Design of an adjustable constant force mechanism based on graphical construction

This article presents the design, analysis, and experimental validation of an adjustable constant-force mechanism. A graphical interpretation of the work-energy equation enables the intuitive generation of kinematic schemes for new, simple one-degree-of-freedom mechanisms that employ a single spring and rigid links. A characteristic feature of these mechanisms is a moving link equipped with two perpendicular linear guides. One of the newly developed structures is analyzed and experimentally validated in detail. Its key advantages include geometric simplicity, ease of manufacturing and assembly, and the capability to smoothly adjust the output constant-force value. The study provides a comprehensive theoretical analysis, including the determination of joint reaction forces, as well as an assessment of how deviations in the pretension and spring characteristics from the manufacturer-specified values affect the accuracy of constant-force generation. The measured constant force and its variation within the operational range are compared with theoretical predictions, and the sources of discrepancies are partially identified and explained. Experimental measurements confirmed that the performance of the proposed mechanism is fully comparable to constant-force mechanisms reported in the literature. The average variation of the constant force is 3.7%, with a force-adjustment ratio equal to 5. The concept of reduced force is applied to estimate friction forces, which were shown to be less than 2.1% of the output force. Consequently, the mechanism can be incorporated into systems intended for gravity balancing and reduction of energy consumption. A direct application example is a counterbalance mechanism for the tabletop in manually adjustable-height desks.