ArticleIn this article, mechanical engineer Brian O’Donel, P.E. discusses crossbow finger amputations. His discussion includes an overview of crossbow anatomy, an explanation of the string motion hazard, and a description of safety precautions to protect users from injury. The experts at Robson Forensic are experienced in investigating and testifying in crossbow injury cases and a broad range of other product liability disputes.
Crossbow Finger Amputations - Expert Overview
Anatomy & Description of Crossbows
The modern crossbow is an amalgam of a compound bow and a rifle. Consisting of a horizontal archery assembly mounted on a frame that is similar in fashion to the stock of a gun, crossbows are designed to feel intuitive to rifle hunters.
The projectiles shot by the crossbow are referred to as arrows, but the terms bolt and quarrel are sometimes used. Throughout this article we will generally refer to them as arrows. The crossbow propels the arrow from rest to a speed of approximately 300+ feet per second (>200mph). To accelerate and propel the arrow, an archery style compound bow assembly containing a series of components including two limbs, pulleys, cables, and a bow string are mounted on the frame.
To fire the crossbow, the bow string is cranked or pulled into the designed drawn position and captured by a trigger mechanism and the arrow loaded. In the drawn position the assembly contains a serious amount of potential energy. When the trigger is pulled, the mechanism releases the bow string, converting the stored potential energy to kinetic energy, and accelerating the arrow forward along the top rail of the frame. A sighting scope is sometimes mounted to the top of the frame for targeting.
The shooter holds the crossbow much like a rifle with the stock against their shoulder, right hand on the rearward grip with index finger on the trigger (right handed shooter), and their left hand cupping and gripping/supporting the forearm (forward handle) for stabilization and support. Crossbows are front heavy, with the limb and pulley assembly located forward of the forearm, which makes the support of the left hand necessary for safe and accurate shooting; this hand placement puts the shooter’s left hand directly below the arrow groove/rest and string path.
Crossbow Amputations - Mechanism of Injury
Crossbows that are currently available have been designed and provided to market with a recognized string motion hazard. By design, the user’s hand is located on the forearm (i.e. forward handle) in close proximity to the string path motion hazard. Users are exposed to the string motion hazard when their fingers cross the plane through which the bow string passes when released.
Injuries and amputations occur when the trigger mechanism is activated while the user’s appendage (usually thumb or forefinger) is exposed to the string hazard. The bow string moves forward with considerable kinetic energy and velocity, sufficient to amputate or severely injure fingers/thumb.
Product Design Hierarchy
Product manufacturers must assure that hazards are engineered out of the product during the design process. If a hazard is inherent and cannot be designed out of the product, system, or process, the hazard must be properly safe guarded. In all cases, the user must be properly warned or instructed about dangers of the product. These steps form the fundamental principles and rules of practice for the safe and appropriate engineering of products, and are sometimes referred to as the hierarchy of hazard control.
The requirement for guarding and, more broadly, product safety is reinforced by a number of government and industry oversight organizations. In keeping with the archery style function of the crossbow, the string motion hazard could not be designed out, but guarding and warning are both available to product designers.
Preventing Crossbow String Motion Injuries
A proper hazard analysis of a typical crossbow would highlight the placement of the forward hand as it relates to the string motion hazard and potential for inadvertent contact. A reasonable design response would be to properly safeguard against the risk of inadvertent contact with the crossbow string motion hazard and injury. Some crossbow manufacturers have addressed the risk of finger amputations by incorporating barrier guards that are negligible in cost and cause no loss of the crossbow’s function or utility. Patents and examples of guards of this type can be found at least as early as 2005.
Additionally, product warnings and instructions, which are a lower order of protection than safeguarding, should be incorporated into the product. Effective warnings should describe the hazard, danger zone, how to avoid the hazard, and the consequence of not avoiding the hazard, as guided by ANSI Z535.4.
Investigating Crossbow Finger Amputations
The experts at Robson Forensic have investigated many incidents involving product or machine related injuries. Our experts on these investigations are licensed mechanical engineers with experience designing, building, and modifying guards for these devices. When appropriate, Robson Forensic can also involve experts specializing in human factors, injury biomechanics, or other specialized areas of science to address issues of concern specific to your case.
For more information submit an inquiry or contact the author of this article.
Brian is a Professional Mechanical Engineer licensed in multiple states. He is an expert in machine safety, machine guarding, and product design and manufacturing. Brian is also an avid outdoorsman and hunting enthusiast with experience inspecting and modifying crossbows and firearms. In his forensic casework, Brian applies his career experience designing and guarding industrial machinery to a range of safety and product design cases, including those involving crossbow injuries and amputations.