EN­ERGY CHAIN SYS­TEM RE­LI­A­BIL­ITY

DEMM Engineering & Manufacturing - - FRONT PAGE -

Treotham pro­vides a large range of high qual­ity igus chain­flex ca­bles that can be used in en­ergy chains for var­i­ous ap­pli­ca­tions.

Due to the in­crease in au­toma­tion and in­dus­trial pro­duc­tion, the use of en­ergy chains is rapidly grow­ing. The re­quire­ments for ca­bles that sup­ply ma­chine parts with me­dia, data and en­ergy are also ris­ing as fac­tors such as speed and ac­cel­er­a­tion be­come more de­mand­ing.

Move­ment in the en­ergy chain spans from hor­i­zon­tal or ver­ti­cal for sim­ple ap­pli­ca­tions, now to mul­ti­ple, com­plex ro­ta­tions for six-axis ro­bot ap­pli­ca­tions. The sys­tem is some­times re­ferred to as the um­bil­i­cal cord of a ma­chine, as both the ca­ble and the chain must per­form ev­ery pre­cise move­ment. There­fore the re­li­a­bil­ity and ser­vice life of mov­ing ca­bles could be the most im­por­tant fac­tor of the whole sys­tem.

It is also ex­tremely im­por­tant that the de­sign and ma­te­rial of the ca­bles are adapted to the en­ergy chains so that they can with­stand the me­chan­i­cal loads from mil­lions of cy­cles with­out fail­ing and suc­cumb­ing to the corkscrew ef­fect.

Ca­bles that are used in mov­ing ap­pli­ca­tions, es­pe­cially en­ergy chains, are sub­ject to in­creased strain. Stan­dard tests that de­ter­mine the dura­bil­ity of the ca­bles of­ten ne­glect to take field ex­pe­ri­ence into ac­count. Field ex­pe­ri­ence shows that even highly flex­i­ble ca­bles of­ten reach their load lim­its while mov­ing in en­ergy chains. There­fore it is ev­i­dent that in con­ven­tional stan­dard tests such as those car­ried out by VDE, IEC or UL, the ser­vice life can­not be pre­dicted.

There are some well rec­og­nized test pro­ce­dures from var­i­ous in­sti­tutes in the ca­ble in­dus­try, how­ever the pro­ce­dures are very ba­sic and do not in­clude spe­cific re­quire­ments for con­tin­u­ous move­ment in the en­ergy chain.

The bend­ing fa­tigue test as per VDE and stan­dard wear test don’t sat­isfy re­quire­ments of a chain-ca­ble com­bi­na­tion. In the bend­ing fa­tigue test the ca­ble has a com­pletely dif­fer­ent move­ment se­quence then the move­ment of the ca­ble in an en­ergy chain. Many ca­ble de­signs that meet these re­quire­ments quickly fail tests con­ducted un­der real world con­di­tions.

Stan­dard wear tests, which use nee­dles, sand pa­per or ra­zor blades to de­ter­mine the wear of a ma­te­rial, can only be used to make a gen­eral com­par­i­son. This test is not good when de­ter­min­ing the dura­bil­ity of a jacket ma­te­rial in an en­ergy chain. It is im­por­tant to test the wear of the ma­te­rial by test­ing the chain and ca­ble ma­te­rial to­gether so that they can adapt to one an­other.

Igus thor­oughly tests the jacket ma­te­ri­als of ca­bles by rub­bing them against chain ma­te­ri­als to de­ter­mine the de­gree of wear. As a re­sult they use ma­te­ri­als such as PVC, PUR and TPE which of­fer min­i­mal wear com­pared to stan­dard PVC ca­bles and pro­vide op­ti­mal per­for­mance when used in an en­ergy chain.

The de­sign of ca­bles for mov­ing ap­pli­ca­tions has also changed dras­ti­cally over time, which has led to ca­bles be­ing braided in bun­dles. In a com­plex process, wires are braided in sin­gle bun­dles con­sist­ing of three, four or five wires, which are then braided with each other. For large ca­ble de­signs, this is done around a strain- re­lief el­e­ment. The re­sult is a ca­ble that is built for move­ment and ideal for use in chains. Un­like a ca­ble with lay­ered braid­ing, each of the wires in the en­ergy chain is moved an equal amount at both the in­ner and outer radii. This helps pre­vent stretch­ing and buck­ling on one side.

For more ex­treme move­ments, ca­bles with more com­plex de­signs are used. One of the ap­pli­ca­tions with the most ex­treme move­ments of bend­ing and twist­ing, are in­dus­trial ro­bots. They use damp­ing el­e­ments to give the wires in ro­bot ca­bles the es­sen­tial free­dom of move­ment within the ca­ble. The tighter the ca­ble is wound and the closer the ca­ble gets to its break­ing point, the more dif­fi­cult it be­comes to twist. Spe­cial shields and outer ma­te­ri­als also en­sure op­ti­mal ca­ble dura­bil­ity.

It is also im­por­tant to con­sider en­vi­ron­men­tal fac­tors when se­lect­ing the right en­ergy chain and ca­ble. De­pend­ing on the ap­pli­ca­tion, they may be ex­posed to chem­i­cals, dirt, dust, mois­ture, im­pacts or ex­treme tem­per­a­tures at times when they are used in harsh in­dus­trial con­di­tions. Un­der these con­di­tions, field ex­pe­ri­ence is ex­tremely im­por­tant for test­ing the re­silience and ser­vice life of the ca­bles. For this rea­son, igus op­er­ates a 1750 square me­ter test lab in which ap­pli­ca­tions are run around the clock on 58 dif­fer­ent test sys­tems. This re­sults in over two bil­lion test strokes per year.

In one ap­pli­ca­tion, a sea con­tainer was spe­cially mod­i­fied to gen­er­ate tem­per­a­tures of - 40°C to 60°C and to test high tem­per­a­ture fluc­tu­a­tions. En­ergy chains and ca­bles were moved mil­lions of times in the con­tainer and the re­sults were eval­u­ated ac­cord­ingly. This is com­pletely dif­fer­ent from the usual cold bend­ing test, where a ca­ble is wrapped once around a man­drel, cooled down to the tem­per­a­ture to be tested and then moved once. If the ex­te­rior man­drel shows no signs of vis­i­ble dam­age, then the ca­ble is con­sid­ered “safe” for the re­spec­tive tem­per­a­ture.

There­fore, the re­li­a­bil­ity of a mov­ing ca­ble de­pends on a large range of fac­tors whose ef­fects can ul­ti­mately only be tested and proven un­der real-world con­di­tions.

TREOTHAM NEW ZEALAND LTD WWW.TREOTHAM.CO.NZ

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