Mechanics of Wood Machining by Etele Csanády & Endre Magoss

Author:Etele Csanády & Endre Magoss
Language: eng
Format: epub
ISBN: 9783030514815
Publisher: Springer International Publishing

The pressure produced by the rollers causes plastic deformation and permanent residual stresses in the blade material. The art of roller tensioning is to introduce these stresses in a manner that increases the stiffness and natural frequency of the blade and improves the cutting accuracy and feed speed of the saw. Different kinds of stresses may be present in a saw blade, such as initial stresses due to tensioning, rotational stresses and thermal stresses. These stresses lie in the saw blade plane and they are called membrane stresses. During deformation of the blade, these stresses do mechanical work and this work counteracts to a transverse displacement under a given external load. Therefore, an increase in the work of deformation of the stresses stiffens the saw blade. This contribution to the blade stiffness is very important with thin saw blades since the deformation work of the membrane stresses increases relative to the work done by the bending stiffness. (Mote 1965).

It is well known that thin-walled structures are very sensitive to compression stresses causing buckling. Thermal stresses are especially dangerous because they dramatically decrease the stiffness of the tips of the teeth. These stresses are the dominant cause of saw instability. Increasing the rim temperature decreases the second and higher nodal diameter natural frequencies. As a consequence, the critical speed is reduced. Tensioning should counterbalance the compressive stresses. Therefore, tensioning is particularly significant for thin and large diameter saws where membrane stress effects on saw stiffness are large when compared to the bending stiffness.

Another important factor in band saws is the blade position on the wheel. Roll tensioning of the saw blade has been shown to significantly improve the precise positioning of the saw blade on the wheels. A stable blade is desirable in a high production band mill allowing high feed rates with minimal blade movement. Precise positioning of the blade due to tensioning does not require significant amounts of overhang, which have been shown to reduce the stiffness of the cutting edge of the blade. Without tensioning, too little overhang would have the risk of the blade moving back on the wheel and damaging the guides.

The roller crown is the fundamental part of any tensioning machine. During the rolling process, the rolling track should be compressed to establish tensile circumferential stresses around the rim in the tooth zone.

Depending on the shape (profile) of the roller, different εt/εr strain ratios can be achieved, where εt is the circumferential strain along the rolling path and εr is the radial strain perpendicular to the rolling path. This strain ratio depends on the crown radius or on the width of the roller and on the depth of the rolling track. Their relationship is shown in Fig. 6.6 (Stakhiev 2001). In all cases the strain ratio εt/εr decreases with increasing rolling depth.

Fig. 6.6Relationship between the depth of rolled track yd and the ratio εt/εr: 1—ρ1 = 350 mm; 2—ρ1 = ∞, b1 = 10 mm; 3—ρ1 = 210 mm; 4—ρ1 = 105 mm; 5—ρ1 = ∞, b1 = 6 mm; 6—ρ1 = 35 mm; 7—ρ1 = ∞, b1 = 4.5 mm

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