The reason of the main failure of Grommets and its prevention and control measures
Grommets design, improper use will accelerate the damage of Grommets, loss of sealing performance. The experiment shows that if the design of the sealing device is reasonable and the pressure is raised simply, it does not cause the damage of the washer. Under high pressure and high temperature working conditions, the main reason of the Grommets is the permanent deformation of Grommets material and Grommets was piled into bite level spacing Grommets seal clearance distortions during movement.
Permanent deformation of Grommets material
As a result of the Grommets with synthetic rubber material is belong to the viscoelastic material, so the initial amount of compression set and springback plugging ability after using for a long period of time, will produce permanent deformation and losing, eventually leak. Permanent deformation and the disappearance of elastic force are the main reasons for the loss of sealing performance of Grommetss. The following are the main reasons for the permanent distortion of o-ring material.
1) compression permanent deformation rate and tensile and Grommets materials Making all sorts of formula of Grommets used for rubber, under the compression state produces compression stress relaxation phenomenon, at this point, the compression stress decreases with the increase of over time. The longer the use time, the greater the compression ratio and the greater the tensile strength, the greater the stress resulting from the rubber stress relaxation, so that the washer is less elastic and the sealing capacity is lost. Therefore, it is advisable to try to reduce the compression rate under allowable usage conditions. Increasing the section size of the Grommets is the simplest way to reduce the compression rate, but it will increase the size of the structure.
It should be noted that when the compression ratio is calculated, it tends to ignore the reduction of the cross-section height caused by the stretching of the Grommets during assembly. The change of the area of the Grommets area is inversely proportional to the change of its circumference. At the same time, because of the force of tension, the shape of the section of the Grommets will also change, which will show its height decrease. In addition, under the action of surface tension, the outer surface of the washer becomes more flat, i.e. the section height is reduced slightly. This is also a performance of the compression stress relaxation.
The extent of the deformation of the Grommets depends on the hardness of the Grommets material. In the case of the same amount of stretching, the stiffened Grommets, the height of the cross section decreases more. From this point of view, the low-hardness material should be selected according to the conditions of use. Under the action of liquid pressure and tension, the Grommets of rubber material will gradually become plastic deformation, and the height of the Grommets will decrease accordingly, so that the sealing capacity will be lost.
2) relationship between temperature and Grommets process
The use of temperature is another important factor affecting the permanent deformation of Grommetss. High temperatures will accelerate the ageing of rubber materials. The higher the working temperature, the greater the compression and permanent deformation of the Grommets. When the permanent deformation is greater than 40%, the Grommets loses the sealing capacity and the leakage occurs. The initial stress value formed in the rubber material of the Grommets due to compression deformation will gradually decrease with the effect of the opening of the Grommets and the decrease of temperature. The initial compression of the Grommets may be reduced or completely eliminated due to the sharp decrease in temperature. In - 50 ~ 60 ℃, under the condition of not good low temperature resistance of rubber materials will completely lose the initial stress; Even good low temperature resistance of rubber materials, at this time of the initial stress is not greater than 25% of the initial stress at 20 ℃. This is because the initial compression of the Grommets depends on the line expansion coefficient. Therefore, when the initial compression is selected, it is necessary to ensure that there is sufficient sealing capacity after the stress reduction due to the relaxation process and temperature.