Wire for coiled spring

It is known that a sectional shape of a spring wire can be formed into an oval made by a semicircle and a semiellipse. However, it has been found that such a shape is not the best obtainable, and that stress distribution can be improved by increasing the diameter of the spring wire slightly in a portion where the stress is greatest, along the semielliptical surface. Parameters are disclosed for determining dimensions that result in increased energy efficiency.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to a coiled spring, and particularly to an 
improvement in a spring were extended for use in making a coiled spring in 
which a wire is generally oval in section. 
2. Description of the Prior Art 
Ordinarily, a conventional coiled spring is made from a wire which is 
circular in section. However, there was a problem in that as is known, 
when an axial load acts on the coiled spring of circular section, a stress 
generated in the peripheral portion is large internally of the coil due to 
the curved coil wire and direct shearing force, and therefore, not only 
the energy efficiency is poor but cracks which lead to snapping are likely 
to occur. 
The maximum stress .tau..sub.max is obtained by Wahl formula: 
##EQU1## 
where C is the spring exponent, and C is given by 
EQU C=D/d 
D is the diameter of coil and d is the diameter of wire. 
Japanese Patent Publication No. 3261/52 and U.S. Pat. No. 2,998,242 have 
been known which improve a disadvantage in that the maximum stress 
increases. In the former, the wire is oval in section, and in the latter, 
the shape is a combination of a semicircle and a semiellipse. The relation 
between the long diameter W and the short diameter t of the wire is 
determined by 
EQU W/t=1+1.2/C C=D/W (2) 
On the other hand, with the recent tendency of reducing the weight of 
automobiles, it has been required, in valve springs, torsion springs and 
the like of the engine, to design light-weight springs. This means that 
the length of close contact when the spring is compressed is minimized and 
the weight absorbing a given amount of energy is minimized, that is, the 
energy efficiency is enhanced. 
The length H.sub.s of close contact of the spring is generally calculated 
by 
EQU H.sub.s =(N-0.5)t (3) 
where N is the total number of turns of the coil, and t is the longitudinal 
dimension of wire. 
That is, to reduce the length of the close contact, the total number of 
turns N is reduced and the longitudinal dimension of the wire is made 
small. To enhance the efficiency of energy, the stress in the peripheral 
portion is made uniform, and the maximum stress .tau..sub.max is lowered. 
In the shape of a wire comprising a semicircle and a semiellipse as 
described above, the length of close contact can be reduced by flattening 
a section of the wire. This meets the aforementioned requirement in 
respect to the fact that the maximum stress is lowered, to which attention 
is recently invited suddenly. 
In the conventional material-dynamics solution, stress analysis of a 
suitable sectional shape is extremely difficult, and the shape of a wire 
is also considered to be relatively simple. However, lately, there is 
established an elastic-dynamics solution (Fourier development boundary 
value average method) in which a boundary in the outer peripheral portion 
in section is divided into a number of wire elements, Fourier development 
is made along each of the wire elements, which are expanded over the whole 
area of boundary, thus making it possible to perform stress analysis of a 
suitable shape. 
As a consequence, it has become clear that the above-described known 
semicircular and semielliptical sectional shapes are not always sufficient 
in respect of evenness of stress and reduction in maximum stress. 
SUMMARY OF THE INVENTION 
In accordance with the present invention, a sectional shape which can lower 
the maximum stress and make stress even as much as possible is found on 
the basis the above-described analysis to reduce the length of close 
contact of a coiled spring and enhance the energy efficiency. 
A known improved sectional shape of a wire is that a portion 1 is a 
semicircle and a portion 2 is a semiellipse as shown in FIG. 1. Thus, the 
following relation is obtained: 
EQU a/b=(2w)/t-1 (4) 
where a is the long diameter of the ellipse forming the semiellipse 
portion, and b is the short diameter. The stress distribution is shown in 
FIG. 2, in which the inside of the coil is indicated at 0.degree. and the 
outside of the coil indicated at 180.degree., and the maximum stress point 
is located at about 55 degrees as shown in the curve B as a function of an 
angle .psi. at the center of gravity G. 
In accordance with the present invention, a so-called build-up is formed in 
the vicinity of the maximum stress point to lower the maximum stress.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
FIG. 3 is a sectional view of a wire showing one embodiment of a spring in 
accordance with the present invention. In the illustrated embodiment, 
there is shown a sectional shape comprising a semicircle 3, an ellipse 4 
and a large circle 5 in contact therewith. The long diameter and short 
diameter of the wire are obtained from formula (2), then the radius R of 
the large circle 5 and the shape of the ellipse 4 are given by 
EQU R=3t (5) 
EQU a/b=7/8(2w/t-1) (6) 
In this shape, the length L from the center O of a semicircle with respect 
to the same coordinate .theta. is longer than L.sub.o of FIG. 1, i.e. 
L.sub.o &lt;L. The stress distribution is nearly even over 
25.degree.-75.degree. as shown by the curve A in FIG. 2, and the maximum 
stress .tau..sub.max is also lowered. 
Generally, since the life of a spring is determined by the maximum stress 
in the periphery in section, it is apparent that the spring of the present 
invention is capable of longer life than that of a semicircle and a 
semiellipse. If the maximum stress is made the same, the wire can be 
reduced in diameter to shorten the length of close contact PG,8 of the 
spring and reduce the weight. 
The following table shows a specific example by which the diameter of wire 
is reduced in the present invention, also showing a circular wire spring, 
and a semicircular and semielliptical wire spring. 
______________________________________ 
Semicircle Present 
Circle 
& semiellipse 
invention 
______________________________________ 
Conditions 
Inner dia 15.6 Same as Same as 
given (mm) of coil left left 
Spring 9.31 
constant 
(kg/mm) 
Load (kg) 100 
Maximum 70 
stress 
(kg/mm.sup.2) 
Shape of 
Diameter .phi.4.65 
-- -- 
spring of wire (mm) 
Long dia w .times. 
-- 5.35 .times. 4.10 
5.2 .times. 4.0 
short 
dia t (mm) 
Effective 6 5.23 5.04 
turn 
Total turn 8 7.23 7.04 
Length of 34.88 27.59 26.16 
close contact 
(mm) 
Weight (gr) 68.09 64.60 60.31 
For cir- 
Shortening -- 21% 25% 
cular percent of 
section close contact 
Reducing -- 5% 11% 
percent of 
weight 
______________________________________ 
In the Table, the shortening as a percentage of the length of close contact 
and the reduced percentage of weight shows the percentages with respect to 
a spring of circular section with the inner diameter of coil, spring 
constant and load being made constant and the maximum stress made the 
same. Considerable improvement is made by using a wire of the shape shown 
in FIG. 3 as compared with the semicircular and semielliptic shape shown 
in FIG. 1. 
As previously described, in the present invention, the length L from the 
center of the semicircle to the semielliptical surface of a wire in FIG. 1 
is made larger than the length L.sub.o in the case of a semicircle and a 
semiellipse of the prior art to thereby lower the maximum stress. In the 
example shown in the above-discribed table, the ratio a/b of long diameter 
to short diameter of the ellipse of the semicircular and semielliptic 
shape is 1.6 according to formula (4) and 1.4 according to formula (6) in 
the embodiment of the present invention. Where the a/b is 1.3, 1.2 and 
1.1, an enlarged comparative view of the sectional shape is shown in FIG. 
4 and the stress distribution shown in FIG. 5. As the a/b lowers, stress 
in the vicinity of 50.degree. lowers, but the evenness is rather worsened. 
On the other hand, as L becomes larger than L.sub.o, the sectional area of 
a wire increases and the weight of the spring increases. FIG. 6 shows a 
stress ratio .alpha. as compared to a circular section of the same 
sectional area, that is, the case where it is standardized to the same 
sectional area. This means that when the a/b is about 1.1, the maximum 
stress reaches that of the same extent as that of the known semicircular 
and semielliptical shape, which fails to achieve the object of the present 
invention which is to lower the maximum stress. 
Accordingly, in the present invention, the following range can be employed: 
EQU 1.1&lt;a/b&lt;(2w)/t-1 
In this range, the shape of a wire is not only formed by a semicircle, a 
partial ellipse and a large circle in contact therewith but formed by a 
combination of a semicircle, a semiellipse and a straight line or by a 
semicircle, a circle having a smaller diameter than the semicircle and a 
large circle in contact therewith. In addition, by employment of a hollow 
wire, the weight reducing percent can be further increased. 
Moreover, in the sectional shape as described above, the sectional 
curvature lengthwise of the coil as shown in FIG. 9, is smaller than that 
of a circular section, and therefore, for example, surface pressure 
generated when a partial coil of the spring is intentionally brought into 
contact in order to obtain a non-linear load characteristic, or surface 
pressure of a contact portion between a wound seat portion and an 
effective coil normally generated is reduced to advantageously lessen the 
wear of said portion to provide a prolonged service life of the spring.