1、13FTM17 AGMA Technical Paper Dynamic Simulations of Radial Lip Seals Followability in an Industrial Gearbox By M. Organisciak, R. Iervolino, M. Sansalone, and S. Barbera, SKF ERC, A. Paykin and M. Schweig, SKF Sealing Solution 2 13FTM17 Dynamic Simulations of Radial Lip Seals Followability in an Ind
2、ustrial Gearbox Michel Organisciak, Rossana Iervolino, Mickael Sansalone, and Stellario Barbera, SKF ERC, Alex Paykin and Matthew Schweig, SKF Sealing Solution The statements and opinions contained herein are those of the author and should not be construed as an official action or opinion of the Ame
3、rican Gear Manufacturers Association. Abstract Industrial gear units are widely used in power transmission systems. They are composed of shafts, gears, rolling elements bearings and dynamic lip seals. The seals performance is critical for a proper functioning of the system. Water or contamination in
4、gress into a mechanical system may lead to a premature failure. Leakage of oil may have the same effect and be harmful for the environment. Depending on the application, seals may need to operate under various dynamic conditions, such as wide range of rotational speed (RPM) and temperatures, shaftto
5、boremisalignment (STBM), shaft dynamic run-out (DRO) or global structure deformations. The prediction of dynamic seal performance is a complex task. The rotating lip seals are usually made in elastomeric materials which display a hyper-elastic and viscoelastic behavior. Combined with the dynamic ope
6、rating conditions, the simulation of the seal performance requires time dependent approaches which are very often time consuming. Innovative modeling methods need to be developed in order to be usable by the development engineering community. This paper presents a novel approach to predict seal dyna
7、mic performance under dynamic conditions. A formulation of viscoelastic super-elements is developed to predict the deformations of the seal lips. It is combined with a contact solver to assess contact force and its distribution around the shaft and other lip contersurfaces (such as other radial or a
8、xial locations). In order to demonstrate functionalities and advantages of the developed method, please consider an example of radial lip shaft seal. The problem addresses prediction of seal performance at cold temperature, large shaft-to-bore misalignment and dynamic run-out conditions. Different m
9、aterial and spring options are assessed in order to improve the performance. This unique modeling capability will allow selecting or developing the shaft seals which would meet and exceed modern gearbox demanding application. It will also enable gearbox manufacturers to bring to the market more perf
10、orming and reliable gearboxes. Copyright 2013 American Gear Manufacturers Association 1001 N. Fairfax Street, Suite 500 Alexandria, Virginia 22314 September 2013 ISBN: 978-1-61481-074-2 3 13FTM17 Dynamic Simulations of Radial Lip Seals Followability in an Industrial Gearbox Michel Organisciak, Rossa
11、na Iervolino, Mickael Sansalone, and Stellario Barbera, SKF ERC, Alex Paykin and Matthew Schweig, SKF Sealing Solution Introduction Industrial gear units are widely used in power transmission systems. They are composed of shafts, gears, rolling elements bearings and dynamic lip seals. The seals perf
12、ormance is critical for a proper functioning of the system. Water or contamination ingress into a mechanical system may lead to a premature failure. Leakage of oil may have the same effect and be harmful for the environment. Depending on the application, seals may need to operate under various dynam
13、ic conditions, such as wide range of rotational speed (RPM) and temperatures, shafttoboremisalignment (STBM), shaft dynamic run-out (DRO) or global structure deformations. The prediction of dynamic seal performance is a complex task. The rotating lip seals are usually made in elastomeric materials w
14、hich display a hyper-elastic and viscoelastic behavior. Combined with the dynamic operating conditions, the simulation of the seal performance requires time dependent approaches which are very often time consuming. Innovative modeling methods need to be developed in order to be usable by the develop
15、ment engineering community. This paper presents a novel approach to predict seal dynamic performance under dynamic conditions. A formulation of viscoelastic super-elements is developed to predict the deformations of the seal lips. It is combined with a contact solver to assess contact force and its
16、distribution around the shaft and other lip contersurfaces (such as other radial or axial locations). In order to demonstrate functionalities and advantages of the developed method, please consider an example of radial lip shaft seal shown in Figure 1. The problem addresses prediction of seal perfor
17、mance at cold temperature, large shaft-to-bore misalignment and dynamic run-out conditions. Different material and spring options are assessed in order to improve the performance. This unique modeling capability will allow selecting or developing the shaft seals which would meet and exceed modern ge
18、arbox demanding application. It will also enable gearbox manufacturers to bring to the market more performing and reliable gearboxes. Figure 1. Example of FEA results from SKF Seal Designer: static seal deformations 4 13FTM17 Why dynamic simulations? Current modeling methods Virtual evaluations of m
19、echanical system performance today are very common in the design process. As a result of extensive numerical analyses performed, during product development and prototyping phases, the risk of failing first prototypes during laboratory or field testing is significantly reduced. In addition, the overa
20、ll time to market may become shorter. The most commonly used method is the finite element analysis (FEA). The body of a mechanical part, such as a seal, is discretized into finite elements. These elements contain equation describing the relation between strain and stress in the material and the mech
21、anical behavior of the materials. For rubber seals this is important because the material exhibits incompressible hyper-elastic behavior, as explained in 1,5,6. In the first phase, using quasi-static simulation, after the seal is designed using SKF internal engineering standards with consideration o
22、f application requirements, the virtual mounting of the seal is performed to verify if the designed product had actually met the specific engineering requirements. SKF has developed SKF Seal Designer, proprietary software allowing this type of simulations. The analysis allows to engineers, among oth
23、er parameters, the determination of: seal deformation seal contact load contact pressure between the shaft and the seal and between the housing and the seal stress and strain levels in various components, such as elastomeric material. Figure 1 displays an example of such analysis, with one of the SK
24、F Standard Line seals installed in the housing and on shaft. The deformations in the material and the contact loads are displayed. This static analysis is a first step in the design of a seal. It allows development engineers to asses seal retention in the housing, the level of contact load and press
25、ure distribution profile to ensure proper sealing ability or the strain and deformation to ensure seal integrity. Seal design can be modified until the required levels are achieved. Getting closer to the real world The seal operating in application is by definition subjected to dynamic boundary cond
26、itions. The shafts of gear units operate at various rotating speeds, Due to the design and the stiffness of the different elements of the system, the seal may experience different effects like: Shaft to bore misalignment (STBM), where the shaft centerline has a constant offset with the centerline of
27、 the seal housing Dynamic Run-Out (DRO) where the actual centerline of the shaft is rotating around its theoretical centerline, which typically happens due to lobbing and other shaft inaccuracies, as well as to the dynamic effects, such as shaft flexing or vibration resulting from dynamic loads. Fig
28、ure 2 illustrates the effects of some DRO on the seal. The main risk is too loose contact between shaft and seal due to a very high DRO or rotational speed for a given seal design and material. This can lead to a leakage from the mechanical system or an ingress of water or contaminant into the mecha
29、nical system. The static Finite Element Analysis is unable to predict this type of effect. Figure 2. Schematic representation of a seal under DRO conditions 5 13FTM17 Operating temperatures can also widely vary in various applications. In some cases a low temperature start-up is required. This is es
30、pecially important since material properties of rubber change dramatically with temperature. Elastomeric materials: General overview Rotating lip seals are usually made of elastomeric materials or rubbery materials. When used in the appropriate range of temperatures, these materials display high fle
31、xibility and elasticity, as well as ability to stretch by more than hundred percent and recover almost immediately to its original shape. In addition, they also have a viscoelastic nature, i.e., they dissipate energy, when they are deformed. An elastomeric material or compound is composed of by a po
32、lymer network, a filler network, plasticizers and numerous additives. The typical elastomers, used for industrial gear units are: NBR (Nitrile butadiene rubber) with a upper operating temperature limit of about 100C; HNBR (Hydrogenated nitrile butadiene rubber) with a upper operating temperature lim
33、it of about 150C; FKM (Fluoroelastomer) with a upper operating temperature limit of about 220C. The filler network ensures good mechanical properties of the compound. The plasticizers and additives improve other properties like thermal resistance, chemical resistance to fluids, wear resistance or fr
34、ictional performance. For extreme conditions or very aggressive lubricants, it is always possible to formulate and develop new compounds with specific properties. While the high end utilization temperature limit of a compound is mainly determined by a chemically driven degradation process, the behav
35、ior towards low temperatures is more related to a mechanically reversible process. The elasticity of the compound is determined by the ability of molecular chains to move and adjust their conformation. This process is energy driven and is dependent on the temperature and rate of deformation. When th
36、e temperature decreases to a value closed to the glass transition temperature, the movements of the molecular chains become limited; the material is “frozen” and the modulus of the material increases dramatically. The glass transition temperature Tg is dependent on the deformation rate of the compou
37、nd. Therefore specific material models are needed to cope with the dynamic behavior of elastomeric compounds. Figure 3 illustrates an elastomeric compound properties change with temperature and frequency. Here you can see how the shear modulus of the material is changing based on temperature and fre
38、quency of variation. Material modeling Several models exist to model the viscoelastic behavior of elastomeric materials 2. They are all composed by an arrangement of spring and dashpots. In this work a multi-branch Zener model is used, It is shown Figure 4. Figure 3. Shear modulus as a function of t
39、emperature (left) and deformation frequency (right) 6 13FTM17 Figure 4. Multi-branch Zener viscoelastic model A single branch is a combination of spring and a Voigt element 2, and the stress strain relation reads: 11i iii 1i i2iEd dEt t (1)where is the deformation; iis the stress in the branch; Iis
40、the relaxation time and E1iand E2ithe modulus of the springs in this given branch of the Zener model. The time dependency of the relation between the strain and stress for a viscoelastic material is clearly expressed in this equation. The integration of this formula for all the branches of the model
41、 gives the shear storage modulus G, the shear loss modulus G” and the value of tan = G”/G for a chosen reference temperature TREF. In order to be able to model the behavior of the materials for any frequency of deformation and temperature, the time temperature superposition is used. The time tempera
42、ture superposition (TTS) principle implies that the same variation in a viscoelastic response obtained by changing the temperature can be also achieved by varying the frequency. In practice, it means that if G is the shear storage modulus at a frequency 1, it can be expressed as a function of G at t
43、he reference temperature TREF: ( ) ( ( ) )11TREFGTGaTT(2)where aT(T)is the shift factor or TTS function value. The most used formula has been proposed by William, Landel and Ferry 3. ()log ( ( )()1REFT2REFCT TaTCTT(3)Dynamic modeling of radial lip seals The computation of the deformation of seal lip
44、 under dynamic condition is based on the viscoelastic material relation (1). Special super-elements describing the relation between stresses and deformations in the seal structure are formulated. They have been used in example below for discretization of the sealing lip, with cross-section shown on
45、Figure 5. An interactive process is needed to solve the equations for all points in the seal cross section and around the circumference. The calculation for a given step includes an elastic term and a history term representing the viscoelastic effects 4. For example for the moment equilibrium for an
46、 element at a given time step, the equation reads: () () ( )ii-1M tKMtMtt (4)7 13FTM17 Figure 5. Discretized seal lip with super elements The computational algorithm for seal deformations is combined with a contact solver. It is indeed necessary to determine at the same time the deformation of the s
47、eal and the contact conditions between the seal and the shaft. The outputs are contact loads distribution and area around the shaft circumference. Application of the dynamic modeling techniques Presentation of considered cases For the demonstration purposes of the capabilities and interest of the de
48、veloped method, a radial shaft seal for a 50 mm shaft is used. Two different material options are used: Compound A with a Tg of -15C Compound B with a Tg of -30C The material properties of these two compounds are presented in Figure 6. The figure includes the material shear modulus and tan . Tan is
49、a good representation of the loss of energy during the deformation of the material and is the highest during the transition between rubbery and glassy region. Here compound A and B have globally the same behavior and the same shape of the curve of tan . The difference between is that the transition between rubbery and glassy state happens at lower temperatures: the curves for G and tan are shifted to the left on the graphs. Compound B remains flexible until much lower temperatures
