THE IMPORTANCE OF A SUITABLE LUBRICATION IN THE REPEATED TIGHTENING OF SCREW CONNECTIONS FOR PLANETARY GEARBOXES

Planetary gearboxes generally consist of a ring gear, or gear body, which is connected with the input and output flanges by means of several screws, equally spaced along the diameter. These screws must provide axial preload between the parts, allowing the assembly withstanding the breakaway torque given by the difference between the output and the input torques applied to the gearbox. For a given screw geometry, the axial preload can be calculated, only if the friction coefficients in the thread and in the underhead are known. Since the tightening torque is the only parameter that can be usually controlled during the assembly and the maintenance procedures, it is really crucial to know the aforementioned friction coefficients, which depend on the different lubrication and wearing conditions of the screw joint. It is known that the addition of a lubricant modifies the tribological behaviour of the joint, thus the wearing evolution of the surfaces across the repeated tightening operations. The present work tackles both these aspects, by means of an experimental campaign, based on an ad-hoc specimen that measures the screw preloading force.