Simple Frictional Analysis of Helical Buckling of Tubing

Previous analyses of helical buckling of tubing have not considered frictional forces. In this paper, differential equations are derived and solved for two simplified cases of interest: downward motion of the tubing-e.g., when buckling occurs during the landing of the tubing-and upward motion of the tubing-e.g., when buckling occurs as a result of thermal and differential pressure loading subsequent to landing. While somewhat more complicated than the conventional frictionless buckling equations, these solutions are still suitable for hand calculations. These solutions, however, do not represent general solutions to buckling with friction. Load reversals and lateral frictional forces add complications that would require computer analysis. Several examples are examined to evaluate the relative importance of friction, which has a significant impact on tubing length change for loaded cases. For instance, the choice of a conservative value for the friction coefficient may allow the solution of a difficult seal-design problem by reducing a large predicted length change. Friction also has an important effect on set-down loads. Frictionless buckling calculations do not give conservative results for this problem.