Specifying cable in applications characterized by repeated motion should allow for four key qualities: allowable bend radius, life cycles under constant flexing, package constraints, and
environmental factors.
Extruded Silicone cable, which is a flat, flexible cable containing multiple conductors encased in silicone, is manufactured exclusively by Cicoil Corporation of Valencia, California. The bend radius of Extruded Silicone cable, like that of any cable, depends on the gauge of the wire type of conductors used in the cable.
In general, the finer the gauge of the conductors, the smaller the allowable bend radius. It should be noted, however, that competing flat cables using PTFE Teflon jackets have a bend radius more than twice as large as Extruded Silicone cables, given that each cable contains the same conductors. Thus Extruded Silicone cables can serve in uses that may not be suitable for PTFE cables, as they will fit in a more compact space.
Most industrial automation equipment today operates 24/7, often with robotic elements that execute rigorous motions repeatedly, sometimes thousands of times a day. These applications stress not only the moving parts of the machine, but also the electrical cabling. Engineers spend considerable time sizing electromechanical movers, but often give little thought to whether the cabling is sized properly. The frequent result is that the cabling won’t handle the rigors of the application, resulting in costly premature failures.
In fact, industry research has shown that wiring and cabling cause over 50% of the quality and reliability issues in automation equipment. For these reasons, designers should plug in a safety factor to ensure cabling will meet flex-cycle needs.
Standard cabling will not last in these applications, as the cables are not designed to flex continuously. Flat cables are best for continuous flexing. Their wire conductors can individually flex in a single plane, which provides optimum flex life.
There are two key considerations for cable that must last a long time in flexing applications: the wire conductors and the cable jacket.
Under continuous flexing, conductors containing multiple strands of fine-gauge wire will last the longest. For example, standard 28 AWG is typically composed of 7 strands of 36 AWG wire, while 28 AWG high-flex wire is composed of 19 strands of 40 AWG wire (or even finer gauge). The finer-gauge base wire exhibits less cold-working under constant flexing. The minimal cold-working lets these wires last through tens of millions of flexing cables.
Given cabling that contains finely stranded wire, the cable jacket (and how the jacket holds wire conductors) is the next determinant of long flexing life. PVC or other thermoplastic jackets are too brittle to withstand continuous flexing, so most high-flex cables use PTFE or silicone rubber as the primary cable jacket. PTFE and silicone rubber can both withstand tens of millions of flexing cycles. However, silicone rubber is a better choice for uses involving extremely tight bend radii, as it is inherently more flexible than PTFE.
Another benefit of Silicone Extruded cables is that the wire conductors are completely encased in the silicone rubber, versus being loose inside a typical PTFE cable. Loose wires will rub against each other, increasing friction, cold working, and reducing wire life. PTFE cables often need additional clamping throughout their length to minimize such effects, which raises the system’s cost and weight/inertia. In contrast, Extruded Silicone cable’s encapsulation of individual conductors optimizes wire life in continuous-flexing applications.
Space is increasingly a factor as companies continue to add features and capabilities to automated machines and processes. The premium paid for industrial floor space has forced machine footprints to shrink.
The result is often a mandate to do more in less space. In this environment, the design of cabling is often an afterthought. Cable runs are frequently ‘stuffed in’ to leftover space. But the usual result is less-than optimum, and it can lead to unforeseen maintenance or troubleshooting issues.
Engineers should consider cabling early in the design process; not only assessing space considerations, but also accessibility criteria. Generally, round cabling will take up more space than flat cable, as the geometry of flat cabling inherently saves space (Fig. 1). Within flat cable, however, not all cable types are equally space-efficient. PTFE flat cable, for example, is limited to pre-formed shapes, as the wires are inserted into ‘pods’ formed by the PTFE jacket.
The result can be less than optimum, as the designer is limited to pre-existing cable profiles.
Silicone Extruded cable, however, can be produced with exactly the size and shape needed for the conductors it contains.
Conductor spacing can be precisely controlled, as can the thickness of the silicone rubber jacketing, to give exactly the dimensions the application demands. In addition, Silicone Extruded cable can be produced with any outside profile. This lets the cable shape be optimized for the specific space on hand.
Another unique feature of Silicone cabling is its ability to be ‘formed’ into S curves and other non-linear shapes (Fig. 2). This lets the designer bend the cable around obstructions to promote extremely compact packaging. This type of ‘curved flat cable’ has been used for many years is supersonic missiles and military aircraft to save space and weight. It also is becoming more frequently used in industrial applications.
Temperature index is the temperature at which a material will sustain burning when the oxygen concentration in the air mixture is 20.9%. A typical silicone rubber base has a temperature index of around 250°C. Extruded Silicone cables are rated for operation in temperatures of -65°C to +260°C (Fig. 3).
Silicone retains room-temperature flexibility and other qualities throughout its -65°C to +260°C operating range. In addition, the linear coefficient of thermal expansion for Cicoil silicone rubber is extremely low (about 0.000185 in/in/°F). This means that extruded silicone cable will retain its package size much better than will PTFE cables throughout wide temperature ranges, which may be important in applications involving extremely tight spaces.
: Design World :
Filed Under: Cables + cable management