Paper making and fiber liberation – Apparatus – Apparatus repair – cleaning or conditioning
Reexamination Certificate
2000-09-14
2003-11-11
Chin, Peter (Department: 1731)
Paper making and fiber liberation
Apparatus
Apparatus repair, cleaning or conditioning
C162S199000, C162S198000, C162S263000
Reexamination Certificate
active
06645349
ABSTRACT:
FIELD OF THE INVENTION
The invention concerns a method for conditioning of a roll, in particular of a roll in a paper machine or in a paper finishing device, in which method the condition of the face/coating on the roll is monitored and the face/coating on the roll is cleaned and/or ground.
The invention also concerns a device for conditioning of a roll, in particular of a roll in a paper machine and/or in a paper finishing device, which device comprises means for monitoring the condition of the face/coating on the roll and for cleaning and/or grinding the face/coating.
BACKGROUND OF THE INVENTION
As is known from the prior art, in paper machines and in finishing devices of paper machines, the rolls in said machines and devices are contaminated, worn and/or damaged during operation, which has the consequence that the properties of the paper that is produced deteriorate. This is why the rolls must be cleaned and/or conditioned from time to time, in which connection the machine has to be stopped for the time of cleaning/conditioning. In some cases, the condition of a roll has deteriorated to such an extent that the roll must be replaced, which is detrimental in view of the production and may result in a standstill in the production in the paper machine.
An unevenness in the roll face, irrespective of whether it arises from contamination or consists of some other exceptional feature in the geometry of the roll face, causes alterations in the profile of the paper web. One particular problem related to coated rolls are the so-called hot spots, i.e. local heating of the roll. Hot spots arise in roll faces at an elevated zone arising from particles of contamination or equivalent, as a result of deformation arising from a higher surface pressure in the nips between the rolls, which results in formation of hysteresis heat, which causes damage to the roll coating at this location. This effect is made particularly problematic by the fact that the progress of a hot spot is rapid, and a hot spot is difficult to locate. Causes of disturbance include debris coming from outside or fibres or other contaminations coming along with the paper web.
One method of monitoring the face of a roll is visual examination of the roll, for example, by means of a camera. By means of detectors that have been fitted stationarily in view of monitoring of the condition of the roll bearings, it is possible to detect disturbance that affects the balance of the roll and also a hot spot that has developed sufficiently far. On the contrary, minor unevenness, single particles of contamination or a hot spot that is in its initial stage in the roll face cannot be detected by means of such methods.
In some prior-art solutions, the operator observes the roll face and, when he notices a hot spot, he eliminates the hot spot manually by means of a doctor. Currently, when recycled stock is used abundantly, adhesive constituents (stickies) contained in said stock increase the problem of hot spots. At present, paper machines are often connected with on-line coating, and the coating material (paste) must be dried before calendering. Sometimes the paste remains moist, which also causes increased hot spot problems.
The coating on a roll can also be damaged by what is called barring, in which case the roll face is subjected to formation of damage in the coating, which causes vibration. Barring means that on the roll face, in the axial direction of the roll, wear traces arise, which extend across the entire length of the roll and which resemble a certain sort of grooves. The process of formation of barring has not been established fully, but it is one explanation that minor variations in the thickness of the web in the longitudinal direction of the web produce wear of the roll face when said variations are repeated at suitable intervals.
The face of a roll is also worn during operation, in which connection the roughness of the face is increased and the quality of the paper deteriorates, in which case the roll must be removed for grinding and replaced by a replacement roll. For example, the thermo rolls used in the prior-art calenders are worn and become rough, in which case the linear load in the calender must be increased, which again reduces the bulk, and/or it is necessary to raise the calendering temperature, which reduces the bulk and also increases the consumption of energy.
A roll can also be worn unevenly, in which case the profile of the paper varies, and the roll must be removed for grinding and replaced by a replacement roll.
In the manufacture of paper, by means of a what is called calender, which is placed after the paper machine or which is connected directly with the paper machine, the face of the paper is processed to make the paper suitable in particular for printing machines so that the paper is passed through one or several roll nips. In the prior art currently applied, the roll nip in the calender is, as a rule, formed between a soft roll and a hard roll. The hard roll is made of metal, for example steel, and its face has been polished. The soft roll has been coated with an elastic synthetic material, for example some polymer. The paper web is passed into said nip formed by the pair of rolls, in which nip the smooth face of the hard roll smooths and glazes the paper web, and the elastic face of the soft roll provides the paper web with uniform density. If it is desirable to glaze both faces of the paper, the machine must have two pairs of rolls, so that one hard roll glazes one face of the paper and the other hard roll the other face. The hard rolls are usually heated rolls. In the U.S. Pat. No. 5,033,373 and in the published EP Patent Application 0,728,867 A2, calenders made of pairs of rolls are described, and the U.S. Pat. No. 5,131,324 describes a group of rolls in which there are one soft roll and two hard rolls. A soft roll differs from a hard roll in the respect that, since the face of a soft roll is made of some resilient material, for example polymers, particular attention must be paid to its servicing. A disturbing factor that is in its initial stage ought to be detected and located immediately in order that it could be eliminated directly. For example, in a two-nip calender, in which there is one variable-crown roll and one thermo roll in each nip, two replacement rolls must be kept constantly on stock, which involves costs. On the other hand, renewed coating of coated rolls is very expensive, so that a coating that has been spoiled, for example, because of a hot spot is quite expensive.
Since, as is well known, the condition of rolls has been monitored mainly by the operator manually in view of contamination, deterioration of condition, damage, for example hot spots, and in view of similar problems as well as by monitoring the quality of the paper, a number of replacement rolls have been necessary, and it has been necessary to renew coatings, which is expensive. Further, the standstills required by cleaning and conditioning operations have increased the costs.
With the use of various doctors, it is possible to clean the face of a roll during operation. Grinding can also be applied. With respect to the prior art related to the present invention, reference can be made, for example, to the FI Patent Applications 971488 and 941620 as well as to the U.S. Pat. No. 5,394,653 and to the EP Patent Application 0,359,304. In the FI Patent Application 971488, a band grinding device for rolls and a method for the control of the band grinding device in the grinding of faces of variable-crown rolls are described. The device and the method known from said published patent application are, however, meant to be used after the roll has already been removed from the machine to a grinding machine, and it is an advantage of said invention that the time taken by the grinding has become shorter. In the FI Patent Application 941620, a method and an equipment in conditioning of the coating on a roll are described, wherein the face of the roll is conditioned periodically and on the site without removing the roll. This arrangem
Koivukunnas Pekka
Mäenpää Tapio
Chin Peter
Hug Eric
Steinberg & Raskin, P.C.
Valmet Corporation
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