Performance

Roberto's unique fibre formulation and special construction ensure excellent performance in metal processing applications.

Wringing & Oiling Performance

The special structure of Roberto roll covers ensures high performance in wringing and oiling applications. Roberto rolls operate under the principle of holding back a liquid dam from in front of the nip and creating a powerful capillary suction condition behind the nip. This combination accurately controls the amount of residual liquid carried forward, which can be varied by adjusting the applied pressure to the rolls.

Under mechanical load in the nip, the porous structure is compressed like a sponge and air and liquid is evacuated from its cavities. Within the nip, the void volume is reduced and the density is locally increased. As the roll turns, the pressure is relieved immediately after the nip and the cover decompresses. As the void cavities open up the Roberto material sucks in additional liquid from the substrate surface.

The suction effect enables a very high squeezing performance and helps to achieve a uniform film layer of the remaining liquid (e.g. oil film).

Removal of Surface Impurities

The porous / open surface structure and flexibility of the Roberto material provide a cleaning effect. Impurities like dust or pieces of metal can be removed from the substrate surface through migration into the material's cavities and remain there. This effect avoids scratches or imprints on the substrate surface during subsequent processing steps (e.g. deep drawing or pressing). To remove these absorbed particles the roll cover can be reground.

Driving and Guiding Performance

Although a compact roll cover (e.g. rubber or polyurethane) has a better coefficient of friction on dry surfaces, Roberto roll covers show a higher coefficient of friction in wet and oily environments (typical values see table below). Under these conditions, compact roll coverings tend to have a reduced adhesion, or even a temporary loss of friction to the substrate surface. This loss usually occurs in periodic intervals and is known as the 'stick-slip effect'. In contrast a porous Roberto roll cover performs like a tyre tread avoiding hydroplaning effects. Roberto maintains a high coefficient of friction resulting in stable driving performance and guiding control, which allows higher line speeds.

Coefficient of friction under various metal surface conditions. 

Mechanical Performance

The porous open structure of Roberto leads to a different behaviour under mechanical stress compared to compact roll coverings (e.g. rubber and polyurethane), as can be seen in the images below. Roberto shows a smoother stress reaction than a compact, closed covering. Stress peaks are on a lower level and are not mainly concentrated in the nip area but are distributed over a larger internal area. In contrast the compact roll covering displays a high degree of compressive strain, concentrated under the nip.  

The stress distribution property of Roberto results in excellent resistance to unusual mechanical stress, e.g. caused by rough/sharp edges, rivets, or splices of sheet metal. Compared to a compact roll, the risk of cutting or tearing out of the roll cover material is substantially lower (see Fig. 5).

In addition, if a cut or local damage does occur, the Roberto cover has a tendency to 'self heal'. Due to its basic flexibility and deformability, localised defects can be levelled out and closed by surrounding material with continued running.  

Typical PU/rubber roll under pressure PU / rubber rolls are non compressible, so the nip pressure causes the roll to bulge out under tension at each side. This tends to cause cuts or damages to enlarge. 

Chemical Performance

The Roberto roll cover range has been specially designed for various chemical environments. The use of different fibres and binding systems allows the chemical resistance to be adapted to specific needs and reduces chemical ageing to a minimum. Each Roberto cover material is tested in several standard media (oils, acids, bases etc) to select a chemically best suited formulation.