| A CABLE IS A CABLE IS NOT A CABLE? |
| Progressive resistance selectorized weight stack equipment traditionally has been fabricated with galvanized wire rope or steel cables that connect the weight stack to the movement arm of the machine through a series of pulleys. These cables have most probably been made of aircraft quality cable of various thicknesses, lengths and load ratings. |
| However, what does all of this mean and what specifications are important? |
| Basically all aircraft cable is fabricated in a spiral weave of bundles of steel fibres or strands. These bundles usually come in 6 bundles of 19 strands, 7 bundles of 7 strands or 7 bundles of 19 strands with the latter being the most common. This winding of the cable is what gives it it’s strength. |
| Additionally, these cables may be uncoated, coated with a clear plastic or coated with a black nylon material. This coating is mainly cosmetic in nature but may decrease friction and resistance when bending the cable around pulley wheels and cams. |
| All cables have a working load limit. These load limits are based on the static tensile strength of the cable. This load limit of the cable is the static maximal weight it will hold when that load is pulled straight down the length of the cable. This load often varies from 1000 to 4400 lbs. What is important to note here is that most fitness equipment does not have static load requirements. In fact the requirements are dynamic, ballistic reciprocal movement of weight stacks of various weights for thousands and even millions of cycles or repetitions. These working load limits must not be exceeded for obvious safety reasons. |
| Another engineering and design feature of a wire rope cable is that of the “safe working load”. This load is based on the “breaking strength” of the cable
which literally means the load at which the wire rope or cable destructs. The breaking strength of a wire rope cable is determined by two factors: a) the sheering forces to which the cable will be subjected such as the diameter and number changes of direction over the pulley wheels it has to go around or the bending arc of a earn; b) the type of terminal attachment hardware applied to the ends of the cable. To |
| determine the breaking strength of a cable when designing cable assemblies a five to one safety factor is used. This means that if the breaking strength of the cable is 2000 Ibs. (118″ 7×19 galv. aircraft cable) then the maximum allowable load to which the cable or assembly is subjected should never exceed 400 Ibs. |
| The terminal attachment wire rope fittings commonly used on fitness equipment cables are eye, lap and stop sleeves, thimbles, shank balls and threaded nut combinations. These fittings are usually made of aluminim, copper and steel. The greatest probable area of risk of cable breakage is at the terminal attachment so that is why when using these fittings the safety factor must be used to determine the breaking strength of the cable. Additionally the plastic coating on the cable must first be stripped and the fittings applied over bare cable to achieve this factor as the fittings will not hold over the plastic leading to integral failure of the assembly. The use of stop sleeves as a stand alone fitting requires that the safety factor be increased to 10 to 1 as they are only capable of holding 1/3 to 1/2 of the rated breaking strength of the cable to which they are attached. |
| The manner in which these terminal attachment fittings are applied to the wire rope cable assembly are varied. The most efficient and safest method is through the use of a hydraulic swager which presses these fittings on with several thousand pounds of force per square inch. In most cases mechanical bench swagers that are basically effective but not as safe are used. Lastly and most commonly hand swagers are used. This is a mechanical hand tool similiar to bolt cutters which when properly used can be effective for static load cables only. The critical success factor in the latter cases is professional training, quality technique and attention to detail on the part of the technician. |
| Due to the inherent design, function and usage of selectorized weight stack equipment as well as its preponderance for abuse it is recommended that when replacing the wire rope cable assemblies on these machines that the manufacturer’s original equipment parts be used or barring that that the cable assemblies be professionally fabricated in a controlled environment for the health, welfare and safe participation of fitness patrons everywhere.
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