Sheathing has three core values for use in fiber optic design:
- Protect the fiber.
- Keep ambient or stray light from creating signal noise (for sensor applications).
- Improve handling of the part.
PROTECT THE FIBER
Being primarily glass, fiber is fragile. When individual fibers break, light transmission and uniformity are reduced. After the first few fibers break at a stress point, a chain reaction occurs, hastening the destruction of the part. Therefore, sheathing helps maintain fiber’s usefulness over the long term.
When designing the part, understanding the end application will help us (and you) select the most effective/least expensive sheathing material.
Consider the following guidelines when specifying a fiber sheath:
For minimal handling, where the application is illumination, and heat exposure is low, consider inexpensive PVC sheathing. PVC offers good protection from corrosive mists and foreign debris, as well as protection from incidental abrasion and contact. This material is also manufactured in corrugated shape, offering some crush and kink resistance. The material is relatively low cost with good availability and selection.
For repeated handling, where the application is illumination, and heat exposure is low, consider monocoil, which provides moderate crush and kink resistance in addition to debris protection. This choice is lighter and more flexible than SL type sheathing. In addition, it’s available in many more ID/OD selections making an efficient and cost effective package. Less expensive than SL or PVC covered SL sheathing.
For repeated handling around big equipment, where heavy objects can fall on, roll over, or simply compress the component, and where the application is illumination, and heat exposure is low, consider PVC covered stainless interlock, which provides the best crush and kink resistance in addition to debris protection. This material is the most expensive selection in the handling application group.
For components exposed to constantly moving environments, where the application is illumination, and heat exposure is low, consider a simple silicone sheathing, which protects the fiber and remains flexible without fatiguing.
HIGH HEAT ENVIRONMENTS
For high heat applications, most covered sheath could melt or burn. When burned, PVC gives off cyanide gas. When used as a component in commercial lighting products, it may be restricted from use in commercial buildings. Bare metal, teflon tubing, or metal/teflon combinations would be recommended for use in high heat (over 70°C continuous) environments.
PREVENTING SIGNAL NOISE
In sensing applications, the potential of signal noise must be eliminated. Sheathings designed to be totally opaque (silicone) should be considered, and in the case of multi-channel construction, both sender and receiver fibers should be individually sheathed inside a larger common sheathing. While it has nothing to do with sheathing, don’t overlook other factors creating noise, including the fluorescing potential of epoxies at the signal-source end of the fiber optic component.
EASY HANDLING & MINIMAL COST
If the fiber component will be installed in equipment and remain static, free from contact with foreign matter, a simple cotton or synthetic mesh might be all that’s required to protect the fiber during shipment and installation.
Sometimes fiber optic cables are routed through and around machinery. A rule of thumb when specifying sheathing: if interlocked metal (plain or covered) sheathing is used, minimum bending radius is 4X the OD of the sheathing. “Soft” sheathing such as PVC or Silicone can withstand bending radius 2X the OD
In addition to the above selection, FTI offers scores of sheathing types, including teflon, metal braided, anti-fungal, tefzel (thin and heavy wall versions), rigid tube and pipe and Cole-Flex™, an all plastic convoluted construction for rugged applications where metal cannot be used.
Some special sheathings are subject to run lot charges, so when possible, specify sheathings normally carried in stock. But of course, FTI has one of the largest sheathing inventories on the planet!
Please check our Sheathing Specs document for more information on available styles.
When designing the part, understanding the end application will help us (or you) select the most effective/least expensive sheathing material.