GB2282981A

GB2282981A - Roll mill and stripping device therefor

Info

Publication number: GB2282981A
Application number: GB9420931A
Authority: GB
Inventors: Herbert Rech
Original assignee: Buehler AG
Current assignee: Buehler AG
Priority date: 1993-10-19
Filing date: 1994-10-12
Publication date: 1995-04-26
Anticipated expiration: 2014-10-12
Also published as: DE4433927A1; DE4433927C2; GB2282981B; CH686934A5; GB9420931D0

Description

2282981 1 ROLL MILL AND STRIPPING DEVICE THEREFOR This invention relates

to a roll mill having a plurality of rolls, particularly for processing pastyliquid material to be ground, and having a stripping device for the processed, ground material associated with the last roll and equipped with a stripping knife. The invention also relates to a stripping device for this roll mill.

It is known that liquid, viscous or pasty material can be removed from the last roll of a multi-roll mill, such as a three-roll mill in particular, with the aid of a stripping knife which is disposed on the last roll after the pasty material has been mixed and comminuted. A stripping knife such as this extends over the entire width of the roll and has an edge adjoining the roll which extends away f rom each end of the roll in the manner of a strip, wherein two edges converge in the manner of a funnel with increasing distance from this roll. This gives rise to a relatively narrow outlet gap via which the pasty material can run off, e.g. into a container.

Products are also being processed to an increasing extent, such as paints in particular, which have an increased viscosity, due to which their ease of comminution is in fact increased, but which results in the disadvantage that these partially thixotropic products remain stuck to the converging edges after being 2 stripped from the roll, so that these are always having to be cleaned afresh.

It has already been proposed in DE-A-38 06 651 that the pumping action between the last two rolls be utilised to maintain the product under pressure in a chamber and to convey it away from there via a channel. This design has in fact been proven for various particular applications, but for very tenacious material it has the disadvantage that the two rolls also draw in air at the same time and the arrangement of the stripping chamber above the two rolls no longer allows the product sufficient time to release the air to the environment again.

It would be desirable to be able to create a roll mill and a stripping device which results in reliable, practically compulsory, conveying of the finished product, and which secondly results in a product which is substantially free from air inclusions.

For a roll mill of the type mentioned initially it is therefore proposed according to the invention that, in the direction of transport of the material to be comminuted, a device for forming a pressure gradient is disposed in front of the stripping device with the stripping knife. By this means the stripped product is advantageously conveyed practically compulsorily to the outlet.

The discharge of the stripped product is particularly effective when, according to a preferred W_ 3 embodiment of the invention, the pressure build-up at the device for forming the pressure gradient and the pressure drop inside a line disposed downstream of the stripping knife are designed so that they cancel each other out. Because the product is taken off at the last roll - the layer is relatively thin here - the opportunity is also advantageously provided for air which has been drawn in to be released to the environment. In this manner the advantages of good de-aeration of the roll are effectively combined with those due to the viscous product being compulsorily conveyed further via the stripping device.

In relation to the stripping device according to the invention for the roll mill described above, it is proposed that the stripping knife is disposed in a chamber formed by the roll and sealing walls, and that a sealing device is provided in front of the stripping knife in the direction of rotation of the roll, which sealing device preferably extends over the width of the stripping knife, is disposed parallel to the latter and extends close to the surface of the product layer situated on the roll. This form of construction results in a simple manner in the creation of the device on the roll mill for building up the pressure gradient.

According to another preferred embodiment of the invention, which is particularly simple but which is nevertheless of universal applicability, the stripping device can be constructed so that the stripping knife and

4 the sealing device are disposed on a slide block, which is preferably common to both, the radial position of which slide block in relation to the roll can be adjusted via an adjustment device. This adjustment can be made in a simple manner by measuring the layer thickness on the roll and then correspondingly adjusting the stripping knife and the sealing device.

So that it is possible to locate the sealing device as close to the surf ace of the layer as possible, but without impeding the transport of the layer, it is proposed according to another preferred embodiment of the invention that the radial position of the sealing device in relation to the roll can be adjusted via an adjustment device irrespective of the position of the stripping knife. A manually operated fine adjustment device for the sealing device is preferred.

According to another preferred embodiment of the invention the sealing device may consist in a simple manner of a sealing body which carries at least one sealing lip. A construction of this type has a low production cost, a high operating efficiency and low operating costs.

The stripping device according to the invention can be constructed in an extremely compact form if, according to another preferred embodiment of the invention, an outlet channel, which conveys the ground material into the line, is provided in the slide block in the region between the sealing device and the stripping knife, particularly between the sealing lip and the stripping knif e.

Finally, it should also be mentioned that it is definitely possible to effect the positioning both of the sealing device and of the stripping knife or the stripping device by means of an electromechanical adjustment device, which may be controlled, for example, by thickness measurement sensors or the like via a computer system.

Further details of the invention follow from the description given below of the examples of embodiments which are schematically illustrated in the drawings, where:

Figure 1 is a numbered, highly schematic view of a roll mill according to the invention, of which Figure 2 is a partial view of a first embodiment; and Figure 3 serves to explain the principles on which the invention is based and illustrates a further embodiment.

Three rolls 1, 2, 3 are mounted, in a manner which is not shown but which is known in the art, on a machine frame (not illustrated) of an attrition roll mill. As is customary in attrition roll mills, the three rolls 1, 2, 3 are pressed against each other by a pressure device, which is not shown, and exert their attrition effect in that the rotational speed of roll 2 is higher than that of roll 1 and the rotational speed of roll 3 is higher than that of roll 2. This difference in speed not only 6 results in the attrition of the pasty product, which may contain paint pigments for example, and which is fed into the roll gap between rolls 1 and 2 in the known manner, but also enables roll 2 to exert a mixing effect in the usual manner. The difference in speed also results in the transfer of the product layer from the slower roll to the faster roll in each case, so that the liquid product reaches roll 3 from roll 1 via roll 2.

A stripping knife 4 is disposed on roll 3 in the usual manner, and lateral, converging edge strips 5 collect the material stripped over the width of roll 3 in order to feed it to an outlet opening 6.

As distinct from known roll mills, the stripping knife 4 is disposed in a chamber 7 which has side sealing walls 8 which cooperate with the end faces of roll 3. A line 9, which may be constructed as a flexible tube for example, is attached to this chamber 7 and its outlet opening 6.

It is essential that - as seen in the direction of rotation of roll 3 - a sealing device 10 is disposed shortly in front of the stripping knife 4, and extends over substantially the entire width of roll 3, as does the stripping knife 4. It has been shown that by this means a pressure gradient is produced behind this sealing device 10, which extends close to the surface of the product layer situated on roll 3. This pressure gradient results in the material flowing over the surface of the 11 7 tripping knife 4 being practically compulsorily conveyed to the opening 6.

At this point it should be mentioned that in this respect Figure 1 is a highly schematic illustration, since the size ratios are reproduced in a distorted manner in order to clarify the function of the chamber 7, i.e. as illustrated, the diameters of rolls 1 to 3 are too small in relation to the size of the chamber 7. An illustration which approximates to the correct size ratios follows from the following description of a first embodiment as shown in Figure 2.

Figure 2 shows part of the machine frame 11, on which roll 3 and the chamber 7 are mounted, amongst other components. In this respect the entire device 7 is radially displaceable towards or away from roll 3 on guide pillars 12 in the manner of a slide, for which purpose an adjustment device 13 with a screw spindle is provided. An adjustment device in the form of at least one adjusting screw 15 is also disposed on the slide block 14 for the adjustment of the stripping knife 4 within its plane, the knife being moved towards or away from roll 3 during an adjustment movement by means of guide screws 16 passing through longitudinal slots (not shown).

The line 9 which was described with reference to Figure 1 extends approximately parallel to the axis of roll 3 here, and runs outwards through the frame 11 at the side.

8 Accurate guidance of the slide block 14 is therefore important, because here not only must the knif e 4 be aligned in the usual manner with the surface of roll 3, but in addition the sealing device 10 is disposed in f ront of the stripping knife 4 - as seen in the direction of transport of the pasty-liquid material situated on roll 3, and comprises a rigid sealing body 10' with a sealing lip 1T Since the thickness of the product layer on roll 3 may assume different values, it is advisable if an adjustment device 17 is associated with the sealing device 10. In practice an adjustment such as this could be made automatically, by measuring the layer thickness on roll 3, for instance. However it is simpler if (as illustrated) the adjustment device 17 has a simple, manually operated adjusting wheel 18, the adjusting shaft 19 of which has an adjusting thread 20 at its end, which engages in a corresponding internal thread of the sealing body 1T. In this respect the adjustment of the sealing device 10 is not overly critical, since the sealing lip 1T' should be situated quite close to the surf ace of the product layer on roll 3, but not right on this surface. As will also be explained below with reference to Figure 3, this sealing device 10 permits the build-up of a pressure gradient behind the seal, due to which the stripped product is fed practically compulsorily to an outlet channel 6' leading to the line 9.

k Z 9 The operation of the arrangement according to the invention will now be illustrated with reference to Figure 3 and with the aid of a few calculations.

If it is assumed that a layer of product of thickness dF is situated on the roll and runs into a gap 22, the thickness of which is greater than or equal to d Ff that the centrifugal force inside the gap 22 plays practically no part in the process, and the material of the layer 21 has a relatively high viscosity with Reynolds numbers <2500 to <1500, a substantially parallel f low occurs in the gap 22. This assumption is also justified in that the gap with ds will normally be much less than the gap length ls.

If it is now assumed that the velocity v, runs in the x direction, to which a y direction is perpendicular, the linear momentum equation in the direction of flow is 1 bp 82 VX v - p 6X 8y2 where p is the density, 8 is the pressure gradient and v. is the velocity in the x direction.

This momentum equation has a parabolic velocity distribution as its solution, corresponding to the formula v. = a + by + Cy2, where a, b and c are constants. The constants a and b can be calculated from the boundary conditions 4.

V.." (Y=O) = V" V. (Y=d,) = 0, whereas the constant c may vary in order to take pressure gradients of different magnitudes into consideration. This therefore leads to the following equation:

VX VW - Vwy - cd,,y + py2 ds Since the pressure gradient in the gap 22 must never result in a back- flow, the range in which c must fall is given by -2 0 < c! vw. ds.

This results in a pressure build-up, with a pressure gradient 6p, over the gap length 1, which is given by the f ormula 8P.5 = 1 s 8P = 2 1. vc 8X The gap width ds is thus preferably selected so that it is in the range given by MF < d,:5 3dF. When d., is selected from within this range, the magnitude of c is given by the formula c = (3 - 6 dF) VW ds d 2 9 It was shown with reference to Figure 2 that the gap between the sealing body 10 and the roll is restricted in 1 01 1 11 the direction of movement of roll 3 by the sealing lip 10' of relatively short length, whereas in the situation illustrated in Figure 3 a relatively longer gap 22 is provided. If it is assumed that the pressure build-up 8p. over the length 1, of the gap 22 and the pressure drop within the line 9 are ultimately to cancel each other out, the relationship for the requisite gap length 1, for steady-state flow can be calculated from the following f ormula ls = 641R( d S) 2 dR bwdF 1 n d 2 R (3 - 6 dF) dc where R is the length of the tube 9, dR is the diameter of the tubular line 9 and bw is the width of roll 3.

Figure 3 also shows that the material emerging from the roll gap between rolls 2 and 3 is spread out on roll 3, over a length which is sufficient for most situations, in the form of a thin layer 21, e.g. 1 pm thick. it therefore has the opportunity to release the drawn-in air to the environment again. A vacuum device 23 may optionally be provided, particularly on the inlet side of the gap 22, in order to promote the release of drawn-in air even further by the application of a vacuum.

1

Claims

12 CLAIMS

1. A roll mill having a plurality of rolls, particularly for processing pasty-liquid material to be ground, and having a stripping device for the processed, ground material associated with the last roll and equipped with a stripping knife, characterised in that, in the direction of transport of the material to be comminuted, a device for forming a pressure gradient is disposed in front of the stripping device with the stripping knife.

2. A roll mill according to claim 1, in which the pressure build-up at the device for forming the pressure gradient and the pressure drop inside a line disposed downstream of the stripping knife are designed so that they cancel each other out.

3. A stripping device for a roll mill according to claim 1 or 2, which is equipped with a stripping knife, in which the stripping knife is disposed in a chamber formed by the roll and sealing walls and a sealing device is provided in front of the stripping knife in the direction of rotation of the roll, which sealing device preferably extends over the width of the stripping knife, is disposed parallel to the latter and extends close to the surface of the product layer situated on the roll.

4. A stripping device according to claim 3, in which the stripping knife and the sealing device are disposed on a slide block, which is common to both, the radial position of which slide block in relation to the 1 4 13 roll can be adjusted via an adjustment device.

5. A stripping device according to claim 4, in which the radial position of the sealing device in relation to the roll can be adjusted via an adjustment device irrespective of the position of the stripping knife.

6. A stripping device according to any one of claims 3 to 5, in which the sealing device consists of a sealing body which carries at least one sealing lip.

7. A stripping device according to any one of claims 4 to 6, in which an outlet channel, which conveys the ground material into the line, is provided in the slide block in the region between the sealing device and the stripping knife, particularly between the sealing lip and the stripping knife.

8. A roll mill substantially as described with reference to and as illustrated by the accompanying drawings.

9. A stripping device for a roll mill substantially as described with reference to and as illustrated by the accompanying drawings.