JPH07138722A - Universal shaft joint for molten metal - Google Patents

Abstract

(57) [Abstract] [Purpose] To provide a universal joint which suppresses periodic fluctuations during torque transmission and improves wear resistance of a contact sliding portion. [Structure] A plurality of claws radially extended at equal intervals are fixed to the shaft end of one shaft, and a plurality of grooves into which the claws are inserted are formed at the shaft ends of mating shafts meshing with the shaft ends. In a clutch type universal joint having a fixed outer cylinder, the universal joint main body is made of an alloy, the engaging portion for transmitting torque of the claw and the groove, the end surface contact portion of the center joint side of both shafts, or the torque transmitting engagement. A molten metal medium universal shaft joint characterized in that only the required thickness is made of ceramic, cermet, or corrosion-resistant wear-resistant alloy.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a continuous hot-dip galvanizing, and more particularly to a universal shaft coupling which is rotated at a certain angle in the hot-dip galvanizing machine to drive a roll which is rotated in the hot-dip galvanizing equipment. .

[0002]

2. Description of the Related Art The construction of a continuous hot-dip galvanizing apparatus is shown in FIGS. A support roll 3 installed in the molten zinc bath 2 immediately below the bath surface and rotating in contact with the steel sheet 1 to be plated suppresses the primary wiping function for controlling the amount of zinc deposited on the steel sheet 1 to some extent and vibration of the steel sheet. With support function for.

As the in-bath universal shaft joint 4 for driving the roll, a cross pin joint 6 made of stainless steel having a structure shown in FIG. 11 has been conventionally used. In the figure, 5 is an intermediate transmission shaft, and 7 is a cross groove sleeve.

[0004]

As is well known, the joint having this structure is a so-called non-uniform velocity joint in which the angular velocity periodically changes in one rotation when the input shaft and the output shaft rotate at a certain angle. And, of course, the torque of the output shaft also periodically changes. Further, since the stainless steel alloy has poor corrosion resistance to molten zinc, the cross pin joint 6 and the cross groove sleeve 7 are worn in a short period of time, causing large backlash.
Therefore, it is an oscillating source for vibrating the support roll 3 due to the two weaknesses of the above-mentioned non-uniform velocity joint and poor wear resistance, and the plating adhesion thickness of the steel sheet 1 to be plated becomes the rotational phase of the support roll 3. This is a so-called chatter mark quality hindrance factor that periodically changes correspondingly. In addition, the life of the joint was short and frequent line breaks were required, resulting in increased mechanical loss, and a large maintenance cost was required for replacement.

As a countermeasure against this, Japanese Patent Laid-Open No. 4-290613
In the cross-pin shaft proposed by No.
There is a joint in which the shaft side material is a carbide cermet or a carbide precipitation type heat resistant alloy, and the yoke hole side is ceramics.
However, this joint has two problems. First, in this joint, the shaft portion of the cross pin and the ceramic ring on the yoke side must be preassembled, that is, the support roll 3 and the intermediate transmission shaft 5 must be preassembled. In the case of a cross pin joint, the rolls can be easily engaged by previously setting the roll on the plating bath and then inserting the cross pin joint 6 mounted on the intermediate conduction shaft 5. However, in this type of joint, the roll and the intermediate transmission shaft must be assembled and then installed and removed from the line, which is extremely inconvenient. The second is that the material of the member is the above, but after that, a joint was actually constructed by combining various ceramics, cermets, and alloys, and a rotation test was performed in molten zinc. Alternatively, the inventors have found that a limited range of materials and combinations among cermets exhibit better performance. In that sense, this joint also has drawbacks.

[0006] As another countermeasure, the molten zinc medium universal shaft joint has a shape in which two of the cross pin joints in Fig. 11 are eliminated and the remaining two claws are projected in opposite directions at 180 ° intervals.
This pin joint is used. On the other hand, the universal shaft joint outside the molten zinc bath is used as a cross pin type joint, and the phase of its claw is matched with the joint in the molten zinc to cancel the non-uniform velocity and to make two joints total, etc. Methods for improving speed are known. However, even with this method, the early corrosion wear of the meshing torque transmitting portion does not change, so that the effect is exerted only for the first few days.

[0007] As yet another measure, JP-A-4-3548
There is a joint presented in Japanese Patent Laid-Open No. 58-58, and even when it is used alone, a ball joint or a trinob type joint having better constant velocity than the combination of the two pins in the bath to the cross pin outside the bath is used, and its constituent members. The whole material is either Cr-Mo super-high strength steel or ceramics.

The former structure, that is, the ball joint is not practical for use in molten zinc. This is because, when all of the inner and outer rings, balls, and cage are made of ceramics, the notch effect due to the complexity of the shape or the inclusion of intermetallic compounds in molten zinc called dross on the rolling surface is extremely Ceramics are damaged in the short term. On the other hand, when only the balls are made of ceramics and the other is made of the special steel, the balls-the inner and outer rings are point-contacted and the surface pressure is high, so the inner and outer rings are corroded and worn out in a short time, and the balls cannot be used .

Although the latter structure, that is, the trinob type joint is structurally simple and practical, if the trinob, that is, the claw and the entire outer cylinder are made of ceramics, they are easily broken due to the complexity of the shape. Further, even if the shape is improved and cracking can be suppressed to some extent, the manufacturing cost becomes extremely expensive, which is not realistic. On the other hand, when the whole is made of special steel, the inventors actually used it on the hot dip galvanizing line, and as a result, the torque transmission meshing part was worn by 7 mm on both sides in total for 10 days. After all, the parts of the world of steel, which are subject to mechanical contact sliding in a harsh environment such as molten zinc, are insufficient in corrosion resistance and wear resistance, and only a short life can be expected, which is insufficient.

On the other hand, in a portion of the molten zinc that is not in mechanical contact with the equipment, Fe-Al, Fe-Zn, which is called dross winding and exists in the molten zinc in a solid state, is generally used.
There is a phenomenon in which intermetallic compounds such as adhere to and grow on the order of several mm in thickness. Therefore, the usable period of the conventional joint is 2
When it is attempted to continuously use for a long period of time significantly exceeding the period of about a week, except for the contact sliding part of the mesh,
In particular, if dross adheres to the front side of the claw and the tip of the groove at the meshing part of the claw to the groove, when the line is stopped and the claw is removed, the dross adhered part gets caught and cannot be removed. . That is, this dross winding becomes another problem.

[0011]

DISCLOSURE OF THE INVENTION The present invention proposes to solve the above-mentioned drawbacks, and in order to impart sufficient corrosion resistance / wear resistance and dross adhesion resistance to necessary parts, (1) A plurality of radially extending claws are fixed to the shaft end of the shaft at equal intervals, and an outer cylinder having a plurality of grooves into which the claws are inserted is fixed to a mating shaft end that meshes with the shaft end. -Type clutch type universal shaft joint, the universal joint body is made of an alloy, and only the meshing portion for transmitting torque of the claw and groove, the end surface contact portion on the center joint side of both shafts, or the torque transmitting meshing portion is required. Made of any one of ceramics, cermet, and corrosion-resistant wear-resistant alloy, and a molten metal medium universal shaft joint, (2) Molten metal in which the number of claws and grooves of the joint described in (1) above is three. Medium universal joint,
(3) Any one or two of ceramics, cermets, and corrosion-resistant wear-resistant alloys except for the ceramics, cermet, and corrosion-resistant wear-resistant alloys of the joint described in (1) or (2) above. A molten metal medium universal joint characterized by being laminated and coated as described above, (4) the above (1) to
A molten metal medium universal shaft joint characterized in that the main body material of the joint according to any one of (3) is stainless steel or stellite alloy, (5) any of the above (1) to (4) The molten metal characterized in that the ceramic material used only in the torque transmission meshing portion of the joint according to item 1 and the center joint side end face contact portions of both shafts, or the torque transmission meshing portion is sialon-based or silicon nitride-based. Medium universal shaft joint, (6) Used only for the torque transmission meshing portion of the joint according to any one of the above (1) to (4) and the end face contact portion of both shafts on the side of the center joint, or the torque transmission meshing portion. Cermet material is WC ・ Cr ₃ C ₂・ TiC
A molten metal medium universal shaft joint characterized by containing one or more kinds of carbides such as NbC, (7) The joint according to any one of (1) to (4) above. Corrosion-resistant and wear-resistant alloy material used only for the torque transmission meshing part, the end face contact part of both shafts on the center joint side, or the torque transmission meshing part is made of stellite or Co-based self-fluxing alloy (8) A molten metal, characterized in that the respective materials of the torque transmission meshing portion of the joint according to any one of (1) to (4) above are a combination of ceramics and cermet. Universal shaft joint, (9) The coating material of the joint according to the above (3) is a Cr ₂ O ₃ based ceramics / Co-based self-fluxing alloy / W
Molten metal universal joint, characterized in that any one or more of C-based cermet / MoC-based cermet / stellite alloy is laminated, (10) Items (1) to (4) above The ceramic / cermet / corrosion-resistant alloy to be used only for the torque transmission meshing portion of the joint and the end face contact portion of the center joint side of both shafts, or the torque transmission meshing portion of either one of the above items is sintered or melted. A fused metal medium universal shaft joint characterized by being processed into a liner shape and mechanically fixing the liner to a joint body with a bolt or the like, (11) any one of (1) to (4) above. Of the joint for torque transmission and the end face contact part on the center joint side of both shafts, or only the portion for torque transmission meshing. A molten metal medium-shaft universal joint characterized by being formed; (12) the torque transmission meshing portion of the joint according to any one of (1) to (4) above; Or use a cermet or corrosion-resistant wear-resistant alloy that is used only for the torque transmission meshing part,
Molten metal medium universal shaft joint characterized by being formed on the joint body by thermal spraying, (13) Torque transmission meshing portion and both shafts of the joint according to any one of (1) to (4) above. Of the central joint side end face contact portion, or a cermet or a corrosion-resistant and wear-resistant alloy used only for the torque transmission meshing portion is formed on the joint body by powder HIP overlaying, in a molten metal medium shaft joint, The ceramic coating material for a joint according to the item (3) is applied with a chromate or chromic anhydride containing CrO ₃ as a main component by coating or dipping, and then heated to 500 ° C. or higher to 1 to 15 μm.
Which has a Cr ₂ O ₃ coating film as a main component, and a cermet or a corrosion-resistant wear-resistant alloy coating material for a joint according to (15) above, which is characterized in that Is formed on the joint main body by thermal spraying, and (16) the combination of ceramics and cermet in the torque transmission meshing portion of the joint according to (8) above, The gist is a universal shaft joint for molten metal, characterized in that the hardness difference in Hv is 30 or more.

[0012]

The operation of the present invention will be described below with reference to FIG. The joint body is made of an alloy to ensure toughness, and torque A of the groove formed between the claw 9 and the outer cylinders 10 and 10, which receives a wear action due to mechanical sliding resistance, and portions A-1 and A-2 and Center end face contact part B-1 and B-2 of both shafts, or A
Only the -1 and A-2 parts are made to have high hardness and a zinc-resistant reaction material to ensure wear resistance, and the other parts are coated with the dross-resistant material. By doing so, the joint as a whole is a fused zinc medium universal joint that has excellent fracture resistance and wear resistance, maintains good constant velocity over a long period of time, and does not hinder the attachment and detachment of the joint during roll replacement. Can be realized.

[0013]

EXAMPLES The present invention will be specifically described based on examples.
In order to determine the material of the meshing torque transmitting part, the inventors first created a 1/3 model molten zinc in-experimental device capable of ensuring the performance of the joint, and variously changing both materials of the meshing torque transmitting part. The test was conducted. The results are shown in Table 1.
Shown in. Each value shown in the table is the total amount of wear on both sides of the meshed part (mm) after the 24 Hr rotation test for the upper left.
Is the magnitude of the stable state of the torque required to rotate the joint, and the torque value (kgf · m) measured and converted by the strain gauge is the torque value for the combination of Sialon and Sialon. Is set to 1.0, and the measured value in the case of the combination of other materials is expressed by the ratio with the value in the case of the combination of Sialon and Sialon.
Therefore, the smaller the ratio of the rotational torque is less than 1.0, the smaller the meshing sliding load, that is, the frictional resistance, and the better.

From the results of the rotation test, it was confirmed that the combination of Sialon or silicon nitride and the carbide cermet, particularly WC-Co, has a small amount of wear and a small torque required for rotation, that is, a frictional resistance. Since the joints intersect at a certain angle, the meshing contact portions constantly move during one rotation. Therefore, it was found that in a combination of ceramics in which no microscopic plastic deformation can be expected, the wear continues relatively because the rotation continues without getting familiar. Further, due to the unfamiliarity and the poor compatibility due to the structure and chemical properties of ceramics, the torque is relatively large and it is not preferable in operation.

The inventors have confirmed in another test that sialon and silicon nitride are similar in composition and exhibit similar tribological characteristics, and silicon nitride may be positioned in the same position as sialon in terms of performance. It should be noted that sialon / silicon nitride is the most resistant to thermal shock among structural ceramics, and there is no risk of cracking during immersion / lifting in molten zinc at about 500 ° C. Furthermore, the mechanical toughness is at a high level, and there is little concern about cracking due to vibration or impact during rotation, which is also preferable.

Although the combination of Sialon and alloy has a small torque, it is not preferable because of a large amount of wear due to the low hardness of the alloy.

WC-Co has a hardness of about Hv900 to Hv1700 depending on the control of the Co content and other factors. Therefore, we selected sialon as a representative of sialon and silicon nitride, and selected WC-
A rotation test was performed in combination with Co. The result is shown in FIG. Looking at this as a whole, WC-C
The torque is stable regardless of whether the hardness of o is higher or lower than that of Sialon, and the total wear amount of both increases as the hardness difference increases. However, it has been clarified that there is an unstable region where the wear is large and the torque is high in a region where the hardness difference between the two is small, and the hardness difference is within ΔHv30.

In the case of the combination of the oxide ceramics and the carbide cermet, the metal parts of the cermet allow micro plastic deformation and the tribological compatibility between the oxide ceramics and the carbide is good, so that the high hardness materials are Although the combination shows good rotation characteristics, it is considered that the same phenomenon as the poor compatibility between the above-mentioned ceramics will occur as the hardness approaches.

It is clear that the same result is obtained even when the sialon is replaced with silicon nitride. Therefore, in the combination of sialon or silicon nitride and the WC-Co material, the hardness of the two becomes closer to each other in the range of the difference ΔHv30 or more. The optimum combination can be obtained by selecting the combination. It should be noted that the combination of sialon or silicon nitride with other carbide cermets also produces WC-C.
Although a good effect similar to that of o can be obtained, even in this case, it is better to provide an appropriate hardness difference as described above.

[0020]

[Table 1]

A specific embodiment of the present invention will be described below with reference to FIG.
~ It describes based on FIG. Claw side shaft 8 and outer cylinder 10 of the joint body
As the material of SUS410, the claw 9 and the groove of the joint, that is, the torque transmission meshing portions A-1 and A-2, are provided with a WC-Co liner 12 having a hardness of Hv1300 on the claw side, which is processed into the joint body, and mounted in the groove. Then, it was pressed by the end plate 16 made of SUS410 and fixed by the bolt 17 made of SUS410. On the other hand, as the high hardness liner 13, a sialon having a hardness of Hv1350 was fixed to the groove side portion of the outer cylinder in the same manner. In the illustrated example, the number of claws and grooves is three, which is the most preferable mode for practical use.

The liner on the claw side may be shaped as shown in 12-2 of FIG. 3 and fixed to the main body with bolts. In this case, since the joint body is made of SUS410, it is preferable that the bolt 17 screwed into the joint body is made of the same material in terms of the thermal expansion matching property with the screw hole of the claw side shaft 8 of the joint body, but in WC-Co and SUS410, the former material is used. Since the coefficient of thermal expansion is small, a gap will be formed under the bolt neck in the molten zinc bath. Therefore, in this case, it is better to manufacture and attach the spring washer 18 with a corrosion-resistant material such as SUS420J1 material that can be easily baked. In addition, the material of the joint body is not made of austenite which is popular stainless steel, but SUS410 of martensite type.
This is because the material has a smaller coefficient of thermal expansion and is advantageous in compatibility with cermet or ceramics, which is a material having a lower coefficient of thermal expansion. Of course, the joint body may be made of Stellite alloy with better corrosion resistance.
Since the coefficient of thermal expansion is larger than that of 410 and the manufacturing cost is also increased, it may be selected according to the situation.

When ceramics is used as the liner, for example, the thermal expansion coefficient of sialon is about 3 × 10 ^-6 / ° C., SU
Since S410 is about 11 × 10 ⁻⁶ / ° C., a gap is still formed in the fitting portion in the molten zinc. The zinc that has entered this gap does not fall out when the joint is pulled up, and it solidifies in the gap, so it becomes a force that pushes out the ceramics.
It causes breakage. Therefore, when the rolls are changed frequently, that is, the joints are pulled up and re-immersed frequently, it is better to increase the thickness as much as possible and to reduce the plane dimension in order to secure the strength of the ceramics. When the torque is small and the wear condition is mild, it goes without saying that both sides may be cermets or a combination of cermet and high hardness alloy, and by doing so, there is no fear of fracture.

When forming a cermet or a high hardness alloy layer by welding / spraying overlay or HIP on the torque transmission meshing portion and the contact portion of the end faces of the central joints of both shafts, SUS41
When 0 is cooled from a high temperature, martensitic transformation occurs and cracking sensitivity is high. Therefore, if SUS430J2, which is a ferrite-austenite two-layer stainless steel, is used as the main material,
There is no fear of cracking during cooling after overlaying, and better results can be obtained.

Contact portions B-1 and B- on the end faces of the central joints of both shafts
No. 2 has low surface pressure, and wear resistance and low frictional resistance are not required as much as the torque transmission meshing portion. 1 material is 5mm thick liner 1
It was set to 4, 15 and attached to the main body with bolts. The same effect can be obtained by forming the Co-based self-fluxing alloy sprayed to a thickness of about 1.5 mm. Of course, another suitable combination of other high hardness ceramics, cermets, or two kinds of ceramics, cermets, and alloys may be used.

The ceramic / cermet / alloy liner was attached to the portions other than the torque transmission meshing portion and the end face contact portions of both shafts on the side of the center joint as a dross adhesion preventing coating material and also for preventing erosion. Then WC
-12Co material with HVOF thermal spray gun, including the end plate and the bolt head, to the entire surface except the torque transmission meshing part and the end face contact part of the center joint side of both shafts.
Coatings with a thickness of 5 to 0.08 mm were carried out. After that,
The entire joint was immersed in chromic acid containing CrO ₃ as a main component to be adhered thereto, and then heated to 520 ° C. to form a 0.01 mm-thick substantially Cr ₂ O ₃ ceramic thin film to form a two-layer coating. Instead of the thermal spray material, MoC cermet or stellite alloy may be used. When a joint is often assembled and disassembled, it often hits, and when peeling of the sprayed coating is concerned, Co-based self-fluxing alloy spraying with higher adhesion may be used. Further, the above-mentioned Cr ₂ O ₃ system ceramics, Co-based self-fluxing alloy, WC system cermet, MoC
Any one or two of the cermet and stellite alloys
Properly stacking two or more species will ensure the full effect.

When this universal shaft joint was used in a continuous hot dip galvanizing line, it was possible to continuously use it for 6 months with the chatter mark of the steel plate being at an extremely low level.

FIG. 4 is a perspective view showing another embodiment of the present invention, showing a two-jaw type long-life constant velocity universal joint, and FIG. 5 is a sectional view taken along the line a--a in FIG. 6 is a sectional view taken along line bb in FIG. 4, and FIG. 7 is a sectional view taken along line cc in FIG. 4 to 7, the same reference numerals as those in FIGS. 1 to 3 indicate members and materials of the same kind.

[0029]

By adopting the joint of the present invention, chatter marks on the surface of the steel sheet, which has been a problem in the conventional non-constant velocity type cross pin joint, are greatly reduced. In addition, the life of the joint can be greatly extended and it has become possible to use it for 6 consecutive months.

[Brief description of drawings]

FIG. 1 is a three-dimensional view showing the structure of a universal joint according to the present invention.

FIG. 2 is a sectional view taken along the line EE of FIG.

FIG. 3 is a view showing a method of taking out only a tip portion of a nail and attaching another high-hardness liner.

FIG. 4 is a perspective view showing another embodiment of the present invention.

5 is a sectional view taken along line aa of FIG.

6 is a sectional view taken along line bb of FIG.

7 is a sectional view taken along line cc of FIG.

FIG. 8 shows the results of examining the amount of wear and the torque by performing a rotation test by changing the hardness of WC-Co using a 1/3 model experimental device in the combination of sialon and WC-Co in the torque transmission meshing portion. Graph.

FIG. 9 is a schematic side view illustrating the configuration of a continuous hot-dip galvanizing apparatus.

FIG. 10 is a front view showing the configuration of a support roll.

FIG. 11 is a perspective view showing a conventional cross pin joint.

[Explanation of symbols]

1 Plated Steel Plate 2 Hot Dip Galvanizing Bath 3 Support Roll 4 Hot Dip Galvanized Universal Shaft Coupling 5 Intermediate Conduction Shaft 6 Cross Pin 7 Cross Groove Sleeve 8 Joint Claw Side Shaft 9 Joint Claw 10 Joint Outer Cylinder 11 Joint Outer Cylinder Side torque transmission meshing part 12 High-hardness liner attached to the torque transmission meshing part of the joint side of the joint 13 Attached to the torque transmission meshing part of the outer cylinder side of the joint High hardness liner 14 Attached to the center end surface contact part of the joint side of the joint High hardness liner 15 High hardness liner attached to the center end surface contact part of the outer cylinder side of the joint 16 End plate of the joint 17 Bolt of the joint 18 Spring washer of the joint

Claims (16)

[Claims] 1. A plurality of claws radially extended at equal intervals are fixed to a shaft end of one shaft, and a plurality of the claws are inserted into a shaft end of a mating shaft meshing with the shaft end. In a clutch type universal shaft joint in which an outer cylinder having a groove is fixed, the universal joint body is made of an alloy, a meshing portion for transmitting torque of the claws and the groove, and an end face contact portion on the center joint side of both shafts, or A molten metal medium universal shaft joint characterized in that only the torque transmission meshing part is made of ceramics, cermet or corrosion resistant wear resistant alloy to the required thickness. 2. The molten metal medium universal joint according to claim 1, wherein the number of claws and grooves is three. 3. A portion other than the portion made of ceramics, cermet, or corrosion resistant wear resistant alloy is
The molten metal medium universal shaft joint according to claim 1 or 2, which is laminated and coated with any one kind or two or more kinds of cermet and corrosion resistant wear resistant alloy. 4. The material of any one of claims 1 to 3, wherein the main body material is stainless steel or stellite alloy.
A molten metal medium universal joint as described in the item. 5. A ceramic material used only for the torque transmission meshing portion and the end face contact portions of the two shafts on the side of the center joint or for the torque transmission meshing portion is made of sialon or silicon nitride. 5. A molten metal medium universal shaft joint according to any one of claims 4 to 4. 6. The material of the cermet used only for the torque transmission meshing portion, the end face contact portion on the center joint side of both shafts, or the torque transmission meshing portion is WC.Cr ₃ C ₂ .T.
The molten metal medium universal joint according to any one of claims 1 to 4, wherein any one or more kinds of carbides such as iC and NbC are contained. 7. The corrosion-resistant and wear-resistant alloy used only for the torque transmission meshing portion, the end face contact portion of the both shafts on the side of the central joint, or the torque transmission meshing portion is made of stellite or a Co-based self-fluxing alloy. Claim 1
4. A molten metal medium universal joint according to any one of 4 above. 8. The molten metal medium universal joint according to claim 1, wherein the respective materials of the torque transmission meshing portion are a combination of ceramics and cermet. 9. A coating material of Cr ₂ O ₃ based ceramics C
4. The universal shaft joint for molten metal according to claim 3, wherein any one of O-based self-fluxing alloy, WC cermet, MoC cermet, and stellite alloy is laminated. 10. Ceramics, cermet, and corrosion-resistant wear-resistant alloy used only in the torque transmission meshing portion and in the end face contact portion of both shafts on the center joint side, or in the torque transmission meshing portion are sintered or melted into a liner shape. 5. The molten metal medium universal shaft joint according to claim 1, wherein the liner is mechanically fixed to the joint body with a bolt or the like. 11. A cermet or a corrosion-resistant and wear-resistant alloy used only for the torque transmission meshing portion and the end face contact portions of both shafts on the side of the center joint or for the torque transmission meshing portion is formed on the joint body by weld overlay. The molten metal medium universal shaft joint according to any one of claims 1 to 4. 12. A cermet or a corrosion-resistant wear-resistant alloy which is used only for a torque transmission meshing portion and an end face contact portion on the center joint side of both shafts, or a torque transmission meshing portion is formed on a joint body by thermal spraying. The molten metal medium universal shaft joint according to any one of claims 1 to 4. 13. A cermet or a corrosion-resistant wear-resistant alloy which is used only for a torque transmission meshing portion and an end surface contact portion on the center joint side of both shafts, or a torque transmission meshing portion is formed on the joint body by powder HIP overlaying. The molten metal medium universal joint according to any one of claims 1 to 4. 14. The ceramic coating is applied to a chromate or a chromic anhydride containing CrO ₃ as a main component by coating or dipping, and then heated to 500 ° C. or higher for 1-1.
4. The universal shaft joint for molten metal according to claim 3, wherein a film containing Cr ₂ O _{3 of} 5 μm as a main component is formed. 15. The molten metal medium universal shaft joint according to claim 3, wherein a cermet or a corrosion and wear resistant alloy coating material is formed on the joint body by thermal spraying. 16. The universal shaft joint for molten metal according to claim 8, wherein the hardness difference between the ceramics and the cermet in the torque transmission meshing portion is 30 or more in terms of Hv.