Abstract:
A face wall for the hitting face of a golf club head is supported on a hollow structural shell. The face wall is formed to realize maximum face strength with minimum face mass. This is accomplished by varying the thickness of the face wall so it is thickest in the general vicinity of the face center and becomes thinner toward the edges of the face. This allows the club head to weigh less, incorporate a large face area and adequate strength while maintaining high moments of inertia of the head.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to a new construction for the face wall of a golf club head. 
     Nearly all modern, popular heads called “woods”, such as the driver and the fairway woods, are in the form of hollow shells, usually of metal. Driver heads must not weigh more than about 210 grams, or there is an unacceptable penalty in maximum distance of drives. The present inventors have done research which indicates that for maximum drive distance, optimum head mass may be as small as 180 grams and the shaft may be longer than usual. This finding is in reasonable agreement with modern trends in driver design. In addition, a large face is highly desirable because it strongly reduces the percentage of hits which are partly off the face (which the present inventors call POF hits). The present inventors have found that large faces are especially important because these POF hits are usually the worst hits a golfer makes. Large moments of inertia of the club head about its center of gravity are also highly desirable because they reduce errors caused by hits which are somewhat off center. Large size correlates closely with large moments of inertia, because this puts mass farther from the center of gravity. 
     These considerations bring about a design limitation in the maximum size of face which will have adequate strength for withstanding impact of club head and ball. The present invention respects this limitation, while concurrently allowing club heads to have larger faces. 
     FIG. 1 (prior art) shows an elevation view of a common design of bridge trusses for illustrative purposes. Supports are indicated at numeral  10  at the ends. At mid-span, the truss is often deeper (thicker) than at the supports as indicated at  12  to accommodate the greater bending stresses in this region. This has limited similarity to the construction of the face wall in the present invention. Such configurations have not been used in connection with golf club faces in the years during which hollow club head construction has been favored. There are other important differences from a beam, such as the club face wall of the present invention being a continuous structure rather than an assembly of beams, the requirement for the ball hitting surface to be an integral part of the structural elements, and the face surface being elliptical in shape, or having other shapes which are used on golf clubs. 
     FIG. 2 (also prior art) is a downward looking cross section of the face wall of a typical modern prior art “wood” type club head which is made of metal. The face wall, which has a hitting surface  13 , has small ribs  14 , extending from top to bottom of the face wall, which are integrally formed and intended to improve the strength of the face wall without much increase in face wall mass. The present inventors have shown that the addition of small ribs such as those illustrated in FIG. 2 actually tend to reduce the strength of the face wall if the face wall mass is maintained constant. 
     U.S. Pat. No. 5,380,010 issued to the present inventors, and U.S. Pat. No. 5,464,211 (C. Atkins), U.S. Pat No. 5,570,886 (Rigal et al), U.S. Pat. No. 4,076,254 (G. Nygren), and U.S. Pat. No. 664,438 all show internal bracing between the inside surface of the face wall and other parts of the club head to provide adequate face strength. In order to maintain total head mass at a desired value, all involve removing peripheral mass and adding at least part of the removed mass at locations nearer to the center of gravity to provide the internal bracing, thus lowering moments of inertia. 
     U.S. Pat. No. 4,903,781 (D. Allen) shows a honeycomb structure to support the face. It has nominally uniform bending strength. 
     U.S. Pat. Re. No. 34,925 (J. McKeighen) shows a construction using a face wall which varies in thickness in an opposite sense, with thicker outer portions and a thinner center portion as compared with the present invention. As a result, it actually requires greater face wall mass for adequate strength. 
     U.S. Pat. No. 5,163,682 (G. Schmidt et al.) describes a face wall structure in which, compared with the center thickness, the face wall is thinner toward the toe or toward the heel, or both. Toward the toe end, it is of constant thickness in the up-down direction. Toward the heel end, there is a thickened region which starts approximately at the face center and runs toward the lower part of the heel end of the face, its purpose being to facilitate the flow of metal into the face wall when the head is cast. The present invention not only uses thickness variation in the toe-heel direction, but also in the up-down direction, and whereas patent &#39;682 specifies the presence of the thickened portion running toward the heel, the present invention does not depend on any such thickened portion running toward the heel. U.S. Pat. Nos. 5,318,300 and 5,474,296, (both to Schmidt et al.) are similar in construction to each other. 
     SUMMARY OF THE INVENTION 
     This invention provides for increasing the maximum size of the hitting face of a golf club that is usable by having a structural configuration which allows increased moments of inertia and better optimizing of the location of the center of gravity. 
     The face wall is made thicker in the central area where bending stresses are greatest and progressively thinner toward the edges of the face, where bending stresses diminish. The face wall remains thick enough near the edges so that shear stresses will not cause failure. In this choice of thickness variations, consideration is given by the present inventors to hits anywhere on the face, not only hits at the face center. Alternately, similar bending strength variation and corresponding mass reduction may be achieved by use of properly designed ribs, a honeycomb structure, or a sandwich structure rather than simple variations of the face wall thickness, wherein such alternate structures do not extend all the way to the edges of the face. 
     This optimum design includes a center of gravity location which is roughly in the vicinity of the geometric center of the club head and favors location of the mass of the club head as far from this center of gravity as practical. 
     The term “perimeter weighting” is ordinary terminology commonly used by golfers, and roughly described the need for proper distribution of the mass. In practical designs, all or most of the walls of modern hollow club heads are much thinner than the face wall to allow the face wall to be thicker so as to have adequate strength to resist impact of club head and ball. The additional mass in the face wall from a thick, uniform wall moves somewhat more mass closer to the center of gravity as a necessary design compromise, and in turn, this reduces the most important moments of inertia. Accordingly, it is important to add no more mass to the face wall than necessary. 
     The scatter of the centers of impact of hits by golfers of various skills has been shown to have a normal statistical distribution as described in some detail in U.S. Pat. No. 5,366,223. All golfers sometimes have hits for which the impact is partly off the face. This problem is much worse for less skilled golfers. These POP hits are probably the worst hits in golf and a large face greatly reduces them, especially for drivers, because a tee is used. For this reason, a large face is very important for a good design, especially for drivers. 
     The present inventors have also found by extensive mathematical analysis that there exists an optimum combination of values for the center of gravity location, the loft angle, the moments of inertia, and the club head speed. Reducing unnecessary mass in the face facilitates approximating these optimum values. 
     The present invention uses local values of face wall thickness which provide adequate bending strength in those areas where bending failure would be most likely to happen and adequate shear strength in those areas where shear failure would be most likely to happen. This leads to greater thickness in the central part of the face and lesser thickness in the outer parts of the face wall, where it joins the heel, toe, top and bottom walls. Alternately, appropriately designed sandwich or honeycomb structures, or ribs may be used in place of, or in addition to, varying the thickness of the face wall. In such cases, the dimensions of such alternate structures vary appropriately with distance from the face center, having less mass toward the periphery and satisfying the need for greater bending strength near the center with adequate shear strength near the periphery; and may even shrink away toward the periphery to a simple homogeneous face wall of adequate thickness. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows for illustrative purposes a span in one kind of a prior art vehicular bridge which varies in thickness somewhat as in this invention; 
     FIG. 2 is a cross sectional view of the face wall portion of a typical prior art driver head, looking downward, and showing small ribs on the inner surface which extend from top to bottom of the face wall; 
     FIG. 3 is an elevation view of a driver face with contour lines representing uniform thicknesses of the face wall to show how face wall thickness varies in an unusually large face driver design embodying the present invention; 
     FIG. 4 is a partial cross section of the hitting face, looking downward along lines  4 — 4  in FIG.  3  and is a representation of a club where there is no curvature of the hitting surface of the club face; 
     FIG. 5 is a cross section of the face wall taken on line  5 — 5  in FIG. 3; 
     FIG. 6 is a top plan view of a typical golf club head with the face wall sectioned similarly to FIG.  4  and for a club face having typical curvature of its outer surface; 
     FIG. 7 is a view taken on line  7 — 7  in FIG. 6 except that it is a club face having typical curvature of its outer surface; 
     FIG. 8 is a top plan view of the club head of FIG. 7 with part of the top wall broken away; 
     FIG. 9 is a fragmentary sectional view similar to FIG. 4 illustrating a honeycombed construction for the face wall; 
     FIG. 10 is a fragmentary front view of the face of the golf club structure shown in FIG. 9 with parts broken away to show interior wall members; 
     FIG. 11 is a fragmentary sectional view similar to FIG. 4 showing a multi-layered composite face wall; and 
     FIG. 12 is similar to FIG. 3, except it is a more conventional face shape. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The design of the present invention provides a desired club head mass together with maximum face size, adequate face wall strength, and with maximum moments of inertia of the club head about the center of gravity of the head for any orientation of the axes of the moments of inertia. The moment of inertia about the vertical axis is more important than about other axes. 
     A consideration in choice of the structure of the face wall of the present invention is that the bending moment per unit width of the face is largest in the vicinity of the center of the face and along a line generally parallel to the largest dimension of the face perimeter (as shown toe-heel). This is because a reasonable approximation for analysis is to model the face structure as a beam extending perpendicular to the largest dimension of the face and considered to span across the shortest dimension of the face. This approximation is reasonable when the face height, (up-down) is substantially smaller than the face width (toe-heel) which is usual with club face designs. This orientation of the modeled beam is much stiffer than a beam which spans the longest dimension and therefore carries the major portion of the impact load. More exact analysis is possible by such methods as finite element analysis, but such analysis would yield generally similar results to the simplified model. 
     FIG. 3 shows calculated optimum thicknesses of the face wall over one representative showing of the face of a driver design having a very large elliptical face perimeter shape when made of 359T6 aluminum. Other materials would have other thicknesses. This face was made as large as practical, consistent with the design goals and limitations explained above. A driver is used by way of example in FIG. 3 because it is a more difficult design problem to realize adequate strength of the face wall as compared with the other clubs. The principles and advantages of this invention apply to other clubs, also. The material of the face wall may be as preferred and may be any structural material such as metal or non-metal. When an alternate structure such as ribs, honeycomb, or sandwich structure is used, such alternate configuration is essentially present in the central zone, and minimal or absent in the outer zone which is illustrated in FIG. 3 as being of constant thickness, because shear strength governs the design of this outer zone. 
     In FIG. 3, the golf club head is indicated at  15 , and the face wall is shown at  16 . On the face wall, the center of the face is at the origin point of the graph (the 0— 0  point) indicated at  30 , and typically, this is shown as having a thickness at the center (See FIG. 4 as well) of 6.86 mm. The general shape of the bulge portion shown at  32  in FIG. 4 is elliptical around its perimeter, and has elliptical contour lines of uniform face wall thickness spaced outwardly from the center essentially as shown. 
     Here and elsewhere in this discussion “contour lines” is used to describe locations on the face where thickness is constant along such lines. 
     By way of illustration, FIG. 4 shows the shape of the face wall resulting from the use of uniform thickness contour lines having the wall thickness indicated in FIG.  3 . 
     FIG. 4 is a partial horizontal cross sectional view of the face wall shown in FIG. 3 along the line  4 — 4  in FIG. 3, for the case where the hitting face surface  17  is flat or planar. The inner surface  18  of the face wall  16  is thus curved or bulged to provide the variable thickness perpendicular to the face but with contour lines of uniform thickness around the center as indicated in FIG.  3 . The thickness between surfaces  18  and  17  smoothly changes, as shown. 
     FIG. 5 is a fragmentary vertical cross sectional view of the face wall described in FIG. 3 taken along the section line indicated at  5 — 5 . It shows a flat face surface  17  as in FIG.  4 . In this view, the loft angle of the club head is shown as “LA” at  19 . 
     The face surface  17  is flat, and the inner surface  18  is smoothly curved between the contour lines of uniform thickness, which again are elliptical as shown in FIG.  3 . The face wall  16  joins a top wall  60  at a junction  62  and the face wall  16  joins the club head sole or bottom wall  64  at a junction  66 . The top wall  60  and the sole wall  64  also then join and are integrally formed with the heel wall  52  and the toe wall  54  to form a hollow, integral club head shell. 
     In practice, there are well-known reasons to use a face that is not flat or planar but is curved as desired for minimizing the errors caused by hits which are somewhat off center. FIGS. 6-8 show the same variations of thickness of a face wall  16 A of a club head  15 A along elliptical contour lines of uniform thickness as those indicated in FIG. 3, but incorporates a face  17 A having a face surface curvature from a heel wall  57  to a toe wall  58 . The curvature of the face surface  17 A provides most of the variation in face wall  16 A thickness so the inside surface  18 A has an approximately planar center portion by chance, in this illustration. The face wall  16 A joins heel wall  57  at a junction  55 , and toe wall  58  at a junction  59 . 
     In FIG. 7, the curved front face surface  17 A is illustrated in vertical section. The loft angle LA indicated at  66  is also shown. This shows the same variations in wall thickness as that illustrated in FIG. 3, but again, the curvature of the front face surface  17 A alters the rear face  18 A, so that in vertical cross section it has a slightly different curvature than wall  18  in FIG.  5 . 
     The face wall  16 A joins a top wall  70  at a junction  71 , and a sole or bottom wall  72  at a junction  73 . The walls are integrally formed at the corners or junctions. The top wall  70  and sole or bottom wall  72  join a heel wall and a toe wall of the club to form the integral hollow head. The face walls  16  and  16 A of the two forms are joined only at their peripheral edges to the top, sole, heel and toe walls as can be seen at the corners. The face walls have a uniform thickness adjacent the junctions where they join the shell outer walls. 
     A hosel  80  is mounted on the club  15 A as shown in FIG. 8, and a club shaft  82  can be mounted in the hosel in a conventional manner. 
     The features described in the present invention are also applicable to club faces having perimeter shapes other than elliptical. 
     FIG. 12 shows a conventional golf driver head strike face shape, having a face wall  119  made in accordance with the present invention. The face outline is at  122 , the center is at  125 , and two of many possible contour lines of equal face wall thickness are indicated at  123  and  124 . The face wall thickness would be constant from contour line  123  to the perimeter  122  of the face. The face wall thickness would vary smoothly from the face center through these contour lines, to the perimeter zone of constant thickness. These contour lines and the perimeter area of constant face thickness are similar to the design described above for a club head having an elliptical face as in FIG.  3 . 
     The contour lines for FIG. 12 are shown only to illustrate the case for face perimeter shapes other than elliptical, but were not accurately calculated for this figure. In general, they are not elliptical contour lines as in the case of FIG.  3 . The same general design considerations apply to FIG. 12 as were described for FIG.  3 . 
     It is apparent that these variations of face wall thickness eliminate unneeded face mass as compared with a face whose thickness is constant at the maximum required thickness (at the face center). In turn, the mass saved from the face wall can be used elsewhere in the club head which provides more freedom for optimizing the location of the center of gravity and for increasing the moments of inertia. As shown, the center portion adjacent center  30  is at least 10% thicker (as shown, 35% thicker in this example) than the average wall thickness adjacent the peripheral edge. 
     Another means of providing adequate strength with minimal mass is use of a sandwich (honeycomb center) structure for the face wall, as shown in FIG. 9, which is a well-known structural configuration. As encompassed by the present invention, it is made appropriately stronger by thicker surface layers or skin and/or greater thickness of the honeycomb in the central portions of the face wall than at the edge portions. The material of the central part of the sandwich between its front and rear surfaces must have adequate compressive strength to withstand the compressive loading of club-ball impact. Further, shear stresses may be difficult for sandwich structures. 
     FIG. 9 illustrates a club head  85  that has a honeycomb type construction face wall  86 . This honeycomb construction is shown schematically, and includes a front face skin  87  forming the ball strike surface, a rear skin  88 , and a honeycomb  89  between the two skins  87  and  88 . The honeycomb members are bonded to the skins  87  and  88  in a suitable manner. The honeycomb  89  is a series of structural tubular members formed with walls  89 A which surround openings  90 , as shown in FIG.  10 . As shown, the cross sections of the openings are square or rectangular, or may be of other shape, but generally speaking, the honeycomb openings would be hexagonal. The square cross sections are used for purposes of illustration. The individual walls  89 A, as can be seen, are varied in length to permit a bulge portion  91  to be formed in the center portions of the club head. The face skin  87  and the rear skin  88  also can be varied in thickness for changing strength characteristics. The cross-sectional area of honeycomb tubes  90  can be smaller in the center portion, so that there are more support walls to provide greater support between the front and rear skins in the center portions where the maximum loads are encountered. 
     In FIG. 11, a modified club  96  is illustrated, and it has a face wall  97  with a striking surface  98  on an outer skin layer. The face wall  97  is made up of a plurality of laminate layers  99  that are bonded together to form a sandwich of solid laminate layers forming a solid wall. The face wall  97  is made up of a plurality of individual layers or laminates  99  all bonded together. The rear surface  100  of the laminated face wall, as shown, can be curved for the purposes stated previously, that is, for greater strength without increasing the mass. The walls shown in FIGS. 9,  10  and  11  are modifications of the present invention that provide alternatives to the solid face wall. The wall  97  shown in FIG. 11, is a sandwich type construction that has the outer layers of material with multiple laminates between them all bonded together. 
     It should be noted that the structures of FIGS. 9 and 10 do not have to be honeycombed or multi-layered all the way out to the supporting heel and toe walls or top and bottom walls. In other words, the center portions shown at  91  and at  101  can be multi-layered, while the outer edge portion shown at  91 A and  101 A can be a solid plate. 
     In the honeycomb structure, which is also a sandwich structure, the center portions of the outer face wall can be a light weight filler between the inner and outer skins, and the same can be true with the laminated structure shown in FIG.  11 . In FIG. 11, the center laminates that make the bulge between the inner and outer skins can be lightweight materials that are bonded to the inner and outer skins, forming a homogenous structure. 
     The simplest and presently preferred design is to make the face wall a solid that is thicker in the central portions and thinner in the outer portions as described in FIGS. 3 through 8. 
     The present invention is intended to encompass adequate bending strength in the central portion of the face by use of thicker face, honeycomb, or sandwich structure, with progressively less bending strength toward the edges of the face, together with such thickness as needed for the shear strength, such that the mass of the face is minimized for the strength of the wall. 
     Although the present invention has been described with reference to preferred embodiments, workers skilled in the art will recognize that changes may be made in form and detail without departing from the spirit and scope of the invention.