MATERIAL AND RESEARCH TREND OF O-RING

Transcription

1 International Journal of Mechanical Engineering and Technology (IJMET) Volume 9, Issue 6, June 2018, pp , Article ID: IJMET_09_06_040 Available online at ISSN Print: and ISSN Online: IAEME Publication Scopus Indexed MATERIAL AND RESEARCH TREND OF O-RING Jun Seop Lee Department of Automotive Engineering, Kongju National University, South Korea Byung Mo Yang Department of Mechanical Engineering, Kongju National University, South Korea Haeng Muk Cho* *Corresponding Author Department of Mechanical and Automotive Engineering, Kongju National University, South Korea ABSTRACT In this article, O-ring used basically among seals has been described, and materials for the O-rings in an automobile are employed in diversified forms with the uses in an automobile being shown in TABLE 1 according to their advantages by identifying the advantages and disadvantages of the material. Through the results of the research article, aging rates of the material have been confirmed to be proportional to temperature irrespective of the material type. In addition, the tightness has been found to be maintained for more than 2,000 hours under the temperature condition under 100 C so that excellence and durability of the existing O-ring could be determined based on the study results. Also, as the study results on the form of O- rings were considered, V-groove O-ring and Peanut O-ring could be confirmed to have a higher maximum compressive stress and maximum contact normal stress compared with the existing O-ring. It has also been proved through the study results that higher contact face pressure and vertical stress occurred as compared with the existing O-rings. Based on such results, V-groove O-ring, with a groove placed, and Peanut O-ring, with the contact face being raised, have been designed to be the shape required for the automotive O-ring, presenting an opportunity to consider the shapes of an automotive O-ring in the future. Therefore, the information on materials for O-ring and on the study status thus far will be shared in this article. Keyword: O-ring, material, Hermeticity, V-groove, Multi-contact O-ring Cite this Article: Jun Seop Lee, Byung Mo Yang and Haeng Muk Cho, Material and Research Trend of O-Ring, International Journal of Mechanical Engineering and Technology, 9(6), 2018, pp editor@iaeme.com

2 Material and Research Trend of O-Ring 1. INTRODUCTION O-ring is the most basic seal and is being used as a key part to conveniently seal a mechanical apparatus in a static or dynamic contact condition or to prevent leakage of fluids and shut off foreign objects from the outside. It is applied in almost areas. O-ring is taking charge of an important role as research institutions including NASA of the US have recognized its importance and incorporated enormous research funds to perform studies on the sealing performance of the O-ring [1, 2] after the Poseidon propulsion engine explosion accident caused by a design error in the O-ring s squeeze as well as the space shuttle Challenger explosion caused by degradation in low-temperature restoration characteristics of the O-ring. For the design of O-rings, it is designed as a structure enabling compressive sealing action by perfectly covering of the O-ring with a cover after forming a groove along the circumferential direction to allow insertion into the inside diameter side or the outside diameter of piping or driving shaft where the O-ring is then assembled. [3] The thus designed O-ring comes to have sealing performance by using repulsive elasticity generated by some deformation, mainly by applying compression. Namely, it makes use of large deformation property and restoration force of the material. [4] However, the repulsive elasticity controlling the sealing performance is affected by temperature. It is because the effects of the temperature are to expand the O-ring while lowering the strength of the O-ring. [5, 10] In addition, diversified loads and deformation conditions act on the O-ring as a function of assembly conditions due to the gap and internal pressure condition since the O-ring is subjected to a compressive force due to assembly and a high pressure from the inside, with such deformation having effects on the hermeticity and the damage of the O-ring. Therefore, contact stress and deformation behavior of the O-ring subjected to an internal pressure and a given squeeze ratio are very important factors for maintaining hermeticity. Major factors affecting stability of deformation behavior of the O-ring include elastic deformation behavior characteristics of the material and shape of the sealing cross section. While variations of deformation behavior hermeticity as a function of material properties and shape changes of the O-ring is natural, deformation behavior safety is greatly changed depending on how the contact cross section shape of the sealing rig and the empty space are designed even if the same rubber material is employed for the elastic body. [6] The contribution with the largest effect on optimum design of the O-ring is made by diameter and material characteristics of the O-ring which should be high compared with width and depth of the groove and squeeze ratio. [7] Thus, to maintain the original functions of the O-ring seal installed in the mechanical apparatus, the shape change of the cross section should be small so that the resistance against expansion should be large and a low squeeze ratio is required. In the present article, study articles were surveyed based on such reference, the extent to which hermeticity of the O-ring is varied with temperature has been identified based on the results from the studies, and excellence of the hermeticity has also been confirmed through Figs. 1, 2 and 3 using the results. In addition, through study results on shape changes taking charge of a part of the hermeticity, comparisons have been made between the hermeticity performance of the existing O-rings and that of the O-rings with shape improvement. Consequently, based on these study results, the shape of automotive O-rings has been designed with presentation of the part for development of the future O-rings editor@iaeme.com

3 Jun Seop Lee, Byung Mo Yang and Haeng Muk Cho 2. IDENTIFICATION OF MATERIAL PERFORMANCE FOR O-RING Nitrile rubber (NBR) Table 1 Material for O-ring and advantages/disadvantages Material Advantage Application example Compatible with oil; wear resistance & Oil seal, packing, O-ring & heat resistance are good and molding is diaphragms easy. Polyacrylate rubber (ACM) Silicone rubber (VMQ) Fluorocarbon rubber (FKM) Polyurethane-Ester (AU) Compatible with oil & heat resistance and in particular, compatibility with gear oil is good. Heat resistance, cold resistance & squeeze resistance are excellent Heat resistance & chemical resistance are excellent Strength & hardness are high and wear resistance & cold resistance are excellent Oil seal for high temperature, high speed & for gear oil Oil seal for high speed, O- ring & diaphragms High-temperature oil seal, medical packing Packing, diaphragms for high pressure The material for O-ring should have the characteristics related to the conditions against environments including heat resistance, cold resistance, compatibility with oil, chemical resistance, etc. and the performance such as wear resistance, squeeze resistance, durability, etc. Depending on use conditions, rubber for seal is made by putting in several additives to the basic material. NBR, ACM, VMQ, FKM and AU are mainly used as the basic material as shown in TABLE 1, and applicable parts can be confirmed to be different depending on this material Materials for automotive O-ring NBR is a material that is very widely used for manufacturing of O-ring Seal due to the excellent resistance against gasoline, mineral oil, fuel oil, ethylene glycol, etc. However, it is not suitable for the use in ester-based fluids. Allowed operating temperatures for NBR are 40~120 C. [8] Considering the chemical structure, it is a copolymer of butadiene & acrylonitrile (ACN), and may be classified into low nitrile (18~20mol %), middle nitrile (28~34mol %), high nitrile (38~39mol %), and ultrahigh nitrile (45~48mol %), etc. depending on the content of ACN. [9] ACM is a monomer ester of alkyl or alkoxy with the major composition consisting of crosslinking points during vulcanization. It is also a rubber material with a good resistance against mineral oil, oxygen & ozone, accompanied by an excellent resistance against hot oil, automatic transmission oil, power steering oil, etc. It is strong against hydrocarbon-based fuels and gas permeation. However, it is very vulnerable to brake liquid containing glycol or aromatic & chlorinated hydrocarbon, steam, etc. VMQ is silicon-based as an element derived from quartz, and attaches organics of pendant such as methyl, phenyl, &vinyl to silicon atoms. FKM, called fluorine rubber, is a copolymer of Propylene hexafluoride & Vinylidene fluoride. Due to the C-F bonds as an inert combination structure with large bond energy, it has an excellent heat resistance and chemical resistance next to FFKM among the currently practical raw materials. It has an outstanding resistance against oil, fuel oil, aliphatic, aromatic hydrocarbon, hydrocarbon solvent, etc. In terms of weather resistance & ozone resistance, it has the highest excellence among rubbers. As it contains a large amount of fluorine, it is difficult to burn, and has self-flammability. However, it is better to not be used for oxygenated solvent, alkali, and esters of low molecular weights, ether, ketones, some amines & chlorosulfonic acid. Operating temperatures for FKM are 24~204 C. [10] editor@iaeme.com

4 Material and Research Trend of O-Ring AU, there are generally two types of Polyurethane (AU) & Polyurethane (EU), with EU polyurethane rubber having an excellent chemical resistance against hydraulic oils. As it has an excellent wear resistance & tensile strength compared with other rubbers, it also has a high elasticity & tear strength so as to be used frequently for the parts exposed to hydraulic oil, hydrocarbon-based fuel, ozone, sunlight, etc. It is not good when used for bases, acids, oxygen-based solvents, hot water, moisture, etc. 3. STUDY STATUS ON O-RING Among the characteristics of the O-ring, hermeticity is important. Therefore, the statuses of study in terms of shapes for the hermeticity & for the life allowing confirmation on maintenance of the hermeticity of O-rings have been considered Maintenance of hermeticity by O-ring For the NBR O-ring used as an actual O-ring of 7.65mm in the inside diameter and 1.63mm in the line diameter, Compression Stress Relaxation results of the O-ring at 80 C, 90 C, &100 C are shown in Fig. 1, where Ft is the sealing force at time t, and Fi the initial sealing force. The breakdown condition was selected as the condition where Ft/Fi becomes 10%, being displayed as a dashed line. According to the measurement result, the time taken for the sealing force of the O-ring to reach the breakdown condition of 10% at 80 C was shown to be 2,360 hours. Additionally, the aging rates were increased as the temperature in heat-accelerated experiments was increased. Since characteristics degradation resulting from temperatures is known to occur frequently in the case of rubber products [11], the phenomenon as shown in the graph of Fig.1 may be determined to appear. Figure 1 Stress relaxation of a NBR O-ring. [11] In addition, NBR O-rings have also been used in the present study as literature among the studies on O-ring for compression set where the inside diameter & the line diameter were 91.67mm and 3.53mm, respectively. This product was subjected to heat-accelerated aging at 100 C, and changes in the sealing force observed accordingly after the aging levels of the NBR O-rings were selected to be Ft/F0 = 1.0, 0.8, 0.6, 0.5, 0.4, 0.2 are shown in Fig. 2. Based on this result, the life of NBR O-ring may be confirmed to exceed 2000 hours when the breakdown condition of 10% is considered. [12] editor@iaeme.com

5 Jun Seop Lee, Byung Mo Yang and Haeng Muk Cho Figure 2 Stress relaxation of a NBR O-ring at 100 C. [12] The study results were considered from the same experiments not only with NBR rubber but also with fluorosilicone rubber or FVMQ as a synthetic rubber of FKM & VMQ. According to the results of consideration, the aging rate was increased with an increase of temperature in heat-accelerated aging experiments s with NBR rubber as shown in Fig. 3. Additionally, the time to reach the breakdown condition of 50% was not reached in the measurement results at 60 C & 80 C, while it could be confirmed to be observed as 2,845 hours, 364 hours, & 172 hours at 100 C, 150 C, & 170 C, respectively. Thus, when the temperature is lower than 100 C, FVMQ could also be determined to maintain hermeticity for 2000 hours. [13] Figure 3 Stress relaxation of a FVMQ O-ring. [13] When considering studies on hermeticity from the characteristics aspect, performance of O-rings can be seen by measurements and hermetic performance of O-rings could be seen to be maintained even for more than 2000 hours. Although it is lowered showing a nonlinear relationship as the temperature is increased, excellence of hermeticity of the O-ring can be seen by observing it being nearly constant even when a difference up to 1/1000 occurs from the initial sealing force. However, inflow of the fluid must occur due to continuous circulation & aging, where the solution measure will be devised through the shape of O-rings Study status on the shape of V-groove As a finite element analysis concerning the sealing behavior and the durable safety of O-rings with V-groove, O-ring structure produced by putting in a v-groove in the middle part of cross section of the shape with two O-rings overlapped can be observed. Since such O-ring with v- groove forms sealed contact faces in two sections on the left & right sides with the V-groove at the center, it may be confirmed that the sealing force is increased further and it comes to editor@iaeme.com

6 Material and Research Trend of O-Ring have the sealing mechanism with improvement in the sealed durable safety. Since the O-ring with V-groove can shut off gas leakage caused by damages in the contact surface due to extrusion that occurs frequently in a sealed apparatus with one O-ring connected to two, the O-ring can be used for a long period of time. The maximum strain produced when a charging pressure of 0.9MPa is applied to the O-ring with V-groove was shown to be higher by 1.13 times than that for the existing O-ring, while the maximum compression stress 1.14 times and the maximum contact normal stress 1.37 times. [14] In addition, when a study concerning the shape of O-rings for improvement of hermeticity for the ball valve is considered in Fig. 4 with reference to the shape, the O-ring with V-groove can be seen to show stable behavior from both aspects of stress & strain. In addition, according to the results produced through the graph of Fig. 5, a tendency occurred where the contact face pressure is dropped by a large deformation in the O-ring with U-groove after maintaining a high contact face, while the higher contact face pressure occurred with V- groove compared with the existing O-rings may be considered through an article. [15] (a) A typical O-ring (b) U-groove O-ring (c) V-groove O-ring Figure 4 A model of O-rings. [15] Figure 5 Results of a finite element analysis. [15] This result signifies that sealing behavior and sealing durable safety is more excellent for the O-ring with V-groove compared with the existing O-ring. While there is a method for enhancing hemeticity by making a groove in the rubber composition in such a way, there is also a method for enhancing the hermeticity by making a gap so as to allow multi-contact for the cross section of peanut shape, i.e. between the faces Study status on Peanut shape Since sealed contact faces are formed at two spots in the multi-contact O-ring having a cross section of peanut shape, the structure makes gas leakage relatively difficult, and it also has an advantage where O-ring damages occurring frequently due to extrusion in a sealed structure with one O-ring connected to two do not occur. Thus, comparison tests were conducted accordingly between the existing O-ring & the multi-contact O-ring, and comparisons were made by analysis of strain, compression stress, & characteristics of contact normal stress editor@iaeme.com

7 Jun Seop Lee, Byung Mo Yang and Haeng Muk Cho behavior using the finite element method. As shown in Fig. 6 for the result, the maximum strain produced when the hermeticity test pressure of 1.8Mpa was applied to the multi-contact O-ring was higher than that for the existing O-ring by 1.77 times, the maximum compression stress by 1.78 times, & the maximum contact normal stress by 1.66 times. As the damage occurring frequently due to extrusion in the exiting O-rings was not observed in the multicontact O-ring of peanut type, the durable safety for sealing action could be seen to be more excellent. [16] (Gas pressure 1.8MPa) (Assembled O-ring) Figure 6 Shape comparisons between the existing O-ring& the multi-contact O-ring. [16] In addition, the study results were observed where a finite element analysis was made based on the analysis models for the existing O-ring & the peanut O-ring as shown in Fig. 7. As a result, both the existing O-ring & the peanut O-ring newly presented in the present study seal the gas in a stable manner under the hermeticity test standard for LPG charging system of 1.8MPa. In the case of Peanut O-ring, the vertical stress was shown to be 1.75Mpa while that for the existing O-ring was shown to be 1.71Mpa, indicating through comparisons between the two shapes that Peanut O-ring showed occurrence of the slightly higher vertical stress than the existing O-ring. [17] The shape is shown by Fig. 8. Figure 7 Finite elements of O-ring. [17] editor@iaeme.com

8 Material and Research Trend of O-Ring Figure 8 Contour bands of O-ring. [17] 4. CONCLUSION Materials for the O-ring put in an automobile are in diversified shapes depending on the automobile products, and the aging rates of the materials were confirmed to have a proportional relationship to the temperature irrespective of the material type. In addition, since the hermeticity is found through study results to be maintained for more than 2000 hours under the temperature conditions below 100 C, excellence & durability of the existing O-ring could be determined based on the study results. In addition, according to the consideration of study results on the shapes of O-rings, it could be confirmed that the O-ring with V-groove in the center and the Peanut O-ring showed more excellent maximum strain, maximum compression stress, & maximum contact normal stress than the existing O-ring, and the contact face pressure & the vertical stress was also proven to have higher values through experiments compared with the existing O-ring. Consequently, the O-ring with V-groove in the center as well as Peanut O-ring with the contact face raised were devised to have the shape required for the automotive O-ring, through which the study directions for automotive O-rings have been presented. ACKNOWLEDGEMENT This work (C ) was supported by Business for Cooperative R&D between Industry, Academy, and Research Institute funded Korea Small and Medium Business Administration in REFERENCES [1] Ellis, R. A. and Keller, JR, R. B. Solid Rocket Motor Nozzles. NASA SP, 8115, [2] Allan, J. M. Lessons Learned but Forgotten from the Space Shuttle Challenger Accident, Space 2004 Conference and Exhibit. AIAA , 2004, pp [3] Richter, B. Perfluoroelastomer O-rings Reduce Risk of Failure. World Pumps, 1995, pp [4] Monthly press technology. November issue, 2002, pp [5] Mark, J. E., Erman, B. and Frederick, R. RUBBER Rubber Science and Technology. Eirch Original Articles Organic Elastomeric Society, 2000, pp [6] Kim, C. K., Cho, S. H. and Kim, Y. G. On the Contact Behavior Analysis and New Design of O-ring Seals. Prpceedomgs pf the Second Asia International Conference on Tribology, 2002, pp editor@iaeme.com

9 Jun Seop Lee, Byung Mo Yang and Haeng Muk Cho [7] Kim, C. K. and Kim, D. H. A Study on the Optimum Design of O-ring in LPG Charging Nozzle. Journal of the Korean Institute of Gas, 10, 2006, pp [8] Anil, K. B. and howard, L. S. Handbook of elastomers. M. Dekker, 2001, pp , [9] Korea Institute of Footwear & Leather Technology. Intro-duction to Rubber Technology. Daejoo Advertising, 2000, pp. 71. [10] Visscher, D. and Gadlage, M. The dichtometik O-ring handbook. Dichtometik North America, pp. 8-9, 12-17, , [11] Han, S. W., Kwak, S. B. and Choi, N. S. Accelerated Life Prediction of Ethylene- Propylene Diene Monomer Rubber Subjected to Combined Degration. Trans. Korean Soc, 38, 2014, pp [12] Lee, J. H., Bae, J. W., Choi, M. C., Yoon, Y. M., Kim, W. H., Park, S. H. and Ju, J. N. A Study on the Correlation Analysis between Airtightness and Permanent Compression Ratio of NBR O-ring. Proceedings of the Korean Society of Propulsion Engineers, 2017, pp [13] Lee, J. H., Bae, J. W., Yoon, Y. M., Choi, M. C. and Ju, J. N. A Study on Life Prediction of Fluorine Silicone O-ring by Intermittent Compressive Stress Relaxation Method. Proceedings of the Korean Society of Propulsion Engineers, 2016, pp [14] Kim, C. G. Finite Element Analysis on Sealing Behavior and Durability of O-Rings with V - grooves. Journal of the Korean Institute of Gas, 17(1), 2013, pp [15] Kim, D. H., Kim, C. K. and Lee, I. K. A Study on the Shape of O-ring to Improve Airtightness of Ball Valve. Proceedings of the Korea Gas Association Conference, 2012, pp [16] Kim, C. K. A Study on Sealing Characteristics Analysis of. Journal of the Korean Institute of Gas, 16(5), 2012, pp [17] Kim, T. H. and Kim, C. K. A Study on the Sealing Performance Improvement of LPG Charge Coupler Using Peanut O-ring. Proceedings of the Korea Gas Association Conference, 2012, pp editor@iaeme.com