2.3 Tensioning

Tensioning is the intentional introduction of a favorable residual stress state in a saw through plastic deformation by hammering or rolling, and through local beating. Tensioning stresses have been introduced during saw manufacture in the heat treatment process [Neunlann, Varhanicek 1970: McKenzie 1971: Borovikov, Orlov 1974].

Two indirect methods of residual stress evaluation tire possible: (a) measurement of saw natural frequencies and critical speed and (b) measurement of saw stiffness. The first determines the influence of initial stress on the saw natural frequencies and critical speed or the stability criterion function itself. The second determines the effect of initial stress on saw stiffness ha specific modes of deformation. Dugdale [1968] and Szyntani and Mole [1977] showed that the approximate modal stiffness can be used as a measure of the tensioning effect on the saw natural frequencies and critical speed. The control of the induced tensioning stresses by the rolling or hammering process is the most important and difficult problem m the tensioning research field. the significant early investigations on hammer tensioning and tension measurement are those of Barz [1962] and Dugdale [1963a, b: i966a] where theoretical and experimental analyses are developed and compared.

The stress distribution resulting from tensioning is dependent upon the method of tensioning and degree of plastic deformation. The elastic-plastic stress analysis of the rolling process is especially complex and work is needed here. Optimal saw design and operation necessitates the introduction of specific initial stress stales and the determination of the initial stress states that actually exist in the saw blade. No research is available on the persistence of the initial stress state with cyclic loading. Are the tension valuations that are observed it1 saws caused by plastic deformation of the saw in the process or by a creep relaxation of the original stress state? Classical results indicate creep is unlikely because the stress state is not large and the blade temperatures are not usually high. Preliminary research findings by the authors on saws that were laboratory and production tested show no tension decay. Two thermal tensioning methods have demonstrated potentional for application. In the first, initial stresses sufficient to cause yielding are induced by local heating using inductive heating or by subjecting the saw blade to an axially symmetric thermal shock by pulsed lasers over a concentric annular area [Hsu, Trasi 1976]. This is an alternative to the rolling or hammering. In the second method, thermal stresses insufficient for yielding are induced during the sawing process. This allows continuous adjustment of the state of stress and stability.

2.4 Clamping and floating collar concept

Tightening the central clamp shifts all the frequencies upwards. Inclusion of a viscoelastic damping layer between the saw blade and a clamping plate in turn, increases the saw blade damping and reduces noise [Cheremnykh, Chizhexskii mvi]. The collarless saw and the saw with collars that float on the arbor which are guided at the periphery is a relatively recent clamping concept [Belts 1969; Thrasher 1972; Mc Lauchlan 1972]. Kerf reductions from 9.5 mm to 2.5 mm or less have been reported possible [Betts 1969]. Theoretical studies on the stability of the floating clamping collar circular saw have been conducted by Mote [1975, 1977] and this work is continuing. These saws have critical speeds that are calculated m the same manner as clamped saw blades. An increase in the collar mass increases the collar resistance to acceleration and eventually the saw becomes effectively clamped. The floating clamping collar boundary always reduces all natural frequencies from those of a clamped saw with otherwise equivalent geometry, but the reduction may be small in the critical speed mode.