Method for manufacturing a ball joint

This manufacturing method comprises a first step in which a molten aluminum alloy is cast into casting dies into which a ball 8 forming a ball portion 1a of a ball-shank 1 in corporation with a shank 1b connected to the portion 1a is inserted as a core so that a holder 2 covering the ball portion 1a is formed leaving the connected portion of the shank 1b as it is; a second step in which the aluminum alloy for forming the holder 2 is subjected to a solution heat treatment; a third step in which the shank 1b is brought into pressure contact with the ball 8 exposed from the holder 2, electrodes 10a and 10b are held in contact with the shank 1b and the holder 2, respectively, and the ball 8 and the shank 1b are projection-welded by turning on electricity between electrodes to thereby form the ball-shank 1; and a fourth step in which an external force is applied to the holder 2 or the ball-shank 1 to thereby provide a gap between the ball 8 and the holder 2.

FIELD OF ART 
The present invention relates to a method of manufacturing a ball joint for 
use with a link motion mechanism such as the suspension arm portion or 
steering portion of an automobile or the cutting blade driving portion of 
a combine. 
BACKGROUND ART 
Conventionally, as a method of manufacturing a ball joint of the above 
type, there has been known one that is disclosed in the Examined Published 
Japanese Patent Application No. 77886/1993. 
As shown in FIG. 17, this method comprises a first step of forming a holder 
102 by die-casting using dies 100 having a steel ball 101 inserted 
therebetween as a core (refer to exploded views a and b), a second step of 
forming a ball-shank 104 comprised of a shank 103 projection-welded to the 
steel ball 101 covered with the holder 102 (refer to exploded views c and 
d); and a third step of applying an external force to the ball-shank 104 
or the holder 102 so that a minute gap is formed between the steel ball 
101 and the holder 102 closely adhered to each other at the time of 
forging by the first step, and since the spherical surface of the steel 
ball 101 is transferred to the holder 102 at the time of casting the 
latter, a ball joint manufactured by such method as shown in FIG. 18 
features that the steel ball 101 and the holder 102 come into sliding 
contact with each other without rattling thereby making a smooth 
rotational or oscillating motion. 
Further, in the first step of the above-described manufacturing method, the 
steel ball 101 serving as a core at the time of casting the holder 102 is 
clamped from on both upper and lower sides thereof by closing the dies 100 
in order to keep the steel ball 101 as a core between the dies 100 at the 
time of casting the holder 102 and the holder 102 is made to have openings 
105 and 106 in correspondence to the upper and lower positions of the 
steel ball 101, respectively. And in the second step, a projection welding 
operation is performed by using the openings 105 and 106 such that the 
shank 103 and an electrode 107 for feeding a welding current are brought 
into contact with the spherical surface of the steel ball 101 through the 
openings 105 and 106, respectively, and under such condition, a projection 
welding operation is performed while feeding a predetermined amount of 
welding current between the steel ball 101 and the shank 103. 
Now, where a projection welding operation is performed by bringing the 
electrode into direct contact with the steel ball according to the 
above-described conventional method, there is a fear that unless a 
sufficiently large area of contact between the steel ball and the 
electrode is secured, a welding resistance to the welding current 
generates between the electrode and the steel ball which results in 
melting the steel ball at the position at which the electrode is in 
contact with the steel ball. Therefore, it is necessary that the opening 
formed to allow the electrode to pass therethrough has some degree of 
wideness. 
However, such opening for the electrode comes to become coaxial with the 
shank welded to the steel ball and if the opening is formed sufficiently 
large, the area of contact between the steel ball and the holder projected 
along the axis of the shank (refer to FIG. 18) can not but be small so 
that there has arisen a problem that the capacity of the shank to bear the 
load acting along the axis thereof, i.e., the axial load, becomes 
insufficient. 
Further, there has also been a problem in order to increase the 
load-bearing capacity of the shank without reducing the diameter of the 
opening for the electrode, the diameter of the steel ball has to be made 
large and the resultant ball joint itself is made large-sized and heavy in 
weight accordingly. 
On the other hand, even when the electrode-passage opening formed in the 
shank is sufficiently large, if the degree of contact of the electrode 
itself with respect to the steel ball is not favorable, a welding 
resistance still generates between the electrode and the steel ball 
resulting in melting the ball at the contact position of the electrode 
with the ball. Therefore, in the case of the above-described conventional 
manufacturing method, a copper plate electrode is formed to have a 
spherical surface after the example of the spherical surface of the ball 
but such arrangement has had the disadvantage that since the configuration 
of the electrode changes gradually while repeating the projection welding, 
it is necessary to reform the electrode every time when a predetermined 
number of welding operations is performed and so the production control 
becomes quite troublesome. 
Further, the above-described conventional manufacturing method has had the 
problems that it mainly uses a zinc-based die-casting alloy (for example, 
a zinc-aluminum-copper type die-casting alloy) as a holder forming 
material but although such die-casting zinc alloy has an excellent degree 
of mechanical strength such as a tensile strength, hardness, tenacity and 
etc., since it lacks anti-corrosiveness and has a large specific gravity, 
the holder tends to become corroded and can not but become heavy in 
weight, the location of using a link mechanism formed by using a ball 
joint manufactured by the method is limited and the reduction of weight of 
the link mechanism is hindered. 
On the other hand, there may be given an aluminum-based material as a 
die-casting alloy having a small specific gravity but such die-casting 
aluminum alloy in its state of being die-cast is inferior to the 
above-described die-casting zinc alloy with respect to its tensile 
strength and hardness and a ball joint manufactured by using this alloy 
can not be used at the structural portion of which a mechanical strength 
is required, for example, the portion like the tie rod at the suspension 
of an automobile. 
DISCLOSURE OF THE INVENTION 
The present invention has been made in view of the above-described problems 
and an object of the invention is to provide a method of manufacturing a 
ball joint which method can manufacture a ball joint having a high level 
of capacity to bear an axial load acting in the direction of axis of the 
shank irrespective of the smallness of the diameter of the ball and is 
easy of production control. 
Another object of the present invention is to provide a method of 
manufacturing a ball joint which method is capable of forming a holder 
having an excellent mechanical strength by die-casting an aluminum alloy. 
In order to achieve these objects, the present invention provides the 
following two methods of manufacturing ball joints. 
That is, the first manufacturing method features that it is adapted to 
manufacture a ball joint having a holder and a ball-shank which are 
coupled to each other so as to oscillate or rotate freely and comprises a 
first step in which a molten electro-conductive alloy is cast into casting 
dies having a ball inserted therebetween to thereby casting a holder 
covering the ball leaving a shank connected portion as it is, a second 
step in which two electrodes are brought into contact with the shank and 
the holder, respectively, and a welding current is fed between the 
electrodes so that the ball and the shank are projection-welded to produce 
a ball-shank and a third step in which an external force is applied to the 
holder or the ball-shank to thereby form a gap between the ball and the 
holder. 
According to such a method of the present invention, since, in its second 
step, the welding current is fed between the shank and the holder through 
the ball thereby projection-welding the two members without bringing the 
electrode into direct contact with the ball, it is not necessary to 
provide an opening in the holder through which the electrodes come into 
contact with the ball. Accordingly, in the first step of casting the 
holder, the ball can be covered with an electro-conductive alloy material 
leaving only the shank-bonded portion as it is so that it is possible to 
manufacture a ball joint having a high-level of capacity to bear an axial 
load acting on the shank. 
Further, even when the welding current is fed between the shank and the 
holder through the ball, the holder cast by the first step is held in 
close contact with the ball and since it is only the shank connected 
portion where the ball is exposed from the holder, the ball and the holder 
have a sufficiently wide area of contact. Accordingly, almost no welding 
resistance to the welding current generates between the ball and the 
holder so that it is possible to connect only the shank and the ball by 
projection welding without allowing the ball to be welded to the holder. 
In addition, according to such method of the present invention, the 
connected portion of the shank and the ball can be quenched immediately by 
re-feeding a current between the shank and the holder after the completion 
of the projection welding so that it is also possible to easily prevent 
the connected portion from cracking resulting from leaving the connected 
portion of the ball and the shank as they are after welding. 
On the other hand, the second manufacturing method of the present invention 
features that it comprises a first step in which an aluminum alloy is cast 
into casting dies having a steel ball inserted therebetween as a core and 
a holder covering the steel ball is cast by die-casting, a second step in 
which the holder made of the aluminum alloy is subjected to a solution 
heat treatment and a third step in which an external force is applied to 
the steel ball or the holder to provide a gap between the steel ball and 
the holder so that the steel ball is made rotatable with respect to the 
holder. 
According to the method of the present invention comprising the 
above-described steps, since the holder covering the steel ball is cast by 
die-casting the aluminum alloy in the first step and the holder is 
subjected to a solution heat treatment in the subsequent second step, the 
mechanical strength of the holder is remarkably improved due to an age 
hardening taking place over several hours after such processes. 
Accordingly, even when the holder is formed by die-casting the aluminum 
alloy which have been considered to be inferior to other materials in its 
mechanical strength, the mechanical strength required of the ball joint 
could be satisfied to a sufficient degree. 
Now, where the function of a ball joint is taken into consideration, the 
steel ball used in the first step is required to have a anti-abrasion 
property and it is general that the surface of the ball is subjected in 
advance to quenching such as carbonic quenching. However, due to the fact 
that the temperature of the solution heat treatment of the aluminum alloy 
covering the steel ball in the first step is remarkably lower than the 
steel ball quenching temperature, when the holder is subjected to a 
solution heat treatment after die-casting, the same result as when the 
steel ball is tempered generates so that there is a fear that the surface 
of the steel ball hardened by quenching with much trouble will become 
softened and the sliding condition of the steel ball with respect to the 
holder will become worse in its early stages. 
FIG. 16 is a graph showing a relationship between the tempering temperature 
and the hardness of a quenched steel ball (stainless steel/440.degree. C.) 
when the steel ball is tempered. From this graph, it will be understood 
that when the tempering temperature exceeds 550.degree. C., the hardness 
of the steel ball drops rapidly and the softening of the surface of the 
steel ball is accelerated. Accordingly, where a special steel such as a 
stainless steel is used as the above-described steel ball, if the solution 
heat treatment temperature for the holder is less than 550.degree. C., it 
is possible to maintain the surface hardness of the steel ball at a 
practically safe level. 
Further, from the points of view of production cost and universality, it is 
preferable to use, as the steel ball, a common steel subjected to a 
carbon-quenching process but in that case, the solution heat treatment 
temperature for the holder must be further lower than the above-described 
tempering temperature of the stainless steel ball. According to the 
confirmation of the present inventors, when the solution heat treatment 
temperature for the holder is lower than 450.degree. C., it is possible to 
maintain the surface hardness of the steel ball at a practically safe 
level. 
At the same time, in view of the fact that the solution heat treatment 
temperature differs depending on the composition of the aluminum alloy, in 
order to keep the solution heat treatment temperature within the 
above-described range, it is the composition of the aluminum alloy that 
matters. In this connection, it is considered that various kinds of 
aluminum alloys the solution heat treatment temperatures for which falls 
within the above-described range may be produced by preparing chemical 
elements to be added to the aluminum alloys and according to what has been 
confirmed by the present inventors, there exists an aluminum-zinc-silicon 
type die-casting aluminum alloy whose solution heat treatment temperature 
is in the order of 360-450.degree. C. and which shows a mechanical 
strength exceeding that of a die-casting zinc alloy due to an age 
hardening. The concrete composition of such die-casting aluminum alloy 
will be described later. 
Further, in the method of the present invention, a ball-shank may be formed 
in such a manner that after the completion of a solution heat treatment 
and quenching of a holder, a shank is projection-welded to a steel ball in 
the same manner as in the case of the conventional method. However, it is 
preferable that a welding current is indirectly fed to the steel ball 
according to the above-described first manufacturing method of the 
invention from the point of view of preventing the steel ball from getting 
injured because of the fact that the surface of the steel ball tends to 
become softened after the solution heat treatment of the holder as 
described above. With such an arrangement, the shank can be 
projection-welded to the steel ball without the necessity of pressing the 
electrode against the steel ball thereby forming the ball-shank without 
damaging the surface of the steel ball. 
Further, the shank is not always required to be welded to the steel ball 
after die-casting the holder but the holder may be welded to the steel 
ball prior to the die-casting. In this case, only the ball portion of the 
ball-shank formed by projection welding is inserted between the casting 
dies and the holder is die-cast in that condition. 
It should be noted that it is not always necessary to weld the shank to the 
steel ball used in the method of the present invention but if a holder is 
die-cast with a steel ball having a through hole, it will be possible to 
use a ball joint by the method of the present invention by inserting and 
fixing the rod or the like of a link mechanism into the through hole of 
the steel ball of such ball joint. 
In addition, in the second step of the method of the present invention, 
since the holder to be subjected to a solution heat treatment has already 
covered the steel ball, if the holder is heated, the steel ball will also 
be heated so that there is a fear that the surface of the steel ball 
becomes remarkably coarse by oxidation and the intrinsic function of the 
ball joint of allowing the holder and the ball to come into smooth sliding 
contact with each other can not be displayed. Accordingly, from such point 
of view, it is preferable that the holder be subjected to a solution heat 
treatment in an oxygen-free environment so as to prevent the surface of 
the steel ball from becoming oxidized due to heating to thereby secure a 
smooth sliding motion between the holder and the steel ball.