EXPERIMENTAL RESEARCH FOR MEASURING FRICTION FORCES FROM ROD SEALING AT THE HYDRAULIC CYLINDERS

EXPERIMENTAL RESEARCH FOR MEASURING FRICTION FORCES FROM ROD SEALING AT THE HYDRAULIC CYLINDERS Petrin DRUMEA1, Corneliu CRISTESCU1, Aurelian FATU2, Mohamed HAJJAM2 1 The Hydraulics and Pneumatics Research, Institute INOE 2000-IHP, Bucharest, ROMANIA, 2 Institute PPRIME, CNRS, Université de Poitiers-ENSMA, UPR, FRANCE cristescu.ihp@fluidas.ro, aurelian.fatu@univ-poitiers.fr ABSTRACT The paper presents experimental equipment and research results conducted by Hydraulics and Pneumatics Research Institute INOE 2000-IHP Romania in cooperation with Institute PPRIME University of Poitiers France, in order to determine the friction forces that occur in mobile rod sealing at hydraulic cylinders. In the first part the device and the experimental stand are presented and in the second part, some obtained results are shown. The graphics illustrate the evolution of the friction forces at the rod seal of the hydraulic cylinders. Experimental research marks the beginning of a major action on the tribological behaviour of hydraulic seals, which will be developed in the next period. Keywords: sealing, friction forces, industrial tribology, measurements, test stands 1. INTRODUCTION The sealing represents assemblies of machine elements which close hermetically a space containing a medium under pressure. The sealing may be fixed or mobile, and the mobile ones, from the point of view of the relative motion between the sealing surfaces may be with rotation or translation motion, the last ones being the subject of the present article [1]. The mobile translation sealing are specific to the hydraulic cylinders Fig. 1, where it realizes the sealing on the rod with diameter d, being in reciprocating translation motion on the stroke, of the fluid with the constant viscosity η and under pressure p. In Fig. 1, d is the rod diameter, S is the rod stroke, v and vr are the velocities, in the both senses and Q represents the leakages through the seal. The leakage represents a fluid loss through the sealing and the component parts towards which it has a relative motion. When the rod, under the action of the working pressure, gets out with the speed v or gets back with the speed vr,, are produced some fluid losses which can be calculated. Fig. 1. The sealing of the hydraulic cylinders rod

11 The process of mobile sealing of the hydraulic cylinder rod is realized in conditions of friction-fluid and rod lubrication [2], when between the surfaces with relative motion with speed v (the sealing sleeve and cylinder rod), is provided continuously portent lubrication. The fluid lubricant adheres to the component parts in motion. The variation of the tangential force, which appears between the surfaces with relative motion, represents the real friction force, which can be calculated by mathematical modeling and computer simulation [3], but must be determined, also, by experimental methods [4]. the measurements. The adopted technical solution was the replacement of the rod cylinder head sealing Fig. 2, with one double sealed guidance bushing, which contains two U type sealings, fixed with the wings facing one another Fig. 3 and Fig. 4. 2. THE EXPERIMENTAL RESEARCH TO MEASURE FRICTION FORCES In order to evaluate the friction forces from the sealing of the hydraulic cylinders, it was developed a real experimental research based on special requires. Fig. 2. The rod cylinder head sealing 2.1. The Basic Conditions for the Experimental Device Design Through the design of the experimental device, several points must be taken into account in order to ensure the optimum evaluation/measuring of the friction forces between the seal and the shaft: - to eliminate, as much as possible, large masses in motion, to avoid the occurrence of the inertial forces; - to ensure coaxiality of the rod and double sealed bush; - to evaluate precisely the additional pressure forces; - to eliminate the mechanical possible bindings, bowing etc; - to provide fluid supply under various operational pressures; - to measure and record the evolution of the friction forces, stroke and pressure; - to control the sealing fluid pressure during the entire working stroke. Fig. 3. The sealing Fig. 4. The double sealed bush 2.1. The Technical Solution for Measuring Friction Forces Firstly, it was projected and realized an experimental device, which can be mounted on the existing stand, to study the real friction forces evolutions [3]. This experimental device is conceived purposefully for working by mounting it on a stand which provides operational strokes. The main element of the experimental device is represented by the technical solution found for maintaining under the pressure of the operational fluid the sealing of the cylinder rod, without generating additional pressure forces on the rod head, which may affect the result of Fig. 5. Entrance of the operational fluid

12 The operational fluid is injected between these two sealing elements (Fig. 4 and Fig. 5), which it strains, in an operational system, identical with that of a hydraulic cylinder running. The only problem which appears is that the friction force measured is given by the friction forces from the two sealings, at the same direction of motion of the rod; one of them is simulating the sealing when the rod gets in the cylinder and the other when it gets out. 2.3. The Determination of the Friction Force These two friction forces from the two sealings are, probably, not equal for the same direction of motion of the rod, but, surely, can be comparable. That is why it is assumed that the friction force, between a sealing and a rod, represents, approximately, half of the force measured at the rod by the force transducer. If it is considered the weight Gt of the rod suspended by the force transducer, and the friction forces from the two sealings Ff1 and, respectively, Ff2,, then the force measured by the force transducer, Fm, will be: Fm = Ff1 + Ff2 + Gt (1) Assuming that the two friction forces Ff1 and Ff2 are, approximately, equal, it may be concluded that the friction force in one sealing Ff, will be: Ff = Ff1 = Ff2 = (Fm Gt)/2 (2) Using these mathematical formulas the friction force values can be determined from one sealing of the rods of the hydraulic cylinders. 2.4. The presentation of the Experimental Device The experimental device, presented in Figure 6, operates in vertical position and need the mounting of the liner of the experimental device on the mobile rod of the hydraulic cylinder of one existing stand. This provides the motion of the cylinder liner, together with the double sealed bush, up and down. The rod of the cylinder, which passes through the double sealed bush, is immobile. In this manner, will be eliminated the risk of additional inertial force which can perturb the force transducer. In the schematic representation (fig. 7), it can be seen that the rod (1) is fixed and suspended on the upper bridge of the stand, by means of a threaded bush (2), by the force transducer (3), connected at the other side by the up catching stationary head (4) of the existing stand. Downside, the rod crosses the double sealed bush (5) assembled by means of a nut (6), from the used cylinder, by its liner (7). The cylinder liner is used just for supporting the double sealed bush and for making pace necessary for performing the operational strokes of the cylinder rod. Downside, the cylinder liner is connected to the mobile rod (8) of one the hydraulic cylinder, of the existing stand where it is mounted, by means of a connection part (9) and of a nut (). This connection provides the up and down motion of the liner together with the double sealed bush. The cylinder rod that passes through the double sealed bush is immobile, which eliminate the risk of additional inertial force perturbing the force transducer. In order to eliminate other additional forces (of binding, bowing etc.), all the conditions must be fulfilled. The measuring of the friction forces will be done in the going down motion of the bush. Fig. 6. The experimental device Fig. 7. Schematic representation According to the principle of operation of experimental device, implies its mounting on a vertical stand (Fig. 8). The mounting of the experimental device (1), on the existing bench (2), is made according to Figure 9. The existing stand (2), available in the Tribology Laboratory of INOE 2000 IHP, by plant hydraulic own (3), provides vertical movement in either direction, the movable rod (4) of the hydraulic cylinder of the stand and, also, the measuring of the work race through race/ stroke transducer (5). The device is manufactured starting from basic components (the cylinder liner, the cylinder rod etc.) of a classical hydraulic cylinder, produced by REXROTH Co., with rod diameter of 25 mm and piston diameter of 40 mm.

13 Fig. 8. The Existing bench test Fig.. The stand for friction force Fig. 9. Schematic representation the double sealed bush and, also, for avoiding the occurrence of unwanted mechanical binding /bowing. During the vertical motion of the bush, the operational pressure at the double sealed bush is continuously provided through a hydraulic hose, by an additional hydraulic installation (Fig. ). In Figure 11 is shown the scheme of the hydraulic and informatics links of the device. This consists of a manual pump Pm, which aspires the oil from the tank T and discharges under the pressure, by means of the unidirectional valve Ss, the pressurized fluid towards the 2 sealings of the bush by the hole existenting between them. The experimental device presented above was mounted on the previously mentioned stand existing in INOE 2000-IHP and was used in order to develop the first series experimental measuring in the sealing field. This will represent a real groundwork for the experimental research performed in our country in the field of hydraulic sealing. 2.6. Experimental Results Fig. 11. The scheme of the hydraulic and informatics links 2.5. The principle of Operation The measurement of the friction forces is performed at the downward stroke of the double sealed bush, for ensuring the alignment of the rod and The experimental research is in its beginning phase. In the first series of measuring, it was tested one type of seal, named U sleeve, for one kind of material [5] and only for the sealing of the rod, with the diameter of 25 mm, but not for piston. There are provided a lot of experimental measuring regarding the different kind of seals, for different materials, working pressure and, also, for different working velocities. One experimental result is given in Figure 12. This experimental research was made in order to measure the friction force which appears between the cylinder rod and the sealing system.

14 Fig. 12. The graphical variation for the main parameters The measurements were made for the working pressure between 0 and 200 bar, in pressure steps for 50 bar, for theoretical working velocities: 0.0 m/s. For each measurement were obtained the variation diagrams for the following parameters: the friction force; the working pressure; the working stroke and the variation of working speed. In Figure 12 it is presented a complex graphical result for these 4 parameters, which includes variation for 150 bar nominal pressure and theoretical working speed of 0.0 m/s. The medium values of the measured forces and the calculated friction forces for one seal, are presented in Table 1, for different nominal pressures. In the second part, the paper presents the development of an original experimental device, designed to analyze real hydraulic rod seals. The device is designed to precisely evaluate the friction force variation in the seals, versus time or operational stroke. The sealing fluid pressure and the working stroke are, also, precisely controlled and measured. There were obtained a lot of interesting results which confirmed the data known from technical references [1]. The stand creates the possibility to develop a wide range of experiments and allow to know the real evolution of the friction forces. Table 1. The measured and calculated values Nominal 50 0 125 150 200 Pressure p [bar] Measured Force 28 48 74 82 88 6 Fm [N] Additional weight G [N] Friction Forces 9 19 32 36 39 48 Ff1 [N], [Ff = (Fm G)/2] REFERENCES 3. CONCLUSION The paper presents the technical solutions and the experimental stand for determining the friction forces from the actual hydraulic rod seals of the hydraulic cylinders. First, there are presented some technical problems specific for the mobile sealing, used in construction of the hydraulic cylinders 1. Cristea V. et al., 1973, Sealings, Technical Publishing House, Bucharest (in Romanian). 2. Cristescu C., Drumea P., 2008, Mathematical modelling and numerical simulation of the tribological behaviour of mobile translation sealing subjected at high pressures, Hidraulica, no. 2, pp. 26-33. 3. Fătu A. et al., 2009, Development of an Experimental Device to Study Real Hydraulic Rod Seals During Fully Transient Operation, 8th EDF-LMS Poitiers Workshop, Futuroscope, France, October 5. 4. Drumea P. et al., 2009, Theoretical research on the development of an experimental device to evaluating the friction forces between seals and rods of hydraulic cylinders. In: Proceedings of The 17-th International Saloon HERVEX-2009, Calimanesti-Caciulata, Romania, 18-20 November, pp.37 45. 5. Fatu A. et al., 20, Evaluation of the elastomeric hyper elastic behavior a U-cup hydraulic rod seal, Hidraulica, No. 3, pp. 41-48.