�tliis force, so that we can distinguish them as springs for tension, thrust, bending or torsion. Bending- and torsion-springs are most often of metal, but also sometimes of wood ; for the thrust- and (less often) the tension-springs india-rubber and other organic materials are much used.
The ease with which the most various forms can be given to springs permits them often to have a form which, considered as a whole, is adapted for working under a force -application quite different from that for which the cross-section of the material from
which they are made would seem to adapt them. The helical spring (Fig. 131) for instance is closed, when used as a whole, by a force in the direction of its axis, in other words, is employed as a tension-organ ; as regards its cross section it is suited for working with torsion.* If the same spring be so formed that its coils are not in contact in its normal position it can be used as a whole as a pressure-organ ; for this pur- pose, however, it must be enclosed in a suitable case to prevent lateral deflection. A spring which is itself adapted for tension can be, and is
used as a whole as a torsion or wrenching spring in the well- known form shown in Fig. 132.
Springs, both simple and compound, working under a bending force are familiar in their many applications to railway work, as are several kinds of torsion-springs, and among the pressure-springs one of a peculiar kind (Strebe-feder) the steel ring used between the tread and tyre of the wheel in the proposed " spring wheels " of Mr. Adams.*)*
Springs are very well suited for supplying the force-closure of
- See Der Ccmstructeur, 3rd Ed. p. 59.
t See Colburn's Locomotive Engineering, pp. 99, 100, etc.