Fig. 32.1 shows two alternative types of compression springs for which drawing conventions are used. Note that the convention in each case is to draw the first and last two turns of the spring and to then link the space in between with a long dashed dotted narrow line. The simplified representation shows the coils of the springs drawn as single lines.
Note: If a rectangular section compression spring is required to be drawn then the appropriate shape will appear in view (E), view (D) will be modified with square corners and the Ø symbol in view (F) will be replaced by □.
A schematic drawing of a helical spring is shown in Fig. 32.2. This type of illustration can be used as a working drawing in order to save draughting time, with the appropriate dimensions and details added.
Fig. 32.3. (A) Closed ends, ground, (B) closed ends, (C) open ends, ground and (D) open ends.
Fig. 32.4 shows a selection of compression springs, including valve springs for diesel engines and injection pumps.
15.2 Helical Compression Springs
The most familiar type of spring is the helical compression spring. In its most common form, it is made from constant diameter round wire with a constant pitch, as shown in Figure 15.5. Other forms are possible, such as the variable pitch, barrel, hourglass, and conical helical compression springs shown in Figure 15.6. In addition to variations on the pitch of the coil and diameter, the formation of the end is important. A variety of common end treatments is illustrated in Figure 15.7. Plain ends result from cutting the spring stock and leaving the spring with a constant pitch. Treatment of the end by some form of machining or pressing can facilitate alignment, and this is the purpose of options (b)–(d) illustrated in Figure 15.7, each of which adds to the cost of production of the spring and influences the performance. The end of a spring can also be formed to improve the connection to mating components by the incorporation of, for example, hooks and rings.
Figure 15.5. A constant pitch helical compression spring.
Figure 15.6. Some additional helical spring configurations. (a) Variable pitch. (b) Barrel. (c) Hourglass. (d) Conical.
Figure 15.7. Common styles of end treatments for helical compression springs. (a) Plain ends. (b) Plain ground ends. (c) Squared ends. (d) Squared ground ends.
The principal dimensions for a constant pitch helical compression spring are illustrated in Figure 15.8. The wire diameter d, mean diameter D, free length Lf, and either the number of coils N or the pitch p are used to define a helical spring's geometry and in associated analysis. The inner and outer diameter are useful in designing the mating and locating components. The minimum recommended diametral clearance between the outer diameter and a hole or between the inner diameter and a pin, according to Associated Spring (1987), is given by 0.10 D for D < 13 mm or 0.05 D for D > 13 mm.
Figure 15.8. Dimensional parameters for helical compression springs.
In addition to the geometrical parameters identified in Figure 15.8 for an unloaded spring, there are a number of useful lengths defined for a spring in use, as illustrated in Figure 15.9. The installed length is the length after installation with initial deflectionδinitial. The operating length is the shortest dimension to which the spring is compressed in use. The shut height or solid length is the length of the spring when the spring is loaded such that the coils are actually touching. This is the shortest possible length for the spring without crushing it beyond all recognition.
Figure 15.9. Various lengths associated with a spring in use.
Springs are subject to failure by yielding due to too high a stress in the case of static loading or by fatigue in the case of dynamic loading. In order to determine the geometry of a spring to avoid such failure, or to determine when failure will occur, it is necessary to consider the stresses experienced by a spring under loading.
The free-body diagram for a helical spring loaded with force F is illustrated in Figure 15.10. There are two components of stress on any cross section of coil: a torsional shear stress due to the torque and a direct shear stress due to the force. The stresses accumulate and the maximum shear stress will occur at the inner fiber of the wire's cross section.
