Safety of Fitness Equipment Cables

There is a family of fitness machines that provides a manual workout task requiring the user to push or pull against a resistance provided by a stack of weight plates. The weight system is usually linked with a single cable to a gripping or user interface device to produce a constant resistance. A fracture of the tensioned cable along its length or at its end connectors causes a sudden acceleration of the grip or other interface device driven by the operator’s push or pull. The sudden loss of resistance often results in an exerciser pulling a heavy bar into his or her face. Because falling weights, accelerating grips and rapidly unloading muscles are all hazardous, manufacturers of exercise machines want to maintain the structural integrity of the cables. To accomplish this, manufacturers usually recommend “scheduled servicing” of their cables. This Preventive Maintenance (PM) strategy is frustrated by nylon sheathing that hides the cable failures. Further, the swedged or silver soldered connectors often fail covertly by internal fatigue fractures. A more effective PM strategy has been adopted by many manufacturers called “Scheduled Replacement”; they advocate annual cable replacement. Here the nemeses are sloth and greed, best expressed by the philosophy, “if it ain’t broke, don’t fix it.” As a first consideration of fault tolerant design, a redundant duplication of the cable system was added to a fitness machine; this is called “active redundancy.” This paper demonstrates the inadequacy of active redundancy for eliminating the catastrophic failure mode. Instead, the adoption of a “dormant/standby” redundancy is shown to provide the requisite safety. The proposed system not only eliminates the “fail-to-danger” mode, it provides the most economical use of the cable in the sense that it never discards a cable until its life is exhausted.