The invention is related to a system (process and device) in order to get a perfect winding structure during the winding of web like products especially made of paper to produce at least one roll on a winding machine, with several drums for supporting and starting the rotation of at least one roll which is positioned in a winding bed made by the supporting drums, at least one of which preferably comprises an elastic flexible surface. There are means to change the tensile stress remaining in the wound part.

In the above mentioned winding machines known in the art, the former supporting drums did not have an elastic flexible surface. Then, the winding structure could be influenced in that way that the distribution of weights on the supporting drums was different and variable during the winding process and the pressure on the rider roll dependent on the diameter of the roll was variable as well. Typically, the load on the supporting drum roll that was not wound by the web was

linearly decresed dependent on the increasing diameter of the roll, whereas the load on the first supporting drum in the winding direction (back drum) was linearly increased. The initial and final values were controllable. It was typical as well that the load exerted by the rider woll to the web roll at first was increased according to the diameter of the roll, then decreased and was even not any more existent of a defined diamter of the roll. The load itself was changeable.

It was typical as well that the tension exerted on the web that had to be wound was constant-independent of the diameter of the roll. Thus, the roll qualities of conventional two-drum winders, the supporting drums of which had a basically unflexible surface, could be improved.

The winding parameters did in no way take into account the friction values between the web and the supporting drums. Thereby, the forces exerted on the supporting drums and to the rider roll sometimes could not be transfered to the web. As a consequence slippage occured with respect to the web and the winding structure of the roll became insufficient.

The two-drum winders with at least one supporting drum with an elastic flexible surface (softnip principle) that were introduced in the market in the mean time were not successful with respect to the means for changing the wound tension of the web.

DE-GM 87 08 849 discloses a winder with two parallel drums and a rider roll that can be tilted as a whole to control the nip pressure. At the beginning of the winding process the rider roll exerts high pressure on the winding roll starting to build up. A tight core is obtained by this pressure, a positive speed differential between the drums and • a corresponding back tension. During the following

winding process the pressure of the rider roll is reduced. The back tension and the tilting of the whole winder is used to control the quality of the rolls, i.e. the hardness. It is possible to provide the drums, especially the front drum with different coatings, i.e. a rubber coating.

A method for controlling the hardness of a winding roll by application of a different torque on back and front drum is disclosed in DE-Al-29 32 396. Hardness and the difference of driving current to apply different torque at the two drums follow to the diameter of the roll. Hardness and current difference are kept at a constant value during a first phase. Afterwards, these values are decreased linearly. During a third phase these values are kept at a constant lower level. The first phase may be omitted. Neither a change of the back tension nor a rider roll is revealed in this document.

GB-A-21 17 395 discloses a two drum winder with a rider roll whereby the web tension is controlled by controlling the speed of each drum in dependence on the difference between given speed signals representative of the speed of rotation of each drum. The torque of each drum follows a curve as indicated depending of the diameter of the winding roll. The curves can be separated in three phases. The torque is constant during the first and the third phase. According to this document it is desired to keep the web tension constant throughout the roll. A method how to control the pressure exerted by the rider roll is not disclosed.

ELP, Ewald G. discloses in the paper Papier- und Kunst- stoff-Verarbeitung, September 1981, page 54 to 59, two drum winders with constant back tension in dependence on the roll diameter. A change of overspeed and different torque of the two drums is discussed. In what manner the torque is to follow the diameter of the roll is not mentioned.

FORSBERG, G. proposes in Paper Trade Journal, April 28, 1969, page 36 to 40, to put almost all the torque into the from drum at the start and then gradually to transfer the torque from the from drum to the back drum.

KLEIN, Hugo discloses a two drum winder, the front drum current of which decreases linearly, while the back drum current increases. Neither a dependence of the back tension on the diameter of the roll nor a dependence of the front drum torque during beginning or end of the winding process is revealed. Further, this document reveals a decrease of the pressure exerted by a rider roll in dependence on the diameter of the roll.

Being aware of the above it is an object of the invention to provide - in a system as mentioned above - the means for changing the tensile stress remaining in the wound web such that a more improved winding structure can be realized with the help of such means - even if the softnip-principle is applied. It is a further object of the invention to prevent wrong adjustment nearly completely.

In order so solve this problem the system according to claim 1 is proposed.

One of the advantages of the invention is that the tensile stress remaining in the already wound roll decreases de- gressively as long as the diameter of the roll increases even in case of the two-drum winders are working according the softnip-principle. An other advant advantage is that an important fault in the roll structure can be avoided by preventing slippage between the roll/s and the supporting drum due to the friction values of the specific web and the specific drums or drum covers.

The tensile stress remaining in the wound web can be changed in different ways when the process according to claim 1 is applied. The subclaims consist of these possibilities that are explained in the following referring to the figures in view of a preferred embodiments of the invention.

Fig. 1 shows in principle a winding machine according to the invention as viewed from one end of the supporting drum;

Fig. 2 shows a tension diagram of the web that is to be wound dependent on the diameter of the roll;

Fig. 3 shows a load diagram of a rider roll dependend on the diameter of the (web) roll;

Fig. 4 a diagram of load distribution to the supporting drums according to fig. 1 dependent on the diameter of the roll and

Fig. 5 an example for a practical curve of the tension of the web to be wound in - dependent on the diameter of the roll (web) .

In a winding machine according to fig. 1 a first supporting roll 2 as seen from the direction of at least one paper web 1, 1', 1'', ... to be wound with a hard surface (not shown) or an elastic flexible surface 2' - which is already known per se - will be partly wound by the web, whereas another supporting roll 3 that has - for instance - a less elastic flexible surface 3' , which is the second according to the winding direction, is not wound by the paper web(s). Both supporting rolls form a winding bed 4, by which the roll(s) 5, 5', 5'', ... which are formed from the web(s) 1, 1', 1' ' , . . . , preferably on a core 6 are carried. The roll(s) is/are rotated by the supporting drums. A rider roll 7, which is

very well known per se, rests with adjustable pressurion on roll(s) 5, 5', 5'', ... and is moved by the roll, i.e. is without own driving device.

The tension on the web 1, 1', 1'' ... that is to be wound and shown in fig. 2 dependent from the roll diameter will, during an initial winding phase, be a little bit linearly decreased, then basically held at the same level or - as shown and in so favoured - also linearly but less decreased than during the initial phase. During the final phase of the winding the tension decreases again to a higher degree with respect to the increasing diameter of the roll until a given final diameter of the roll is reached and this particular winding process of winding is finished.

As the two or four arrows in the figures show (see fig. 2 in the beginning and at the end of the initial phase and of the final phase) the tension and the change of the tension can in an adjustable way be changed - favourably at these points - according to the increasing diameter of the roll. Favourably, the initial phase is finished at the end of the acceleration phase of the supporting drum, and the final phase starts with the beginning of the break phase of the supporting drums. In order to make that more clear, the speed diagram of the supporting drums 2 and 3 is also shown in fig. 2 (in broken lines).

As one can see in fig. 4 the change of the wound-in-tension (as explained in connection with fig. 2) can be achieved or supported by changing the load distribution on the supporting drums 2 and 3 in that way that the load distribution during an initial winding phase changes to a relatively high degree, especially in a linear way, whereas in the following main winding phase the distribution of load changes slowlier than in the initial phase according to the increasing diameter of roll. During the final winding phase

the load distribution changes to a higher degree again. During this phase it is favourably provided to vary the absolute load distribution each time at the beginning and at the end of the initial phase and of the final phase. The sum of the load of both supporting drums is shown as the zero line.

The uninterrupted line in fig. 4 shows the load decrease of (second) supporting drum 3 (during increasing diameter of roll D) . The load increase of the (first) supporting drum 2 (shown as an interrupted line) follows automatically as long as the roll diameter D increases.

According to the invention, the tension of the web as in principle shown in fig. 2 is the basis for the steps to be undertaken. Therefore, the distribution of load between the supporting drums is affected in a way which assists the build-up of a tension in the web according to fig. 2.

In order to make sure that the desired load distribution according to fig. 4 on the at least one roll 5, 5' 5'' ... is effected during all winding phases, the load of th rider roll which is depending on the roll diameter can also be changed in many ways. That is indicated in fig. 3 by multiple arrows. Especially the point of maximum load is changeable with regard to its absolute value as well as in relation to the roll diameter. It is typical that a certain load on the at least one roll 5, 5', 5'', ... remains until the desired winding diameter is obtained.

It is important to make sure that no slippage occurs between front drum 3 and winding bed 4. Slippage would disturb the winding structure of the roll. To avoid such slippage, the preselected set of values of web tension, load and load distribution in relation to the roll diameter (see fig. 3 to 4) are continuously surveilled by a computer which compares

these set of values with actual sensored process data. Said computer recalculates a new load or load distribution which makes sure that no slippage occurs in cases where the sensored data show that slippage might otherwise occur. For instance, the actual load (fig. 3) may be too small to transfer the choosen load distribution or the maximum transferable load is nearly reached, so that slippage between back drum 3 and winding bed 4 may occur. In such cases - only shown as an example - the computer will recalculate the curve for the load and/or the load distribution in relation to the roll diameter in order to make sure that no slippage occurs. The new conditions are automatically used by the winding machine without the need of any action of the operator.

For the recalculation of said set of values for the load in the load distribution it is helpful to know the friction factors between the roll 5, 5', 5'', ... and the supporting drums, especially the back drum 3. This friction value can be stored for each type of paper and material of the supporting drum cover in the computer or automatically be measured and received by the computer.