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That may be out of the question but the common thread that runs through all of our products is their ability to withstand deep deflection over numerous cycles without fatigue or loss of performance.
It is well known that the purpose of the bow limb is to store and release energy. Bows function by converting elastic potential energy stored in the limbs into kinetic energy and movement of an arrow. Energy stored in the limbs of the bow as they are deflected is transformed into rapid motion when the string is released, transferring the motion to the arrow. In forming the composite for a spring or bow limb, the trick (both art and science) is to get the fibers perfectly aligned and thoroughly saturated with resin. Voids or misaligned fibers negatively affect the composite properties. Alignment is achieved by having all of the fibers under a precise amount of tension as they go into the forming area; the saturation is achieved at Gordon Composites with a proprietary roll-forming process developed through decades of experience. The goal is a composite in which each fiber is surrounded and bonded with the matrix at a specific ratio so it will work efficiently as it stores and releases energy. |
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The advantage of the original composite bows over bows made from a single piece of wood was their combination of smaller size and high power. They also had disadvantages. Their construction required much more time and a greater variety of materials, and the animal glue traditionally used could lose strength in humid conditions and be ruined by submersion. Despite those drawbacks, composite bows had significant advantages and were quickly adopted by others, touching off a competitive quest for improvement that continues today. The nomads who developed the first composite bows would hardly recognize today’s compound bow with fiber-reinforced composite limbs. 
University-trained engineers now use the latest in computer-driven design technology to develop bow systems that are light, draw smoothly and efficiently release energy when sending an arrow on its way. Among the most critical components of a modern bow are the limbs that store and release that energy. And while those limbs may not look very sophisticated, they are actually complex material systems within the bow assembly. Limbs can be as intricate as the compound bow itself, and while making them is still an art, it has also become a science. Bow limb fabrication begins with the selection of raw materials. The most commonly used are fiber glass reinforcements and epoxy resin. That sounds simple enough until you find that there are hundreds of types of glass fibers, hundreds of epoxy resins and dozens of ways to put them together.
The raw materials used for the glass will determine a variety of factors such as the temperature needed to melt the glass and the strength of the resulting fibers. Without getting too technical, glass compositions with higher silica content and metallic oxides will require a higher melting temperature and be harder to make into fibers, and thus more expensive, but they will also impart higher performance characteristics to the fibers. As soon as each filament of glass emerges from the bushing and cools, a chemical sizing is applied to the surface of the glass. The sizing has two functions: keep the fibers from abrading and breaking, and help the fibers bond securely with the resin. If the glass is melted at a higher temperature and is less viscous, and/or the glass is pulled faster by the forming winder, the glass fibers will be thinner. Lower temperatures and/or slower pull will produce thicker glass fibers. Fiber glass producers, like textile makers, have for centuries referred to fiber diameter with terms like yield or tex. For example, a product with a 450 yield will have 450 yards of fiber in a pound. The strands of glass are wound onto a temporary core to form a package that can be shipped to a composite fabricator. Pound for pound, glass fibers are stronger than steel. They have a high ratio of surface area to weight and an amorphous structure that gives them the same properties both along the fiber and across the fiber. Key performance attributes include tensile strength, stiffness, modulus and fatigue resistance. While other fibers may have one attribute that is better than glass, glass often provides the best combination of properties. For example, carbon is known for being light and strong, but it can also be brittle in some applications. Compared to carbon, glass can undergo more elongation before it fails.
Epoxy or polyepoxide resin polymerizes and crosslinks when mixed with a catalyzing agent or “hardener.” In general, epoxies are known for their excellent adhesion, chemical and heat resistance, and good to excellent mechanical properties. When epoxy is mixed with the appropriate catalyst, the resulting reaction is exothermic – meaning that it generates heat – and the oxygen molecule on the epoxy monomers is “flipped.” This occurs throughout the epoxy and a matrix with a high stress tolerance is formed and “glues” the materials together. Epoxies are classified as “structural adhesives” or “engineering adhesives.” These high performance adhesives are used in the construction of aircraft, automobiles, bicycles, golf clubs, skis, snow boards, and other applications where high strength bonds are required. Epoxies are exceptional adhesives for wood, metal, glass, stone, and some plastics. They can be made flexible or rigid, transparent or opaque/colored, fast setting or extremely slow. Epoxy adhesives are almost unmatched in heat and chemical resistance among common adhesives. There are more than 100 commodity epoxy manufacturers in the world and countless “formulators” who blend or customize epoxies for specialty applications. The resin used in the manufacture of bow limbs comes from the second category, the custom epoxies designed to achieve specific performance requirements.
In the case of a bow limb, the process begins when glass fiber roving doffs – the packages of glass fibers – are arranged in a creel, which is simply a holding device with racks of shelves. The ribbons or “ends” of the glass fibers are pulled from the creel into the area where the composite is formed. 
At Gordon Composites, the creel for making bow limbs is one of the largest in the world with up to 3,000 ends pulled for a single sheet of composite material. When you consider that each end may be made up of 800 or more individual glass filaments, a bow limb could contain more than a million glass fibers. As stated above, the trick in forming the composite for a bow limb is to get the fibers perfectly aligned and thoroughly saturated with resin. Alignment is achieved by having all of the fibers under a precise amount of tension as they go into the forming area; saturation is achieved with a proprietary roll-forming process. The result is a composite in which each fiber is surrounded and bonded with the matrix at a specific ratio so it will work efficiently as it stores and releases energy.
Bar stock is generally made with standard E-glass and shipped to the bow manufacturer as a solid block of material in a specific geometric shape called a “billet.” Bow manufacturers generally do their own fabrication, converting the billet to meet their specific design criteria. Gordon Composites can support the manufacturer with engineering, machining and construction. Limbs made from billets are less expensive than laminated bow limbs and are very easy to fabricate and build into prototypes. They are able to achieve deep deflection and high-stress performance and are used very successfully in the majority of bow-limb applications. 
A typical laminate construction or lay-up generally starts with a composite billet that is machined to a specific shape. Thin layers of composite material are then laminated to the bar stock, giving this type of bow limb its name. The added layers can contain high-performance glass or carbon materials, sometimes arranged in a weft or cross direction. Reinforcements can also be embedded as a scrim or fabric layer. This blending of materials and properties takes the design and engineering of bow limbs to the next level, by not only allowing high and lasting performance, but also the complex shapes that make modern bows so smooth and accurate.
This process begins by winding continuous glass fibers that have been impregnated with epoxy around a set of pins to align the fibers. The combination is placed in the cavity of a matched-metal tool-and-die press. Heat and pressure are applied to shape the limb as it “cures.” There is generally less finishing cost with compression-molded limbs because the part emerges from the press in a near-net shape. The process is used very successfully in many applications and is attractive to bow manufacturers who want to produce their own limbs. On the flip side of the equation are high tooling costs and a lack of flexibility driven by the desire to offset those costs by using molds for a long time.
After deciding what the key performance elements of a targeted limb design are — e.g. high fatigue resistance or high speed — it is imperative to stay within design guidelines.
And while an archer might draw and release a bow several thousand times, manufacturers often test bows up to a half-million cycles to make sure there is no fatigue or “creep” in bow limb performance. An archer might have the strength to move the ends of the limbs several inches closer to each other but mechanical testing devices apply enough force to have them nearly touch each other. Manufacturers want to be sure the failure point is well beyond an individual’s ability to draw the bow. Archery bows function by converting elastic potential energy – today stored in parts of the modern bow frame called limbs – into kinetic energy and movement of an arrow. Energy stored in the limbs of the bow as they are deflected is transformed into rapid motion when the string is released, transferring the motion to the arrow. By fusing various layers of composite material together, bow designers are able to precisely tailor the physical properties of their bow limbs. For example, a designer can move the neutral axis on a limb so it is stiffer on one side. Today’s engineer is able to optimize the tensile and compressive strengths – along with sheer strength – and put them exactly where they are needed during very deep deflection to maximize performance. A typical laminate construction generally starts with a composite billet that is machined to a specific shape. Thin layers of composite material are then laminated to the bar stock, giving this type of bow limb its name. The added layers can contain high-performance glass or carbon materials, sometimes arranged in a weft or cross direction. Reinforcements can also be embedded as a scrim or fabric layer. This blending of materials and properties takes the design and engineering of bow limbs to the next level, by not only allowing high and lasting performance, but also the complex shapes that make modern bows so smooth and accurate. Stress is applied to a bow limb when the string is drawn. One side of the composite is actually stretched while the other is compressed. The fibers resist these two forces and the matrix spreads the load throughout the limb. When the stress is released, the “elastic” composite quickly “springs” back to its original shape. This action converts the elastic energy into kinetic energy and sends the arrow on its way. Today, engineers at Gordon Composites use computers and software to help them plot the stress-strain curve for a given material or composite. The programs actually color code the internal forces, enabling the designer to see how much and where stress is occurring. “’If we can see it and understand it, we can develop ways to make the limbs even better,” says Steve Johnson, General Manager and Vice President.
The following excerpt is reprinted with permission from Inside
Archery magazine. The article appeared in the September 2007 issue. While more recent innovations may not have been quite as monumental, they have been more frequent. Hunting releases, 3D targets, fiber-optic sights, expandable broadheads, carbon arrows, drop-away and total-containment arrow rests, laser rangefinders, scent-containment clothing, portable ground blinds, scouting cameras and much more have all made archery participation more attractive and enjoyable. Through it all, thought, how performance has remained a central theme. As bows became more dependable, more enjoyable to shoot and more accurate, the sport and the industry prospered. Evolving bow performance has hinged on a variety of factors, including refinements in risers, eccentric systems, bowstrings and cables. But in no area have the improvements been more enabling of archery progress than in the evolution of bow limbs. After all, it is the bow’s limbs that allow everything else to happen. If the limbs aren’t up to the task, no new high-performance cam, riser or cable system will carry the day. From the very beginning of the modern archery industry in the late 1940s, limb development has been central to bow improvement. And surprisingly, a single longstanding company has been at the forefront of bow-limb innovation over the past five decades. That company, now called Gordon Composites, has been and continues to be one of the most influential companies in the industry. To read the complete text, download the article by clicking here.
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