Лесотехнически Университет

Summary

During DAAD 2013 research stay of the first author in Rosenheim Faculty of Wood Technology, joint research started into dynamics and strength of bandsaw blades. This paper is, in fact, the first one under the above title resulting from cooperation with Professor Scholz and his team. They have dealt with problems concerning dynamic instability of new sawblades with special crowned tooth shape. Studies had been carried out under various blade-tensioning and guide-system conditions. Now, FE analysis has been involved by means of Autodesk Inventor software. An approach has been proposed and developed for FE modelling the complicated buckling of bandsaw blades by substituting it with a sufficiently equivalent eccentric compression. Research results and conclusions on the reasons for the instability of blades with crowned tooth shape are presented.

Summary

The problem of the internal forces and stresses in the blade of a horizontal band-saw is statically indeterminate in the case of pre-tensioning by a spring. After our solution in a preceding article, in this paper the internal forces and stresses in the different parts of the blade are considered. Some remarks are addressed to the interpretation of such stresses in the bandsaw references reviewed. As well, stresses are analyzed relating to bending the blade over the wheel contours. Additional stresses are also outlined: at the guide rollers, other more bending stresses, as well as stresses due to torsion of the blade, etc.

Summary

As a continuation from a previous paper, an original model is proposed for the variable tensile stress in the blade (the saw band) which girdles the two wheels of a band-saw. The case of a spring mechanisms for tensioning the saw band is treated. The internally statically indeterminate problem of the closed band’s contour is solved in a way as required by the course of Strength of Materials. At an instant position of the blade, the tensile stress in all the cross-sections along the band is searched for, taking into account: the assembly pre-tension; the cutting force; the bending of the saw band at the wheel periphery; the heating; the centrifugal inertial forces due to the curvilinear blade’s motion as a new point in the development of the model proposed now. Another new, basic point is that the spring is not ideally stiff but “normal” now. An equation for the tensile internal force at an „opening“ in the band is deduced by involving differential and integral analysis. The equation is solvable by iteration. In case of zero cutting force, the L'Hôpital's rule is applied.

Summary

The closed contour of a band-saw blade is a bright example to the theme of the internally statically indeterminate schemes in the Strength of Materials syllabus of the students in Wood Technology at Sofia University of Forestry. An original model is proposed aimed at strength and deformation calculations of the blade which embraces the two wheels of the band-saw. The internally statically indeterminate structure problem is solved in a way in which the students are taught. At an instant position of the blade, the tensile stress is determined at various blade’s points taking into account the assembly pre-tension, the cutting force, the bending of the blade at the wheel periphery, and the heating. The stress at any point, while traveling onto the blade contour, varies and follows an oscillogram with complicated, random variations. Under such conditions, the blade is threatened by metal fatigue. A basic, approximate oscillogram has been theoretically obtained which will be complicated with additional waves in next studies. Application of the Integration of Damage Differentials (IDD) method is envisaged for fatigue life assessment as developed in the Faculty of Forest Industry. As well, the study done is envisaged to continue into possible diploma and doctoral theses of the students instructed also by professors in next school subjects. Then, additional dynamic phenomena can be involved in the calculation model, and loading spectra under different regimes of cutting can be built, etc.

Summary

ABSTRACT

In traditional textbooks in woodworking machines, a model is suggested for strength and deformation calculation of a saw-blade frame which is superfluously simplified: each of the two beams of the frame is considered as a simple one on two articulated supports that are swivels of the vertical frame’s stands. However, the joints between the beams and the stands are not articulated but rigid. Therefore, the saw-blade frame should be definitely considered namely as a frame with rigid joints. In the textbooks in Strength of Materials, there is a classical theme about solving internally statically indeterminate closed frame with rigid joints. The author has always stated while teaching and writing textbooks that the saw-blade frame is a very suitable example to the classical theme mentioned. In the first author’s textbook in Strength of Materials, the solution of the statically indeterminate scheme of a similar frame has been demonstrated based on the strain-energy methods. Following the classical manner, the internal bending moment has only been envisaged while the strain energy is composed. A corresponding calculation scheme has been created and adopted in a book of Professor Obreshkov. Now, a more precise calculation model is proposed taking also into account the compression of the stands while the strain energy is formed. Correspondingly, deformational shortening the stands is involved. Besides the solutions of the statically indeterminate problem, an equation has been deduced for the deformational shortening the distance between the mean cross sections of the two beams. The paper throws light on the question of how well-grounded the simplification is to consider each of the two beams as a simple one on two articulated supports.

Summary

An original model is proposed for determination of the statically indeterminate tension (internal tensile force) in the blade (saw band) of a horizontal bandsaw with a tensioning spring mechanism. The statically indeterminate problem is solved in a way missing in the bandsaw references but required by the Strength of Materials science: based on a deformation compatibility equation. In the case considered, in that equation the spring parameters are also involved. The model proposed is a generalization of a published previous model (Stefanov 2013) regarding a vertical bandsaw blade tension.

Summary

As a continuation of two accompanying papers of ours, in this paper calculation is done relating to operational stresses in blades of horizontal mobile bandsaws as a part of their operational characteristics. In the case of tensioning the blade by a spring mechanism, when the problem of the tensile forces and stresses is statically indeterminate, our solution (from the first accompanying paper) applies. This requires iterations which are programmed in an excel-file. In it, a number of formulas for stress calculation are programmed which are derived in our second accompanying paper. Several types of stress-components, and stress conditions, are involved. A series of additional computational results are obtained automatically with varying the input data in the excel-file. Stress oscillograms have been drawn, as well. They show that, under equal other conditions, the horizontal bandsaw blade is more threatened by fatigue than a vertical bandsaw blade.