Experimental Study of the Dynamics of Weakening of Retention Force of Movable Wooden Structures in Friction Gripping Devices

When using friction gripping device with a screw drive to move designs from wood there is relaxation of tightening force of the screw with time due to rheological characteristics of wood. Weakening of tightening leads to a reduction of friction forces, holding the floating structure, which can lead to failure and drop of structure. To ensure reliable holding of wooden construction by gripping device within the specified time, prior increase in torque taking into account dynamics subsequent weakening is required. The article presents description of experimental setup to study the rate of weakening of the torque of the screw clamp and development of elastic deformation of samples of wood and plywood of different thickness with clamps of different hardness. Dependence of elastic strain and associated weakening of tightening force from time which samples stay under load, is revealed. The dependence has general nature for all the samples of wood and derivative materials. Regression analysis of the data and graphs of weakening the tightness of the clamps and development of elastic deformations is performed for each experiment. It is established that approximation to exponential function reflects the dependence of attenuation of torque settings on time in the interval up to 24 hours more accurately, than exponential function. To identify factors influencing the rate of weakening of the torque, regression analysis is made. The dependence of the rate of weakening of the torque by on stiffness of the clamp, thickness of wooden construction in the place of installation of gripping device and of the modulus of elasticity of wood is revealed. With the increase in rigidity of clamp and size of cross section of structure, weakening speed increases. Method of determining the rate of increase in force of pre-tightening of friction gripping device with screw drive to move wooden structures is suggested.