Metal working – Method of mechanical manufacture – Obtaining plural product pieces from unitary workpiece
Reexamination Certificate
2000-08-17
2003-04-22
Vidovich, Gregory M. (Department: 3726)
Metal working
Method of mechanical manufacture
Obtaining plural product pieces from unitary workpiece
C148S228000, C148S230000, C148S589000
Reexamination Certificate
active
06550122
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method of manufacturing a laminated ring for use in a continuously variable transmission or the like.
2. Description of the Related Art
Heretofore, laminated rings for use in continuously variable transmissions or the like have been manufactured as follows: The ends of a thin sheet of maraging steel which is an ultra-high strength steel are welded to each other, producing a cylindrical drum, which is subjected to a first solution treatment. Then, the drum is cut into a ring having a predetermined width. After the ring is rolled, it is subjected to a second solution treatment. The ring is corrected to a predetermined circumferential length, and then aged and nitrided. A plurality of rings thus treated are laminated into a laminated ring.
The first solution treatment that is effected on the welded drum in order to uniformize the hardness that has been locally increased due to the heat applied when the thin sheet of maraging steel was welded. The first solution treatment allows the ring cut from the drum to be rolled with ease. Generally, the solution treatment is carried out by heating the maraging steel to a temperature equal to or higher than the recrystallization temperature of the maraging steel. The maraging steel contains aging precipitation strengthening elements of Ti, Al, Mo, etc. If these elements, particularly Ti, are oxidized, then a desired hardness may not be achieved by the subsequent aging process. In order to avoid oxidization of the aging precipitation strengthening elements, the solution treatment is performed in a vacuum furnace.
The rolled ring has a rolled structure in which metal crystals are crushed by the rolling process. If the rolled structure remained unchanged, then nitrogen would not easily penetrate the ring in the subsequent nitriding process, and the ring would not uniformly be nitrided. The second solution treatment is effected on the rolled ring in order to recover the original metal crystal grains prior to being deformed for facilitating the nitriding process.
The second solution treatment is generally carried out by heating the maraging steel to a temperature equal to or higher than the recrystallization temperature of the maraging steel, and the aging precipitation strengthening elements should not be oxidized in the second solution treatment. Since the vacuum furnace is expensive, the second solution treatment is performed in a heating furnace which contains a reducing atmosphere. The reducing atmosphere comprises, for example, a nitrogen atmosphere containing 1 to 10% of hydrogen. While the nitrogen atmosphere contains a small amount of oxygen, the oxygen is removed from the system by reacting with the hydrogen. Therefore, the concentration of oxygen in the nitrogen atmosphere is low enough to suppress oxidization of the aging precipitation strengthening elements. According to the above manufacturing process, since the second solution treatment is performed in the reducing atmosphere using the heating furnace, the number of vacuum furnaces that are used is reduced for cutting down on the cost of the vacuum furnaces.
However, an increase in the cost incurred in manufacturing the laminated ring is unavoidable because of different furnaces used in the first and second solution treatments.
The circumferential length of the ring which has been subjected to the second solution treatment is corrected as shown in
FIG. 8
of the accompanying drawings. In
FIG. 8
, a ring
4
is trained under tension around a drive roller
21
and a driven roller
22
. While the ring
4
is being rotated by the drive roller
21
, a correction roller
23
disposed between the drive roller
21
and the driven roller
22
is moved in the direction indicated by the arrow. As a result, the ring
4
is loaded by the correction roller
23
in a direction perpendicular to the direction of travel between the drive roller
21
and the driven roller
22
, and elongated to a predetermined length. In order to correct the circumferential length of the ring
4
appropriately, the ring
4
should preferably have an annular shape which is as close to a true circle as possible.
However, depending on the conditions of the second solution treatment, the ring
4
may suffer a thermal strain in the second solution treatment, and may be shaped to a complex irregular form, e.g., a substantially criss-cross shape as viewed in plan as shown in FIG.
10
(
a
) of the accompanying drawings, or an extremely slender elliptical form as viewed in plan as shown in FIG.
10
(
b
) of the accompanying drawings. If the ring
4
after the second solution treatment is of a shape as shown in FIG.
10
(
a
) or
10
(
b
), then when the ring
4
is trained around the drive roller
21
and the driven roller
22
for correcting the circumferential length thereof, the ring
4
may have portions that will not be brought into full contact with the rollers
21
,
22
. If the ring
4
thus shaped is corrected for its circumferential length, then the ring
4
may develop thickness irregularities as ring portions held in contact with the rollers
21
,
22
and ring portions held out of contact with the rollers
21
,
22
tend to have different thicknesses, and the ring
4
may have different circumferential lengths at opposite edges thereof.
SUMMARY OF THE INVENTION
It is therefore an object of the present invention to provide a method of manufacturing a laminated ring at a reduced cost by suppressing a thermal strain caused in a solution treatment after the ring has been rolled, thereby to correct the circumferential length of the ring appropriately with ease.
To achieve the above object, the inventors have studied conditions for a solution treatment to be effected on a welded cylindrical drum and a solution treatment to be effected on rolled rings. As a result, the inventors have found that if the conventional conditions for the solution treatment to be effected on rolled rings are made stricter, then such stricter conditions also applicable to the solution treatment to be effected on the welded cylindrical drum, and has completed the present invention based on the finding.
According to the present invention, there is provided a method of manufacturing a laminated ring, comprising the steps of preparing a cylindrical drum by welding opposite ends of a thin sheet of maraging steel, effecting a first solution treatment on the cylindrical drum, thereafter, severing the cylindrical drum into a plurality of rings each having a predetermined width, rolling the rings, effecting a second solution treatment on the rings which have been rolled, thereafter, correcting the circumferential length of each of the rings, thereafter, aging and nitriding the rings, and stacking the aged and nitrided rings into a laminated ring, the arrangement being such that the first solution treatment and the second solution treatment being effected using the same heating furnace in the same atmosphere in the same temperature range.
In the method according the present invention, a cylindrical drum is prepared by welding opposite ends of a thin sheet of maraging steel, and then placed into a heating furnace where a first solution treatment is effected on the cylindrical drum. Then, the cylindrical drum is severed into a plurality of rings each having a predetermined width, and the rings are rolled. The rolled rings are then placed into the heating furnace that has been used to effect the first solution treatment on the rings. In the heating furnace, a second solution treatment is effected on the rolled rings in the same atmosphere in the same temperature range as those of the first solution treatment, Thereafter, the circumferential length of each of the rings are corrected, and the rings are aged and nitrided. The aged and nitrided rings are stacked into a laminated ring.
By making stricter conditions for the second solution treatment to be effected on the rolled rings, the first solution treatment and the second solution treatment can be performed using the
Imai Hitoshi
Karasawa Hitoshi
Nakajima Katsuyuki
Cozart Jermie E.
Honda Giken Kogyo Kabushiki Kaisha
Vidovich Gregory M.
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