Actual flex-life testing is performed at Northwire to prove designs and is an ongoing activity. The flex test protocol has been standardized over the years and is known as the Northwire Standardized Flex Test Protocol (NSFTP). Flex-life testing is performed to prove new designs, to improve existing designs and to evaluate new materials and constructions.
There are now two test protocols in use:
1. "Mode A" the Tick-Tock Bending Test
The cable is flexed 180° over Nylon blocks that have a 3” radius. The conductors in the cable are daisy-chained together and the continuity of all conductors is monitored continually. A fault in any conductor will stop the tester. There is 1 lb. of weight hanging on the cable as it is flexed. The tester runs at a rate of just under 30 cycles per minute or a little over 40,000 cycles per day. (One cycle is a 90° bend in each direction)
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2. "Mode B", the Rolling/Torsion Flex Test
The cable is pulled approximately 18” over a 3” radius wheel and twisted 360°. All of the twisting takes place in the 18” between the grip and the wheel. There is a 9 lb. weight hanging on the cable as it is twisted. The amount of weight is determined by the size of the cable and inner conductors. (The machine pulls it up and gravity pulls it back down. As above the conductors are chained together and continuity is continually monitored. The speed is the same as above. (One cycle is one up and one down stroke resulting in two 360° twists)
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- Install in accordance with all local regulations and building codes.
- Unwind the cable from the outer layer of the spool without applying excessive tension on the cable. Do not unwind off the end of an upturned spool or cable bundle. It is not necessary to “rest” the cable, or “shake it out”. Northwire EnduroFLEX may be installed immediately.
- The recommended bend radius of EnduroFLEX XM® and CRXM is 10 x the cable OD, for maximum flex-life.
- Cable installed in cable carrier-track must not be affixed or tied internally to the track. The cable must be laid out flat, parallel with other cables and not twisted or interwoven with other cables and hoses within the track. Follow the carrier-track manufacuturers’ recommendations.
- Cable installed in cable carrier-track should have an abrasion-resistant outer jacket that may need to withstand other environmental factors such as oil, water, chemicals, etc.
- Cable installed in bundles, or when tied to other supports, need to slide freely under the cable ties or brackets.
- In general, maximizing the installed bend radius in any installation will significantly improve the cable’s flex-life.
Continuous Motion Applications Need High performance Cable
In robotic or automated manufacturing equipment, the repetitive motion of machinery destroys conventional cabling. Cable specifically designed for repetitive flexing is the answer.
- Material Handling
- Assembly Machinery
- Packaging machinery
- Semiconductor manufacturing
Where the cable is subjected to constant or continuous motion and flexing. Ordinary cable cannot withstand the constant bending, rolling and twisting motion that is found in many of these applications.
In-stock high flex-life cable now available from NorthwireDirect:
- EnduroFLEX XM – Industrial grade cable for robotics and other continuous motion applications
- EnduroFLEX CRXM – Industrial grade cable for robotics and other continuous motion applications – extreme service, highly cut resistant and oil/fluid resistant
What Happens in Continuous Flexing Situations?
4 Basic Kinds of Flexing
- Bending, where the cable bends back and forth. One end may be in a stationary position and the end in motion may be bending around a fixed object.
- Variable Flex, where slacked cable is fixed in two positions along its length, surrounding a free moving apparatus such as a robotic head.
- Torsional flex. The cable is twisting around its axis. The torsional flex may also involve pulling or tension stressing on the cable, combined with bending and/ or rolling flexing.
- Continuous or rolling flexing. This type is commonly found in cable track systems where a flexible cable carrier is rolling back and forth over some length in a linear motion. An added hazard to cable is the constant rubbing of the cable jacket against the carrier track itself and other cables or hoses in the track. There are even some carriers that can twist…
What Happens to the Cable?
Internal conductor bundle corkscrews and eventually one or more conductors break.
- Internal copper conductors “cold-harden” and break.
- Inner conductor insulation fails due to internal heating and/or abrasion and then short-circuits.
- Cable outer jacket fails due to heat and/or abrasion, exposing the inner conductors to the outside environment and then failure.
- The presence of other additional environmental factors may accelerate cable failure in constant motion applications, such as oil or chemicals which may cause premature failure of the cable outer jacket. Welding spatter introduces heat and highly abrasive material to the environment.
- Water, sunlight and temperature extremes are additional hazards.
How to Prevent Failure – Use the Correct Cable
Some Cable Construction Features Continuous-Flex Cable
- Inner conductor insulation materials and outer jacket materials possess sufficient memory. That is, they tend not to maintain a set deformation when stressed.
- Overall cable construction is loose and internally slippery, whereby the conductors can freely move within the bundle without generating enough heat and abrasion to cause failure.
- Inner conductor copper is an alloy that can withstand flexing without cold hardening.
- Design of the copper stranding minimizes internal bending strain.
- Optimizing the lay of the cable bundle to minimize strain.
- Testing and rating for anticipated flex-life.