1. Field to the Invention
The present invention relates to a golf club shaft and more particularly to a golf club shaft, made of a fiber reinforced resin, in which the structure of prepregs is specified and which is lightweight and has an appropriate rigidity and a high strength.
2. Description of the Related Art
In the case where prepregs in which fibers are impregnated with a resin is used as the material of the golf club shaft, the prepregs are molded by winding on a mandrel a so-called straight layer whose reinforcing fiber is parallel with the major axis of the golf club shaft or alternatively an angular layer whose reinforcing fiber forms a certain angle with the major axis of the golf club shaft.
In recent years, the art for making lightweight shafts have been developed. The lightweight shaft is required to have a proper degree of rigidity and a high strength. Therefore there are various proposals made to improve the material of prepregs composed of the lightweight shaft and the laminated structure thereof and to improve the strength of the lightweight shaft.
For example, in the shaft disclosed in Japanese Patent Application Laid-Open No.5-49717, the straight layer has a two-layer structure consisting of an inner layer and an outer layer. The inner layer is made of carbon fiber having a high elasticity, whereas the outer layer is made of carbon fiber having a high strength. Thus the shaft has a weight of less than 63 g when the entire length of the shaft is 45 inches. That is, a shaft which is lightweight and has a proper degree of rigidity is proposed.
The shaft disclosed in Japanese Patent Application Laid-Open No.5-49717 uses prepregs of different kinds in combination, namely, a prepreg having a high elasticity and an intermediate strength and a prepreg having a high strength and an intermediate elasticity. Thus to achieve a shaft having an appropriate rigidity and a high strength, it is inevitable that the number of prepregs increases and hence the shaft becomes heavy. Thus although it is described in the specification that the prepreg has a high elasticity and a high strength, in fact, the prepreg has an insufficient strength. Thus the flexural strength is affected by carbon fiber having a low strength and a high elasticity. That is, the shaft having the above-described structure is incapable of providing sufficient strength.
To make the shaft lightweight, it is necessary to decrease the amount of the reinforcing fiber to be used. A decrease in the amount of the reinforcing fiber leads to a reduction of the strength of the shaft. The reinforcing fiber for the conventional shaft has a problem that one reinforcing fiber has a high tensile modulus of elasticity and hence a high rigidity but has an insufficient strength and other reinforcing fibers have a high tensile strength and hence a high strength but a low rigidity. There is proposed a laminated structure in which the advantage of both reinforcing fibers are utilized by combining both reinforcing fibers appropriately with each other. However, it is difficult to make the shaft lightweight. As such, there is a demand for the development of a shaft that has the smallest possible amount of reinforcing fibers and that of prepregs to be layered one upon another and yet has a sufficient strength and an appropriate degree of rigidity (flexure).