Material selection and processing routes will each have a strong effect on the properties of a finished golf shaft. The section that follows summarises the manufacturing process for steel and graphite shafts.
2.3.1.1 Steel shafts
From the 1920s until the introduction of graphite shafts in the 1970s, the majority of golf shafts were manufactured from steel. A detailed description of the manufacturing process of steel shafts can be found in Maltby (1995, pp. 550-556). Briefly, Maltby describes the manufacturing process as follows: the process begins with the forming of high-alloy steel strips into tubes. High frequency welding is then used to close each tube. The diameter of this tube is bigger than the diameter of the finished shaft, so diameter and wall thickness are reduced on a draw bench. This process is repeated six to eight times until the butt diameter of the finished shaft is reached. A series of dies then produces the stepped tapering of the shaft. Next, a heat treatment improves the hardness and the strength of the shaft. After it is straightened (if necessary), the shaft is cleaned and plated with nickel and chrome for corrosion protection.
2.3.1.2 Graphite shafts
Despite their name, graphite shafts are typically made from Carbon Fibre Reinforced Polymers (CFRPs). Graphite shafts can be either sheet-wrapped or filament-wound. Maltby (1995, pp. 634-341) and Cheong et al. (2006) describe the sheet wrapping manufacturing route as follows. It begins with the production of crystalline carbon fibres from Polyacrylonitrile (more flexible) or pitch (highest carbon content, less flexible). These fibres are pre-impregnated with epoxy
resin and woven to form pre-pregs. These pre-pregs are cut into sections (flags) with different fibre angles. These pieces of pre-preg material are rolled around tapered steel mandrels to form the shaft, resulting in a total number of approximately seven layers (see Figure 3). Usually, each flag is rolled around the mandrel more than once. The ends of the pre-pregs form seams, which have been found to result in inconsistencies in the mechanical properties of the shaft (Huntley, Davis, & Strangwood, 2004). Furthermore, micro-structural analysis (Huntley, 2007) has shown that manufacturers usually roll some of these layers simultaneously and other layers consecutively. When two layers are rolled simultaneously, this will result in an alternating order of pre-pregs (see inner layer 1 and 2 in Figure 4(b)). When two layers are rolled consecutively, this will result in a different sequence of layers (outer layer 1 and 2 in Figure 4(b)). In terms of fibre orientation1 and order of plies, Cheong et al. (2006) presented a model of a shaft with ±45° fibres as inner layers and 0° fibres as outer layers. This is in agreement with the majority of shafts sectioned by Huntley (2007) as well as shafts described by Sabo (1995) and Zako et al. (2004), so it will be assumed here that this is the typical construction of sheet- laminated shafts.
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Fibre orientations are described relative to the longitudinal axis of the shaft, with 0° meaning that the fibres are parallel to the longitudinal axis of the shaft.
Figure 3: Lay-up process for sheet-laminated shafts (adapted from Cheong, Kang, & Jeong, 2006, p. 465).
When all pre-pregs are in place on the mandrel, it is covered with tape providing the necessary pressure during the curing process. After the curing process, the mandrel is removed, and the outer surface of the shafts is sanded and polished to finish. This manufacturing route leaves four options for deliberately altering the mechanical characteristics of the shaft: (1) the number and order of pre- preg layers, (2) the fibre orientation of each layer, (3) the fibre type of each layer and (4) the geometry of the mandrel. Through the work of Huntley (2007) it is evident that changes in other aspects of shaft manufacture can also lead to changes in the mechanical characteristics of the shafts, for instance the amount of interfacial material between plies.
For filament-wound shafts, the only difference is in the method used to place fibres on the mandrel. Rather than rolling plies on a mandrel, a machine wraps two layers of pre-preg tape around it. After this, a filament winding machine weaves carbon fibres around this mandrel (Maltby, 1995, p. 626). In the case of filament winding, the angle of the fibres is configured by varying “the distance the winding head travels down the length of the mandrel per mandrel revolution” (Howell, 1992, p. 1397). Furthermore, it is possible to change the number of circuits in a layer, which is the number of times the winding head travels up and
down the shaft before finishing one layer. In contrast to the lay-up process, filament winding allows the manufacturer to vary the amount of tension on the fibres while they are wound on the mandrel (Howell, 1992).
The differences in the manufacturing process of filament-wound and sheet- laminated shafts manifest themselves in the mechanical shaft properties. Whilst filament winding involves less manual labour, creates more flexibility in the design of the lay-up and avoids seam effects at the end of pre-pregs sheets (Howell, 1992), it is also more expensive and leads to a decreased fibre content (Huntley, Davis, Strangwood, & Otto, 2006).
(a)
(b)
Figure 4: (a) Typical composition of a sheet-laminated shaft from a number of layers2 of carbon/epoxy pre-pregs (adapted from Cheong, Kang, & Jeong, 2006, p. 469). (b) Resulting lay-up of carbon/epoxy layers.
Fibres of the inner layer are typically oriented at ±45°, fibres of the outer layer at 0° (adapted from Huntley, 2007, p. 140).
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