Fig. 56 is the drawing of a helical spring of round wire, the side view only being drawn, as this is all that is necessary to give all the dimensions. To draw the spring we must know the pitch, the diameter of the wire, and either one of the dimensions A, B, or C. If A is given, we subtract from A the diameter of the wire to find B; and if C is given, we add to C the diameter of the wire to get B; then, knowing B and the pitch, we can draw the helix (which is shown in dotted lines) exactly as the helix was drawn in Fig. 52. This is the helix formed by a line in the center of the wire. Now draw a series of circles with centers on this helix and of a diameter equal to the diameter of the wire. Smooth curves drawn tangent to these circles, as shown in the figure, will give the projection of the spring. Fig. 57 shows a helical spring of square wire. The drawing of this is simply the drawing of four helices, starting from each of the corners of the square ACML; this square being the cross section of the wire of which the spring is made. AH four of the helices have the same pitch, equal to AB for, since the square BDPN is the same as ACML, the distance CD is the same as AB; and since the points L, M, N, and P are vertically under A, C, B, and D, respectively, the distance LN is equal to AB, and MP is equal to CD. The helix AFB has a diameter equal to that of the circle IE, and is drawn by dividing the circle IE and the pitch AB, as in Fig. 52; and the helix CGD, having the same diameter as AFB, is drawn by dividing circle IE and pitch CD. The helix LHN has a diameter equal to that of the circle KR, which is IE minus twice the thickness of the wire, and is drawn by dividing up the circle KR and the pitch LN; and the helix MJP, having the same diameter as LHN, is drawn by dividing circle KR and pitch MP. Since the two circles are drawn about the same center, the divisions on circle KR can be found by drawing radial lines from the points of division on circle IE. The vertical lines drawn from the divisions of the pitch AB can be used for the divisions of LN; and those drawn from divisions of CD can be used for MP.
Fig. 55. Diagram of Right-Hand Triple Helix.
Fig. 56. Accurate Diagram for Helical Spring of Round Wire.
Fig. 57. Accurate Construction for Helical Sprint of Square Win.
After the four parallel helices are drawn, it is necessary to study the drawing carefully, to decide what lines will be visible (full lines) and what invisible (dotted lines). Dotted lines should be used from H to Jt N to P, etc., and full lines from F to G, B to D, etc. The line ST is the end of the spring, and consequently any part of a helix which goes outside of that line should not be left on the finished drawing. It is better, however, to draw in the whole of the square ACML, and to draw the helices starting from A to L, in order to draw those parts of the same helices which lie to the right of ST. The parts to the left of ST are shown in the figure by light, dotted lines to indicate that they are construction lines, and not a part of the projection of the spring itself.
Drawing for Spring of Round Wire.
To draw springs by the method just explained involves considerable work and would consume a great deal of time if many were to be drawn; therefore, in working drawings, the draftsman commonly uses a conventional method. This conventional drawing is similar to the true projection, except that straight lines are used in place of curved lines. Fig. 58 shows the conventional drawing of a spring of round wire; and Fig. 59, of a spring of square wire. Springs are often shown in half-section, as in Fig. 60, this method involving less work than the method of Figs. 58 and 59.
Fig. 59. Conventional Drawing for Square Wire Spring.
Fig. 60. Half-Sections of Round and Square Wire Springs.