23rd International Colloquium Tribology - January 2022 335 A novel measurement procedure to analyse the friction of rod seals in relation to pre-defined shear rates and starved lubrication conditions Oliver Feuchtmüller Institute of Machine Components, University of Stuttgart, Stuttgart, Germany Corresponding author: oliver.feuchtmueller@ima.uni-stuttgart.de Lothar Hörl Institute of Machine Components, University of Stuttgart, Stuttgart, Germany Frank Bauer Institute of Machine Components, University of Stuttgart, Stuttgart, Germany 1. Introduction Linear actuators such as hydraulic or pneumatic cylinders are used in a variety of applications. One critical component of every hydraulic or pneumatic cylinder is the rod seal, since it influences the performance of the whole machinery. If a rod seal fails, leakage or machine downtime is unavoidable. Moreover, modern applications demand friction-optimized sealing solutions. Low friction rod seals are required to optimize the efficiency and remain competitive. In high precision positioning actuators, the seal’s friction is of crucial importance, since its influence on the dynamic performance and precision of linear actuators. Consequently, friction properties of rod seals shall be considered when designing actuators for certain operating parameters. Despite decades of research, predicting a seal’s friction remains a challenging task due to numerous influence parameters. One main factor influencing the friction is the lubrication condition in the sealing gap. At outstroke and instroke, the hydraulic rod drags oil into the sealing gap resulting in a thin lubricant film in the nanometer range [1], [2]numerically, and experimentally. The analyzed sealing system consists of an unmodified, commercially available U-cup, a polished rod, and mineral oil. The inverse theory of hydrodynamic lubrication (IHL. A serious drawback of conventional test rigs for analysing friction of rod seals is that the film thickness remains unknown. This limits conclusion which can be drawn on actual tribological mechanisms in the sealing gap. We developed a novel measurement procedure to analyse the influence of film thickness and shear rate on the friction of reciprocating rod seals directly. It is possible to analyse the friction of almost every rod seal and lubricant in relation to the fluid shear and thin film lubrication conditions. The procedure offers a deep insight into the influence of parameters such as the seals material, geometry, surface roughness or rheological properties of the lubricant on the friction. One unique feature of the procedure is that a pre-defined lubricant film in the nanometer range is generated on the rod to achieve certain lubrication conditions and shear rates in the sealing gap. 2. Novel measurement procedure The procedure is based on a film thickness measurement technique using ellipsometry and a recently-developed test rig. The test rig was built to simulate the outstroke and measure the dynamic friction of the seal. The main advantage of the new test rig is that the rod and seal can be removed easily and without effort. Ellipsometry is a technique for measuring thin films in the nanometer range. Ellipsometry was employed to achieve the required accuracy for the film thickness measurements in the nanometer range. Due to the use of the ellipsometer, the hydraulic rod must be polished to a smooth surface finish. The novel measurement procedure contains several steps, which are illustrated in Figure 1. After outstroke at certain operating conditions , the film thickness on the hydraulic rod is measured using ellipsometry as described in previous studies [1], [3]. Therefore, the seal is removed from the rod carefully. Then, the same rod with the measured oil film is used for a second outstroke with increased rod speed . Since the speed is increased at the second outstroke, the oil film on the rod cannot be wiped off by the seal and remains constant. This is proofed by a second film thickness measurement afterwards. 336 23rd International Colloquium Tribology - January 2022 A novel measurement procedure to analyse the friction of rod seals in relation to pre-defined shear rates and starved lubrication conditions Figure 1: Illustration of the measurement procedure Assuming full fluid lubrication conditions, friction depends on the fluid shear in the sealing gap. Then, fluid friction F fluid is a function of the film thickness , speed u, dynamic viscosity η and contact area A as defined in Eq. 1. (1) The outstroke speed u can easily be controlled by the test rig and the film thickness h O on the rod is measured using ellipsometry. Assuming a linear Couette-flow in the sealing gap, the gap height is twice the measured film thickness on the rod. Thus, the shear rate in the sealing gap can be determined. Considering the viscosity η of the lubricant, the shear stress τ in the sealing gap can be calculated. The product of the shear stress τ and the contact area A between the seal and the rod results in the fluid friction F fluid as defined in Eq. 1. Aside, the apparent friction at outstroke is measured by a force transducer. For further discussion and comparison of various sealing systems, the measured apparent friction of rod seals can be analysed as a function of fundamental parameters such as the shear rate, film thickness and viscosity. 3. Results of the empirical study In a first empirical study, the friction of a typical polyurethan U-cup was analysed at different operating and lubrication conditions using the new measurement procedure. The friction was measured at shear rates in the range from approximately 10 5 to 10 7 s -1 which were achieved due to the combination of various pre-defined film thicknesses (1 to 200 nm) and rod speeds (10 to 200 mms -1 ). Furthermore, mineral oils of various viscosity classes (ISO VG 15 to 460) were used for variation of fluid shear stress in the sealing gap. The contact area between the U-cup and the rod was measured using a hollow glass rod in advance. All measurements were carried out at room temperature. The measurement results indicate a significant influence of the analysed parameters on the measured friction. In Figure 2, the measured friction F R,O at outstroke is plotted against the calculated fluid friction F fluid in accordance with Eq. 1. As demonstrated in Figure 2, it was possible to confirm a linear influence of the shear rate and viscosity in the sealing gap on the measured friction for a wide range of viscosities and shear rates. However, the directly measured friction was somewhat higher than the calculated fluid friction based on the speed, film thickness, viscosity and contact area. A possible reason for those discrepancies is the assumption of pure Newtonian fluid friction without considering boundary friction or asperity contacts which may appear in such thin gaps. 23rd International Colloquium Tribology - January 2022 337 A novel measurement procedure to analyse the friction of rod seals in relation to pre-defined shear rates and starved lubrication conditions Figure 2: Measured friction as a function of rod speed, gap height , dynamic viscosity and contact area of a urethane U cup at starved lubrication conditions 4. Conclusions The originality of the new measurement procedure is that friction of commercially available rod seals can be analysed as a function of film thickness, rod speed and viscosity. Moreover, the new procedure can be used to analyse further sealing systems with different components. For example, the tribological properties of hydraulic fluids based on different base oils with different chemical and rheological properties can be analysed in narrow gaps at high shear rates. The novel measurement procedure can be adopted in the development process of new rod seals for fine-tuning of the seal’s geometry, surface topography and the resulting lubrication conditions. Furthermore, the procedure can be used for the validation of soft EHL-simulation models in general. In conclusion, a new perspective on thin film lubrication and friction of practical relevant rod seals is provided by the empirical analysis of pre-defined lubrication conditions. References [1] O. Feuchtmüller, L. Hörl, and F. Bauer, “Oil film generation of a hydraulic rod seal: an experimental study using ellipsometry,” Tribol. Int., vol. 162, p. 107102, May 2021, doi: 10.1016/ j.triboint.2021.107102. [2] O. Feuchtmüller, N. Dakov, L. Hörl, and F. Bauer, “Remarks on Modeling the Oil Film Generation of Rod Seals,” Lubricants, vol. 9, no. 9, p. 95, 2021, doi: 10.3390/ lubricants9090095. [3] L. Hörl, W. Haas, and U. Nißler, “A comparison of test methods for hydraulic rod seals,” Seal. Technol., vol. 2009, no. 12, pp. 8-13, Dec. 2009, doi: 10.1016/ S1350-4789(09)70594-X.