Storage, operation and maintenance of rubber covered rolls

Receipt and storage of the roll

The case in which the roll is shipped should be thoroughly inspected for shipping damage and then opened. The roll will be covered with a paper or other type of wrapper. Inspect to be sure that the cover is not touching or resting upon any part of the case. The roll wrapper should then be carefully removed and set aside to see whether or not any surface blemishes which may be present in the cover have been inflicted after the application of the paper. If there is any damage noted, notify the cover supplier immediately and save the wrapping paper to await further inspection.

At this time, the hardness, centerline diameter and crown should be measured and recorded on the appropriate forms.

The roll should then be installed in the machine, or put in the proper type of roll storage.

A closed shipping box is a good storage place. During storage, the roll should be wrapped with a suitable wrapping material - kraft paper is excellent - and should be stored away from heat, electrical discharges, hazards of oil dripping, splashing with chemicals and possibility of mechanical damage. The roll should never be stored by laying on the rubber face, but should always be supported and moved by the journals.

If rolls are to be stored for long periods of time, over 6 months, it is good practice to regrind them before installing the machine.

Since rolls tend to sag, turn them 180° angle occasionally, every 2 months, during storage. This is especially important for rolls run under light or no loading - table rolls, lumpbreakers, blade coater, etc.

Be extremely careful about moving rolls which are cold, i.e., below 32°F. A sudden blow may cause splitting of the cover, especially on light cores with hard covers such as table rolls.

If there is any question about the proper amount of crown, at the time of installing the roll when it has a freshly ground surface is the ideal time to take a nip width, in order to judge the proper crown.

Care should be taken to be sure that the roll is the proper roll for the position in which it is being installed. For example, top press rolls and size press rolls are frequently interchangeable for size, but not for cover composition.

Note the position in which the roll is placed, the date, the loading, the speed and other expected running data on the roll record card or in the roll record book.

Operation

During any shutdown be sure that the roll is either kept turning or disengaged.

Inspect the roll during each interruption of running, if time allows, looking for:

• Variation of temperature across the face. This is almost invariably an indication of variation in load across the face and, in almost all cases, will result in a peculiar wear pattern, or if severe enough can result in adhesion loss of the cover.
• Fatigue cracks. These cracks are straight and parallel to the roll axis. This is seldom seen across the entire face of the roll. It is most often seen near the ends and is an indication of undercrowning. If this is encountered, it would be wise to take another nip width impression.
• Smash types of cracks. These are irregular and not parallel to the axis. They are due to an excessive wad condition and should be corrected, if at all possible.
• Note any changes in the appearance of the cover, which could be in the form of wash outs, areas of wear which could evidence themselves by roughness, groove dimensions on Venta-Nip® rolls, cupping around the holes on suction press rolls and so forth.

If bouncing of the roll occurs, remember the roll may be the victim rather than the source of the vibration. In addition to irregularities in the cover, bouncing may be due to out of balance felts, bearings, universal joints, etc. Bouncing may also be due to flat spots or raised spots due to resting the roll on the cover instead of the journals.

Corrugations are almost always due to an external cause. They can be alleviated, but not eliminated, by going to a harder cover.

Removal for maintenance

The same precautions should be taken during removal for grinding or other types of maintenance as were observed for the roll installation.

In addition, care should be taken such that the roll is not laid on the floor on the rubber surface before grinding. This will result in a set which will lead to bouncing after the roll is reinstalled.

At this time, the hardness should be taken in several places across the face of the roll. The centerline diameter should be taken and the crown contour should be plotted. Notes on the general appearance of the cover should also be made.

Changes in the hardness indicate, if a softening of the cover, a chemical condition which can probably be rectified by a change in cover composition.

Comparison of the out crown contour with the in crown contour can yield important information as to the distribution of the load across the face of the roll, and perhaps might point the way to a change in crowning procedure.

Localized wearing, wash outs, or cracks indicate variation in nip conditions across the face of the roll.

For rolls softer than 0 P&J, grind at least 1/32" off the cover thickness reducing the roll diameter by 1/16". Be certain that all crack or damage of any kind is completely removed.

Analysis of your roll records will point to ways to improve rubber roll performance. The rubber cover itself is a measuring instrument which can indicate that changes in chemicals, crowns or other conditions are needed.

Before shipping

At this time care should be taken to be sure the case is still satisfactory. Cases are frequently stored out of doors where they are subjected to rotting and attack by insects. Remember, the case that you send the roll to the rubber coverer in is also used to return the roll to you and the rubber coverer will repair the case at your expense, if it is not suitable for shipping the expensive recovered roll.

Before ordering a recover

If you are for any reason dissatisfied with the performance of the previous cover, or if conditions have changed at that roll position, discuss the change in conditions with your rubber coverer. This type of change could involve chemical conditions, speeds or loadings.

The nip width or roll "foot print" is a potent tool for judging the amount of crown to be put into a pair of rolls. There are various ways of determining the nip width – using carbon paper, embossed foil paint, etc. After the nip widths have been obtained, the crown is determined. Valmet can assist with crown calculations.

For more information on proper roll handling of all types of rolls, contact your Valmet representative.