How Does a Compound Bow Work?

What is a Compound Bow?

A compound bow is similar to a traditional longbow or recurve bow, except its design and material give it significant advantages, among them increased power and accuracy. Like all bows, compound bows store energy in their limbs, which are compressed as the string is drawn back. When the string is released, the limbs snap to their original shape, transferring the potential energy into acceleration of an arrow.

Features

The primary feature of a compound bow is the use of one or two pulleys to compress the limbs. Not only do the pulleys magnify the force applied to the string by the archer, they also provide an advantage known as "let off." A standard longbow requires nearly as much force to hold it fully drawn as it does to draw it back. In a compound bow, once the string is about 50-80% drawn, the pulleys reduce the amount of force required to continue drawing the string.

Design

The cables and pulleys make the compound easier for the archer to use, but the real secret to its power is in the composite materials from which it's constructed. The central part of the bow where the arrow is nocked is called the riser. It's usually made of a sturdy aluminum alloy.

Accessories

Archers using compound bows often use a variety of accessories. Unlike traditional bows, which are often fired bare-fingered, archers using a compound bow often use a release, a small mechanical pincer with a Velcro strap, which transfers the force of the draw to the wrist instead of the fingers.
source : eHow

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Advantages over traditional bows

Technical advantages

• As the bow is drawn, the draw weight increases to a peak and then "lets off". The let-off is usually between 65% and 80% of the peak weight, and one manufacturer (Concept Archery) produces a compound bow with 99% let-off. The "let off" is a term that describes what happens as the cam rolls all the way over. The photo on the right shows the axle attaching the limb to cam is mounted at the edge of the cam as opposed to the center. As the string is drawn the cam turns and imparts force to compress the limb. Once the cam turns all the way around the least amount of force needs to be applied to the string to keep the limbs bent. This is known as "let off". This enables the archer to hold the bow fully drawn and take more time to aim.
• This let-off enables the archer to accurately shoot a bow with a much higher peak draw weight than most could manage with a longbow or recurve.
• The bow is resilient to temperature and humidity changes giving the bow superior accuracy, velocity, and distance in comparison to bows made of natural materials.
• The pulley system usually will include some rubber-covered blocks that act as draw-stops. These provide a solid "wall" that the archer can draw against. These draw stops can be adjusted to suit the archer's optimum draw-length. This helps the archer achieve a consistent anchor point and a consistent amount of force imparted to the arrow on every shot, further increasing accuracy.
• The design of the pulleys (cams) directly controls the acceleration of the arrow. What is termed a "soft cam" will accelerate the arrow more gently than a "harder" cam. Novice archers will typically shoot a soft cam whereas a more advanced archer may choose to use a harder cam to gain speed. Bows can be had with a variety of cams, in a full spectrum from soft to hard.
• Some pulley systems use a single cam at the bottom of the bow and a balanced wheel at the top of the bow instead of two identical cams. This design eliminates the need for buss cables and instead uses a single string that begins at the cam on the bottom of the bow, travels over the wheel on top, around the bottom cam again, and ends attaching to the top limb.

Circumstantial advantages

• Compound archers usually use a mechanical release aid to hold and release the string. This attaches to the bowstring near the point where the arrow attaches, and permits the archer to release the string with a squeeze of a trigger or a slight increase of tension. The use of a release aid gives a more consistent release than the use of fingers on the string.
• In tournaments, compound archers are usually allowed the use of a magnifying sight. This is not allowed for other bow classes.
• Stabilizers and dampers are particularly well-developed for the compound bow. They allow the archer to shoot even more accurately, by reducing the movement of the bow when the string is released.

Circumstantial disadvantages

• The relatively low holding weight of a compound bow compared to a recurve bow makes the compound more sensitive to certain form faults when the archer is at full draw. In particular, it's easier for the archer to torque the bow around the vertical axis, leading to left-right errors.

source : wikipedia

Construction of compound bow

The central riser of a compound bow is usually made of aluminium or magnesium. Many risers are made of the aircraft-grade 6061 aluminium alloy. Risers are designed to be as rigid as possible. The riser is the central mount for other components such as the limbs, sights, stabilizers and quivers.

Limbs are made of composite materialsand are capable of taking high tensile and compressive forces. The limbs store all the energy of the bow — no energy is stored in the pulleys and cables.

In the most common configuration, there is a cam or wheel at the end of each limb. The shape of the cam may vary somewhat between different bow designs. There are several different concepts of utilizing the cams to store energy in the limbs, and these all fall under a category called bow eccentrics. The four most common types of bow eccentrics are Single Cam, Hybrid Cam, Dual Cam and Binary Cam. However, there are also other less common designs, like the Quad Cam and Hinged.

Compound bow strings and cables are normally made of high-modulus polyethylene and are designed to have great tensile strength and minimal stretchability, in order that the bow transfers its energy to the arrow as efficiently and durably as possible. In earlier models of compound bows, the cables were often made of plastic-coated steel.

source : wikipedia

What is Compound Bow ?

A compound bow is a modern bow that uses a levering system, usually of cables and pulleys, to bend the limbs.

The limbs of a compound bow are usually much stiffer than those of a recurve now or longbow . This limb stiffness makes the compound bow more energy efficient than other bows, but the limbs are too stiff to be drawn comfortably with a string attached directly to them. The compound bow has the string attached to the pulleys (cams), one or both of which has one or more cables attached to the opposite limb. When the string is drawn back, the string causes the pulleys to turn. This causes the pulleys to pull the cables, which in turn causes the limbs to bend and thus store energy.

The use of this levering system gives the compound bow a characteristic draw-force curve which rises to a peak weight and then "lets off" to a lower holding weight.

The compound bow is little affected by changes of temperature and humidity and gives superior accuracy, velocity, and distance in comparison to other bows. The compound bow was first developed in 1966 by Holless Wilbur Allen in Missouri, and a US patent was granted in 1969. The compound bow has become increasingly popular. In the United States, the compound is the dominant form of bow.

In literature of the early 20th century, composite bows have been described as "compound".