Following simple design guidelines is key to an efficient cable-carrier system
Executive Vice President
Sophisticated and high-speed automated equipment calls for top-notch performance from cable and hose-management systems. Cable carriers guide and protect the lines that relay energy and data from fixed sources to moving mechanisms on everything from small desktop printers to offshore oil rigs. Yet cable carriers are often an afterthought for equipment manufacturers.
Typically, designers address cablemanagement issues late in the productdevelopment cycle. And because cable carriers usually represent less than 1% of a machine’s cost, even the most diligent purchasing agent realizes there is little potential for significantly reducing the total cost of the product.
But an inexpensive component is not necessarily a cheap commodity. A well thought-out cable-carrier design offers a number of benefits that can significantly reduce operating costs over the life of a machine.
First and foremost, for almost any automated system, good cable and hose management extends machine life and enhances operator safety. Minimizing cable wear reduces failures and maintenance requirements. In addition, newer carriers have features such as snap-open hinge bars that provide easy access to cables, speeding repair or replacement. Many of today’s cable-carrier systems also feature modular components, so individual links, bars, and clamps are quickly changed. This differs from older designs where it was often necessary to substantially disassemble the carrier or install a completely new unit.
But even with the latest hardware, up-front engineering often makes or breaks a design. These do’s and don’ts can help specify a better cable-carrier system.
Room to move
The first step in designing a carrier system is determining space requirements for cables and hoses. Probably the most common mistake is overfilling the carrier cavity. This leads to abrasion, twisting, and binding of the cables — exactly the conditions a wellengineered system should eliminate.
Fill the carrier cavity no more than 60% to prevent damage. Each cable or hose should have clearance equal to 10 to 20% of its own diameter. Place cables, whether round or flat, loosely side by side with clearance around each. Stacking cables on top of one another, or placing round cables of different diameters side by side, creates a variety of problems such as twisting, cork-screwing, binding, and snaking.
Use frame-stay dividers, or bars with internal partitioning, to keep cables and hoses apart and ensure each has the necessary clearance. Partitions and proper clamping are especially important in systems with tight bends in continuous operation, particularly when space is at a premium. Clamping also becomes critical in vertical-hanging applications to ensure the cables ride freely through the system and not bunch at the bottom of the track.
Design enough free space into the carrier to allow for linear variations, to prevent hose abrasion and ensure safe operation. High-pressure hoses, for example, require special consideration because their length can vary by as much as 2 to –4% with pressure fluctuations. Hose manufacturers provide detailed data on linear expansion and contraction.
Highly flexible cables and hoses with small diameters — typically <0.375 in. — also require special attention. Loosely bind the cables or place them inside flexible conduit before positioning in the carrier. Conduit diameter should be at least 20% larger than the cross section of the cable/hose bundle. The clearance prevents the cables from twisting and tangling.
These guidelines, and the number and sizes of cables and hoses, determine the minimum cavity height and width. The latter dimension is often limited by the available mounting width on a machine.
Cable and hose manufacturers also provide specifications for minimum dynamic bending radii for their products. The carrier’s minimum bend radius should be no smaller than that recommended for the largest hose or cable inside. In operation, all cables and hoses should pass through the neutral bend radius (the centerline of the carrier’s chainbands) without binding against the carrier ID or OD.
Sizing up carriers
The next design step determines the total weight of the hoses, fluid, and cables. The best configurations distribute weight symmetrically within the carrier cavity. Position heavy conduits or large hoses toward the outside of the carrier with lighter, more-flexible cables in the middle.
Then determine if supports are necessary. 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 50 ft, which equates to more than 80 ft of travel in center-mount applications.
Once a carrier exceeds the unsupported span length, it sags and eventually contacts and rides on itself. In a steel carrier, this is catastrophic and necessitates a support system to prevent excessive friction, wear, and damage.
Plastic carriers generally rely on plastic glide shoes — bearing surfaces that ride inside fabricated-steel guide channels and provide even wear and alignment. Metal carriers typically use rollers or rolling carriages for added support.
Operating speed also affects the design, because it can hasten wear. Applications running at speeds above 5 fps often require more-rigid, durable systems designed for higher g-loads. Modular systems with removable glide shoes are also an option. Replacing worn shoes is less expensive than replacing the complete system. Standard glide shoes are usually constructed of glass-filled nylon. Filled-Teflon shoes are often called on to cut friction at higher speeds and loads.
For clean-room applications, PTFEcomposite carrier systems reduce wear on cable jackets and carrier wear surfaces. Typically, PTFE composites and heat stabilizers offer 15 times the wear resistance of a typical nylon with a resulting 15-fold reduction in generated particulates.
Noise is another issue in high-speed applications. One key to quiet operation at elevated speeds is reducing the “polygon” effect, by using a carrier with short links and a correctly matched bend radius. Links with too large a pitch — the distance between pivot points — increase the potential for noise by creating high momentary loads as they pass through the carrier’s bend radius and seat themselves onto the running surface.
Most carriers on the market are either all plastic, all metal, or hybrids that combine the advantages of both materials. An overriding concern in many applications today is the carrier weight, because heavier systems require larger, power-hungry motors. Lightweight plastic carriers are often preferred for this reason. The downside is that plastic carriers lack the strength and fatigue life of steel.
In some instances, added weight is desirable because a heavier system provides stability and fluid operation when heavy cables are prone to shifting within the carrier.
Nylon, generally a first choice for moderate loads, allows long travel lengths, and high speeds or rapid acceleration. The material costs less than most others and provides good wear characteristics.
Glass-reinforced nylons are stronger still. With directionally oriented glass fibers, the material can reduce wear on cables and hoses. But reinforced nylons become brittle in extremely cold conditions (–40°F) and are porous enough to absorb water, making them unsuitable in cold and high-humidity environments.
Carriers are also fabricated with nylon sidebands and wear surfaces of ultrahigh-molecular-weight (UHMW) plastic, PTFE, or similar materials. Constructing rollers and rolling surfaces of UHMW or Delrin provides enhanced abrasion resistance and excellent life in most applications.
Another potentially serious problem is plastic-on-plastic abrasion caused by nylon carriers rubbing against PVC and polyurethane cable jackets. Hybrids are often the solution here. One common hybrid carrier features plastic sidebands with steel or aluminum crossbars that substantially reduces cable wear.
Steel is usually the material of choice in heavy-duty applications — those with severe shock and vibration. The same holds for hot, aggressive environments, such as steel mills — where temperatures reach 1,000°F. Surface treatments and hardened pins and plates also add to performance. Zinc-plating improves corrosion resistance, for example, and nitrogen-quenched steel provides ultrahard plating, resulting in loadbearing capacity and deformation resistance three times that of plain carbon steel. Stainless-steel carriers handle the most extreme or corrosive cases.