[Illustration: _Fig. 88. Helico-Volute Spring_ _Fig. 89. Double Helico-Volute_ _Fig. 90. Helical Spring_ _Fig. 91. Single Volute Helix-Spring_ _Fig. 92. Flat Spiral or Convolute_ _Fig. 93. Eccentric Rod and Strap_ _Fig. 94. Anti-Dead Center for Foot-Lathes_]
Fig. 88. HELICO-VOLUTE SPRING.--This is a form of spring for tension purposes. The enlarged cross-section of the coil in its middle portion, with the ends tapering down to the eyes, provides a means whereby the pull is transferred from the smaller to the larger portions, without producing a great breaking strain near the ends.
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).
A workman is able to select the right metals because he knows that each has some peculiar property which is best adapted for his particular use. These with their meaning will now be explained.
ELASTICITY.--This exists in metals in three distinct ways: First, in the form of _traction_. Hang a weight on a wire and it will stretch a certain amount. When the weight is removed the wire shrinks back to its original length.