For the coating of a moving base, such as a paper board web, with a coating agent, various size presses are commonly used. In size presses, the paper or board web is passed through a nip formed by size press rolls, in which nip the coating agent is transferred onto the face of the paper or board web.
In the prior art, a number of size presses of different types are known, of which one application, which has been in use for a long time, is one in which the rolls in the size press are arranged substantially on the same horizontal level so that the paper or board web runs through the size press nip substantially vertically from the top downwards. In such an application, as a rule, in the nip between the size press rolls, there is a size pond, through which the paper or board web runs. When the web passes through the pond, the size adheres to both sizes of the web, whereupon, in the nip, the size press rolls spread and smooth layers of suitable thickness onto the web faces.
A second embodiment of a size press is one in which, by means of some suitable coating means, such as bar or blade coaters, size films of suitable thickness are first spread onto the faces of the size press rolls, from which said size films are then transferred onto the web faces in the roll nip. A third prior art embodiment of a size press is a so-called gate roll size press, which is, in two-sided surface sizing of a paper, composed of size rolls, which are in nip contact with each other. A gate roll size press first comprises applicator rolls proper, the web being passed through the nip between said rolls. Size films are spread onto the faces of said applicator rolls by means of gate rolls, which comprise a metering roll and a transfer roll. The transfer roll is in nip contact both with the metering roll and with the applicator roll proper. The coating agent is fed into the nips between the gate rolls, i.e. between the metering rolls and the transfer rolls, where it forms a pond. From both ends of the ponds, as a rule, an overflow is provided to the return circuit. By the effect of hydrodynamic forces, coating agent passes through the nip between the metering rolls and the transfer rolls, forming size films both on the metering rolls and on the transfer rolls. The size film placed on the transfer roll is transferred into the nip between the transfer roll and the applicator roll, which nip smooth and thins the film onto the applicator roll. In the nip between the applicator rolls, the film are transferred further onto the web.
The present invention is expressly related to the size presses of the last-mentioned gate roll type. The construction of conventional gate roll size presses is usually such that one of the applicator rolls is journalled fixedly on the size press frame, whereas the other applicator roll is arranged displaceable so that its bearing bracket is linked pivotally to the frame of the size press. The transfer roll that is in nip contact with the fixed applicator roll is linked pivotally to the size press frame by the intermediate of a loading arm and, further, the metering roll that is in nip contact with said transfer roll is linked pivotally to the size press frame by the intermediate of a loading arm and, further, the metering roll that is in nip contact with said transfer roll is linked pivotally to the bearing bracket or loading arm of the transfer roll by the intermediate of its own loading arm. In a corresponding way, in conventional solutions, the transfer roll that forms a nip with the displaceable applicator roll is linked pivotally to the bearing bracket or loading arm of the applicator roll by the intermediate of its own loading arm or by its bearing bracket and, further, the metering roll that is in nip contact with said transfer roll is linked pivotally to the bearing bracket or loading arm of the transfer roll. On the other hand, the loading cylinders of each roll are linked to the loading arm of search roll and, at the opposite end, to the machine member to which the roll concerned is linked pivotally.
In wide and high-speed paper machines in particular, the prior art solutions are associated with considerable problems of vibration of the gate rolls, resulting from the mode of linkage of conventional gate rolls. For example, the metering roll at the side of the displaceable applicator roll, i.e. the outermost roll in the system, is fixed to the frame by the intermediate of a chain formed by three separate articulated joints. By means of the construction concerned, it is very difficult to provide a sufficiently rigid fastening of the rolls in view of vibrations. Besides wear of the constructions of the frame, the vibrations also cause quality defects in the coating band an increased wear of the roll coatings. The wear of the roll coatings is one of the most important problems of the gate roll size presses.