Rotary shafts – gudgeons – housings – and flexible couplings for ro – Coupling accommodates drive between members having... – Tripod coupling
Reexamination Certificate
1999-12-21
2002-05-21
Browne, Lynne H. (Department: 3629)
Rotary shafts, gudgeons, housings, and flexible couplings for ro
Coupling accommodates drive between members having...
Tripod coupling
C464S902000
Reexamination Certificate
active
06390924
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a power transmission shaft used in apparatuses such as automobiles and industrial machines to transmit torque via a constant velocity joint. The present invention also relates to a constant velocity joint used in apparatuses such as automobiles and industrial machines to transmit driving power.
A power transmission shaft, for example the drive shaft of an automobile, is usually made of carbon steel or carburized steel, and is ensured to have a specified strength by setting proper surface hardening and effective case depth achieved by a heat treatment.
Recently, as the automobiles tend to have increasing output power and the vehicle weight increases for higher safety requirements, the drive shaft is required to have higher strength. On the other hand, the drive shaft is required to be lighter in weight in order to improve the fuel efficiency, that also imposes a pressing need to increase the strength of the drive shaft.
In order to increase the load capacity of the shaft, it is common to increase the carbon content of the material thereby to achieve a higher material strength or increase the effective depth of hardened layer (case depth). However, the former approach leads to decreased strength in notched parts, and lower workability, such as the ease of forging and cutting, due to the increased hardness of the material. The latter approach, on the other hand, leads tot very narrow range of case depths that can be obtained in the case of carburized steel. Also in the case of a shaft made of carbon steel, it becomes more difficult to apply deep hardening as the shaft diameter increases, and it is very difficult to carry out deep hardening with the ratio of effective case depth to shaft radius (hereinafter denoted &ggr;) higher than 0.4 since it may lead to defects such as quenching crack. Recently carbon steel that contains boron B added has been used to enable deep hardening. However, even though the effective case depth is increased with the use of this material, only an increase in the strength up to about 15% is possible since the static strength and the torsional fatigue strength reach the plateau at &ggr;>0.65 and &ggr;>0.5, respectively (Japanese Patent Application Laid-open No. Hei 5-320825). Also in the case of a material with B added, hard nitrogen compounds such as TiN are formed that may lead to lower cutting workability.
The constant velocity joint used in the power transmission shaft falls roughly into two classes of fixed type that allows displacement only in the angle between the two shafts, and sliding type that allows both angular displacement and axial displacement, which are selected according to the operating conditions, purpose and other factors. The fixed type includes the Rzeppa type constant velocity joint and the sliding type includes the double offset type onstant velocity joint and tripod type constant velocity joint as the representative ones.
Applications of the constant velocity joint include the power transmission system of the automobile. Recently, as the automobiles tend to have increasing output power and the vehicle weight increases for higher safety requirements, constant velocity joints of the drive shaft are required to have higher strength. On the other hand, the drive shaft is required to be lighter in weight in order to improve the fuel efficiency, that also imposes a pressing need to increase the strength of the constant velocity joint.
An outer member (outer race) that is a constituent element of the constant velocity joint is made of carbon steel or the like, that is forged into a predetermined form and subjected to heat treatment such as induction hardening in order to ensure the required levels of strength, durability and wear resistance, followed by grinding of portions that require high precision thereby finishing the part to the predetermined dimensions and completing the product. High strength requirement in this case may be satisfied by either increasing the carbon content thereby to increase the material strength or increasing the effective case depth. The former method, however, lowers the machinability for such processes as forging and cutting and leads to increased manufacturing cost. The latter method, on the other hand, is limited in the effect of increasing the strength since the anticipation of defects such as quenching crack makes it difficult to apply further deep hardening.
Constituent elements (inner member, cage, tripod member, etc.) of the constant velocity joint are made of carbon steel or the like, that is machined to a predetermined form and subjected to carburizing treatment in order to ensure the required levels of strength, durability and wear resistance, followed by grinding of portions that require high precision thereby finishing the part to the predetermined dimensions and completing the product.
When a part is carburized by heat treatment, however, the part undergoes a significant deformation caused by the heat treatment with variations in the dimensions. Thus it has been necessary to finish the parts by grinding after the heat treatment. Also pocket surfaces on both sides of the axis among pockets of the cage, for example, must have a certain surface accuracy in order to regulate the positions of torque transmitting balls, but the grinding process after the heat treatment is sometimes omitted in order to simplify the machining process. When the grinding process is omitted, parts that have large deformations caused by the heat treatment are rejected, resulting in higher reject ratio.
Accordingly, an object of the present invention is to provide a power transmission shaft that has high workability for such processes as forging and cutting, and high strength.
Another object of the present invention is to increase the strength of an outer race of a constant velocity joint while simplifying the machining processes for lower manufacturing cost and increase the accuracy.
Further another object of the present invention is to simplify the machining processes for the components of the joint such as the inner member, the cage and the tripod member and cut down on the manufacturing cost of the constant velocity joint.
SUMMARY OF THE INVENTION
The power transmission shaft will first be described below.
According to the present invention, in order to achieve the object described above, in the power transmission shaft using the constant velocity joint, graphite steel is subjected to induction hardening thereby to increase the surface hardness, and a 2-phase structure of ferrite and martensite is formed in the core. Graphite steel is made by graphitization annealing to turn the cementite included in the carbon steel into graphite, and such properties as high cutting machinability due to the inclusion of graphite that is a free cutting element and favorable property for cold forging and warm forging due to softness. Consequently, graphite steel maintains high machinability even when treated to include a high concentration of carbon for the purpose of increasing the strength.
While majority of the conventional power transmission shafts have been manufactured by applying induction hardening treatment to carbon steel, the core is not subjected to the influence of heat in many cases in order to avoid defects such as quenching crack. Even in such cases as the core is subjected to the influence of heat, most of the core has turned into martensite and therefore the residual compressive stress on the surface has diminished. According to the present invention, on the contrary, effect of the heat by induction hardening not only hardens the surface layer but also reaches the core to form solid solution of graphitewith ferrite, thereby turning the core into 2-phase structure of ferrite and martensite. As a consequence, residual compressive stress remains on the surface thus making it possible to achieve higher strength and high resistance against fatigue. In order to give the effect of heat treatment to the core, it is preferable to carry out induction hardening a
Kondou Hideki
Makino Hiroaki
Wakita Akira
Yoshida Kazuhiko
Dunwoody Aaron M
NTN Corporation
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