This study performed sealing ring experiments and scanning electron microscopy (SEM) analysis to reveal the influence of sealing materials on frictional behavior and sealing performance. A test rig was built for experiments on the sealing performance under oil-lubricated conditions. A series of test results was obtained, compared, and discussed to understand the effects of sealing materials on surface friction and sealing performance.
| 1. Experimental details |
|---|
| 1.1 Experimental device |
| 1.2 Material and specimen |
| 2. Results and discussion |
| 2.1 Variation of friction coefficient of different composites |
| 2.2 Influence on surface temperature |
| 2.3 Influence on leakage |
| 2.4 Wear mechanisms of the sealing materials |
| 3. Conclusion |
A special test rig was built and used for the experiments on dynamic sealing performance under high pressure and high speed, which is shown in Fig. 1. This test rig was composed of a transmission system, a seal test system, and a control and data acquisition system. The transmission system consisted of a variable frequency motor, a shaft, a synchronous belt, and other ancillary equipment. The seal test system included a test chamber, a hydraulic pressure station, an oil recovery device, and other components. The control and data acquisition system involved sensors in the test rig, test software, and other electronic devices.
The shaft passed through the test chamber, and the other end of the shaft connected the motor. A torque/speed sensor, which measured torque and speed, was connected to the shaft as a whole. Oil pressure was supplied by the special hydraulic pressure station. The temperature measurement and adjustment system in the test chamber was composed of oil temperature sensors, heaters, and controllers. The DRFL series torque sensor, which was made by ETH Company, was selected for the test rig to measure torque and speed simultaneously. The rated torque was 30 N m, and the rated speed was 6000 r/min. The maximum torque measurement error was 0.1% of the full scale, and speed measurement accuracy was 71 r/min. Pressure was measured by the pressure sensors produced by Hydro Technik Company. The sensor's measurement ranged from 0 MPa to 6 MPa, and the sensor's maximum pressure measurement error was 0.5% of the full scale. Oil leakage was measured by an ultrasonic fluid meter made by Dynasonics Company. The sensor's measurement ranged from 0 L/min to 40 L/min, and the sensor's maximum flow measurement error was 0.1% of the full scale. The meter was installed on an oil inlet pipe. The leakage amount in the test chamber was the same as the amount of oil supplied. Therefore, the meter could measure the leakage rate of the sealing ring in the test chamber. A Pt100 series temperature sensor made by Jumo Company was chosen to measure the temperature of the sealing rings. The sensor's measurement ranged from 30 °C to 250 °C, and the maximum measurement error was 1% of the full scale.
The sealing ring was located between the groove in the rotating shaft and the housing in the test rig. The sealing rings maintained the oil pressure between the stationary housing and the rotating shaft. The sealing ring employed in this study had a joint. The design of the sealing ring was quite similar to that of the piston ring. However, the sealing ring rotated instead of performing a reciprocating motion. The schematic of a sealing ring is shown in Fig. 2(a). The sealing principle of the sealing rings in the transmission of a heavy vehicle is presented in Fig. 2(b). The sealing principle indicated that the end surface “BC” was the main sealing face, and the cylindrical surface “AB” was the auxiliary sealing face. Under working conditions, the shaft rotated, whereas the sealing ring remained at rest. That is, the sealing ring did not rotate with the shaft.
The main parameters of the sealing ring were the outer diameter D1, inner diameter D2, and thickness L0. The outer diameter of the sealing ring corresponded to the rotating shaft diameter. The parameters of the sealing ring used in this study were as follows: outer diameter D1 was 125 mm, inner diameter D2 was 119.2 mm, and thickness L0 was 2.6 mm. Several sealing rings worked together in the transmissions of heavy vehicles. The appropriate surface roughness should be established to reduce power loss and enhance transmission efficiency. Therefore, the average surface roughness (Ra) of the sealing ring was set at 1.6 μm. The average surface roughnesses of the shaft and that of the sealing ring were the same. The practical experiences and references explained that polytetrafluoroethylene (PTFE), polyimide (PI), and polyetheretherketone (PEEK) matrix materials were chosen because of their excellent material properties. Prior to the present study, the three matrix materials abovementioned were studied with different filler materials. The most suitable of the three sealing composite materials from the various filler materials of the three matrix materials was selected through experiments. This study compared these matrix composite materials to select one or two materials that were suitable for sealing ring application. The composites included in this paper were bronze filled PTFE, MoS2/glass fiber-filled PEEK, and MoS2/glass fiber/graphite-filled PI, which were proved to have the best frictional characteristics. Basic information on the tested materials is listed in Table 1. All the materials tested were in the form of sealing rings. Custom-made materials were manufactured into sealing rings by Saint-Gobain Company, China.
Fig. 3 presents the variation in friction coefficient of the sealing ring with speed for the three sealing composites when pressure p was at 2.0 MPa and oil temperature To was at 60 °C. The data point in the figures was the average value of six test data, and the error bars represented the precision error from the six tests.
Fig. 3 shows that the friction coefficient of the PTFE composite changed from 0.064 to 0.038 with the increase of speed from 500 r/min to 5000 r/min. The test values of the PTFE composite initially decreased with increasing speed, reached a minimum, and then increased with speed. This finding was consistent with the empirical formulas proposed by Zhang et al. [15] with regard to the PTFE composite in the oil-lubricated conditions. The test results of the PTFE composite shared the same changing tendency as those of the PI and PEEK composites.
The critical speed for the three sealing composites, where the friction coefficient reached a minimum, was between 4000 r/min and 4500 r/min, which proved that hydrodynamic lubrication was essential and in accordance with the calculated results [16]. Hydrodynamic effect had a positive effect on the operation of sealing rings because high pressures were compensated by the hydrodynamic force caused by the rectangular groove structure of the sealing ring. The friction coefficient slowly increased with speed when the rotating speed was greater than 4500 r/min because of the increase in viscous shear effect of the oil film.
The analysis of the variation in friction over the course of the test provided insight into the material's behavior in an actual application. The test results clearly indicated that bronze-filled PTFE consistently provided the lowest levels of friction. Moreover, MoS2/glass fiber-filled PEEK provided low friction compared with the PI composite. The friction coefficients of the PTFE and PEEK composites were generally 79–90% of the PI composite.
Fig. 3 shows that the friction coefficient had relatively high values at low speeds. Poor lubrication appeared on the sealing surface at low speeds and promoted the excessive wear of the sealing ring with high dominant values of friction force. Thus, the life of the sealing ring was diminished [17]. The three composites provided low friction at high speed. The film between the sealing surfaces was formed easily at high speed and was relatively thick and stable.
The difference between the minimum and maximum friction coefficients during the course of the test is shown in Fig. 3. The minimum and maximum friction coefficients of the PI composite were 0.0458 and 0.0710 respectively. The minimum and maximum friction coefficients of the PEEK composite were 0.0385 and 0.0681 respectively. The minimum and maximum friction coefficients of the PTFE composite were 0.0366 and 0.0642 respectively. Minimal difference was observed between the minimum and maximum friction coefficients of the PI composite. The friction of the PTFE and PEEK composites significantly varied with the changes in speed.
The results of the sealing surface temperature in relation to the changes in rotating speed at oil temperature To¼60 °C and pressure p¼2.0 MPa are displayed in Fig. 4.
All the sealing materials demonstrated an increase in surface temperature with increasing speed. The surface temperature of all the sealing materials significantly changed between 1000 r/min and 4000 r/min but only slightly increased at speeds above 4000 r/ min, especially for the PI composite. The bronze-filled PTFE produced the lowest surface temperature as the rotating speed increased. The MoS2/glass fiber-filled PEEK exhibited low surface temperature, especially at low rotating speeds. The PTFE composite had a much lower temperature than the PEEK composite at high rotating speeds. The PI composite consistently yielded higher surface temperature than the other composites.
All the tested sealing materials demonstrated increasing trends in temperature with the increase in sliding friction. The PI composite yielded the highest surface temperature among the three sealing materials given that it had the highest friction coefficient. The increasing surface temperature with increasing speed was most likely a result of the decreasing lubricant viscosity, which may allow the hard filler materials to come into contact with the counter surface [18]. The increase in surface temperature caused by frictional heating was also responsible for the changes in the friction and wear behaviors of materials [19]
Fig. 5 shows the variation in volume leakage rate with rotating speed in the case of the constant pressure p¼2.0 MPa and oil temperature To¼60 °C. The leakage rate of the PI composite was higher than those of the PTFE and PEEK composites. The sealing ring in this paper was an elastic ring with a joint, which caused considerably more leakage than the remaining parts of the sealing ring. The joint size should be designed according to the elastic force requirement. The elastic force was related to the modulus of the sealing material [16]. If the material and material formula were determined, the modulus of material could be determined, and then the elastic force of the sealing ring itself could be determined also. Thus, the modulus of the sealing material had an effect on the ideal joint size and affected the leakage rate of the sealing ring.
The difference in leakage rates among the PTFE, PEEK, and PI composites was small. Leakage increased with rotating speed and was more sensitive to speed. Hence, the centrifugal effect in the fluid was responsible for the increased leakage rate [20]. Speed also had an effect on the joint opening, which in turn affected the leakage rate. Furthermore, the effect of speed on the leakage of the sealing ring was more significant than that of the wear of the sealing surface.
Further investigations utilizing SEM revealed unique tribological characteristics of the sealing materials under sliding contact. The sliding direction in all SEM images was approximately parallel to the horizontal direction.
The wear characteristics of bronze-filled PTFE were consistent for high and low temperatures. The bronze particles in all cases had scratches parallel to the sliding direction. A representative image of the worn PTFE surface is displayed in Fig. 6a, which shows that the surfaces of the worn bronze particles and the matrix material were approximately level with rough transitions between the filler and matrix material. The gaps could be distinguished around the edges of the filler particles. The bronze particles appeared flattened and sank well into the matrix. Wear scars were seen only on the bronze particles. The presence of filler material improved the resistance of the PTFE composite to adhesion wear by changing the structure of the composite. The bronze particles at high pressure loading occupied a large area of the contact region. The bronze particles were more wear resistant than the PTFE material. Small displacement of the bronze particles was observed in the bulk matrix, which did not lead to severe wear of the PTFE composite.
SEM evaluation of the worn PI composite surface revealed significant wear scars with grooves cut into the material by the steel counter surface, which is shown in Fig. 6b. The counter surface particles were observed in the composite, which is often the case in glass fiber-filled materials. The sharp glass fibers tended to collect other softer materials in the sliding direction and polish on the counter surface. Several corrugations perpendicular to the motion direction of the friction pairs could be observed. The repeated sliding against hard steel could lead to fatigue at the contact peak points of the PI surface. The surface of the PI composite experienced shear deformation, and the deformation effects accumulated gradually in the friction process [21]. The corrugations during the friction process expanded perpendicularly to the direction of the sliding motion. The frictional heat increased the temperature at the counter surface when the sliding speed increased. The increased temperature weakened the connection between the filler materials and the matrix material and led to plastic plow and adhesive wear.
The analysis of the worn PEEK composite surface revealed that some patches of the deformed layers covered the worn surface of the PEEK composite as a result of a softened matrix material (Fig. 6c). The increase in temperature caused thermal effects, which resulted in the softening of the matrix material. The frictional shear stress was low at high temperature; however, the frictional shear stress was probably sufficient to cause filler material pull-out owing to the weak bond. The thermal and mechanical combined effects resulted in the debonding of the filler materials, and, sometimes, in the loss of structural integrity. However, the PEEK composite displayed the smoothest worn surface among the three sealing materials in this study; only slight abrasion scratches and some pitting marks were seen. The PEEK composite had a small scar width. Abrasive wear was not noticeable, and the wear mode changed to the production of flaky debris. The wear scars were smooth and almost indistinguishable by SEM. The production of flaky debris instead of grooving indicated that the wear mechanism has probably changed to micro-cracking or surface fatigue. However, wear resistance was greatly improved despite presenting a failure mode because of the added glass fibers [22,23]. The smooth surface of the PEEK composite yielded the low friction coefficient observed for all materials in this study. The mating metal wear surface of the shaft groove was studied by optical micrograph. The shaft groove surface only had the slight abrasion compared with the sealing ring surface. The scratches on the shaft groove surface were parallel to the sliding direction.
SEM examinations further confirmed the experimental findings on the suitability of materials for seal application. Good mechanical and tribological properties of polymers with respect to practical applications were required under particular conditions. The sealing ring currently used in the transmissions of heavy vehicles was made of PTFE composite filled with graphite, MoS2, and bronze [24]. Compared with the sealing ring used, the sealing rings in this study significantly improved the frictional behaviors especially for the frictional coefficient and surface temperature in the middle and high speeds.
The tribological characteristics and sealing performance of the sealing ring were investigated under sliding contact in the presence of an oil lubricant for PTFE, PI, and PEEK composites. Experiments were conducted on a special test rig of the sealing ring. A comparative study on the frictional analysis of the three sealing composites was presented and took into account several aspects of the tribological behavior of the materials under analysis.
(1) Increased speed generally resulted in an increase in friction. The highest friction levels were provided by the PI composite, which was followed by MoS2/glass fiber-filled PEEK, and finally, bronze-filled PTFE.
(2) Surface temperature and leakage rate were influenced by material characteristics and test conditions. No significant relationship was found between surface temperature and leakage rate.
(3) Wear mechanisms were dependent on material composition. Plastic deformation, in particular, the sliding grooves typical of micro-ploughing, was observed in the wear track of the PI composite. Slight wear scars were observed on the surface of the PEEK composite. Significant wear scars were observed only on the bronze particles of the PTFE composite.
(4) From the comprehensive indicators, the PEEK and PTFE composites were better suited as the materials for sealing rings under pv factor from 8 MPa m/s to 65.4 MPa m/s.