Condition Monitoring Of Nylon And Glass Filled Nylon Gears S. Senthilvelan and R. Gnanamoorthy Department of Mechanical Engineering Indian Institute of Technology Madras Chennai 600 036 ABSTRACT Gears and bearings made of advanced reinforced polymers find increasing application in home, office and industrial equipments. Structural polymers like Nylon, Delrin and PEEK are used for manufacturing gears through injection molding and other processing techniques. However there is a lack of clear understanding on the performance of gears at different operating conditions. The current project is aimed at understanding the performance of polymer-reinforced gears. The condition monitoring of polymer composite gear is carried out using the gear tooth temperature and vibration monitoring techniques. Significant rise in the gear tooth surface temperature was observed during testing. Vibration levels also vary as the severe gear tooth wear occurs in the polymer base gears. Increase in power spectral density was observed as the gear tooth wear occurs. Condition monitoring of polymer gears using temperature and vibration monitoring can be used effectively for assessing the performance of the polymer gears. INTRODUCTION Polymers have low strength and modulus, better tribological properties and poor thermal resistance compared to metals. Therefore machine parts like gears, bearings, shafts, etc., made of polymer base materials behave unlike metals during service. Metal gears fail by different modes, gear tooth fatigue breakage, pitting, scoring, uniform wear, etc., depending upon the load, speed, lubrication, and other operating conditions during service [1]. However less is known about the behavior and performance of polymer and other non-metal gears during service. Gearbox condition monitoring is carried out using different monitoring techniques [2]. Sound and vibration level variation of the gearbox are used for understanding the condition of gears in many cases [1-4]. Fujita et al [3] have investigated the possibility of early detection of gear tooth failure in case hardened and induction hardened spur steel gears using noise and vibration analysis. Ferrographic oil analysis techniques are also used for condition monitoring of oil lubricated gears [5]. Few research works were carried out on the condition monitoring of the polymer gears [6-7]. Hooke et al [6] measured the surface temperature of polymer gears and related to the gear wear. Yousef and Burns [7] measured the gear tooth running temperature and
correlated fatigue strength of thermoplastic gears. Gang et al [8] have proposed the surface temperature index for surface strength evaluation of alloy steel gears. This paper discus about the gear condition monitoring techniques and relationship to the gear failures for nylon 6 and glass filled nylon gears. Results of the ongoing research work carried out using the power absorption type gear test rig are reported. CONDITION MONITORING Condition monitoring studies were conducted on the injection-molded gears made of engineering polymers, nylon 6 and 20 % glass filled nylon 6. Gears of 2 mm module and 17 number of teeth with 20 0 pressure angle were made in the laboratory. Test gears were injection molded after preheating at about 353 K for 4 hours. Gears were molded in a carefully prepared and polished die at a constant injection pressure. Nylon 6 gears were molded at 498 K and glass reinforced gears was molded at a marginally higher temperature, 518 K. All the gears made are thoroughly inspected using gear tooth caliper, weight, and visual inspection before testing. Power absorption type gear test rig is used for the current investigation. Details of gear test rig is described elsewhere [9]. The test gear is driven by the DC motor. Power resistor connected to the generator loads the test gear. The power resistor offers a quiet dissipation mechanism, and thus do not contribute any additional sound and vibration to the test rig. The speed of the drive motor and the load on the gear can be varied smoothly to accommodate the range of speed/torque testing conditions. Gear tests were conducted at a constant speed of 1000 rpm, which corresponds, to a pitch line velocity of about 1.8 m/s. Gears are tested at different torque levels ranging between 0.8 and 3 Nm. All the tests were conducted under unlubricated dry conditions. Test gears were mated with the machined mild steel gears. Specimens were carefully assembled in the test rig after measuring the gear tooth thickness using gear tooth micrometer of 1 µm accuracy and weighing using the electronic balance of 0.1 mg accuracy. The speed of the test gear is gradually raised to the test speed. Vibration levels of the bearing support near the test gear are measured periodically using the piezoelectric accelerometers. One accelerometer is fixed along the direction of rotation for measuring the vibration level in the axial direction and another accelerometer is fixed in the radial direction. Vibration level and gear tooth surface temperature are continuously measured and recorded during testing. At the time of storing data for vibration analysis, data were sampled at 2400 Hz. Gear tooth surface temperatures of test and standard gears are measured using non-contact infrared temperature sensor, which are placed near the test and mating gears. Tested gears were observed using optical microscope.
RESULTS AND DISCUSSIONS Gear Tooth Surface Temperature In the case of metal gears in mesh, frictional forces between tooth surfaces contribute to the major amount of heat generation. In general, metal gears run under lubricated condition. The lubricant dissipates most of the generated heat. Good thermal conductivity of metal also aids in faster heat dissipation. The gear tooth surface temperature in un lubricated gears depends on the transmitted load, sliding velocity, mean rolling velocity, surface roughness, thermal conductivity, etc. [7]. Since thermoplastics are temperature sensitive materials, heat generation in running gear is an important parameter. Gear temperature results from equilibrium between the heat generated and the heat dissipated during operation [10]. Figure 1 shows the variation of gear tooth surface temperature of the nylon 6 gear tested at a torque of 2 N-m and speed of 1000 rpm. The gear tooth surface temperature varies continuously throughout the gear life. Exposure to high temperature for prolonged duration softens the material leading to severe plastic deformation and tooth shape distortion. Fig 1. Gear tooth surface temperature rise in unfilled nylon gear tested at 2 N-m torque at a speed of 1000 rpm. Heat generation and heat dissipation in a polymer gear pair is quite different compared with metal gears in mesh. Since polymers are viscoelastic in nature, a large amount of internal friction (due to hysteresis effect) is generated within the polymers due to repeated mechanical deformation [11]. Another cause of rise in temperature of polymer gear is due to high force of adhesion and intimate contact between surfaces [11]. Since the compression modulus of polymer gear is relatively low compared to steel [12], it is possible to have intimate contact between surfaces, which will lead to high frictional forces and hence higher heat generation.
Addition of glass fillers improves the strength and modulus of the material. Flexural modulus of the 20 % GF nylon is higher than unfilled nylon. Increase in stiffness reduces the energy lost due to hysteresis effect. The improved strength and modulus affects the contact conditions in the polymer and reinforced polymer gears. Reduced contact area leads to the reduced heat generation compared with un reinforced gear. High thermal conductivity of the glass filled nylon also contributes to increased heat dissipation. This causes the less gear surface temperature in reinforced gear compared with the un reinforced polymer gear (Fig 2). The data corresponds to the test torque of 0.8 N-m and pinion speed of 1000 rpm. Fig 2. Gear tooth temperature profile of unreinforced and glass reinforced gear tested at 0.8 N-m and 1000 rpm. Vibration Characteristics On set of failure in the test gear during contact fatigue tests is difficult to notice. Severe wear of gear tooth is one of the major gear failure modes, which will lead to high vibration and sound in the transmission unit [1]. Particularly at lower test torques, gear tooth wear causes increase in backlash, which in turn causes dynamic imbalance in the system [1]. From the raw vibration signal of both the accelerometers (Fig 3 (a) & (b)), it is inferred that higher amplitude of waveform is observed from the accelerometer fixed in the radial direction than accelerometer fixed in the axial direction. Hence signal from accelerometer fixed in radial direction is further processed to obtain power spectral density at various stages. By knowing the speed of rotation 1000 rpm and time history 330 milliseconds, waveform signal for 5.83 number of cycles (0.330/(60/1000)) is theoretically found out.
15 15 Accelertaion (m/s 2) 5-5 -15 0 100 200 300 400 Accleration( m/s 2) 5-5 -15 0 100 200 300 400 Time (ms) Time (ms) (a) (b) Fig 3 Acceleration signals after 1.4 million cycles for gear tests carried out in UF nylon from accelerometer fixed at the (a) radial direction and (b) axial direction. Fourier analysis is extremely useful for data analysis, as it breaks down a signal into constituent sinusoids of different frequencies. This technique is used to obtain power spectral density. Signals were sampled at 2400 Hz to satisfy Nyquest criteria (Minimum sampling frequency must be two times the higher frequency). The power spectrum of the signal measured consists of peaks located at integer multiples of the meshing frequency f g = Nf s, where N is the number of teeth of the gear and f s is the shaft frequency (Figure 4). In the present study f s = 16.66Hz and N= 17, hence f g = 283.3 Hz. It is observed that the maximum magnitude of power spectral density is obtained at fundamental gear mesh frequency (283.3 Hz). From this figure it is clear that the amplitude increases with cycle time. Significant uniform wear occurring in this polymer base gear leads to continuous increases in gear wear. When unfilled nylon-6 gear is tested at 1.5 N-m and 100 rpm, a weight loss of 60 mg and decrease in tooth thickness of 46µm was observed after 1.4 million cycle. This indicates the significant wear in the polymer gears, which causes increase in backlash leading to high vibration levels.
CONCLUSIONS Fig. 4. Power spectral density at various stages of life Different condition monitoring techniques for polymer-based gears are considered for use in gear tests. Gear tooth surface temperature using non-contact infrared temperature sensors indicates significant variation in gear temperature depending upon the trque transmitted, operating speed and material properties. Uniform wear encountered in service causes increase in backlash and contributes to increased vibration. Monitoring the performance using accelerometer connected predicted the damage occurring in the gear unit. ACKNOWLEDGEMENTS Authors acknowledge the support provided by the Robotics and Manufacturing Group of the Department of Science and Technology for carrying out the project.
REFERENCES 1. Dudley D.W., Gear Handbook McGraw-Hill New York, 1962. 2. Derek Smith J, Gear Noise and Vibration, Marcel Dekker, Inc, 1999. 3. Fujita K, Yoshida A and Ota K, On the possibility of early detection of gear tooth failure by noise and vibration, Proceedings of the Sixth World Congress on Theory of Machines and Mechanisms, 1983. 4. Wilson.Q., Ismail, F., and Golaraghi. F., Assessment of gear damage monitoring techniques using vibration measurements, Mechanical Systems and Signal Processing, 15, 905-22, 2001. 5. Bower R, Scott D, Seifert W and Westcott V. C, Ferrography, Tribology International, 109-115, 1976. 6. Hooke. C.J, Mao. K, Walton.D BreedsA.R, and Kukureka. S.N., Measurement and prediction of the surface temperature in polymer gears and its relationship to gear wear, Journal of Tribology, 115, 119-24, 1993. 7. Yousef.S.S, Burns.D.J and Mckinlay.W., Techniques for assessing the running temperature and fatigue strength of thermoplastic gear, Mechanism and Machine Theory, 8, 175-85, 1973. 8. Gang Deng, Tsutomu Nakanishi, and Masana Kato, Surface temperature calculation and is application to surface fatigue strength evaluation, Journal of Mechanical Design, 124, 805-12, 2002. 9. Senthilvelan S and Gnanamoorthy. R., Reinforced Polymer Gear Fatigue and Failure Analysis, to be presented 4th Conference on Creep, Fatigue and Creep- Fatigue Interaction Indira Gandhi Centre for Atomic Research, Kalpakkam, 2003. 11. Koffi D, Gauvin and Yelle H, Heat generation in thermoplastic spur gears, J of Mechanisms, Transmissions, and Automation in Design, 107,31-7,1985. 12. Rosato Dominick, Plastics Design Handbook, Kluwer academic Publishers, 2001. 13. Vishu Shah, Handbook of Plastics Testing Technology, John Wiley & Sons, 1984.