Misalignment effects on the load capacity of a hydraulic cylinder

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Abstract

Analytical and experimental investigations of typical hydraulic cylinders have indicated that their load capacities are significantly different from those obtained from simple buckling analysis of idealized systems. In any case, an increase in the friction coefficient at the restrained ends changes the actuator's limit load, while an increase in the initial maximum deflection (initial misalignment) decreases the limit load. A common practice of most cylinder manufacturers is to use a safety factor (between 2.5 and 4) to determine the service load after the critical load (buckling) is obtained by simple analytical procedures treating the cylinder as a perfect stepped column. The intricate aspects of friction effects have been deliberately left aside in this present work. Nevertheless, friction and interaction between mechanism and actuator in the buckling characteristics will be presented in the ongoing paper, which will follow this work. Authors know that, in a real system, the cylinder tube–rod interface is not rigid. Due to the flexibility of guide rings and clearances between components, misalignment (an angular deflection which increases with increasing axial load) exits at the interface. When initial imperfection angle exists, there is no sudden buckling. Then, stresses and deflections increase with increasing load. After repetitive use, the tolerance between the parts will become larger, consequently increasing the initial deflection, which has been proved to considerably decrease the load capacities of the power cylinders. From this analysis, a theoretical and experimental work has been carried out in order to show the advantages and disadvantages of the current design methods, characterizing the critical factors that cause the collapse and proposing useful design criterions. The present work aims to describe the behaviour of actuators under load capacity with experimental validation.

Introduction

Hydraulic cylinder is one of the most used components in the hydraulic industry. Traditionally, hydraulic cylinder manufactures design them for a buckling load capacity by using Euler equation and safety factor, assuming that the cylinder is an ideal column under concentric load. Euler's criterion and others of this kind do not take into account real factors that normally appear in different cylinder applications, 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 concentric load, tends to facilitate the generation of the collapse of the cylinder, they must be considered.

In the majority of real applications, cylinder external load is hardly concentric. Perfect alignment between piston-rod and cylinder tube does not exist causing misalignment. Misalignment can mainly be attributed to geometric tolerances from the manufacturing processes, seals and wear of guide rings which will increase with working cycles and other factors, like the action of the pressure inside of the cylinder tube.

The remainder of the paper is subdivided as follows: Section 1 presents the introduction of the work carried out in this paper. Section 2 presents the current theories of buckling and load capacity in hydraulic cylinders. From the studied theories, the analytical model is developed based on the principal factors. Section 3 develops the buckling load capacity theoretical model from a misalignment approach. Results of initial imperfection angle versus fluid pressure are presented. Section 4 details the experimental work by designing, constructing and using a hydraulic cylinder test bench for buckling purposes. Section 5 presents an application test. Results of the maximum rod stress and critical and limit load are obtained through the analytical model. Experimental results are compared with those obtained through the analytical model. Finally, conclusions for the load capacity by using a misalignment approach are discussed.

Section snippets

State of the art

A hydraulic cylinder cannot be considered as a column. For the last 50 years, technical papers related to load capacity of hydraulic cylinders have been prolific. Hoblit [1] is probably the first author who proposed the importance of calculating the instability of the critical and limit load taking the hydraulic cylinder as a structural element. Ideal buckling, Euler's criterion and others of this kind, are not appropriated to calculate the load capacity of a hydraulic cylinder. He demonstrated

Buckling load capacity theoretical model: misalignment approach

The majority of the proposed models fails to consider the possible influences of the actuator working under real conditions involving kinematics of the mechanism. The general objective of the present work is to obtain a new theoretical model for load capacity analysis of hydraulic cylinder capable of describing the effects of all those parameters involved in these phenomena such as guide rings, seals in piston-rod and cylinder, supporting elements related with linked mechanism layout,

Experimental work

A testing phase is also taken into account by bi-articulated hydraulic cylinders. There is no doubt that the imperfection angle owing to misalignment is an important factor. However, mountings friction torques is also an important factor. In order to avoid the camouflage of both phenomena jointly misalignments and mountings friction torques—an experimental work has been conducted in this present work to show separately the incidence of misalignment on the load capacity of hydraulic cylinders.

Specific unit of actuator

Experimental tests of cylinders with dimensions 50×30×502 mm (Table 3) have been carried out till collapsing them applying an axial load. The cylinder is articulated in both sides. In this particular cylinder, the own weight of the cylinder tube including fluid is 100 N/m and of the rod is 60 N/m.

The actuator critical load (buckling instability) is plotted out in Fig. 11. This load is calculated by taking the determinant of the coefficient matrix of Eq. (14). The determinant becomes equal to zero

Conclusion

In general, a common practice of most cylinder manufacturers is to use a safety factor to determine the service load and the critical load (buckling). Their values are obtained by simple analytical procedures treating the cylinder as a perfect stepped column. Reliability of frictionless models or formulas (including Standards) is questionable when experimental and theoretical results are compared. The design should be based on knowledge and experimental work.

An analytical method based on

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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