Investigation of Seal Pumping Rate by Using Fluorescent Method L. LOU * Y. SHIRAI * *Research & Development Center, Electronic Systems Development Department A new technique for the measurement of seal pumping rate using the fluorescent substance contained in mineral oil originally has been established. In this paper, principles of the measuring technique and system are explained in detail. Furthermore, the selection of the collecting liquid and the method of uniform mixing of oil in the collecting liquid, the optimization of excitation wavelength, and the calibration method are also explained. Finally the relation between seal pumping rate and pressure and the relation between seal pumping rate and lip wear width, which are difficult to measure by conventional methods, are investigated using this new measuring technique. The results show that the helix lip seal has a large pumping rate compared with the plain lip seal under all test conditions. Therefore, it can be said that the helix lip seal is very useful for counteracting leakage when internal pressure exists. Key Words: helix lip seal, pumping rate measuring method, lip wear, fluorescent 1. Introduction Oil seals (hereinafter referred to as seals) are widely used to prevent leaks of lubricating oil used in various types of machines. Higher performance and longer life machines have been developed recently. In order to keep pace with this progress of machine technology, there is a need for performance improvement in seals. Therefore, the authors have been very energetically involved in research to raise the dynamic sealing effect (hereinafter referred to as pumping effect) of seals 1~6). The objective of this paper is to aim for improved sealing performance of seals exposed to pressure (hereinafter referred to as pressure seals). Taking advantage of the fluorescent substances contained in oil, a pump action measurement method of pressure seals has been established. This measurement had been difficult to obtain using conventional measurement methods to investigate whether or not helix lip seals can be used in place of the plain lip seals that account for the majority of current pressure seals, and to investigate whether or not there is merit to this. Here pump action is the act of returning oil leaked to the air side of the seal from the oil side by rotation of the shaft. Maximum oil quantity returned per time unit is generally referred to as "pumping rate." The "weighing method" was previously used to measure pumping rate. The "weighing method" converts to oil volume by measuring the weight of the oil collected. Because the "weighing method" requires oil to be manually collected, it can only be measured at atmospheric pressure. Therefore, the pumping rate of the pressure seal could not be measured. 2. Measurement Method Petroleum lubricatants contain aromatic hydrocarbons such as chrysene, floren, and pyrene 7). These aromatic hydrocarbons emit fluorescent light by ultraviolet or visible light radiation. The more oil quantity there is, the higher the intensity of the Fluorescent light intensity 8 6 4 2 35 38 41 44 Fluorescent wavelength, nm Oil quantity Fig. 1 Fluorescent spectrum Low Medium High emitted fluorescent light (Fig. 1). This research involved quantitative measurement of oil utilizing these characteristics. 2. 1 Measuring System Fig. 2 shows a schematic diagram of the measuring apparatus. The measuring apparatus is equipped with an oil tank and an oil collecting tank for collecting the oil displaced from the oil tank. The lubricating oil actually used is placed in the oil tank, and liquid not containing fluorescent substances is placed in the oil collecting tank. The system is designed so that the oil that flows from the gap between the seal lip and shaft flows into the oil collecting tank, and the oil in the oil collecting tank automatically mixes uniformly with the collecting liquid. A spectrofluorophotometer was used to detect concentration of the oil in the oil collecting tank. In order to save sampling time, the measuring apparatus is equipped with a pump that sends sample liquid automatically sucked up from the oil collecting tank to the spectrofluorophotometer. After completing measurement of fluorescent light intensity, the KOYO Engineering Journal English Edition No.157E (2) 7
sample liquid is once again automatically returned to the oil collecting tank by the pump. Precision of the fluorescent light measurement method is affected by many factors. Next, the selection of collecting liquid, method of mixing collecting liquid, setting of excitation light wavelength, and measurement of calibration curve shall be introduced. Oil tank Seal Oil collecting tank Lubricating oil Shaft Collecting liquid Flow direction Pump Fig. 2 Principle of measuring system 2. 2 Selection of Collecting Liquid Because the collecting liquid allows oil to be collected, and furthermore has limitation conditions whereby seal rubber must not be negatively affected, the following are considered when selecting: q Minimal negative effect on rubber properties w Good compatibility with oil e Does not contain fluorescent substances r Good detergent characteristics t Low cost In this research, synthetic oil was selected and used. 2. 3 Mixing of Collecting Liquid In order to detect the amount of oil in the oil collecting tank accurately, the oil must be made uniform. In this research, oil was made uniform by taking advantage of viscosity of the collecting liquid and rotation of the shaft. In other words, the collecting liquid having viscosity constantly flows around the shaft due to rotation of the shaft. This makes the oil that flows out from the oil tank mix uniformly in the oil collecting tank. If the collecting liquid did not have adequate viscosity, a separate mixer would have to be used. 2. 4 Setting of Excitation Light Wavelength When measuring the fluorescent spectrum of collecting liquid containing oil, selecting wavelength of excitation light is extremely important. The fluorescent spectrum changes largely according to wavelength of excitation light. In this research, the excitation light wavelength when fluorescent light intensity is maximum (345nm) was used. Spectrofluorophotometer 2. 5 Creation of Calibration Curve In order to conduct quantitative measurement of oil that flows into the oil collecting tank, a calibration curve that expresses the relationship between oil quantity and intensity of fluorescent light in the collecting liquid must first be created. By creating a calibration curve, the concentration (oil quantity, in other words) of measurable oil can be known. In this research, the calibration curve shown in Fig. 3 was created by mixing a known oil quantity into the oil collecting tank a little at a time. Also, under measurement conditions of this research, it was learned that minimum detectable oil concentration was 1 ppm. Density of oil, ppm 5 4 3 2 1 Seal dimensions (outer shape inner diameter width) 2 4 6 Table 1 Measurement conditions 3 2 7 Fluorescent light intensity Fig. 3 Calibration curve 3. Measurement Conditions Main measurement conditions are given in Table 1. Pressure application method Air pressure Type Helix lip seal Pressure,,, 196, 392kPa Seal material FKM Seal lip wear width,.3,.7mm Lubricating Engine oil Wear method Forced wear oil 7.5W-3SE Room Excitation light Temperature 345nm temperature wavelength Collecting liquid Synthetic oil Assembly eccentricity Max..5mm Amount of Shaft 4 mr Max..5mm collecting liquid eccentricity Measurement time 3 minutes Rotational speed 45~3 min 1 4. Results and Consideration 4. 1 Change of Pumping Rate According to Pressure Figs. 4 and 5 show measurement results for new plain lip seals and helix lip seals. From Fig. 4, in the case of plain lip seals, it can be seen that the pumping rate decreases significantly according to pressure increase. When pressure exceeds kpa, the pump rate becomes almost zero. In the 8 KOYO Engineering Journal English Edition No.157E (2)
case of helix lip seals, on the other hand, from Fig. 5, no large change in pumping rate due to pressure was observed. This means that, in the case of helix lip seals, the pumping effect of the rib is not reduced even if pressure is applied. It can also be seen that the pumping rate itself is 2 times greater for helix lip seals than for plain lip seals..2 12 1 8 6 4 (Wear width :.3mm) 1 2 3.1 New product 1 2 3 2 1 2 3 4 Pressure, 1kPa Fig. 7 Pumping rate vs. pressure (Helix lip seal).2 1 2 3 4 12 1 Pressure, 1kPa Fig. 4 Pumping rate vs. pressure (Plain lip seal) 8 6 4 2 New product 1 2 3 4 Pressure, 1kPa 1 2 3 Fig. 5 Pumping rate vs. pressure (Helix lip seal).2.1 (Wear width :.3mm) 1 2 3 1 2 3 4 Pressure, 1kPa Fig. 6 Pumping rate vs. pressure (Plain lip seal).1 (Wear width :.7mm) 1 2 3 4 Pressure, 1kPa 1 2 3 Fig. 8 Pumping rate vs. pressure (Plain lip seal) 12 1 8 6 4 2 (Wear width :.7mm) 1 2 3 4 Pressure, 1kPa 1 2 3 Fig. 9 Pumping rate vs. pressure (Helix lip seal) Figs. 6 and 7 show measurement results when wear width is.3mm. From Fig. 6, whereas in the case of plain lip seals, when wear width is.3mm, pumping rate is extremely small, from Fig. 7, in the case of helix lip seals, there is a large pumping rate. Furthermore, in the case of plain lip seals, whereas pumping rate drops due to the rise in pressure, it oppositely increases in the case of helix lip seals. KOYO Engineering Journal English Edition No.157E (2) 9
Figs. 8 and 9 give the measurement results when wear width is.7mm. When wear width is.7mm in the case of plain lip seals, pumping rate is almost zero, but in the case of helix lip seals, there is a comparatively large pumping rate. Even in this case the pumping rate of helix lip seals increases due to the rise in pressure. The reason the pumping rate changes completely differently for plain lip seals and helix lip seals under the condition of pressure can be explained from the effect of the following two opposite effects. When pressure is applied, the pressure gradient that tries to leak oil from the oil side increases. Also, due to the increase in seal radial load according to the pressure increase, it becomes hard for oil to pass between the lip and shaft. This works as an obstacle that reduces pumping rate. On the other hand, when pressure is applied, the seal radial load increases. This makes the contact area of the rib designed on the seal lip larger, and causes the pumping effect of the rib to increase. Hence, the pumping rate of the seal increases. In the case of plain lip seals, because no rib is designed on the lip, the pumping rate is reduced by the previously mentioned effect. In the case of helix lip seals, the two opposite effects act simultaneously, and whether pumping rate will increase or decrease therefore depends on which of the two effects will prevail. Under the experimental conditions of this research, it is believed that the pumping effect of the rib prevails. Also, the reason why the pumping rate of helix lip seals becomes extremely large in comparison to plain lip seals can be explained as follows. In the case of plain lip seals, the pumping rate is produced according to surface roughness in contact area, but because the surface roughness is generally extremely small, the pumping effect is also extremely small. If the surface roughness becomes large, static sealing effect drops, and because the possibility that it will cause oil leaking when still or at low speeds becomes large, surface roughness in contact area cannot be made large. On the other hand, the pump action of helix lip seals consists of that of roughness in contact area and that of the rib designed on the lip. Normally, because the rib is extremely large compared to surface roughness, the pump action also becomes overwhelmingly large. Fig. 1 shows measurement examples of rib contact width change due to pressure. From Fig. 1, it can be seen that rib contact width increases together with the increase of pressure. Here rib contact width was measured by mounting a seal on a hollow transparent shaft and observing the contact area within the transparent shaft with a microscope. Fig. 11 shows an observation example of a seal contact area. Rib contact width, mm 1.8.6.4.2 Lip contact width 1 2 3 4 Pressure, 1kPa Fig. 1 Lip wear width vs. pressure Contact area Contact area 4. 2 Change in Pumping Rate Due to Progress of Wear Figs. 12 and 13 show the change in pumping rate according to wear width when rotational speed is 1 min 1. Here, wear width is obtained from forced wear. Fig. 12 shows the case of plain lip seals, and Fig. 13 shows the case of helix lip seals. As can be seen from Fig. 12 and Fig. 13, the pumping rate drops for both plain lip seals and helix lip seals due to the progress of wear. However, the reduction in pumping rate due to wear is larger for plain lip seals. In the case of helix lip seals, even though the pumping rate drops due to the progress of wear, the reduction is comparatively small..2.1 Pressure : low Lip Lip contact width Fig. 11 Contact situation of lip 1 min 1 Pressure : high Lip.2.4.6.8 Fig. 12 Pumping rate vs. lip wear width (Plain lip seal) 1 KOYO Engineering Journal English Edition No.157E (2)
12 12 1 1 8 6 4 1 min 1 8 6 4 3 min 1 2 2.2.4.6.8 Fig. 13 Pumping rate vs. lip wear width (Helix lip seal) Figs. 14 and 15 show the change in pumping rate according to wear when rotational speed is 3 min 1. Just as in the case of 1 min 1, in the case of helix lip seals, reduction of pumping rate due to the progress of wear is extremely low compared to plain lip seals, and the pumping rate value also is overwhelmingly large compared to plain lip seals. Whereas in the case of plain lip seals, after a wear width of.5mm, the pumping rate is almost zero, but the pumping rate of the helix lip seals is 2mr/h or more. From these results, in the case of pressure seals, the pumping effect of helix lip seals is extremely large, even when the seals are worn. Therefore, from the view of pumping effect, using helix lip seals instead of plain lip seals would probably largely improve the sealing performance of the pressure seal. By doing so, it would probably be possible to extend the life of pressure seals largely as well..2.1 3 min 1 5. Conclusion A quantitative method of pumping rate measurement of pressure seals using fluorescent substances contained in oil has been established. As a result of considering change in pumping rate due to wear under conditions where pressure is applied and change in pumping rate for which conventional consideration had been difficult, it was found that the pumping rate of helix lip seals was extremely large compared to that of plain lip seals, and reduction of the pumping rate due to wear also was comparatively small. Therefore, it is believed that there is much merit in using helix lip seals as pressure seals. References.2.4.6.8 Fig. 15 Pumping rate vs. lip wear width (Helix lip seal) 1) L. Lou, K. Yamamoto, K. Ikeuchi : ASME Journal of Tribology, 12, 7 (19) 476. 2) L. Lou, M. Mitsumaru : Proceedings of JAST Tribology Conference Tokyo (1996) 418. 3) L. Lou, M. Mitsumaru, K. Ikeuchi : Proceedings of JAST Tribology Conference Tokyo (1997) 197. 4) L. Lou, M. Mitsumaru, K. Ikeuchi : Proceedings of JAST Tribology Conference Osaka (1997) 4. 5) L. Lou, M. Mitsumaru, R. Okada, T. Tamura : Proceedings of JAST Tribology Conference Tokyo (19) 462. 6) Perutz : Industrie der Mineralöl (1886), 68..2.4.6.8 Fig. 14 Pumping rate vs. lip wear width (Plain lip seal) KOYO Engineering Journal English Edition No.157E (2) 11