Polymer Cable Carriers Take Center Stage In Metalworking

Then total force is Ft = Fa+Fp (lb). Total force must not exceed the load rating of the carrier. For reference, a typical plastic cable carrier with a 2.5-in. inner height handles about 1,800 lb. Some manufacturers make several grades of carriers within a given size range. Selecting a light-duty carrier for a high-acceleration application might cause carrier for a high-acceleration application might cause carrier damage and machine downtime. Conversely, a heavy-duty carrier for a low-stress application wastes money.

Cable carriers also come with different bending capabilities and each has a suggested minimum bend radius. The carrier’s minimum bend radius should equal or exceed the recommended minimum bend radius of the stiffest cable or hose it will carry. A larger bend radius puts less stress on cables so they last longer.

As a rule, keep the bend radius at least 8 to 10 times the outer diameter of the largest cable or hose. (Bend radius 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.)

Another common concern is conduit tangling and abrading. As the carrier reciprocates thousands of times, smaller conduits tend to work their way between larger ones. Cables of different cross sections have different bending characteristics. If cables and hoses are placed in a carrier randomly, they will almost surely interfere with one another, eventually causing conductor breakage, accelerated jacket wear, or both.

The solution is to install separators in the carrier. Put cables of different cross sections and diameters in separate compartments, and segregate hoses from cables altogether. Some manufacturers offer flexible systems of vertical separators and horizontal shelves to let users customize the carrier interior layout. Less flexible systems require custom-ordered separators for a specified conduit layout. They may not accommodate alternative arrangements should requirements later change.

Some carrier designs are inherently more “cable-friendly” than others, with interiors carefully designed to minimize pressure points and abrasion. Moreover, carriers with easy-open crossbars or covers permit more frequent inspection of conduits and simplify replacement. Choose a specific interior design based on carrier type and number and variety of conduits.

Carrier length depends on mounting and machine configurations, but typically length is approximately half the total travel plus the curve length. Usually on a machine, one end of the cable track moves and the other is fixed. Two basic mounting configurations are center and off-center mount. Center mount means that the track moves half to the left and half to the right of the fixed point. Or in other words, the fixed point is in the middle. This arrangement uses the shortest possible carrier.

An off-center mount configuration means that the movement is not equidistant form the fixed point. Sometimes the track can’t be mounted in the center because of machine restrictions. In this case more t rack is needed to compensate for the fact that the fixed end isn’t centered.

Failure to support and guide the carrier in long travel applications is another common problem. For short travel lengths and light loads, most carriers are self-supporting. Manufacturers supply data on a carrier’s unsupported span as a function of travel and weight. For example, plastic carriers generally are rated for unsupported spans of 2 to 15 ft, depending on the size. Large steel systems can have unsupported spans approaching 5o ft.

Once a carrier exceeds the unsupported span length, it sags. For many applications, some sag is acceptable. But eventually the carrier contacts and rides on itself. In a steel design, this can be catastrophic. Where metal unit doubles back on itself, such as in center-mounted installations, if the upper run drags on the lower run, the carrier will quickly saw itself to pieces.

This necessitates a support system to prevent excessive friction, wear, and damage. A carriage support system consists of rollers conveyor supports, and a moving framework that supports the carrier over the entire length of travel. Channel guides ensure accuracy and dependability, even at high loads and velocities. Plastic carriers have a much lower coefficient of friction, and they are often permitted to glide on top of themselves. Plastic carriers can also use plastic glide shoes — bearing surface that ride inside guide channels and provide even wear and alignment.

Regardless of the design, however, when a carrier is used in any arrangement besides horizontal flange-fixed, the manufacturer should be consulted, because the application will often require specialized supports.

For many demanding linear-motion application, the conduit itself must withstand the external stresses of its environment. For example, most electrical cable is not designed to handle the constant tensile, compressive, and shear forces that carriers impose. Continuous bending causes most cables to “corkscrew”— a major reason conductors break — and the spiral twist may entrap and damage other cables.

The cable’s interior structure should be specifically designed with isolated conductors to counteract the corkscrew effect. Also, cable jacket material must be abrasion resistant and slick enough to slide easily against other cables and against the carrier itself. Few cables are designed for carrier applications, but taking the extra effort to select the most “carrier-friendly” cable will pay for itself many times over in longer life.

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.

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