Noise and friction characteristics of polymer ball bearings

Abstract

Polymer bearings are preferred in certain areas due to their superior properties to metal bearings. They have high corrosion and chemical resistance, they are light, they can work without oil. In addition to this, low friction coefficients and high vibration damping properties are the advantages that polymer bearings stand out. With these qualities, it becomes widespread in areas where the use of metal bearings such as the medical and food industry is not suitable. However, despite the advantages of polymer bearings, there are also negative aspects that limit working conditions. Polymer materials have lower temperature resistance than metal materials do. For this reason, changes in temperature, speed and loading conditions can change the tribological behavior of polymer bearings. The complex relationship of the friction behavior of polymers with temperature, speed, and load makes it necessary to test with various conditions in order to understand the running behavior of polymer bearings. In this experimental study of ball bearings made of polymers, variation in friction moment and noise characteristics in different operating conditions are of interest. Three types of polymer ball bearing were tested at three different rotational speed, four different temperature value, and three different radial loading condition. There are two aspects considered in this study: How the working conditions (i.e. speed, load and temperature) affect the sound pressure level and friction torque of the polymer ball-bearing; the effect of the material content of the components of the ball bearing on the sound pressure level. The three types of bearings used to examine the material effect in the study are: 1) POM rings, steel ball, and PA cage; 2) POM rings, glass ball, and PA cage; 3) PEEK rings, steel ball, and PA cage. The influence of the ball material was investigated by comparing the noise characteristics of the bearing-1 and the bearing-2. The effect of ring materials was investigated by comparing the data of bearing-1 and bearing-3. The operating speeds were selected as 450 rpm, 900 rpm, and 1300 rpm. The determination of the test loads was made by calculating the light, medium and heavy load zones according to the dynamic load capacity given in the manufacturer's catalogue of the polymer ball bearings. The temperature conditions were chosen to remain within the working range of the polymer materials. The test temperatures were T0 (22ᵒC±3ᵒC), T1(50±5ᵒC), T80(100±5ᵒC), and T2(100±5ᵒC). Also, B3 was tested under T3(135ᵒC±5ᵒC) condition as its ring material PEEK and resistant to higher temperatures than POM. A test apparatus was designed in order to carry out the experiments. In the apparatus, the shaft to which the polymer bearing was mounted was driven by an electric motor. It was aimed to reduce the effect of possible axial misalignment by placing a flexible coupling before and after the moment sensor at the motor output. The shaft was supported by two ball bearings. The polymer test bearing was mounted into the bore of an aluminum hub. The polymer test bearings were heated up to the desired temperature levels by heating the aluminum hub. The aluminum hub has four bores for the purpose of fixing up the flat cartridge heaters. It has also hook having a screw which enables us to apply a radial load on bearing. The temperature measurement was performed with a K-type thermocouple placed in the hole from outside of the aluminum hub to a distance close to the outer ring of the polymer ball bearing. An acoustic box having 85x170x170 cm dimensions had been used for sound level measurements. The acoustical box isolates the test bearing from the environmental sound which is over 400 Hz. Sound pressure level measurements were carried out between the frequency of 6,3 Hz and 12,5 kHz in a 1/3-octave band with a hand-held sound level meter fitted with a ½ "microphone. The test bearing and the aluminum hub were inserted into the acoustic box. The sound level meter was placed on a tripod so that the sound level meter will measure from a fixed point of the acoustic box. In addition, the torque sensor connected to the motor output was used to measure the torque fluctuations due to the friction. As a result of the study, the changes in the sound pressure level and friction torque were compared, and the reasons were interpreted. For this purpose, theories and studies related to the tribological properties of polymers were utilized. However, the effect of operating conditions on tribological properties as well as bearing dynamics was taken into account in the change of sound pressure levels. The first and clear finding of the study was that the sound pressure level increases with increasing speed in all frequency bands and all temperature conditions for each bearing in accordance with the information in the literature. The second parameter of the study was radial load. The bearings were radially loaded. It was observed that radial load did not have a significant effect on sound pressure levels of bearings. However, SPL of the polymer bearings increased with increasing radial load. After a point, the rate of increase decreased and remained almost constant. A difference was obtained in the SPL values of B1 and B2 at T2 (100 ᵒC). This differentiation of the curves in B1 and B2 bearings at T2 was not considered correct since the bearing disabled its function when it loaded radially at 100ᵒC. The third parameter which was the temperature didn't have a direct and simple relation with the sound pressure levels of the bearings. Temperature effect was evaluated at different rotating speeds. While B1 and B3 reacted nearly in the same way B2 was differentiated by its inconsistent and incomprehensible results. The B3 bearing which consists of PEEK rings and steel balls had the highest SPL values at all rotational speeds and temperature values. In addition to this, having glass balls made the SPL values increase at higher rotational speeds.