Friction effects on the load capacity of a column and a hydraulic cylinder

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Abstract

Over the last years, a better comprehension of hydraulic cylinder performance is observed to be a prime objective of many owing to demanding applications. By observing these hydraulic cylinder applications, we see currently proposed models and design criteria do not take into account real factors, and further there are misalignment and boundary conditions in the end supports, mechanism interaction, dust, clearance and imperfections. Since these boundary conditions can develop progressive moments, the ideal simple supported bi-articulated configuration of the actuator is no longer valid, and virtual clamped configuration appears with friction moments that substantially modify the buckling analysis. In a previous work, misalignment effects were studied and discussed. In the present work a study of the influence of friction moments when misalignment effects are deliberately left aside is carried out. In order to separate these two phenomena (friction and misalignment) boundary conditions in a column are analysed. From this analysis, a theoretical and experimental work has been carried out for columns and an actuator characterizing the critical factors that cause the collapse. The aim of this paper is to describe the behaviour of actuators under load capacity with experimental validation when friction moment is taken into account and becomes an unknown variable. Experimental and theoretical results point out the importance and influence of the friction effects in columns and hydraulic cylinders. Then, it is recommended that knowledge of the hydraulic cylinder application to predict its load capacity owing to that direct extrapolation of results from current theories and criterions could lead to incorrect estimations.

Introduction

Hydraulic cylinder applications are extremely wide and can be found, among others, in industrial, mobile, agriculture and public work machinery. A good comprehension of hydraulic cylinder performance is necessary to improve its characteristics under working conditions. From hydraulic cylinder manufacture point of view, design of a hydraulic cylinder for buckling load capacity is based on Euler's criterion, and others of this kind, with a safety factor, assuming that the cylinder is an ideal column under concentric axial load. By observing the conventional study of hydraulic cylinders, it seems reasonable to consider them as a bi-articulated configuration assembly, which means that this actuator is under an axial load and ideal boundary conditions at the end supports: no friction. However, this study has to be done taking into account the boundary conditions of hydraulic cylinder applications.

Currently proposed models and design criteria do not consider real factors that normally appear, which include among others friction on the supports or elements in contact, the actuator's own weight, clearances and imperfections in the connection piston rod and cylinder tube. Since any of these factors can develop progressive moments, which when combined with the axial load tend to facilitate the generation of the collapse of the cylinder, they must be considered. Experimental results confirm that this theoretical and ideal interpretation has no correspondence with reality and point out that friction in the end supports has an important influence.

Taking into account all these considerations, it is necessary to develop a new theoretical model capable of integrating the influence of the generated moments in both articulations of the hydraulic cylinder as a consequence of an interaction between hydraulic cylinder and the mechanism. In a previous work ([1]) misalignment effects were considered. In the present work, in order to separate misalignment and boundary conditions in the end support effects, the first study concentrates on columns. Hence, a theoretical model is developed to evaluate the influence of the generated moments in both articulations of a column whereupon misalignment effects are left out.

The remainder of the paper is subdivided as follows: Section 1 presents the introduction to the work carried out in this paper. Section 2 presents the current theories of buckling and load capacity in hydraulic cylinders and columns. From the studied theories, Section 3 develops the buckling load capacity theoretical model from a friction approach based on the principal factors of a hydraulic cylinder and a column. Results of virtual clamped and friction moments in columns versus axial load are presented through the analytical model. Section 4 details the experimental work by designing, constructing and using a hydraulic cylinder and column test bench for buckling purposes. Results of bending moment distribution along the rod mark the existence of a virtual clamped moment owing to friction effects. Section 5 presents experimental and model results for a specific hydraulic cylinder and three columns. Results of the column axial load versus column diameter are obtained through the analytical model and are compared with those obtained through experimental tests. For a hydraulic cylinder, results of virtual clamped moment versus friction moments from analytical model and experimental tests are also presented and discussed. Finally, conclusions in Section 6 for the load capacity by using a friction approach are stated.

Section snippets

State of the art

Since Euler to von Kármán, the theoretical and experimental approaches of the rigid columns have been carried out with ideal boundary conditions in the end supports. In ideal conditions, without friction in the supports, the cylinder will hold up a compression load. The cylinder is simply supported and bi-articulated. But under working conditions which involve dust, tight fit between pin-end supports bushing and wear of the union elements can generate (because of interaction of the mechanism)

Buckling load capacity theoretical model of a hydraulic cylinder: friction approach

A theoretical model for load capacity analysis of a hydraulic cylinder was presented in a previous work ([1]). This model described the effects of all those parameters involved in these phenomena, e.g. guide rings, seals in piston rod and cylinder, supporting elements related with linked mechanism layout, eccentricities in application of the load and misalignments. However, the intricate aspects of friction moments in mounting supports had been deliberately left aside.

The general objective of

Experimental work

An experimental methodology was prepared for testing actuators and columns. Ten tests have been carried out for the actuator and columns.

Columns

The diameters of the columns have been 22, 25 and 30 mm (Table 1). Fig. 9 shows results of axial load of three column diameters (22, 25 and 30 mm) through Euler's criterion (simple supported and clamped), experimental results for two different pin diameters (22 and 25.6 mm) and axial rod results through the previously defined model (25.6 mm pin diameter and 0.1 friction coefficient).

The limit of the axial load in the columns is established by Euler's criterion, where the maximum is for clamped

Conclusions

Currently proposed models and design criteria do not consider real factors that commonly appear in hydraulic cylinder applications. Reliability of frictionless models or formulas (including Standards) is questionable when experimental and theoretical results are compared. Friction moments in the joint of the end supports of an actuator and a column appear to be critical and characterize the load capacity.

In order to separate friction and misalignment effects, an analytical model and

Acknowledgments

The authors would like to acknowledge the support of EU by co-financiering this work within the Sixth Framework Programme. Project: NMP FP62002-NMP-2-SME “New Design and Manufacturing Processes for High Pressure Fluid Power Products” (2004–2008). Project acronym: PROHIPP. The authors would also like to extend their acknowledgments to all partners who have taken part on the project, but especially Pedro Roquet (Spain) for providing the necessary support to accomplish this work.

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