Page 547 - Shigley's Mechanical Engineering Design

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Mechanical Springs 521
Figure 10–2

Types of ends for compression + +
springs: (a) both ends plain;
(b) both ends squared; (c) both
ends squared and ground; (a) Plain end, right hand (c) Squared and ground end,
(d) both ends plain and ground. left hand

+ +

(b) Squared or closed end, (d) Plain end, ground,
right hand left hand

Table 10–1 Type of Spring Ends
Formulas for the Plain and Squared or Squared and
Term Plain Ground Closed Ground
Dimensional
Characteristics of End coils, N e 0 1 2 2
Compression-Springs. Total coils, N t N a N a 1 N a 2 N a 2
(N a = Number of Active Free length, L 0 pN a d p(N a 1) pN a 3d pN a 2d
Coils) Solid length, L s d(N t 1) dN t d(N t 1) dN t
Source: From Design Pitch, p (L 0 d) N a L 0 (N a 1) (L 0 3d) N a (L 0 2d) N a
Handbook, 1987, p. 32.
Courtesy of Associated Spring.

used without question. Some of these need closer scrutiny as they may not be integers.
4
This depends on how a springmaker forms the ends. Forys pointed out that squared
and ground ends give a solid length L s of
L s = (N t − a)d
where a varies, with an average of 0.75, so the entry dN t in Table 10–1 may be over-
stated. The way to check these variations is to take springs from a particular spring-
maker, close them solid, and measure the solid height. Another way is to look at the
spring and count the wire diameters in the solid stack.
Set removal or presetting is a process used in the manufacture of compression
springs to induce useful residual stresses. It is done by making the spring longer than
needed and then compressing it to its solid height. This operation sets the spring to the
required final free length and, since the torsional yield strength has been exceeded,
induces residual stresses opposite in direction to those induced in service. Springs to
be preset should be designed so that 10 to 30 percent of the initial free length is
removed during the operation. If the stress at the solid height is greater than 1.3 times
the torsional yield strength, distortion may occur. If this stress is much less than 1.1
times, it is difficult to control the resulting free length.
Set removal increases the strength of the spring and so is especially useful when
the spring is used for energy-storage purposes. However, set removal should not be
used when springs are subject to fatigue.

4 Edward L. Forys, “Accurate Spring Heights,” Machine Design, vol. 56, no. 2, January 26, 1984.