficult to recover, and the recovery after deformation is slowly completed.
Regardless of the material, its permanent deformation is more or less related to temperature. Generally, the compression permanent deformation is minimal near room temperature, and the low and high temperature parts increase the permanent deformation.
The mechanism by which compression permanent deformation increases at low and high temperatures is different. The increase in permanent deformation during low-temperature compression is due to the slow movement caused by molecular freezing during compression, resulting in residual deformation in a short period of time. Once the room temperature is restored, the deformation value at room temperature will be restored. So the residual deformation at low temperatures is a reversible deformation.
On the contrary, the compression permanent deformation in the temperature range from room temperature to high temperature is the result of chemical changes in the compressed state. Therefore, even if left at room temperature for a long time, there is almost no deformation recovery, which is an irreversible deformation. The compression amount of materials in use generally does not exceed 30%; The stretching amount after installation shall not exceed 5%. Otherwise, permanent deformation will occur and the seal will fail.
Measuring compression permanent deformation is relatively simple. A cylinder with a standard thickness of 12.5mm can be taken as the specimen, and in practice, an O-ring with a thickness similar to the actual product can also be used as the specimen.
Considering the time effect of compression permanent deformation, test the low-temperature compression permanent deformation, compress for a certain period of time at the test temperature, release the pressure at the original temperature, let it stand for 30 minutes, and then measure the thickness of the specimen at the test temperature.
Test the permanent deformation of high-temperature compression, maintain it in the compressed state and at the test temperature for a certain period of time, release the pressure and let it sit at room temperature for 30 minutes, and measure the thickness of the specimen at room temperature. The compression permanent deformation at high temperatures can serve as a lifespan indicator for rubber materials.
5. Wear resistance
For dynamic seals, wear resistance is also an indicator of material life. The wear resistance of materials is generally evaluated through wear tests, which measure the amount of wear over a certain period of time.
Actual wear is a complex process that is greatly influenced by usage conditions such as lubrication status, roughness of sealing surfaces, working pressure of media, load, sliding distance, movement speed, and temperature.
From the perspective of the material itself, the wear resistance of the material is closely related to its hardness. The harder the material, the more wear-resistant it is, and it is also related to its tensile strength.
6. Friction coefficient
When the dynamic seal moves at low speeds, frictional resistance is the main cause of unstable motion, which has a negative impact on the performance of components and systems. Therefore, for dynamic seals, frictional performance is one of the important performance indicators, and the friction coefficient is an evaluation index of frictional performance.
The friction coefficient of synthetic rubber is relatively high, but for synthetic rubber used in hydraulic seals, it is not meaningful to examine the material friction coefficient alone, because the seal is usually in a mixed lubrication state with working oil or lubricant during motion.
Lubrication conditions have a significant impact on the friction coefficient, such as the dynamic friction coefficient of NBR, which can vary between 0.5 and 3 depending on the measurement conditions.
The lubrication conditions of pneumatic components are relatively poor during operation. For oil-free cylinders, lubricating grease is only applied during installation, and no additional lubricant is supplied during use. For such seals, the friction coefficient of the material needs to be carefully selected.
The hardness of synthetic rubber is related to its friction coefficient, with higher hardness resulting in lower friction coefficient; The friction coefficient of synthetic resin is generally lower than that of rubber; The smallest friction coefficient is polytetrafluoroethylene, with a non lubricated friction coefficient of 0.04.
In addition, the friction coefficient is also related to many factors such as surface condition, contact stress, and motion speed, which are very complex. It is difficult to directly measure the friction coefficient, and the general experimental method is to measure the friction force under a certain standard state.
Static friction is affected by various factors mentioned above, resulting in significant measurement errors, and the measured values can only be used as references; Compared to this, dynamic friction can obtain more stable and repeatable measurement values. 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 failure cannot be completely ignored during use. When using seals for sports, especially in situations with vibration, if the shape of the seal repeatedly changes, attention should be paid to the fatigue damage of the seal.
In pneumatic seals that are sensitive to the influence of friction, in order to reduce frictional resistance, the seals are often made into shapes that are easy to deform. This way, if the lubrication condition deteriorates, the seals will repeatedly deform and fatigue will occur. So, in this situation, it is important to master the bending fatigue strength of the material.
The bending strength can be tested by fracture test. The method is to apply repeated bending deformation to the specimen, record the number of bending times and fracture propagation speed at the time of fracture, in order to reflect the bending strength.