Smooth wire feeding, smooth welding

Questioning wire-dispensing practices produces big payoffs

THE FABRICATOR® MARCH 2009

March 24, 2009

By: ,

A recent study analyzed wire feed practices, tested variables, and took initial steps at developing a scientific knowledge base that ultimately could take the trial and error out of wire feed setup.

Birdnesting causes downtime

Figure 1

Welders often take inconvenience for granted. If a wire feeds inconsistently, or if setup takes forever and a day, so be it. It's the way it's always been, so why question it? Nevertheless, questioning everything is a key tenet of lean manufacturing, and it need not be limited to reducing work-in-progress and finished-goods inventory. Wasteful practices abound inside workcells, welding included.

Some activities within the welding cell aren't questioned very often. Consider wire feeding. Few ask if the wire could be fed more efficiently. But wire feeding problems often can cause significant downtime. Such problems can emerge, ironically, as workers become better welders. One company, for instance, stepped up training efforts and, in some cases, nearly doubled the average fillet welding arc speed. Average 1/8-inch fillet welds, for example, jumped from 20.8 to 40 inches per minute (IPM). To achieve higher productivity, welders used higher wire feed speeds, changed to a richer argon blend to reduce fumes, and welded in the spray-transfer mode. Soon after changes were implemented, however, wirefeeding problems emerged.

Wire is dispensed in various ways depending on how the wire is packaged—be it on spools, reels, or drums—and poor, improper setup can create problems and, ultimately, significant waste. Cumbersome wire changes may add unnecessary minutes to the operation. Every time a wire birdnests, the welding operation can stop for 10 to 15 minutes, or even more (see Figure 1).

Wire travels from the drum or spool, through a conduit, inlet guide, feed rolls, into the gun, the liner, and out the contact tip—and no element should be ignored. Many take wire dispensing for granted, and because no good scientific data exists, setup often happens through trial and error, without adequate engineering for perfect first-time quality.

A recent study may help matters. Conducted at various fabricating facilities across North America, the study analyzed wire feed practices, tested variables, and took initial steps at developing a scientific knowledge base that ultimately could take the trial and error out of wire feed setup.

Attacking Friction

Various frictional forces must be overcome to achieve a uniform, efficient wire feed. First, the feed motor must overcome static friction and, once in motion, provide the force necessary to overcome the kinetic friction along the entire wire transport system. Rolling friction, the kind that hinders the wire motion along a surface, is small in wire feeding compared to kinetic friction. But any variation in these forces will affect the uniformity of feeding (see Figure 2). Because the coefficient of kinetic friction is usually less than that of static friction, wire feed initiation requires high, instantaneous torque.

Figure 2
Different conduit materials have different coefficients of friction. Teflon® has a very low coefficient of friction, yet Teflon is never used to feed steel wires because the steel quickly abrades the soft Teflon.

Material in Contact
Coefficient of

Static Friction

s

Coefficient of

Kinetic Friction

k

Steel on steel (dry)
0.7
0.6
Steel on steel (lubricated)
0.12
0.07
Steel on aluminum
0.61
0.47
Steel on Teflon
0.04
0.04

As part of the wire feed study, engineers at ELCo Enterprises and Air Liquide designed a test bench to measure frictional force needed to feed various wires through conduits to the feeder, then through the welding gun whip past the contact tip. The initial tests established the force needed to buckle several wires. Various conduits were tested, and friction force varied from 0.05 to 9.69 pounds, depending on the makeup of the conduit. For example, nonmetallic conduits performed much better than steel-lined conduits. The tests showed that the frictional force could be limited to less than 4 lbs. at the feeder by selecting the correct type of conduit for the job at hand. If the frictional force exceeds 4 lbs., an air motor assist can help transport wire from the drum to the feeder over a distance of 150 ft. or more with no difficulty.

As the test showed, the smaller the wire's diameter, the more difficult it is to feed. Thin wires have lower column strength and will birdnest at lower feeding force. Aluminum wires are susceptible to birdnesting at forces less than 4 lbs. while 0.045-in. steel wires will take more than 50 lbs. of force before buckling and birdnesting. The test proved that for a reliable wire feeding operation, frictional forces must be kept significantly below the wire buckling strength (see Figure 3).

This test measured the amount of force that can be applied before different wires buckle.

Figure 3

Welding Wire
Wire Type
Buckling force lbs.
0.035" Aluminum
ER 4043
2.96–3.48
0.031" Stainless
ER 308LSi
8.76
0.045" Aluminum
ER 5356
13.14–14.76
0.035" Steel Wire
ER 70S-6
21.12–28.82
0.045" Steel Wire
ER 70S-6
>50.00

Optimal Feeding

The level of friction in wire feeding depends on the quality and length of the gun liners, conduits, and their layout; the brake setting on the wire spool; as well as the cast and helix of the wire—as shown by in-house tests as well as numerous field tests, described later. Specifically, all of the following factors can contribute to irregular wire feed, causing vibration, burnback, and eventually birdnesting:

  • Brake on the Spool. The brake on the spool mount must be adjusted so that the spool or reel stops rotating when the welder stops his weld. If not, the wire will come off the spool and tangle. If the brake is too strong, the feed rolls may slip, requiring more pressure and eventually damaging the welding wire and jamming the gun liner.
  • Conduit Type, Length, and Setup. The conduit transporting the wire from the wire drum, reel, or other type of large packaging up to the feeder must be robust and suited to the wire's diameter. The frictional force will increase with the length of the liner. Feeding the wire through too small a conduit will induce more friction and eventually jam, as the wire's lubricant rubs off and builds up inside the liner. If the conduit is not stiff enough, it will sag and introduce more friction. Nonmetallic conduits may not be hard or tough enough and may wear or snap off prematurely.

    Also, excessive drag at the wire source will compound itself throughout the conduit. Minor drag at the beginning of the conduit will become much more significant at the end of a long conduit, particularly if it turns and twists its way to the welding cell. Simply put, the more turns in the wire, the more drag on the wire. A conduit with droops and twists will require more force to pull the wire through. Minimizing those droops and twists can help minimize friction.

    Robotic applications bring up special considerations for conduit. Its material, whether polymer or flexible steel, must be tough enough to prevent breakdowns. Also, the conduit's connection points, where the greatest fatigue occurs from the moving robot, should be examined. Strong, reinforced connection points can prevent conduit breakage and unnecessary downtime.

  • Wire, Gun, and Conduit Liners. Flat steel liners, round spring liners, oval spring liners, steel braided liners, and nonmetallic liners all will induce varying degrees of frictional force. The force will vary more if the liner is straight, bent, or kinked during the welding operation. Spring liners made with an elliptical wire cross section have the lowest coefficient of friction, compared to round and flat cross sections. Also, wires with sharp cut edges or small apexes tend to shave during feeding, causing small birdnests, high friction, and, ultimately, complete blockage of the liner. Elliptical torch liners and conduits tend to shave the least amount and, therefore, produce a low friction coefficient.
  • Cast and Helix. Because welding wires are mechanically formed or drawn, they contain a natural springback characterized by cast and helix. Using wire with a small cast (more windings) is, in effect, like trying to pull a spring through a tube. The small-cast wire will flip more frequently in the gun and exit in a curved pattern out of the contact tip, creating peak frictional load every time it flips. On the other hand, large-cast or torsionless wires from a drum may be so straight that they are unable to pick up electrical current at the contact tip, making wire feeding erratic. For these reasons, the cast and helix should be large enough to limit friction, but not so large as to prevent electrical conductivity at the contact tip.

Testing on the Shop Floor

Field tests of actual welding setups used a digital wire-draw gauge to measure frictional force just before the feeder and at the contact tip. The force varied widely, from 2.5 to 17 lbs. at the feeder and from 4.2 to 45 lbs. at the gun (see Figure 4 and Figure 5).

One company had a truly optimized setup. A structural shop where welders used both GMAW and FCAW for in- and out-of-position work, its welding cells involved simple, boom-mounted wire feed with no significant kinks in the conduit (see Figure 6 and observation 1 and 14 in Figure 5). The pull force at the feeder was less than 4 lbs., and the pull force at the gun was less than 12 lbs.—two of the study's benchmark measurements. Other shops in the study varied from this ideal. But after implementing some small changes, many made significant improvements.

For instance, one structural steel shop switched to bulk wire drum packaging to make wire replacement easier and safer. The solid wire was provided in a drum, and the wire was then fed into an electrical feed assist motor to feed the wire from the drum on the ground, through a 50-ft.-long conduit installed through an articulated jib crane. The conduit carried the wire to a feeder positioned just over the welder. A dual-feeder setup allowed FCAW to be used if needed. Fume extraction equipment was also installed on the articulated jib crane. Welders were experiencing vibration in the gun and erratic wire feeding.

Two stations with a 50-ft. liner had varying force readings, with one showing 6.3 lbs. and another showing 17 lbs. at the feeder (see Figure 7 and observation 2, 3, and 4 in Figure 5 ). At the gun, the friction force at the second station was 42 lbs. After the gun liner was changed, the force dropped to 26 lbs.—much better, but not ideal. Electric feed assist motors were well-justified. But to reduce friction further, the shop could replace its electrical feed assist motor with an air motor to improve wire feed consistency, because air motors automatically adapt to required torque.

In another structural shop making bridge sections, wire was supplied in a torsionless drum and fed through a conduit 23 ft. long with no feed assist (see Figure 8 and observations 9, 10, and 11 in Figure 5). Both stations experienced low pull force, despite the complicated wire feed system with fume extraction equipment and a column-and-boom arrangement to hold the wire feed overhead. In all cases, the friction force at the gun was more than 12 lbs. The friction force increased from 14 lbs. when the feeder was far away from the column to 45 lbs. when the feeder was moved close to the column. As the feeder was moved closer to the column, the conduit carrying the wire bent and kinked, causing more friction. In this case, an air wire assist motor could greatly help the wire feed.

Ask Questions, Save Money

One electric transformer component manufacturer had a straightforward welding installation, yet wire feed problems persisted. Feeders were installed on the welding machines, and all the guns were 15 ft. long. Two stations were tested. One had a gun liner that was too long, resulting in a high frictional force of 22 lbs. at the gun. The other station had the wire brake set too high. When the brake was adjusted, the frictional force dropped from 17 to 7.5 lbs. at the gun (see observations 12, 15, and 16 in Figure 5 ).

This last example shows the need for welder training—and the importance of asking questions. Of all the setups studied, this was the simplest, yet inconsistent wire feed made it very inefficient. It also shows how a laissez-faire approach can affect operations: It's always been that way, why question it? The reason is the same as any good business decision: Asking questions can save money. Even the smallest change in duty cycle can amount to many hundreds, if not thousands, of dollars in annual savings (see Small Improvements Produce Big Savings sidebar). A 1 percent change in duty cycle might not seem like much, but the savings over time can have a real impact—all the more significant during these challenging economic times.

Viwek Vaidya, P.E., is director of welding technology for Air Liquide Canada Inc., 1250, boul. Rene-Levesque W., Suite 1700, Montreal, QC H3B 5E6, Canada, 514-933-0303, www.ca.airliquide.com. Ed Cooper is president of ELCo Enterprises Inc., 5750 Marathon Drive, Jackson, MI 49201, 517-782-8040, www.wire-wizard.com. Figures used in this article were presented by the authors during the AWS Professional Program at the Fabtech Intl. & AWS Welding Show, Oct. 6-8, 2008, Las Vegas, Nev.



Related Companies

More in Arc Welding from TheFabricator.com

Published In...

The Fabricator®

The FABRICATOR® is North America's leading magazine for the metal forming and fabricating industry. The magazine delivers the news, technical articles, and case histories that enable fabricators to do their jobs more efficiently. The FABRICATOR has served the industry since 1971. Print subscriptions are free to qualified persons in North America involved in metal forming and fabricating.

Preview the Digital Edition

Subscribe to The Fabricator®

Read more from this issue

comments powered by Disqus