Metal treatment – Stock – Ferrous
Reexamination Certificate
2001-10-10
2004-01-06
Yee, Deborah (Department: 1742)
Metal treatment
Stock
Ferrous
C148S902000
Reexamination Certificate
active
06673167
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, and more particularly to a power transmission shaft used in a constant velocity joint.
The present invention also relates to a power transmission shaft that transmits torque via splines or serration.
A power transmission shaft, for example the drive shaft of an automobile, is usually made of carbon steel that is subjected to heat treatment in order to harden the surface thereof, thereby ensuring a predetermined level of strength. Recently, as the automobiles tend to have increasing output power and the vehicle weight increases in order to meet higher safety requirements, the drive shaft is required to have increasingly higher strength. On the other hand, the drive shaft is required to be lighter in weight in order to improve the fuel saving efficiency, which also imposes a pressing need to increase the strength of the drive shaft.
In order to increase the load bearing capacity of a shaft, it is common to increase the strength of the material by increasing the carbon content of the material or other means. With this approach, strength can be increased in smooth-surfaced portions although notched portions such as serration tend to be subjected to quenching crack or other abnormality, leading to lower strength, contrary to the intended purpose. A torsion test was conducted on a serration shaft, and breakage mode was analyzed. The analysis showed that the material is subject to shear fracture when the carbon content is low, but the principal stress becomes dominant as the carbon content increases and the material breaks due to the principal stress (ratio of inter-granular rupture increases). This result also shows the necessity for further inter-granular enhancement of the steel structure. Increasing the carbon content also leads to deterioration of machinability such as forging and cutting.
Accordingly, a first object of the present invention is to further increase the strength and decrease the weight of the power transmission shaft used in a constant velocity joint, without compromising the machinability.
Power transmission shafts for transmitting torque are used in many mechanical components in such apparatuses as automobile and industrial machinery. Among power transmission shafts, spline shafts and serration shafts that transmit high torque, in particular, are manufactured by applying surface hardening treatment such as carburizing, induction hardening or nitriding and/or heat treatment such as thermal refining to medium-carbon steel or low alloy steel to increase the strength of the shaft, while taking into consideration the ease of plastic processing, machinability and cost.
Recently, as the concern about the global environmental issues increases, stricter regulations on the automobile emissions and improved fuel saving efficiency are being called for, and accordingly measures to reduce the weight of the automobile have been taken in order to meet these requirements. Spline shafts and serration shafts are widely used for the drive shaft and propeller shaft of automobiles for coupling with constant velocity joints, and reduction of the weight of the spline shafts and serration shafts makes a great contribution to the weight reduction of an automobile. Thus there is a strong requirement for increasing the strength of these shafts both in terms of static strength and fatigue strength.
Hence a second object of the present invention is to increase the static strength and fatigue strength of a power transmission shaft having torque transmitting teeth such as spline or serration.
SUMMARY OF THE INVENTION
In order to achieve the first object described above, the present invention provides a power transmission shaft which is suitable for a power transmission shaft having notched portion such as torque transmitting teeth, particularly for a power transmission shaft used in a constant velocity joint, and which is made of a carbon steel that is subjected to induction hardening to achieve a hardening ratio in a range from 0.25 to 0.50. As the carbon steel described above, such a material is used that contains 0.39 to 0.49% of C, 0.4 to 1.5% of Si, 0.4 to 1.0% of Mn, 0.025% or less S, 0.02% or less P, and 0.01 to 0.1% of Al by weight as basic components, with the rest comprising Fe and inevitable impurities.
The hardening ratio is given by the effective depth of hardened layer divided by the radius of shaft. When this ratio is lower than 0.25, strength of the shaft decreases because breakage starts inside the shaft (core). When the ratio is higher than 0.50, on the other hand, quenching crack occurs in notched portions such as serration.
Among the elements described above, C content lower than 0.39% leads to insufficient strength due to a low surface hardness achieved by induction hardening, and C content higher than 0.49% leads to lower strength due to excessively high hardness that results in increased notch sensitivity of the notched portion.
FIG. 3
shows measurements of the static torsional strength as a function of carbon content, indicating that sufficient strength (about 1600 Mpa of failing stress) can be obtained when the carbon content is within the range described above.
Si is added as a deoxidizing agent and for the purpose of enhancement of grain boundary during the steel making process. When Si content is lower than 0.4%, the effect of grain boundary enhancement cannot be obtained. When Si content is higher than 1.5%, cold workability (ease of forging and cutting by turning) lowers significantly.
Mn content is required to fix sulphur content of the steel in the form of MnS and diffuse it. When Mn content is lower than 0.4%, hardenability becomes lower (sufficient depth of hardening cannot be obtained). When Mn content is higher than 1.0%, effect of hardenability reaches a plateau resulting in lower cold workability.
S, existing in the form of MnS inclusion by bonding with Mn, may become the start point of cracking during cold working, and therefore the content thereof is kept 0.025% or lower. Content of P, that precipitates in the grain boundaries of steel thereby to lower hot workability and decrease the material strength, is kept to 0.02% or less.
Al, used as a deoxidization agent to remove oxygen included in the steel by being oxidized during the steel making process and for the purpose of controlling the grain size, is contained with a concentration not less than 0.01%. Since a high concentration of oxide lowers the toughness and the oxide may become the start point of crack during cold working, Al content is kept within 0.10%.
When ferrite grains in steel structure are too large, quenching crack sensitivity increases remarkably and therefore ferrite grain size number (JIS G0552) of the carbon steel is set to 7 or higher. The grain size number refers to a unit representing a number of grains included in unit area or unit volume of a polycrystal material. It is generally represented by the grain size number determined from measurement of the unit area. The grain size can be measured in, for example, a core portion of the shaft where the thermal effect of induction hardening has not reached.
The carbon steel includes 0.001 to 0.004 weight percent of B, 0.02 to 0.05 weight percent of Ti and 0.008 weight percent or less N, with the ratio of Ti/N not less than 3.4.
B is added for the purpose of improving the hardenability, grain boundary enhancement and reduction of quenching crack sensitivity. When B content is less than 0.001%, these effects cannot be obtained sufficiently and, when B content is higher than 0.004%, BC is formed in the grain boundaries thus lowering the strength. Ti is added for the purpose of fixing N through the formation of TiN, and thereby preventing the formation of BN. When Ti content is lower than 0.02%, formation of BN cannot be prevented and, when Ti content is higher than 0.05%, cleanliness of the steel deteriorates thus leading to lower strength. While N is included in the st
Makino Hiroaki
Wakita Akira
Yoshida Kazuhiko
Arent Fox Kintner & Plotkin & Kahn, PLLC
NTN Corporation
Yee Deborah
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