Influence of friction in joints on force characteristics of scissor rolling bridge jack

The object of the study is the operation of a scissor rolling bridge jack used for vehicle maintenance under service conditions. The subject of the study is the influence of friction in the joints and the geometric parameters of the scissor mechanism of the rolling bridge jack on the force in its drive. The paper analyses the kinematic and force schemes of an automotive scissor rolling bridge jack with a hydraulic drive. The mechanism was calculated taking into account the friction angles in the joints of its links. Based on the force analysis, an analytical relationship was obtained between the load applied to the crossbeam of the scissor rolling bridge jack and the axial force acting on the rod of the hydraulic cylinder. Force characteristics were determined as functions of the friction coefficients in the mechanism joints and the specified geometric parameters. A skeleton and a parameterized three-dimensional model of the scissor rolling bridge jack were developed in the Creo Parametric 12 environment, which made it possible to perform computer simulation of the mechanism operation while taking into account the specified geometric parameters and friction in the joints. The simulation results yielded force characteristics that are in qualitative and quantitative agreement with the theoretical calculations. The parametric model made it possible to assess the influence of the geometric parameters of the mechanism, the position of the drive hydraulic cylinder, and the friction coefficients on the drive force. For the adopted quantitatively defined structure, interferences between the mechanism elements in the folded position occur at a link arm ratio of L/(2l) >= 2.76 and an offset of x >= 20 mm. Under unfavorable operating conditions, the contribution of friction (μk′ = 0.42) in the joints of the scissor mechanism may amount to 25-30% of the total force on the hydraulic cylinder rod.