Experimental study on stress relaxation of rubber O-ring for mechanical seal

The stress relaxation process and its variation law under multiple stresses have an important impact on the performance of rubber O-rings. An O-ring experimental device was developed, the end contact forces of the O-ring under the conditions of the single and multiple stresses were tested, and the contact force retention rate was calculated based on the test results. The change processes and laws of the end face contact force and the retention rate were obtained by the analyses of the experimental data under the different impact factors. The experimental results indicated that the compression ratio of the O-ring can increase the end contact force, but the greater compression ratio results in greater attenuation of contact force and longer attenuation time. The medium temperature hurts the contact force retention rate, and the medium pressure is conducive to stabilizing the contact force retention rate.


Introduction
Rubber O-rings are widely used for static or dynamic sealing of various mechanical equipment due to their simple structure, easy installation and disassembly, and strong interchangeability, to prevent the leakage and invasion of liquid or gas media with a certain temperature and pressure inside and outside the equipment.With the development of production equipment technology, new requirements have been put forward for the application parameters and quality of rubber O-rings.On the one hand, the use of high-parameter equipment requires high-performance rubber O-rings for long-term operation as a sealing guarantee; on the other hand, the failure or damage of O-rings often leads to device leakage and inability to function properly, resulting in material loss, energy waste, environmental pollution, and even leading to fires, explosions, and endangering personal safety.
Nam et al. [1] proposed a new model to develop a transparent photoelastic experimental loading device to study the behavior and stress changes of O-rings under uniform extrusion rate and internal pressure over time.Kenneth et al. [2] proposed a new model to use isothermal experimental methods to test and analyze the rubber performance aging law of O-rings under compressive stress relaxation.Shankar et al. [3] proposed a new model to design and develop a performance test bench for reciprocating hydraulic seals.Bernstein et al. [4] proposed a new model to use an air circulation oven, a compressive stress relaxation instrument, and a thermocouple-connected continuous recorder to measure the long time required for fluorosilicon materials to achieve physical relaxation.
Gu, et al. [5] proposed a new model to develop an O-ring sealing performance test device for static rings and also analyzed the relationship among the contact stress, leakage, medium pressure, and precompression rate.Tsinghua University et al. [6] proposed a new model to design the testing equipment including a test cylinder, linear guide rail, electric cylinder, force sensor, displacement sensor, and control cabinet; and tested the sealing and friction performance of sealing rings with different shapes (such as O-rings and Y-rings).Yang [7] proposed a new model to design a micro-motion test rig of the auxiliary seal ring to assess the friction of the auxiliary seal ring of the fluid dynamic pressure type mechanical seal for the nuclear main pump and analyze the influence of the reciprocating frequency and reciprocating displacement amplitude on the friction law of the auxiliary seal ring in the normal operating conditions.Beijing University of Chemical Technology [8] proposed a new model to design a test system for determining the friction force of auxiliary O-rings in mechanical seal compensation mechanism and obtained the friction force of the rubber O-ring lubricated by water.Qu et al. [9] proposed a new model to design an experimental device for detecting and studying the aging status of a typical rubber O-ring sealing structure.Hu [10] proposed a new model to use instantaneous creep testing technology to study the aging behavior of rubber materials under the effects of humidity and temperature The above studies have provided great help in exploring the performance of O-rings under different working conditions.However, the research on the performance testing device for O-rings lags behind the development requirements of industrial equipment.The main shortcomings are as follows: (1) During the medium loading process, the medium pressure acting on the medium cavity will affect the initial sealing pressure of the tested O-ring.(2) It cannot reflect the sealing performance of the dynamic and static rings when they are worn and followed by the O-ring, as well as the stress relaxation state of the O-ring used for the static ring; (3) The axial load loading of the O-ring is unstable and inaccurate.(4) It is difficult to test the friction performance, sealing performance, and stress relaxation performance of the O-ring on a testing machine.
This work develops a rubber O-ring stress relaxation performance testing device with the function of measuring contact force and compression under multiple stresses simultaneously.Based on the multiple stress effects of different medium pressures, medium temperatures, and compression amounts, the stress relaxation experiments of rubber O-rings are carried out, the formation principle of rubber Oring performance is explored, and the stress relaxation law of O-ring is obtained.

Bench system
The stress-loading experimental system mainly consists of a stress-loading test bench, sensors, heating oil tanks, high-temperature circulating oil pumps, pressurized nitrogen gas cylinders, etc.The various components are connected through copper pipes, and the structural diagram is shown in Fig. 1.The stress loading experimental platform can accurately measure the contact force of the O-ring sealing surface and accurately control the compression amount of the O-ring.It mainly includes the core shaft, pressure column, sliding table, tested O-ring, force sensor, non-contact displacement sensor, and stepper motor.To measure the temperature and pressure of the medium, the temperature sensors and pressure sensors are installed at the inlet of the stress loading test bench.The medium temperature is heated by a heater in the heating oil tank, and the medium pressure is pressurized by nitrogen gas output from the nitrogen cylinder.

Automatic measurement and control system
The automatic measurement and control system is mainly composed of an upper computer, a PLC control cabinet, sensors, and driving equipment.The measurement and control software developed by Labview is installed on the upper computer, and the control instructions and data can be sent to the PLC to control the operation status of the equipment by operating the software.During the control process, it mainly controls the start and stop of the high-temperature oil pump and the rotation angle of the stepper motor, which can adjust the heating power of the heater, and the operation interface is shown in Fig. 2.During the O-ring acceleration experiment, thermal oil is used as the sealing medium.The thermal oil is injected into the test bench through a high-temperature oil pump and copper pipe connection and then flows back to the heating oil tank to complete the medium circulation.The heating modes of heat transfer oil are manual and automatic.The manual mode allows for setting a heating coefficient of 0-100%.After setting the heating coefficient, the heater will heat as a percentage of the heating power.In automatic mode, the control system will form a closed-loop PID cascade control based on the inlet temperature and set temperature, and then adjust continuously the heating coefficient of the heater to ensure that the temperature of the heat transfer oil rises to the setting temperature steadily.

Contact force measurement
The O-ring is installed below the pressure column and connected to the upper thread of the force sensor by the core shaft.The lower thread of the force sensor is connected to the upper thread of the sliding table, and the lower thread of the sliding table is connected to the differential screw through a sleeve and the stepper motor shaft.Under the action of the guiding flat key, only the axial movement of the sliding table is generated.When the motor shaft rotates, the sliding table drives the pressure column to move axially downwards.so the axial tension generated by compressing the O-ring is the measured Oring contact force, which can be measured by an intermediate force sensor.

Compression measurement
The upper end of the differential screw rotates with the sliding table to generate a pitch P1; the lower end of the differential screw rotates with the bracket to generate a pitch P2, and two threads are righthanded.When the stepper motor shaft rotates counterclockwise, the differential screw moves down P2 relative to the bracket, and the sliding table moves up P1 relative to the differential screw.At this point, the core shaft and pressure column installed on the same vertical centerline move down P2−P1, which forms the compression of the O-ring, thus the compression of the O-ring can be measured by a noncontact displacement sensor.

Experimental plan (1)Performance attenuation parameters
The contact force retention rate can be as the performance degradation parameter of rubber O-rings, and the calculation formula is: ε= where  is the contact force retention rate,%;  is the initial contact force between the O-ring and the sealing surface, N;  is the measured contact force of the O-ring sealing surface at t, N.

Experimental results analysis (1)
The effect of compression rate on the contact force of sealing surfaces Fig. 3 shows the changes in contact force of the O-ring sealing surface with compression rate at different temperatures, and the influence of medium pressure is not considered.
After comparing the contact force of the O-ring at different experimental temperatures and compression rates, it is found that the higher the compression rate, the greater the initial contact force of the O-ring, and the attenuation of contact force is faster at an early stage of attenuation.The contact force tends to be stable after 18000 s at 10% and 15% compression rates, but there is still a significant attenuation trend at a 20% compression rate.This indicates that reaching the stable point of contact force needs more time when the compression rate of the O-ring is higher, and the variation between the contact force and the initial contact force is greater at the stable point.
At the same compression rate, the higher the experimental temperature, the greater the expansion deformation of the O-ring, which results in a greater initial contact force.After 18000 seconds, the contact force with high temperature is still greater than that with low temperature.This indicates that the higher the temperature, the faster the stress relaxation and the less reliable the O-ring is.
(2)The effects of medium pressure on contact force and contact force retention rate Fig. 4 shows the contact force changes of the O-ring with different medium pressures at a 10% compression rate and room temperature, and Fig. 5 shows the changes in contact force retention rate with different medium pressures.
By comparing the changes in the contact force of the O-ring sealing surface with different media pressures, it can be found that the contact force of the O-ring sealing surface increases continuously with the increase of media pressure.
The retention rate of contact force under medium pressure is much greater than that without medium pressure.During the process of decreasing contact force retention rate, the higher the medium pressure,   (3)Comparison of contact rates with different experimental parameters According to the above experiments, it can be seen that when the compression rate is 10%, the initial contact force on the sealing surface of the O-ring varies with medium temperatures and pressures.So the attenuation of the contact force can be reflected by comparing the contact force retention rate, as shown in Fig. 6.
When the medium pressure is constant, the higher the medium temperature, the lower the retention rate of the O-ring contact force.On the contrary, when the medium temperature is constant, the higher the medium pressure, the higher the retention rate of the O-ring contact force.Therefore, it can be seen that the temperature of the medium hurts the contact force retention rate, however, the pressure of the medium is conducive to stabilizing the contact force retention rate and reducing the stress relaxation phenomenon of the O-ring.When comparing the experimental data with medium pressure, it is found that the contact force retention rate is the highest under the working conditions of 0.7 MPa and 80℃ even after 25000 seconds.The contact force retention rate is at the lowest position and the attenuation rate is also the fastest under working conditions of 0.3 MPa and 120℃.

Conclusions
Based on the characteristics of the O-ring, a stress relaxation of rubber O-ring experimental device was developed, which can monitor the attenuation process of O-ring contact force under different medium pressures, medium temperatures, and compression rates.
(1)The greater the compression rate of the O-ring, the greater the initial contact force, the faster the attenuation rate, the greater the attenuation amount, and the longer the time for the contact force to reach the stable point.
(2)The temperature of the medium hurts the contact force retention rate, while the pressure of the medium is conducive to stabilizing the contact force retention rate.

Figure 2 .
Figure 2. Upper computer operation interface.The start and stop of the high-temperature oil pump and the rotation angle of the stepper motor are both controlled through digital signals output by the PLC.The rotation angle of the stepper motor is determined by the number of high-speed pulses sent by the PLC to the stepper motor driver.By setting the parameters of the stepper motor, the rotation angle accuracy of the stepper motor can reach up to 0.03°, therefore the precise control of the loading and compression amount of the O-ring contact force can come true.During the O-ring acceleration experiment, thermal oil is used as the sealing medium.The thermal oil is injected into the test bench through a high-temperature oil pump and copper pipe connection and then flows back to the heating oil tank to complete the medium circulation.The heating modes of heat

Figure 4 .
Figure 4. Contact forces at different medium pressures.

Figure 5 .
Figure 5. Contact force retention rate at different medium pressures.

Figure 6 .
Figure 6.Contact force retention rate with different parameters.
the slower the decrease of retention rate.Secondly, the lower the medium pressure, the longer it takes for the contact force retention rate to reach a stable stage.