[Fig. 89]. Double Helico-Volute.—This form, so far as the outlines are considered, is the opposite of [Fig. 88]. A compression spring of this kind has a very wide range of movement.
[Fig. 90]. Helical Spring.—This form of coil, uniform from end to end, is usually made of metal which is square in cross-section, and used where it is required for heavy purposes
[Fig. 91]. Single Volute Helix-Spring.—This is also used for compression, intended where tremendous weights or resistances are to be overcome, and when the range of movement is small.
[Fig. 92]. Flat Spiral, or Convolute.—This is for small machines. It is the familiar form used in watches owing to its delicate structure, and it is admirably adapted to yield to the rocking motion of an arbor.
[Fig. 93]. Eccentric Rod and Strap.—A simple and convenient form of structure, intended to furnish a reciprocating motion where a crank is not available. An illustration of its use is shown on certain types of steam engine to operate the valves.
[Fig. 94]. Anti-Dead Center for Foot-Lathes.—A flat, spiral spring (A), with its coiled end attached to firm support (B), has its other end pivotally attached to the crank-pin (C), the tension of the spring being such that when the lathe stops the crack-pin will always be at one side of the dead-center, thus enabling the operator to start the machine by merely pressing the foot downwardly on the treadle (D)