Wire slip on the capstan: causes, effects and the role of the pulling band

On a wire drawing line the capstan has one clear job: pulling the wire through the die at the required speed, wrap after wrap. That pull is transferred by friction on the pulling band, the part of the capstan the wire wraps around. When this contact loses stability, the wire moves across the surface more than it should. That is slip.

Slip is not always a fault. On some drawing machines it is part of the design and of the way the machine works. It becomes a problem when nobody has planned for it: it marks the wire, alters its diameter, wears down the pulling band and forces operators into constant adjustments.

This article looks at how a capstan transmits pulling force, at the most common causes of unwanted slip and at why surface condition matters as much as line settings. It follows on from our pieces on roughness and galling, from a different angle: on the capstan, friction above all has to stay consistent. At Paganoni we have been working on these surfaces for the wire drawing industry since the 1970s.

Is wire slipping on the capstans of your line? Talk to one of our technicians: together we will assess the pulling band and identify the right coating.

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How a capstan transfers pull to the wire

After leaving the die, the wire wraps around the pulling band for a set number of turns. The force the capstan can transmit depends on two factors: the friction between wire and surface, and the wrap angle. The relationship is described by a classic equation of mechanics, the Euler–Eytelwein formula, better known as the capstan equation: the tension the wire can carry without slipping grows exponentially with the friction coefficient and the wrap angle.

This has two practical consequences. Adding or removing a single wrap noticeably changes pulling capacity. And small changes in friction, caused by surface wear or lubricant residue, have a bigger effect than you might expect.

There is also a speed constraint. The wire elongates in every die, so each capstan has to turn faster than the one before it, in proportion to the reduction in cross-section. If the peripheral speed of the capstan does not match the actual wire speed, the difference is absorbed at the contact point and the wire slides over the pulling band.

Want to check that the capstans on your line are running under the right conditions? Get in touch for a technical discussion.

Slip and non-slip machines: two different principles

Before deciding whether slip is a problem, you need to look at the type of machine.

On slip-type drawing machines, widely used for wet drawing of copper and aluminium and on multi-wire machines, the wire runs over stepped cones or capstans that always turn slightly faster than the wire itself. Slip absorbs the differences in elongation between one pass and the next, and is part of the process. What matters here is material resistance: cones, rings and rollers work under continuous sliding, often immersed in lubricant, and their surfaces must wear as little as possible.

On non-slip machines, typical of dry drawing of steel, each capstan pulls the wire without sliding, and speeds are synchronised by accumulation systems or dancers that compensate for changes in tension. Here slip is an anomaly: a sign that something in the system has changed.

The two situations call for different solutions, and the choice of surface follows the logic of the machine.

Running slip or non-slip machines? Ask for information on the materials best suited to your drawing machines.

The most common causes of unwanted slip

When wire starts slipping on a machine that is not designed for it, there is rarely a single cause. These are the most frequent ones.

• Pulling band wear: the passing wire cuts grooves and score marks, or polishes the surface unevenly. The contact area changes shape and the pull is no longer spread evenly across the wraps.
• Lubricant build-up: in dry drawing, soap carried by the wire accumulates on the pulling band and forms a layer that changes friction.
• Too few wraps: if the number of wraps is low for the pull required, the slightest variation is enough to make the wire slide.
• Speed mismatch: die wear changes wire diameter and therefore elongation; if capstan speeds are not corrected, the difference turns into slip.
• Insufficient or unstable back tension: the tension at which the wire reaches the capstan has a direct effect on how much pull it can transmit.

Some of these causes can be dealt with by adjusting the line. Pulling band wear cannot: as long as the surface is damaged, every adjustment is only temporary.

Seeing score marks or shiny patches on the pulling band? Talk to our technical team: assessing the surface condition is the first step.

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What slip does to the wire and to the line

The first effects show up on the product. Wire sliding over the pulling band picks up surface marks and scoring and, in more severe cases, diameter variations. On wire headed for further processing or for applications with tight tolerances, these defects end up as scrap.

Then there is heat. Sliding generates heat right in the contact zone, which is already the most heavily loaded area. As temperature rises, so does the risk of galling: wire material sticks to the capstan surface, builds up and makes the contact even worse. We covered the mechanism in detail in our article on galling.

Finally, there is wear. Slip wears down the pulling band, and a worn band encourages more slip. The cycle ends with a machine stoppage and the replacement or reconditioning of the capstan. In the meantime, operators chase the problem with repeated changes to speeds and wraps, taking time away from production without removing the cause.

Want to break the cycle of wear and slip? Contact us to find the right solution for your capstans.

Coatings and materials for consistent pull over time

A new, properly set capstan runs without trouble. The real question is how long the pulling band keeps the geometry and finish it had when it left the workshop. Much of the answer lies in the coating: a surface that wears very little keeps contact with the wire consistent and makes line settings last.

Within the Paganoni range, the choice depends on the drawing process and on the component.

• Coldside®: a coating based on tungsten carbides, with high resistance to friction and to wire slippage. It is particularly suited to capstans used for drawing ribbed wire for reinforced concrete, and it can also be applied to finished or worn parts. You can read more in our dedicated article on Coldside®.
• Kerblack® chromium oxide coating: designed for dry drawing of ferrous and non-ferrous metals on capstans and reels, rings and cones. It sharply reduces the friction coefficient, eliminates galvanic corrosion issues and allows higher production speeds while preserving the surface quality of the wire.
• Wolside®: a super-compact hard-metal coating based on special tungsten carbides, hot-fused using HIP (hot isostatic pressing). It works in both dry and wet drawing on rings and cones, idler pulleys, pulling capstans and guide rollers, with negligible wear and high impact resistance.
• Kersint®: a sintered technical ceramic based on zirconium oxide. Rather than a coating, it is a material used to make complete parts or inserts: drawing cones, rings for copper and aluminium breakdown machines, multi-groove rollers for multi-wire machines. It is the natural choice for slip-type machines in wet drawing, where sliding is continuous and excellent corrosion resistance in acid baths is required.

Reducing friction where the wire is meant to slide and keeping pull stable where it is meant to be gripped are compatible goals. In both cases what counts is that the surface stays the same for the whole life of the component. For a broader view, see our article on how to extend the service life of your capstans.

Not sure which solution suits your capstans? Request technical advice and we will help you identify the right coating or material.

Inspecting and recovering the pulling band

Slip rarely appears overnight. Recurring score marks on the wire, shiny or dull patches on the pulling band, having to add wraps or correct speeds more often than usual: these are warning signs worth taking seriously. Regular dimensional checks on the pulling bands let you act before the damage reaches the wire and the machine.

When the pulling band is already worn, replacing the capstan is not the only option. With our capstan and pulley recovery service we restore the original geometry by depositing Kerblack®, Wolside® or Coldside® on the most worn area and, as a rule, we also clean the inside of the capstan to remove limescale deposits. A recovered and coated part outperforms a new uncoated capstan, with a clear cost advantage over buying new. The full process is described in our article on recovering worn capstans and pulleys.

Every job goes through our in-house departments for dynamic balancing, dimensional inspection and roughness testing, and ends with a final test certificate. It is the approach we have followed since Paganoni, a division of Mario Nava S.p.A., started working for the wire drawing industry in the 1970s, and today we offer it to machine builders and wire producers in Italy and worldwide.

Is wire slipping on your capstans, or does the pulling band show signs of wear? Write to us: we will review your case and propose the right solution, from coating to recovery.

Il filo slitta sui vostri cabestani o la fascia di tiro mostra segni di usura? Scrivici: analizziamo il caso e vi proponiamo la soluzione più adatta, dal rivestimento al recupero.