Method and equipment for surface-hardening treatment of steel balls for a ball bearing

An apparatus for surface-hardening steel balls includes a container having projections protruding inwardly from and extending longitudinally along an inner wall; a support shaft having projections extending outwardly from and longitudinally along the support shaft; a container driving mechanism which rotates the container in one direction; and a support shaft driving mechanism which rotates the support shaft in an opposite direction. In operation, the container's projections transfer the balls from a lower portion of the container to a higher portion where the balls are dropped to the lower portion while the support shaft's projections strike the steel balls when the steel balls are dropping to the lower portion.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a method and apparatus for 
surface-hardening steel balls produced of hardened steel. 
2. Description of the Related Art 
Japanese Patent Laid-Open Application No. 195069/1993 describes approaches 
for surface hardening steel balls (e.g. after quenching and tempering) 
used as bearing balls which are produced of a high carbon chromium bearing 
steel (JIS G4805) represented by SUJ2 or a martensitic stainless steel 
(JIS G4303) represented by SUS440C. The surface hardened steel balls have 
a surface layer with greater residual compressive stress and hardness than 
untreated steel balls. 
Referring to FIG. 8, Japanese Patent Publication No. 12813/1989 describes 
steel balls 101 charged into a regular octagonal steel barrel 100 through 
a port 103 to, at most, about two-thirds the inside capacity of barrel 
100. The barrel 100 is rotated in the direction of arrow P about a center 
axis 104 supported on a frame 102. Upon rotating barrel 100, the steel 
balls 101 move upwardly, and subsequently fall in the direction of arrow 
Q. The falling steel balls strike against the steel balls 101 lying below 
and against the inner wall of barrel 100. By continuously repeating this 
operation, the entire surface of the steel balls 101 are surface hardened. 
The finished-steel balls are discharged through port 103. 
The method described above, however, has the following disadvantages: 
(1) If the barrel rotates at an excessively high speed, the steel balls are 
held against the inner wall of the barrel by a centrifugal force, and thus 
will not strike against the inner wall. Therefore, it becomes necessary to 
decrease the revolution number of the barrel to 80 rpm or lower. 
At this lower speed, the frequency at which the steel balls strike against 
each other or against the inner wall is less, such that a longer time is 
needed to achieve a desired surface residual compressive stress and 
hardness. 
(2) The smaller the diameter of the steel ball, the smaller the striking 
force per unit time. For example, in the case of a 25 mm steel ball, the 
net weight of the ball is 0.62 N (63.57 gf), whereas in the case of a 3 mm 
steel ball, the net weight of the ball is 0.001 N (0.11 gf). 
Since the revolution number of the barrel is limited for the reason 
described above (1), a much longer time is required for surface-hardening 
small-diameter, e.g., 3 mm steel balls. 
(3) To surface harden 3 mm diameter steel balls in a predetermined time, it 
is necessary to increase the height from which the steel ball is dropped. 
This, however, requires a larger size barrel, and correspondingly a larger 
size surface-hardening apparatus, resulting in a lower operation 
performance. 
Consequently it is extremely difficult to surface harden very small steels 
balls measuring 1 to 3 mm in diameter. 
(4) Recently, steel balls for ball bearings used in e.g., automotive 
transmissions are required to have a long service life and to remain 
usable even if foreign substances become introduced in the lubricating 
oil. The steel balls employed for such service must have a greater 
residual compressive stress and hardness in the surface layer. 
To produce steels balls (particularly those of smaller diameter) having 
surfaces with the required residual compressive stress and hardness, it 
becomes necessary to increase the barrel size and/or to prolong the 
treatment time. 
SUMMARY OF THE INVENTION 
In a general aspect of the invention, an apparatus for surface-hardening 
steel balls includes a container having projections protruding inwardly 
from and extending longitudinally along an inner wall; a support shaft 
having projections extending outwardly from and longitudinally along the 
support shaft; a container driving mechanism which rotates the container 
in one direction; and a support shaft driving mechanism which rotates the 
support shaft in an opposite direction. In operation, the container's 
projections transfer the balls from a lower portion of the container to a 
higher portion where the balls are dropped to the lower portion while the 
support shaft's projections strike the steel balls when the steel balls 
are dropping to the lower portion. Related aspects of the invention 
include the apparatus itself and a method of surface-hardening steel 
balls. 
Among other advantages, both the projections on the container and those on 
the support shaft cause the steel balls to strike against each other and 
against the inner wall of the container with a greater frequency, as 
compared with containers and support shafts that do not have projections. 
Also, the projections on the container agitate the steel balls. 
Consequently, the treatment time to produce steel balls having a surface 
layer with a desired residual compressive stress and hardness is reduced. 
Moreover, the force at which the steel balls are struck by the projections 
of the support shaft can be adjusted by changing the revolution number of 
support shaft and its projections. This enables the precise control of the 
amount of compressive stress produced in the surface layer of the steel 
balls. 
Embodiments of the above aspects of the invention may include one or more 
of the following features. The container may be a cylindrical barrel that 
includes projections protruding radially inwardly from its inner wall. 
The support shaft can be configured to rotate at a revolution number 
greater than 1.5 times that of the container. Each of the projections on 
the support shaft may have a steel ball striking region that is hardened 
by quenching. The striking region may be an additional attachment affixed 
to each of the support shaft's projections to lower the costs of 
production since it may be more cost effective to surface-harden the 
attachment rather than the projection itself. There can be three equally 
spaced projections on the support shaft, and the projections can be shaped 
as flat blades. The container may include six equally spaced projections. 
The method of surface-hardening steel balls may include stopping the 
rotation of the container and the support shaft after surface hardening of 
the steel balls is finished. 
All the above aspects provide substantial advantages for a wide variety of 
applications, including those employed in the automotive and aerospace 
industries. 
Other features and advantages will become apparent from the following 
description and from the claims.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Referring to FIG. 1, a cylindrical barrel 1 includes six projections 4 
equally spaced around an inner wall of the barrel and each extending from 
the inner wall in a direction parallel to the longitudinal axis of the 
barrel. Barrel 1 also includes a port 8 for charging and discharging steel 
balls 3, and a cover 9 removably attached to port 8. A support shaft 2 is 
coaxially mounted in the cylindrical barrel 1. Support shaft 2 includes 
three flat blades 5 extending in the longitudinal direction within the 
cylindrical barrel 1, the blades being equally spaced around the support 
shaft 2 and reinforced with a plate 7. Each blade 5 includes a fixedly 
attached surface hardened beating (or striking) section 6 for beating 
steel balls 3, the beating section 6 having a surface hardened by 
quenching. Projections 4 and blades 5 are arranged so that a predetermined 
space exists between the projections and the blades. Furthermore, the 
longitudinal ends of blades 5 are spaced from the end walls (not shown) of 
the cylindrical barrel. In alternate embodiments, support shaft 
projections shaped differently than flat blades 5 may be used. Also, 
non-cylindrically shaped containers may be used in place of cylindrical 
barrel 1. 
In operation, steel balls 3 of hardened steel are charged in the lower part 
of cylindrical barrel 1 to about one-third or less the inside capacity of 
cylindrical barrel 1. As cylindrical barrel 1 rotates in the direction of 
arrow A, projections 4 carry steel balls 3 from the lower part of barrel 1 
until the balls reach a point where they are dropped back into the lower 
part of barrel 1. During rotation of the cylindrical barrel 1, the steel 
balls 1 are constantly agitated by the projections 4 in the lower part. 
Simultaneously, support shaft 2 with blades 5 rotate in a direction 
(designated by arrow B) opposite that of barrel 1 and at a revolution 
number 1.5 times (or higher) than that of barrel 1. Steel balls 3 dropping 
from projections 4 are struck by beating sections 6 of blades 5, and thus 
scattered in the direction of arrow C while also being struck against 
other steel balls 3 moving in the C direction and/or against the inner 
wall of cylindrical barrel 1. 
Rotation of barrel 1 and support shaft 2 is continued until the entire 
surface of each steel ball 3 acquires a specific residual compressive 
stress and hardness as desired. 
The number and position of blades 5 and projections 4 were determined 
experimentally and the arrangement of the three equally spaced blades 5 on 
the support shaft 2 and the six equally spaced projections 4 on the inner 
wall of the cylindrical barrel 1 has been found to be a presently most 
preferred embodiment. 
FIGS. 2 to 5 illustrate a surface-hardening apparatus 10 applied to 3 to 17 
mm diameter steel balls. 
The surface-hardening apparatus 10 includes a cylindrical barrel 20, a 
support shaft 30 supporting the cylindrical barrel 20, a cylindrical 
barrel driving mechanism 40 for rotating the barrel in one direction, and 
a support shaft driving mechanism 50 for rotating the shaft in a direction 
opposite that of cylindrical barrel 20. 
Cylindrical barrel 20 is assembled by tightening a side plate 22 by bolts 
23 to a hollow cylindrical body 21. Cylindrical body 21 has an outer 
diameter of about 1200 mm, a 16 mm wall thickness, and a width of 1300 mm. 
On the inner wall of hollow cylindrical body 21 are six projections 24 
equally spaced around the circumference of body 21, and extending in the 
longitudinal direction. The cylindrical barrel 20 is supported on the 
support shaft 30 through the pillow blocks 26. The hollow cylindrical body 
21 includes a steel ball charge-discharge port (not shown). 
Support shaft 30 is supported on a frame 60 by pillow blocks 36. On the 
outer periphery of support shaft 30 are three equally space flat blades 
32, extending in the longitudinal direction within cylindrical barrel 20. 
Affixed to each blade 32, by bolts 35, is a hardened beating section 34 
for beating steel balls. Blades 32 are spaced from projections 24 as they 
pass by a distance of about 100 mm and are spaced from side plate 22 by a 
distance of about 40 mm. A plate 33 mechanically reinforces blades 32. 
The cylindrical barrel driving mechanism 40 includes an electric motor 41, 
a belt power transmission section 42, and a roller chain power 
transmission section 43 that includes a sprocket 44 which is fastened to 
the pillow block 26 and the cylindrical barrel 20 by bolts 46. 
Support shaft driving mechanism 50 includes an electric motor 51, a belt 
power transmission section 52, and a roller chain power transmission 
section 53, including a sprocket 54 mounted on support shaft 30. 
Belt power transmission mechanism 42 and the roller chain power 
transmission mechanism 43 are provided as examples of power transmission 
mechanisms. However, the present invention is not limited to these types 
of mechanisms, for example, gears or wire ropes may be used for the same 
purpose. 
The operation of surface hardening apparatus 10 will now be described. 
After a predetermined quantity of quenched and tempered steel balls are 
charged through the steel ball charge-discharge port into cylindrical 
barrel 20, the steel ball charge-discharge port is closed. Cylindrical 
barrel 20 and support shaft 30 are then rotated at a specified revolution 
number in the directions indicated by arrows D and E, respectively. An 
outer ring 28a of a ball bearing 28 installed in each pillow block 26 
rotates in the D direction, while an inner ring 28b rotates in the E 
direction. 
After the surface hardening operation is completed, the steel ball 
charge-discharge port is opened and cylindrical barrel 20 and supporting 
shaft 30 are manually rotated to enable the discharge of the steel balls 
to a ball receiving cover 62. The ball receiving cover 62 is inclined as 
shown in FIG. 3 so that the steel balls roll into a holding vessel. 
Thereafter, the steel balls undergo a polishing process. 
The respective revolution numbers (i.e., rotational speeds) of cylindrical 
barrel 20 and support shaft 30, and the surface-hardening treatment time 
are determined according to results obtained from measurements of surface 
layer hardness, X-ray measurements of residual compressive stress and 
amount of retained austenite, and rolling fatigue life tests. 
Described below is a comparison of the processing conditions and parameters 
between the prior art illustrated in FIG. 8 and the present invention 
(shown in FIGS. 2-5) used to produce 5/16 inch nominal diameter steel 
balls with a residual compressive stress within a preferable range of 400 
MPa to 800 MPa (from surface to 200 .mu.m depth). 
The parameters for the conventional regular octagonal steel barrel shown in 
FIG. 8 are: 
______________________________________ 
Barrel size: 1000 mm between opposing flat sides 
1200 mm in width 
Steel ball holding 
480 kgf 
capacity by weight: 
Revolution number of 
65 rpm 
barrel: 
Processing time: 
2.5 hrs 
______________________________________ 
The parameters for the apparatus shown in FIGS. 2-5 are: 
______________________________________ 
Barrel size: 1200 mm outside diameter; 16 mm 
wall thickness; 1300 mm in wide 
Steel ball holding 
480 kgf 
capacity by weight: 
Revolution number of 
20 rpm 
barrel: 
Revolution number of 
65 rpm 
blades: 
Processing time: 
1.5 hrs 
______________________________________ 
Thus, the processing time with the apparatus illustrated in FIGS. 2-5 is 40 
percent less than that of the apparatus illustrated in FIG. 8. Thus it is 
shown above that the present invention can impart a specific uniform 
residual compressive stress and hardness to the surface layer of steel 
balls in a shorter period of time than by a known conventional apparatus. 
In some applications, bearings (e.g. automotive transmission bearings) are 
required to be usable for a prolonged period of time when used in 
lubricating oil contaminated with foreign substances. To meet this demand, 
the steel balls of the bearings must have a surface layer with a higher 
residual compressive stress. For example, steel balls with a nominal 
diameter of 5/16 inch having a residual compressive stress of 1000 MPa can 
be produced with the apparatus (having the apparatus parameters described 
above) shown in FIGS. 2-5 operating under the following processing 
conditions: 
Revolution number of barrel: 20 rpm 
Revolution number of blades: 80 rpm 
Processing time: 3 hrs 
An alternate embodiment is shown in FIGS. 6 and 7 in which a 
surface-hardening apparatus 70 is used to treat very small steel balls, 
for example, those with a diameter between 1 to 3 mm. Described below are 
the different characteristics of this embodiment as compared with that 
illustrated in FIGS. 2-5. 
(1) The cylindrical barrel 72 is smaller, having an outside diameter of 
about 400 mm and a width of about 750 mm. 
(2) There is about a 20 mm spacing between projections 73 and blades 74. 
(3) A steel ball beating surface 74a of the blades 74 has been 
surface-hardened, rather than having an attached hardened beating section. 
(4) Ball bearings 76 are used in place of pillow blocks . 
Elements shown in FIGS. 6 and 7 similar to those shown in FIGS. 2-5 are 
designated by the same reference numerals and will not be described. 
Small steel balls having a desired residual compressive stress are produced 
with the apparatus shown in FIG. 6 and 7 employing the following 
processing parameters: 
______________________________________ 
Barrel size: 400 mm in outside diameter; 
750 mm in width 
Steel ball holding capacity 
12 kgf 
by weight: 
Revolution number of barrel: 
400 rpm 
Revolution number of blades: 
65 rpm 
Processing time: 4 hrs 
______________________________________ 
It is noted that the striking force on the steel balls produced by the 
blades can be adjusted by changing the revolution number of the blades, 
and thus 
(1) it is possible to surface-harden 1 to 3 mm diameter steel balls which 
have been difficult to process by heretofore known methods; and 
(2) it is possible to impart a residual compressive stress and hardness to 
the surface layer of the steel balls according to the desired use of the 
steel balls.