Method and Machine for Refining Wood Chippings, Shavings, Paper Pulp and the Like

The present invention relates to a method and apparatus for refining wood chippings, shavings as well as paper pulps in higher non-pumpable concentrations by the use of a refining machine comprising two disc-shaped grinding means (hereinafter referred to as "grinding discs") rotatable relative to one another and each provided with grinding bodies, forming between them a narrow, slot-shaped grinding chamber. The grinding discs are so arranged that they are displaceable towards and away from one another for adjustment of the size of the slot-shaped grinding chamber in order to compensate for the wear of the grinding bodies.

Refining machines of this kind, an advantageous design for which is described in copending U.S. Pat. application Ser. No. 412,464, filed Nov. 2, 1973, assigned to the same Assignee as the present application, have for several decades been employed in, inter alia, paper manufacture. Although during this period the refining machines have undergone constant development to improve their performance as to quality, quantity, reliability, etc., the known refining machines suffer from the common disadvantage that the quality of the refined material or the degree of refinement varies in the course of time. This is because the grinding bodies of the grinding discs wear during use, so that the distance between them increases. To compensate for this, the refining machines have for a long time been provided with manually activated adjustment devices for intermittent adjustment of the relative position of the grinding discs. Adjustment has usually been effected by means of screws or by electro-hydraulic means. At best, adjustment has resulted in optimum refining being obtained immediately after effecting the adjustment operation, which then, however, deteriorates until the next adjustment. The degrees of refinement will thus vary in the form of a saw-tooth-shaped curve.

These manual adjustments, which have until now -- although their imperfection was well-known -- been considered the only possible practical way of compensating for wear on the grinding bodies, have also led to other problems and risks. Among other things, an adjustment can be neglected or be too small, resulting in poor quality of the refined material. Alternatively, too large an adjustment can be made causing a metallic contact between the grinding bodies, which can lead to mechanical break-down. To reduce these risks and to achieve refining results as good as possible, it has been necessary to make high demands on the accuracy and professional skill of the operator.

The main objects of the present invention are to achieve a method of and a machine for refining which provides a uniform and high quality of the refined material, and in which all of the above disadvantages are eliminated.

According to the invention these objects are obtained by effecting the adjustment of the size of the grinding slot by displacing the grinding discs relative to each other continuously at a predetermined rate during the period the machine is in operation.

The invention will be described in greater detail below with reference to the embodiment shown diagrammatically in the accompanying drawing of a control system according to the invention.

Reference numeral 1 designates a fixed grinding disc of a refining machine and reference numeral 2 a corresponding movable grinding disc. The grinding discs are shown only diagrammatically, and can in practice comprise two flat ring-shaped members forming a narrow grinding slot between them. The movable grinding disc 2 is displaceable in relation to grinding disc 1 by means of a piston 4 in an hydraulic cylinder 3. The position of the piston 4 in the cylinder 3 is controlled by a flow-regulating servo-valve 5, which is actuated by an electrical signal received from an amplifier 6. The valve 5 is so designed that it allows a fluid flow proportional to the signal from the amplifier 6 to pass to one side of the piston, the other side of the piston being connected to a collecting vessel (not shown).

The grinding discs 1 and 2 are provided on their facing surfaces with grinding bodies, which can have the form of bars or ribs extending in various directions, so that to obtain the desired quality of the material processed between the grinding discs it is very important to keep constant at a certain pre-determined value the distance between the grinding bodies of the two grinding discs, in spite of their becoming worn.

For adjustment of the desired initial distance when fitting new grinding discs, for which the height of the said grinding bodies is known, the position of the grinding disc 2 relative to the fixed grinding disc is sensed, for example by means of a differential transformer 7, and this value is compared in the amplifier 6 with a predetermined desired value set by a potentiometer 8. At the output of the amplifier 6, an amplified difference signal is generated which actuates the servo-valve 5 in such a direction to move piston 4 so that the actual position value of piston 4 as sensed by the transformer 7 is made to correspond to the predetermined desired value as set on the potentiometer 8. The signal received from the transformer 7 can be supplied via a control circuit 10 to an instrument 9, such as a meter, to indicate the size of the slot between the grinding discs 1 and 2.

To compensate for the progressively increasing distance between the grinding bodies caused by wear, the control system described above has been supplemented, according to the present invention, by a compensating circuit which includes a compensator 11, for example in the form of a potentiometer, which, via a reduction gear 12, is driven by a synchronous motor 13. The total voltage applied across the potentiometer 11 is set by means of a potentiometer 14. The movable terminal contact on the potentiometer 11 is connected to an input of the amplifier 6. To read the rotation of the potentiometer 11 and to reset it, a wheel 15 provided with a dial is connected to the shaft of the potentiometer.

The function of this circuit is as follows. The reduction gear 12 is so calculated that it gives a rotation to potentiometer 11 corresponding to minimum wear. If therefore it is assumed that, with minimum wear, the same grinding discs can be retained for six months, the reduction must be so selected that the potentiometer runs through one cycle during this period of time.

By means of potentiometer 14, the output signal gradient, voltage/unit of time, is set that corresponds to the expected wear for the relevant application. Alternatively, the voltage across potentiometer 11 can be kept constant and the rotational speed of the potentiometer set in dependence upon the rate of wear. A combination of these methods is also feasible, but from a practical aspect it is however most suitable for the rotational speed to be kept constant and for the voltage across potentiometer 11 to be altered. The output signal from potentiometer 11 will have, in the case described, the form of an inclined or ramp function, which is added to the other signals supplied to the amplifier. As compensation for wear of the grinding bodies begins to have an effect, the difference between the voltages received from transformer 7 and from potentiometer 8 will increase and balance the increasing voltage received from potentiometer 11. This results in the output signal from the amplifier 6 becoming substantially constant and so setting the valve 5 such that a flow fluid corresponding to the balancing value set on potentiometer 14 is supplied to the pressure side of the piston 4. This results in grinding disc 2 continuously drawing closer to grinding disc 1, this movement wholly corresponding to the wear of the grinding bodies known from experience and set on potentiometer 14.

When using a compensating circuit according to the above in connection with the refinement of material, for which the rate of wear is unknown, the compensation conditions can be established quite simply by a short test run with a "safe" value for the rate of wear, and after this test period the wear concerned can be measured and the correct value set on potentiometer 14. In certain cases, the test run can be divided into several periods for a progressive approach of setting to the correct value.

When replacing the grinding discs or changing the rate of wear, the position-giving differential transformer 7 is reset, and also the compensator 11, by means of the wheel 15. The synchronous motor 13 is connected to the driving motor of the refining machine, so that the compensator is only connected during the periods the refining machine is in operation. For exact reading of the compensation introduced, the position of wheel 15 should be determined in the light of the voltage set on potentiometer 14.

The control system described above should be regarded only as an example of one system according to the invention, which can be varied in a number of ways within the framework of the claims. If, for example, wear which is not time-linear is to be compensated, potentiometer 11 can be replaced by another suitable device whose output signal has a form corresponding to the actual wear. For this purpose, the potentiometer 11 could perhaps be run at a varying speed.