Page 429 - Analysis and Design of Machine Elements

Springs
expected life under operating conditions. Additional considerations include natural 407
frequency, shock absorption, corrosion resistance and so on. A well-designed spring
functions properly and is manufactured easily.
The manufacturing of coil springs mainly includes four steps. First, straighten coils
of spring wire and wind on a mandrel to form springs. Spring ends are then processed.
For compression springs, two ends are ground to ensure end supporting surfaces are
vertical to the spring axis, while for extension and torsion springs hooks are made for
connection or loading. Followed by proper heat treatment to reduce residual stresses
induced by the winding process, impact or fatigue tests are required to check spring
quality [4].
After springs are formed, a shot peening or presetting operation [1] may be called for
in important applications to improve fatigue strength by inducing favourable compres-
sive residual stresses. Coating, plating or painting are used to enhance corrosion resis-
tance. During manufacturing process, scratches on spring surfaces should be avoided.

14.4.2 Materials and Allowable Stresses
While selecting spring materials, material properties, application conditions, manufac-
turing process, as well as costs and availability are factors that need to be considered.
For reliable functioning of springs, candidate spring materials should have high strength
(including ultimate, yield and fatigue strength), high resilience and good resistance to
corrosion and creep.

Application conditions include load magnitude and cyclic features, operating tem-
peratures and environments. Hard-drawn or oil tempered carbon steel spring wire is
adequate for statically loaded springs, while an alloy steel with restricted surface quality
is preferred for cyclically loaded springs [9]. When it is required to prevent corrosion
and magnetism, nonferrous materials are preferred. For springs operate at elevated tem-
peratures, nickel alloys are the best choice.
Spring materials may be formed into bar, wire or strip by hot-working or cold-working
processes. Normally, cold-working is used for spring wires with diameters less than
8–10 mm, while hot-working is used for greater wire diameters. Square or rectangular
wire may also be used.
A great variety of spring materials are available, including high carbon steels, alloy
steels and stainless steels, as well as nonferrous materials such as brass, phosphor
bronze, beryllium copper and nickel alloys.
Table 14.2 lists material properties and applications for commercially available spring
wires with good surface finishes.
The allowable stress of spring materials depends on the type of load, materials, pro-
cessing methods and wire size. Both torsional and tensile strengths of spring material are
inversely proportional to wire diameter. Due to limited data, tensile elastic limit strength
in Figure 14.16 can be roughly used to start a spring design.
Since elastic limit is between 60 and 90% of tensile strength [1], the tensile
e b
strengths of various spring materials can be roughly derived from elastic limits from
Figure 14.16. The allowable stresses expressed by tensile strength are listed in
b
Table 14.3. More detailed and precise data can be found in design handbooks or
manufacturer’s catalogues [7].