At starting of the refiner, the driving motor, a synchronous motor, excites strong oscillating moments, which immediately before the working number of revolutions, the synchronous speed, has been achieved, passes a resonance frequency of the mechanically coupled shafts. The excitation moment from the synchronous motor increases substantially at the passage of the resonance frequency, and a maximum moment develops which can exceed 200 kNm. In view of the maximum moment at the start, the refiner must be overdimensioned in relation to what is required with respect to the other operation conditions.

The present invention solves this problem and implies, that the oscillation moment can be reduced to a level close to the other operation conditions. According to the invention, the torsional oscillations are damped in that a friction damper is provided between the motor and refiner shaft. The characterizing features of the invention are defined in greater detail in the attached claims.

The design of the device according to the invention is simple with few parts, which makes it cheap and easy to install in existing refiners. Further, the function of the device is independent of outer drive or control means.

The invention is described in greater detail in the following, with reference to the accompanying drawings showing some embodiments of the invention.

Fig 1. shows a refiner with a driving motor according to the invention;

Fig 2. shows the coupling principle between motor and refiner according to the invention;

Figs. 3-5 show three embodiments of the friction damper according to the invention.

The refiner shown in Fig. 1 is of the single-disc type, i.e. one refining member is stationary and the other one rotary. The rotary refining member is located on a refiner shaft 2, which is coupled to the rotor shaft 5 of the motor via an intermediate shaft 3 by means of a first and, respectively, second coupling 6 and, respectively, 7. The motor is a synchronous motor which in operation maintains a constant speed irrespective of the shaft moment. This is essential at the refining of cellulose material. The motor output can vary between 1 and 20 W or more, depending on the refiner type. The synchronous motor for a refiner normally has four poles, rendering the motor a synchronous speed of 1500 and, respectively, 1800 rpm at 50 and, respectively, 60 cps alternating current frequency.

The intermediate shaft 3 connecting the motor 4 to the refiner 1 preferably is coupled by bolt joint for taking up axial moments. The intermediate shaft 3 has the object both to move down the neutral frequency of the system where the resonance amplitudes are lower and to act as a safety device at breakdown.

An outer pipe 8 stiff against torsion is provided about the intermediate shaft 3 and concentrically therewith. One end of said pipe is rigidly connected to the first coupling 6 at one end of the intermediate shaft 3. The other end of the pipe 8 is connected by a friction damper 9 to the second coupling 7 at the other end of the intermediate shaft 3.

The friction damper 9 comprises friction surfaces which are pressed against each other. It implies, that the strong torsion oscillations arising at resonance passage can be removed as friction energy in the friction surfaces of the friction damper.

At the start of a refiner, increasing torsion osciallations arise after some seconds due to resonance. When the torque increases to a magnitude causing the friction surfaces to start sliding relative one another - the tear-off moment - the increase in torque is limited to continue. Due to the torsion oscillations, repeated slidings take place thereafter forth and back in the friction damper while

simultaneously heat is developing. After some second, the resonance has been passed and the torsion osciallations have ceased.

By using a friction damper of this kind, it was found possible, for example, to reduce the maximum torque at the start of a refiner from 210 kNm to 80 kNm. By optimizing the equipment it should be possible to reduce the maximum torque even further.

The parameters defining the function of the friction damper are a.o. the tear-off moment, the friction moment and the rigidity of the intermediate shaft. A suitable size of the tear- off moment is 30-100% of the effective output of the motor at full load. The diameter of the intermediate shaft is determined in view of the transferred moment. By choosing a suitable length of the intermediate shaft, a desired angle of distortion between the couplings can be obtained.

At a motor output of 15 MW it was found suitable to have a tear-off moment of 20-50 kNm, preferably 30-40 kNm. The friction moment normally amounts to appr. 60% of the tear-off moment. The friction area should be 0,2-0,5 m ² , preferably 0,3-0,4 ² . The rigidity of the intermediate shaft 3 should be between 1,2 and 3,2 MN /line, preferably 2,0-3,0 MNm/line. It can be dimensioned for a maximum moment during the start of about 80 kNm. At the dimensioning, regard has to be paid to the heat development due to friction. A lower tear-off moment and a lower rigidity of the intermediate shaft, thus, result in increased heat development.

The outer pipe 8 stiff against torsion shall have a rigidity substantilly exceeding the rigidity of the intermediate shaft, suitably a rididity 3-15 times higher and preferably 8-10 times higher.

According to the embodiment shown in Fig. 3, the torsion damping is achieved by means of a brake disc 10, which is connected to the pipe 8 stiff against torsion and is held by spring loaded brake blocks 11, which in their turn are located in a yoke 12 on the second coupling 7. The friction surface is oriented substantially radially. By choosing spring-load and brake disc diameter, the damping effect can be determined entirely independent of other operation data in order to bring

about a desired damping. Instead of spring-load, another mechanical or hydraulic tightening-up can be arranged.

According to Fig. 4, the friction moment is produced by external brake blocks 13, which are pressed against the second coupling 7 by means of a bellows 14, which expands by pressurizing an incompressible fluid in the bellows. Alternatively, a mechanical tightening-up can be arranged. The friction surface here is oriented axially. According to other alternatives (not shown) , the friction surfaces, of course, can be arranged at different angles between radial and axial.

According to Fig. 5, the friction moment is produced by means of a sleeve 15 composite with the pipe 3 and having an oi gap 16 extending all about. The sleeve 15 extends about the second coupling 7 so as to form a friction surface between them. The tightening is effected by hydraulic pressure in the oil gap 16 via a valve 17. This design can easily be made rigid. It comprises few parts, has little inertia mass, and the friction moment can be adjusted easily.

The invention, of course, is not restricted to the embodiments shown, but can be varied within the scope of the invention idea.