Unprotected cables can twist, bind, and wear out fast.
East Providence, R.I.
Edited by Lawrence Kren
Machine tools, coordinate-measuring machines, palletizers, robots, ship-to-shore cranes, and laboratory medical equipment are just some of the applications for cable carriers.
Cable carriers keep cables and hoses on translating and rotating axis from tangling and exceeding minimum bend radii. Carriers also help expel debris and prevent cable and hose jackets from touching the machine itself.
A cable carrier consists of hinged plastic or metal links that articulate as a machine axis moves. The most common carrier arrangement is horizontal and unsupported. This configuration works well for short travel distances and gives the longest possible service life. Here, the upper run of the carrier moves without touching the lower run throughout the entire travel length. Maximum unsupported length is typically about 20 to 30 ft and depends on carrier type and cable fill weight. When travel length exceeds that of an unsupported installation, a guide trough and glide bar supports the upper run. Cable carriers can also hang or stand vertically. Side-mounted, rotary, multiple-nested carriers and combination motions are also possible.
SPEC'ING A CABLE CARRIER
Cable carriers are an integral part of a machine and should be considered early in the design process. Gather necessary data about the installation including travel length, size and weight of cables and hoses, the operating environment (debris, heat, chemicals), and machine speed and acceleration.
Select a carrier material. Metal carriers have higher tensile strength than plastic and hold up better under heavy fill weights. Metal carriers also withstand high temperatures. Plastic, in turn, weighs much less than metal. This cuts the amount of power needed to move the carrier and may eliminate complex support systems. Plastic also resists corrosion, is quieter in operation than metal, nonconductive, and needs no lubrication. Several plastic-carrier designs snap together without metal fasteners. Metal fasteners can shake loose, embed in a cable or hose jacket, or jam inside the carrier and prevent proper movement.
Size the carrier. Add 10% to the diameter of the largest cable and 20% to the diameter of the largest hose. The resulting dimension is the minimum inner height of the carrier. For example, certain cable carriers in ATM machines have an inner height of 0.20 in.
Choose a carrier design. Snapopen versions permit access to cables via crossbars along the carrier length. A nonsnap-open design may be appropriate for highly cost-sensitive applications, though users must feed cables through one end of the carrier. A tube-style cable carrier replaces crossbars with lids that fully enclose and protect cables from debris such as wood or metal chips, Other access designs include open, split, or hinged crossbars, and zippers. With a split crossbar, users install and remove cables and hoses through a pair of cantilevered plastic live hinges in the carrier top. Open crossbar carriers are lightweight and ease inspection and replacement of cables. Hinged nylon crossbars flex for attachment to side links. Zipper-type carriers, as the name implies, employ interconnected lids that pull back like a zipper to remove the carrier top section.
Modular cable carriers with four or more removable parts per link target heavy-duty uses with long travel. Modular designs come with hinged crossbars that open on either the inner or outer radius, or with lids that form a tube to expel debris. Special designs include carriers that generate little vibration or noise; multiaxis units for robots; "twister" chains that accommodate rotational movement; lowcost, single-piece molded carriers; and carriers with integrated wheels for long travel lengths and reduced wear.
Many applications have limited space for a cable carrier, so be sure to factor in carrier camber when checking available height. Camber is the curvature of the upper portion of the carrier along its unsupported length. Most cable carriers come standard with camber. No-camber designs are available, but they carry less load than cambered units.
Cable carriers come in different bend radii and each has a suggested minimum bend radius. As a rule, keep the bend radius at least eight to 10 times the outer diameter of the largest cable or hose. A larger bend radius puts less stress on cables so they last longer. Bend radius, by this definition, is measured from the center of the curve loop to the center of the pivot pin on the carrier side link, not the overall curve height.
Cables and hoses should lay in the carrier according to diameter, in most cases, side by side and evenly distributed by weight. Cables and hoses with different outer-jacket materials may stick to each other so they should lay separately. Do not stack more than two cables on top of each other. If necessary, use vertical separators and shelves to create compartments that keep cables from tangling. Also use separators when travel speeds exceed 1.64 ft/sec (0.5 m/sec). In all cases, cables and hoses should move freely within the cable carrier to prevent damaging them or hindering movement of the carrier itself.
Select the cable carrier length. Ideally, the fixed end should locate at the center of travel because this arrangement uses the shortest possible carrier. Calculate the force needed to accelerate the filled cable carrier from rest, Fa:
Fa = Wa/32.17 (lb)
where W = weight of carrier and fill (lb), and a = acceleration (ft/sec2). Calculate the push force, Fp, needed to move the carrier along its support surface:
Fp = μW (lb)
where μ = the coefficient of friction between the carrier and support surface or glide bar. For a plastic carrier sliding on a metal surface, μ = 0.5. For a plastic surface, μ = 0.2.
Then total force, Ft is,
Ft = Fa + Fp (lb)
Total force must not exceed the load rating of the carrier. For reference, a plastic cable
carrier with a 2.5-in. inner height handles about 1,800 lb. Regardless of the application, always strain relieve cables at the moving end, and whenever possible, at both ends. The exception is hydraulic hoses; they should be strain relieved only at the moving end. Improper, or lack of, strain relief is a common cause of cable and hose failure. Strain relieve cables and hoses using profile rails, clamps, tie wraps and tie-wrap plates.
Bundled cables flex better
Cables with layered conductors work for certain short-travel applications. But they tend to fatigue and lose tensile strength in demanding, flexing applications, especially with long travel lengths. Failure of the cable core releases torsional stress and makes the cable "corkscrew." In contrast, cables with bundled conductors balance torsional stress, which stabilizes the core under high bending stress. Wires twist together using a special pitch length. The resulting conductors are then bundled into cables.