Coaxial Sealing System: History
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Subjects: Engineering, Manufacturing
Contributor:
Tudor Deaconescu

Coaxial sealing systems are widely used for both pistons and rods of hydraulic cylinders.
A coaxial sealing system consists of a seal that can have various cross-section forms and is in direct contact with the mobile surface and with a pre-stressing ring that can also have a number of different cross section geometries (circular, rectangular, complex). The pre-stressing rings do not engage in contact with the mobile surface and are, thus, not subject to wear; their role is merely to generate the pressure required for tensioning the entire system. Pre-stressing rings are also vibration dampers. A pre-stressing ring is mounted in its seat in pre-compressed state, with an initial specific radial deformation of 10–25%. As the pressure of the fluid in the hydraulic cylinder builds up, the deformation of the pre-stressing ring increases and generates a larger radial force.

  • coaxial sealing systems
  • hydraulic cylinders

1. Introduction

Research on the energy efficiency of industrial actuation systems cited in the literature [1] shows a mere 21% efficiency of hydraulic drives, thus revealing an insufficiently optimized field. Despite their very low energy efficiency, hydraulic actuation systems have been deployed heavily in industrial applications that require large forces, rigidity and endurance. Reducing energy consumption in hydraulic drives can be achieved mainly by increasing the efficiency of the components. Thus, hydraulic cylinder efficiency can be improved by reducing the level of friction present in the system. Besides the large quantity of dissipated energy, friction also affects the control, precision and repeatability of the hydraulic cylinder’s motions.
Many applications of hydraulic cylinders entail small velocities of just several millimeters per minute. Included here are, for example, the feed motions of machine-tools, precise positioning motions by means of hydraulic cylinders or remote manipulation needed in applications on land, sea or in space.
It is known that for small velocities, in the case of dry friction, as well as of limit or mixed friction, the motion of the surfaces forming the so-called friction couple can be accompanied by intermittence or jolts. In the case of hydraulic cylinders, displacement of the mobile element at small velocities can cause two forms of jolty sliding, namely sliding accompanied by eigen-vibrations and intermittent sliding, known as stick-slip [2]. Sliding accompanied by eigen-vibrations is characterized by small amplitude variations of the friction force and passing from its maximum to its minimum values within a relatively large interval of time. Stick-slip is characterized by large amplitudes of the oscillatory phenomenon and large friction forces. Passing from maximum to minimum values takes place in very short time intervals, causing high frequency jerking. The sliding velocity at that stick-slip occurs is smaller than that typical for eigen-vibrations [3].
Non-uniform, jerky and uncontrolled motions at small velocity operation of hydraulic cylinders has numerous causes, including deviations from co-axiality of piston and rod, lack of component manufacturing precision, air pockets in the hydraulic oil and most importantly the inadequate selection of the sealing system. Hydraulic cylinder sealing system performance is influenced by numerous variables that cannot be simultaneously taken into consideration for obtaining generalized mathematical relationships. Speeds or properties of the sealing system components materials, roughness of the contacting surfaces, and temperature of the sealed fluid are only a few such quantities that affect the operational behavior of sealing systems [4][5][6]. The importance of an optimum sealing system design follows also from the conclusion of the study published by Oprean et al. [7], which asserts that 44% of the operational malfunctioning of hydraulic cylinders is due to flawed sealing.
While the literature features numerous reports on hydraulic cylinder behavior at small velocities, such research concerns the particular applications of sealing systems and the respective results do not lend themselves for generalization. Thus, a study by Eclipse Engineering Inc. presents the effects of stick-slip on seals (softening, swelling, and premature wear) as well as means of prevention (correct selection of materials, optimum roughness of the contacting surfaces, suitable fluids deployed at adequate temperatures) [8].
Sealing systems manufacturer, Trelleborg Sealing Solutions of Sweden, addresses stick-slip from a pragmatic perspective and proposes introducing dampers in order to absorb vibrations. Thus, by integrating an elastomer element into the sealing system the micro-movements created by the stick-slip phenomenon at the sealing contact area can be dampened to eliminate vibration and noise [9].
Several scientific papers describe experimental set-ups used for studying the stick-slip phenomenon. Pan et al. present an experimental apparatus developed to measure the friction forces for hydraulic cylinders under different operating conditions [10]. Puglisi et al. also discuss in [11] the experimental identification of friction effects defined by the parameters of the LuGre model.
Research reported in [12] demonstrates that stick-slip is caused by the transfer of carbon monoxide from carbon steel to the sealing surfaces. This leads to noise and accelerates the wear of sealing elements.
Further numerous research papers focus on the selection of sealing system materials. In [13], the authors recommend the selection of certain seal materials by area of application (food and beverage or oil and gas applications). In [14], Tran et al. assert that the type of friction occurring in the sealed area depends on pressure and seal material.
Most frequently, the materials used for hydraulic cylinder sealing systems are polymers, predominantly elastomers, plastomers or thermoplastic elastomers. These materials meet best the multiple requirements for a sealing system, namely, to achieve the best possible packing while ensuring maximum energy efficiency and motion precision.
One of the polymer materials most frequently used for seals is PTFE (virgin polytetrafluoroethylene) [12][15][16] or filled PTFE [16][17]. Research showed that polytetrafluoroethylene presents a small friction coefficient (0.05 … 0.1) and is also nearly completely chemically inert to the substances it comes into contact with. These characteristics render PTFE eligible for tribological applications designed to reduce energy consumption in friction-intensive machinery, as well as for reactive and corrosive applications [4]. Another polymer material is polyurethane, which, when combined with certain solid lubricants, ensures small friction coefficients.

2. Coaxial Sealing System

Coaxial sealing systems are widely used for both pistons and rods of hydraulic cylinders (Figure 1).
Polymers 14 00290 g001 550
Figure 1. Coaxial sealing systems.
A coaxial sealing system consists of a seal that can have various cross-section forms and is in direct contact with the mobile surface and with a pre-stressing ring that can also have a number of different cross section geometries (circular, rectangular, complex). The pre-stressing rings do not engage in contact with the mobile surface and are, thus, not subject to wear; their role is merely to generate the pressure required for tensioning the entire system. Pre-stressing rings are also vibration dampers. A pre-stressing ring is mounted in its seat in pre-compressed state, with an initial specific radial deformation of 10–25%. As the pressure of the fluid in the hydraulic cylinder builds up, the deformation of the pre-stressing ring increases and generates a larger radial force.
The relative motion between the seal and the mobile surface of the hydraulic cylinder (the interior wall or the rod of the cylinder) causes the appearance in the contact area of a variable thickness fluid film that is wedge shaped. Depending on the velocity v, the thickness of the fluid film determines the type of friction between the seal and the surface of the cylinder, that can be dry, fluid or mixed. A greater velocity causes a thicker fluid film, and the consequent fluid type friction ensures a high level of energy efficiency. This is, however, conflicting in relation to the sealing process, as the hydrodynamic separation of seal and cylinder surface leads to an increased loss of fluid that flows towards the lower pressure side [4].
At small velocities the thickness of the fluid layer is also smaller, even zero, hence the type of friction will be dry. Responsible are the adhesion forces between the contacting materials and the asperities of the two surfaces. Dry friction must be avoided as it can cause uncontrolled motions of the hydraulic cylinder. This can be achieved by a correct selection of the seal materials.
Correct selection of the compounds for the sealing system components is significantly facilitated by knowing the chemical structure of the various materials and fluid that are used, as well as their reciprocal reactions.
The pre-stressing rings of coaxial sealing systems are generally made from elastomers of various types: nitrile (NBR), hydrogenated nitrile butadiene rubber (HNBR), silicone rubber (Q), fluorocarbon (FKM), ethylene propylene diene monomer (EPDM) and fluorosilicone (FVMQ). For high temperature working situations, up to 325 °C, the material recommended for these rings is perfluoroelastomer (FFKM), while for very low temperatures, as low as −75 °C, the recommended material is VMQ [4][18].
The second element of the sealing system, the actual seal, which is in contact with the mobile elements of the hydraulic cylinder, is made from plastomers of polytetrafluoroethylene (PTFE) type. This material is used as it combines a number of important properties, such as chemical inertness, ageing strength, the ability to fill the irregularities of the sealed-off surface and, mostly, the smallest friction coefficient in contact with various other solid materials.
Depending on the concrete operational conditions, seals are made from virgin PTFE or PTFE with added bronze, glass fibers, molybdenum disulfide (MoS2) or carbon fiber (graphite). Bronze endows the seal with increased extrusion strength, molybdenum disulfide facilitates low-friction and ensures long seal life, while the carbon fiber is responsible for a long wear life.
Another material used for seals is polyurethane, due to its resistance to high pressure and low friction coefficient. Variants of polyurethane compounds are filled with different solid lubricants and are used especially for dry running applications. Solid lubricants decrease the friction between the cylinder and the seal.
The performance of coaxial sealing systems at small linear velocities was tested for seals made from three different materials, Polytetrafluoroethylene (PTFE) and a self-lubricated thermoplastic polyurethane elastomer (H-PU). In all cases, the pre-stressing ring was an O-ring made from nitrile butadiene rubber (NBR) with a hardness of 70 Shore A. Table 1 shows the essential characteristics of the studied materials [19].
Table 1. Characteristics of seal materials.
Material Composition Shore D Hardness Utilization
Virgin PTFE 100% PTFE 55 ± 3 Resistant to almost all chemicals
PTFE 25% Glass 25% clean milled glass fibers and 75% virgin PTFE 58 ± 3 Resistant to almost all chemicals
H-PU 55D PUR 55 ± 3 Resistant to oil, petrol, hot water, hot air, ozone
The analyzed coaxial sealing systems were mounted on the piston of a hydraulic cylinder with a barrel made from OLC45 steel (Romanian standard) (heat-treated quality carbon steel; DIN EN AISI 1.0503 C45-1045). For each of several tested polymer-steel pairs of materials the static and kinetic friction coefficients were determined. Consequently, recommendations are made for the range of velocities within that these seals can function without occurrence of the stick-slip phenomenon.
Figure 2 displays the essential dimensions (in mm) of coaxial sealing systems of cylinder pistons.
Polymers 14 00290 g002 550
Figure 2. Essential dimensions of sealing systems.

This entry is adapted from the peer-reviewed paper 10.3390/polym14020290

References

  1. Hydraulics & Pneumatics. Available online: http://hydraulicspneumatics.com/blog/how-efficient-are-your-hydraulic-machines (accessed on 12 September 2021).
  2. Owen, W.S.; Croft, E.A. The Reduction of Stick-Slip Friction in Hydraulic Actuators. IEEE/ASME Trans. Mechatron. 2003, 8, 362–371.
  3. Deaconescu, A.; Deaconescu, T. Low Friction Materials Used in the Construction of Hydraulic Sealing Systems in the Case of Small Velocities. J. Balk. Trib. Assoc. 2016, 22, 454–463.
  4. Deaconescu, A.; Deaconescu, T. Tribological Behavior of Hydraulic Cylinder Coaxial Sealing Systems Made from PTFE and PTFE Compounds. Polymers 2020, 12, 155.
  5. Nikas, G.K. Eighty years of research on hydraulic reciprocating seals: Review of tribological studies and related topics since the 1930s. Proc. Inst. Mech. Eng. Part J J. Eng. Tribol. 2010, 224, 1–23.
  6. Skowrońska, J.; Kosucki, A.; Stawiński, Ł. Overview of Materials Used for the Basic Elements of Hydraulic Actuators and Sealing Systems and Their Surfaces Modification Methods. Materials 2021, 14, 1422.
  7. Oprean, A.; Ispas, C.; Ciobanu, E.; Dorin, A.; Medar, S.; Olaru, A.; Prodan, D. Hydraulic Drives and Automation; Technical Publishing House: Bucharest, Romania, 1989. (In Romanian)
  8. Eclipse Engineering, Inc. How to Avoid Stick-Slip in Your Seal. Available online: https://eclipseseal.com/blog/seals/avoid-stick-slip-seal/ (accessed on 14 September 2021).
  9. Trelleborg Sealing Solutions. The Stick-Slip Solution. The Importance of Damper Integration in Dynamic Sealing. Available online: TSS-stick-slip-whitepaper.pdf (accessed on 14 September 2021).
  10. Pan, Q.; Zeng, Y.; Li, Y.; Jiang, X.; Huang, M. Experimental investigation of friction behaviors for double-acting hydraulic actuators with different reciprocating seals. Tribol. Int. 2021, 153, 106506.
  11. Puglisi, L.J.; Saltaren, R.J.; Garcia Cena, C.E. Experimental Identification of Lu-Gre Friction Model in a Hydraulic Actuator. In Advances in Automation and Robotics Research in Latin America. Lecture Notes in Networks and Systems; Chang, I., Baca, J., Moreno, H., Carrera, I., Cardona, M., Eds.; Springer: Cham, Switzerland, 2017; Volume 13.
  12. Muraki, M.; Kinbara, E.; Konishi, T. A laboratory simulation for stick-slip phenomena on the hydraulic cylinder of a construction machine. Tribol. Int. 2003, 36, 739–744.
  13. McBride, T. Seals for Hydraulic Cylinders. The Hydraulics & Pneumatics Article 2019. Available online: https://www.hydraulicspneumatics.com/technologies/seals/article/21118898/seals-for-hydraulic-cylinders (accessed on 15 September 2021).
  14. Tran, X.B.; Hafizah, N.; Yanada, H. Modeling of dynamic friction behaviors of hydraulic cylinders. Mechatronics 2012, 22, 65–75.
  15. Heipl, O.; Murrenhoff, H. Friction of hydraulic rod seals at high velocities. Tribol. Int. 2015, 85, 66–73.
  16. Golchin, A.; Simmons, G.; Glavatskih, S. Breakaway friction of PTFE materials in lubricated conditions. Tribol. Int. 2012, 48, 54–62.
  17. Kowalski, K.; Złoto, T. Exploitation and Repair of Hydraulic Cylinders Used in Mobile Machinery. Teka Comm. Mot. Energetics Agric. 2014, 14, 53–58.
  18. Trelleborg Sealing Solution: Aerospace Sealing Systems. Product Catalogue & Engineering Guide. 2011. Available online: www.tss.trelleborg.com (accessed on 26 September 2013).
  19. DMH. Solution for Seals. Available online: https://www.dmh.at/materials/ (accessed on 20 October 2019).
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