Abstract: The article conducted experiments on cushion materials such as nylon, PTFE, copper based powder metallurgy, and bronze using a self-made high-frequency heavy-duty rod end joint bearing friction testing machine. The experiment shows that PTFE material has the best friction performance, followed by nylon, and has good thermal stability and thermal conductivity.
Keywords: self-lubricating rod end joint bearing; Friction performance; Mechanical properties; PTFE material; nylon
Self lubricating rod end joint bearings are a type of spherical sliding bearings that play an important role in tilting and rotating swing mechanisms. They have been widely used in aerospace, industrial production, metallurgy, machinery and other fields, and have achieved significant results. According to incomplete statistical results, the vast majority of connection control and adjustment equipment introduced in China in recent years belong to self-lubricating rod end joint bearing equipment. In order to ensure that the bearing cushion can withstand a large amount of friction, it is necessary to choose a material cushion with strong anti friction performance. Therefore, the experiment in this article has very practical significance.
1. Friction performance experiment
1.1 Experimental Materials
The material commonly used in this experiment is the rod end joint bearing shown in the following figure. The basic parameters are as follows: the inner diameter of the material inner ring is Φ 16 millimeters, 21 millimeters wide; Outer diameter of outer ring Φ 40mm, 15mm wide. The length of the internal threaded rod is 44 millimeters and the depth is 28 millimeters.
Three experiments were conducted using nylon, bronze, PTFE, and copper based powder metallurgy. There are four oscillation frequencies in each material experiment, and 48 experiments are required for each one. Table 1 shows the properties of four materials:
1.2 Experimental methods and equipment
This experiment uses a self-made experimental machine, and the specific schematic diagram is shown in Figure 2.
As shown in Figure 2, a lever, torque sensor, thermocouple, etc. were used to record the wear amount, friction torque, temperature, and other data of the material in the experiment. The specific experimental steps include the following four aspects: (1) Firstly, install the experimental bearing into the testing machine and keep it under static pressure for 15 minutes; (2) Adjust the lever dial indicator, torque sensor and other testing instruments to their working state, and then prepare for startup; (3) Detect the entire experimental process and record parameters such as material friction temperature, wear amount, and friction torque; (4) The bearing undergoes 25000 oscillations at a certain frequency.
During the experiment, the rod end joint bearing was subjected to a load through a hydraulic loading system, mainly due to the advantages of convenient pressure regulation and stability of the hydraulic loading system. In the experiment, the instrument for measuring swing parameters online is a lever dial indicator. Before loading the rod end bearing, the pointer is positioned against the bearing plane, and one end of the thermocouple is connected to a paperless recorder. The two ends of the thermocouple are placed in small holes in the bearing end face. The specific location of the small hole is shown in Figure 3:
2. Analysis of experimental results
2.1 Relationship between friction coefficient and oscillation frequency
The variation pattern of friction coefficient of four materials under a load of 16MPa and a certain oscillation frequency is shown in Figure 4.
From Figure 4, it can be seen that as the oscillation frequency increases, the friction coefficients of the four materials gradually decrease. The main reason for this phenomenon is due to the heat generated by friction, which causes the material to soften and reduce its friction coefficient. From the curve of friction coefficient variation, it can be seen that the friction coefficient decreases with the increase of oscillation frequency. When the oscillation frequency is 1.5-2.5Hz, the friction coefficient Z of PTFE is small, and the rate of friction coefficient reduction is also small. Nylon ranks second, and copper based powder metallurgy has a higher friction coefficient Z. When the oscillation frequency is 3.0Hz, the friction coefficient of PTFE material begins to slowly increase again. Overall, PTFE materials have a lower friction coefficient Z, followed by nylon, and copper based powder metallurgy has a higher friction coefficient Z.
2.2 Relationship between swing frequency and wear amount
Figure 5 shows the relationship between wear and oscillation frequency of four materials under a load of 16MPa.
As shown in Figure 5, under certain load conditions, with the increase of bearing oscillation frequency, the wear of nylon bearings gradually decreases, and the change speed is faster at 2.0Hz. With the continuous increase of oscillation frequency, the wear change tends to be gentle. The other three types of bearings have different trends compared to nylon bearings. At 3.0Hz, there is a intersection point between the wear change curve of PTFE bearings and nylon bearings, and the change curve of friction coefficient is similar. This is related to the thermal stability and thermal conductivity of nylon materials, which improve the wear resistance of nylon bearings. As shown in Figure 5, the wear of PTFE bearings is relatively small among the four types of bearings, with a trend of around 14%; The bronze bearing exhibits a turning point at a swinging frequency of 2.0Hz, and as the frequency continues to increase, the wear of the bronze bearing shows a decreasing trend; Compared to the other three types of bearings, copper based powder metallurgy bearings have a larger wear amount Z. The rate of increase in wear amount gradually increases between 1.5-2.5Hz, and wear debris appears during the experimental process, indicating that the wear resistance of copper based powder metallurgy bearings is very low. A turning point appears at 2.0Hz, and the change in wear amount gradually tends to be gentle.
Overall, the wear resistance of the four materials of bearings is in descending order: PTFE bearings, nylon bearings, bronze bearings, and copper based powder metallurgy bearings.
2.3 Relationship between oscillation frequency and friction temperature
Under certain load conditions, as the bearing speed increases, friction heat generation becomes increasingly severe, and the surface temperature of the bearing increases, reducing the strength of the material. Figure 6 shows the trend of bearing friction temperature changes for four types of materials under a load of 16MPa.
From Figure 6, it can be seen that the friction temperature of the bearing gradually increases with the increase of oscillation frequency. When the oscillation frequency is 1.5Hz, the friction temperature Z of PTFE bearings is low, followed by nylon bearings, and the temperature Z of copper based powder metallurgy bearings is high. When the oscillation frequency reaches 2.0Hz, the upward trend of the friction temperature of the bronze bearing gradually increases and intersects with the temperature change curve of the copper based powder metallurgy bearing. The friction temperature of the bronze bearing is Z higher at this oscillation frequency. The friction temperature between nylon bearings and PTFE bearings gradually increases, but the upward trend of PTFE bearings is more gradual. In summary, the heat dissipation of the four materials used for bearings is in descending order: PTFE bearings, nylon bearings, bronze bearings, and copper based powder metallurgy bearings.
2.4 Analysis of friction mechanism
Using microscope technology, four materials of bearings were photographed under a load of 16MPa and different oscillation frequencies, and SEM photos were provided. According to the photos, it can be seen that under the oscillation frequency of 1.5Hz, the nylon bearing liner material was not damaged, and the substrate material only showed certain wear marks. This is due to the discontinuity of the transfer film on its surface and the scratching caused by the inner ring material. As the oscillation frequency increases to 3.0Hz, the surface of the liner is intact. Due to the increase in friction temperature, the polymer and its composite materials on the surface of the coupling are more likely to form a transfer film, which reduces the micro cutting and scratching effects on the nylon matrix, thereby helping to reduce its wear amount. At 1.5Hz, the PTFE bearing material showed slight wear. When the frequency increased to 3.0Hz, the cushion material began to peel off slightly, and the matrix material gradually appeared; The wear of bronze bearings is severe at 1.5Hz, and groove shaped wear marks and pear wrinkles are also observed on the surface of the cushion. As the oscillation frequency increases, the wear of the bronze bearing cushion material becomes more severe at 3.0Hz, leading to the appearance of spots and turbidity; At a frequency of 1.5Hz, copper based powder metallurgy bearings exhibit furrows and discontinuous crack patterns on the surface of the material. As the oscillation frequency increases, the wear of the cushion layer gradually increases, and even sticky and molten phenomena occur.
3 Conclusion
In the experiment, PTFE bearings had good friction performance and excellent heat dissipation among the four materials used in the bearings; Nylon bearings are second; Copper based powder metallurgy bearing Z difference. According to wear analysis, the thermal conductivity and stability of nylon bearing material are good, so it can operate under high-frequency and heavy-duty conditions.
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