The influence of lubrication on the frictional characteristics of threaded joints for planetary gearboxes

Abstract

Planetary gearboxes generally consist of a ring gear, or gear body, connected with the input and output flanges by means of several screws, equally spaced along the diameter. The ring gear is manufactured with steel, whereas the flanges are usually made of cast iron. These screws must provide axial preload between the parts, allowing the assembly withstanding the breakaway torque given by the difference between the output and input torque applied to the gearbox. For a given screw geometry, the axial preload can be calculated, provided that the friction coefficients in the thread and in the underhead are known. Most often, the tightening torque is the only parameter being controlled during assembly and service operations. Hence, it is mandatory to know the friction coefficients of the joint. These depend, among others, on the hardness, roughness and texture of the mating surfaces, as well as on the lubrication state of the joint. In fact, the addition of a lubricant modifies the tribological behavior of the joint, thus the wearing evolution of the surfaces across repeated tightening operations. The present work tackles the following two aspects: (i) the characterization of the preloading force–tightening torque relationship on the actual component by means of a dedicated specimen, (ii) the evaluation of the influence of lubrication on the evolution of the frictional characteristics of the joint across several re-tightening operations. The present work has been carried out by means of both numerical finite element analyses and experimental stress analysis techniques.

Tribological Properties of Connecting Rod High Strength Screws Improved by Surface Peening Treatments

“…The number of replications was set to ten with regard to the trials in dry conditions, which were performed first; therefore, the three-factor plan involved an overall number of 180 tests. In the campaign in lubricated conditions, which, consistently with the outcomes in [9], exhibited a much lower scattering than that without lubrication; the number of replications was reduced to five. This number seemed to be reasonable to properly estimate the experimental uncertainty affecting the results, also based on previously published studies dealing with screw tribological assessments [9,11], in agreement with Standard [18].…”

“…In the campaign in lubricated conditions, which, consistently with the outcomes in [9], exhibited a much lower scattering than that without lubrication; the number of replications was reduced to five. This number seemed to be reasonable to properly estimate the experimental uncertainty affecting the results, also based on previously published studies dealing with screw tribological assessments [9,11], in agreement with Standard [18]. Therefore, 90 further trials were run under lubrication, which led to the overall number of tests of 270.…”

“…Previous studies from the same group [9][10][11] have indicated that the bearing and the thread frictional coefficients have a strong impact on the achievable preload for a fixed tightening torque being controlled by a torque wrench upon fastening. In addition, specifically regarding con-rod screws, it must be emphasized that the usually recommended fastening procedure involves a retightening, meaning that they are tightened, untightened, and retightened again.…”

A Methodology for the Lightweight Design of Modern Transfer Machine Tools

“…The terminal portion of the shank and the "threaded" hole on the rotating table have been joined by means of a bonded contact. The analysis has been divided into two steps: at the first step, a preload of Fi = 70 kN has been assigned to each screw: this preload is calculated, based on a tightening torque T = 106 Nm and oiled surfaces, μm = 0.10 [20][21][22][23], as for the company's assembly specification. The displacements retrieved at this step are shown in Figure 12a.…”

An experimental study on the relationship between torque and preload of threaded connections

“…C Friedrich and T Hoernig 18 conducted an experiment of tightening screws with nut components made from aluminum alloys to analyze their frictional behavior and gave an advice for thread reinforcement. M De Agostinis et al 19 conducted a research to evaluate the effect of lubrication on the evolution of the frictional characteristics of the joints across several re-tightening operations by means of both numerical finite element analysis and experimental stress analysis techniques. Nassar et al 20 studied the influence of the thickness of the coating on the friction coefficient and torque coefficient of the bolts by experiments.…”