Background of the Invention High turbulence pressure screening apparatus of the kind shown, for example, in U. S. Pat. No. 5.064.537. issued November 12.1991 to Chupka. et al.. and assigned to the same assignee as this invention, the disclosure of which is incorporated herein by reference, is used in the preparation of papermaking stock. A suspension of liquid and paper fibers, which may in varying degrees contain undesirable rejects or contaminate particles, is supplied to the inlet of the apparatus as disclosed, for example, in the'841 patent, where it is applied to an annular cylindrically shaped screen having specifically designed slots or perforations therethrough. Typically, the paper stock is fed to the interior of a vertically oriented cylindrical screen, and the rejects are withdrawn from one end of such a screen cylinder, while the accepts pass through the slots or perforations in the screen and are collected at a location outwardly of the screen. A rotor has foils or vanes positioned to move at or adjacent the inlet (usually inside) surface of the screen surface, in close relation to the surface, to reduce plugging of the screen slots or holes.

Typically, a screen cylinder of the kind described above is formed by machining the required slots or grooves in a flat plate of metal material, such as stainless steel, then rolling the material into the shape of a cylinder, and welding the rolled plate at the abutting ends to form a welded seam. The material left between the slots defines bars or wedgewires. Thereafter, end rings, formed from bar stock, are rolled, welded at their respective ends, and then attached by welding to the rolled cylinder plate, at its respective axial ends so as to form attachment flanges. The screen cylinder is attached to the screening apparatus housing by bolts through one or

both of the end rings.

A further common style of a screen cylinder to which the invention may be applied is known as a wedgewire screen in which the body, instead of being formed from a single plate that has been perforated or slotted, is formed rather by an array of side-by-side positioned wedgewires that are welded together to form a cylinder body.

One or more intermediate reinforcing rings. also formed from rolled bar stock, are attached by welding at longitudinally spaced locations on the outlet (usually the outer surface) of the rolled screen plate or wedgewire array. The spaced intermediate rings define longitudinally spaced circumferential zones therebetween and with the end rings. An example of a fabricated screen cvlinder with reinforcing rings is shown in Hatton et al., U. S. Pat. No. 4.017.387, issued April 12,1977. and assigned to the same assignee as this invention. An example of a wedge wire screen with reinforcing rings is shown in Fig. 1 of U. S. Patent No. 5,472,095.

Screen cylinders as used in pressure screening apparatus are exposed to a number of different mechanical loads, primarily in the radial and in the circumferential direction. In part, the loads are due to the pressure drop across the screen cylinder which induces the main flow, from the inlet to the accepts side.

Other radial loads are due to the rotating element. which operates near the cylinder surface and creates turbulence, shear, and positive and/or negative pressure pulses.

A circumferential load is imposed by the fluid shear and turbulence. generated by the rotor. Most of the torque which is required to turn the rotor at a given speed is transmitted through the fluid to the screen cylinder.

The above loadings occur during normal operation of the screen.

Additional torque load may result from irregular operation, such as, for example, when the screen is plugged. On such an occasion, the space between rotor and screen cylinder may fill with increasingly thicker pulp. The torque required to turn the rotor increases and this entire torque is transmitted through the thick pulp into the screen cylinder. In the extreme, the rotor stalls and the motor stalling torque, plus any dynamic torque load, are transmitted into the screen cylinder.

Most of the circumferential torque loading is introduced more or less uniformly over the entire screen cylinder surface. However, depending on rotor

design and on the clearance between rotor and cylinder, locally higher shear stress areas are possible. The cylinder is restrained from rotating in response to the torque by bolts or torque restraints at one or both end rings. One ring is bolted to the screen housing, to hold the cylinder in place against rotation and axial displacement. The other end ring, if used for this purpose, may be axially floating but keyed to provide torque transmission into the housing.

Fig. 1 illustrates a prior art arrangement showing the fragment in elevation of a conventional slotted cylinder having slots 25 cut through the plate material defining small metal bars 26 between the slots. This illustrates the case where an end ring 30 is fixed so that the entire torque T must be transmitted through the screen plate, in the direction shown by the arrow 28. Reference numerals 34 and 36 represent intermediate reinforcing or stiffening rings that are applied in surrounding relation to the screen cylinder at either the inlet or outlet surface thereof.

In prior art Fig. 1, the torque T is transmitted by the bending of the small metal bars 26 between the slots, in the circumferential direction, and results in a displacement of the bars from the rest position as represented by the broken lines to the moved position, shown in exaggerated manner, by the unbroken or full lines, and a corresponding displacement of the slots 25. Accordingly, all the bars in just one circumferential slotted zone, that is the zone between the reinforcing ring 34 and the fixed ring 30, must act together to withstand the total torque load, and the stresses developed in such bars can be significant and can lead to the failure of the screen cylinder. While Fig. 1 shows axially discontinuous slots, the same issue also applies for screen cylinders with substantially continuous slots, or with wedgewire arrays that form screening slots.

Uniform slot spacing is also important. If there are occasional wider bars between slots, these would tend to carry a much greater share of the total load than the remaining bars and may be particularly endangered.

The issue of high stresses due to torque transmission has been recognized before and past attempts for correction include, for example, the patent to Hatton et al., above. Hatton et al. proposed that several areas of the screen plate remain unslotted, leaving them strong for transmission of the forces to the end ring.

More recently, a major supplier of slotted screen cylinders (Fiedler) has arranged a

full cylindrical jacket around the outer rings of a wedgewire screen cylinder. The jacket has many large holes to permit passage of the accepts pulp flow. The jacket is welded to both top and bottom end rings so that only half of the total circumferential force is transmitted through the plate to the first end ring. The other half goes into the second end ring and from there through the jacket into the first. As previously noted, another approach to reduce stresses by a factor of two includes bolting down the first end ring and providing a keyway in the second end ring for torque transmission to the housing. None of the conventional approaches is fully satisfactory. The first occupies a significant portion of the active screen surface.

Both reduce the stresses only by 50%.

U. S. Patent No. 5. 718.826 also pertains to screen cylinders but fails to disclose the specific issue of torque loading. Nevertheless. Fig. 13 of the'826 patent tends to show a design which could assist in the transmission of torque loads through axial land areas between openings 52. Note however that this design is another example of a complete jacket or cage, such as that marketed by Fiedler. The cage is an undesirable solution, as explained above.

U. S. Patent No. discloses various bars which could act as torque transmission elements, even though they are not intended as such. Note that all bars in this patent are arranged on the inlet surface of the screen cylinder, for the purpose of creating turbulence and to facilitate passage of fibers through the screening slots. There is no mention of the issue of the transmittal of torque, or circumferential forces, from the cylinder screening areas to the end ring.

U. S. Patent No. 5,791,495 discloses axial tie rods to hold various ring elements of a cylinder together. The tie rods serve to compress the cylinder elements so that torque is transmitted by friction between the slotted elements and the rings.

Since the tie rods are going through clearance holes at intermediate rings (as shown in Fig. 2 of the'495 patent), they do not serve to transmit tangential forces. It is a strong detriment that the relatively fragile bars between the slots are put into axial compression.

In summary. none of the known prior art satisfactorily addresses the transmission of tangential forces. In order to provide a functioning cylinder, these designs must strengthen the bars between the slots unnecessarily which leads to the

loss of open area, unnecessary costs. and perhaps premature mechanical failure of these cylinders.

SUMMARY OF THE INVENTION The improvement according to this invention is to provide suitable torque transmitting elements like bars, beams or plates which are fastened to the reinforcing rings, on the accepts side of the screen cylinder. Such a measure would dramatically reduces torque-induced stresses, i. e., 80% to 95%, depending on the number of circumferential zones of the cylinders. The bars can be shaped to minimize the flow restrictions. so that accepts pulp can freely pass by them. The bar elements may connect all rings or only some of them, depending on general design considerations and on the circumferential loads to be transmitted. Advantageously. the bar or beam elements are designed so as not to produce secondary stresses in the screen cylinder, like axial stresses. They can be shaped and arranged to transmit primarily tangential forces from one ring to another, without significantly increasing stiffness of the cylinders in another, i. e. the radial direction. In this connection the bars or elements are relatively stiff in a circumferential direction and comparatively more flexible in a radial direction.

The bars may also be chosen to connect selected intermediate rings with one or both of the end rings which would then transmit the torque into the screen housing. Conceivably. even one or more intermediate rings may be selected to transmit the torque to the screen housing.

An axial bar extending from the first to the second end rings and being connected to the end rings and one or more intermediate rings, is a potential configuration. Note however, that this design would provide locally increased stiffness against radial deflection of the screen cylinder, almost like a box beam or I- beam would, whereby the bar and the screen plate would serve as flanges. The remainder of the screen cylinder would have its original radial flexibility. Such a variation of radial stiffness may not be desirable.

The invention will be further described in conjunction with the following detailed description and appended drawings.

BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is an enlarged partial elevation view of the exterior of a prior art screen cylinder showing circumferential deformation of the cylinder in response to a torque T acting thereon; Fig. 2 is an enlarged partial elevation view of the accepts side of a screen cylinder in accordance with a first embodiment of the invention; Fig. 3 is an enlarged partial cross-section view of the screen cylinder of Fig. 2 taken along the line 3-3; Fig. 4 is an enlarged partial elevation view of the accepts side of a screen cylinder in accordance with a second embodiment of the invention ; Fig. 5 is an enlarged partial elevation view of the accepts side of a screen cylinder in accordance with a third embodiment of the invention; Fig. 6 is an enlarged partial cross-section view of a screen cylinder in accordance with a fourth embodiment of the invention; and Fig. 7 is an enlarged partial elevation view of the accepts side of a screen cylinder in accordance with a fifth embodiment of the invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS A first preferred embodiment of the configuration is shown in Figs. 2 and 3 in which like parts have been provided with like numbers corresponding to Fig.

1. Fig. 3 illustrates a modified screen plate design in which small generally rectangular plates 50 forming torque transmitting members are welded between the first end ring 30 and the first intermediate ring 34 at the accepts side of the screen. In a similar manner, two or more of such plates 52 (one being partially shown in Fig. 2) may be welded between the first intermediate ring 34 and a second intermediate ring 36, preferably offset rotationally 90° from the first pair of such plates 50.

Alternatively, one, three, or more of such plates 50 or 52, which are essentially identical in configuration, could be located between others of the adjacent rings in neighboring zones, and such plates should be rotationally offset from each other to avoid axial alignment.

Fig. 3 is a fragmentary sectional view through the screen plate, showing a cross-section of the fixed end ring 30 and a section through one of the

plates 50 which is provided with a generally large cylindrical opening 53 or'center hole, to reduce flow restriction through the slots of the screen body plate 55. The plate 50 is welded at the regions in common with the rings 30 and 34, and the same is the case for the additional such plates 50 or 52.

In a further embodiment, the plates may be generally X-shaped or hourglass shaped as shown in Fig. 4, wherein a torque transmitting element or plate 60. in accordance with a second embodiment of the invention, is welded to the end ring 30 and the first intermediate reinforcing ring 34. A second generally X-shaped plate 62 is welded between the first intermediate ring 34 and the second intermediate ring 36. The plates 60.62 are spaced radially outward from the cylinder plate so as to avoid restricting the flow of accepts through the screen cylinder slots. The plates 60 and 62 are circumferentially offset from each other.

In a third embodiment in accordance with the invention, the torque transmitting element is a round bar stock, shaped like a V, as shown in Fig. 5.

Torque transmitting elements 70,72 are formed from round bar stock by trimming to a desired length and then bending into a V shape. The torque transmitting element 70 is fixed in place between the end ring 30 and first intermediate reinforcing ring 34 by welding the vertex 71 of the element 70 to the end ring 30, and welding the ends of the legs 72 of the element 70 to the facing radial side wall 73 intermediate ring 34.

Of course, the element 70 may be inverted with its vertex 71 welded to the intermediate ring 34 at the wall 73 and the ends of its legs 72 welded to the first end ring 30. Moreover, the torque transmitting element may be formed into other shapes as desired, such as, for example, an N, M, or W shape. Torque transmitting element 74 is similarly fixed between the first intermediate ring 34 and the second intermediate ring 36. but circumferentially offset from element 70 so as to avoid alignment along a common longitudinal axis. Of course, elements 70,74 may be formed from square or rectangular bar stock, if desired. Again, as in all the embodiments, it is desirable that the circumferential stiffness of the elements exceeds their radial stiffness.

Referring now to Fig. 6, there is shown an enlarged partial cross- section of a fourth embodiment in accordance with the invention. A torque transmitting element 80, which may be in the form of a plate or bar stock in

accordance with any of the embodiments discussed above, is welded to the opposed radial faces of non-adjacent rings. In the example shown in Fig. 6. the element 80 extends between the radial face 81 first end ring 30 and the opposed radial face 82 of second intermediate ring 36 so as to be spaced radially outward from a first intermediate reinforcing ring 83. Ring 83 may be formed so as to have an exterior radius everywhere shorter than the minium radial distance from the longitudinal centerline of the screen cylinder to the element 80. Alternatively, ring 83 may be formed in a fashion similar to ring 36, but be provided with slots or keys shaped to permit element 80 to pass over or through ring 83 in non-contacting relation thereto.

Moreover, element 80 may skip over more than one ring so as to connect together every third or fourth ring. In addition, ring 36 may be provided with a slot or key so as to permit another torque transmitting element (not shown). circumferentially offset from element 80, to connect together the first intermediate ring 83 and a third intermediate ring (not shown).

Referring now to Fig. 7, there is shown an enlarged partial elevation view of the accepts or outward side of a screen cylinder in accordance with a fifth embodiment of the invention. A torque transmitting element 90, in the form of a square or rectangular bar stock, is wrapped around the screen cylinder in spiral fashion. The element 90 is butt-welded at one end 91 to the underside of the first end ring 30 such as at the surface 81 shown in Fig. 6. The other end of the element 90 is likewise welded to a second end ring (not shown). The radial extent of the intermediate rings is modified such that their outer surfaces come into contact with the inner surface of the element 90 and then the element 90 at these points of contact. is welded to such outermost surfaces or faces of the intermediate rings 34,36, etc.

Alternatively, the element 90 need not be welded to every ring so that the element 90 connects non-neighboring rings together while allowing non-connected rings to slide relative to the element 90. Alternatively, the intermediate rings may be provided with grooves or notches to receive the element 90 therein.

While the invention is described in terms of a screen cylinder for screening a suspension of paper fibers in water, it will be understood that the invention is not limited to a pure cylinder form of a screen and that the screen may have other shapes such as frustro-conical. Also. the invention may be applied to

screen cylinders where the stock is applied at an outer surface and the accepts are collected at the inner surface.

It will be noted that in each of the embodiments described herein, the torque elements are relatively stiff in a circumferential direction to transmit torque from one axial end to the other, but are relatively flexible in the radial direction.

Therefore, the screen cylinder is not particularly restricted by the addition of the torque elements from bending radially somewhat in response to the rotating impulses and forces developed by the rotor. Also. the elements are designed such that they interpose a minimum impedance to the flow of accepts or accepted stock through the screening openings or slots 25. It may be also noted, in each embodiment, that the torque elements are themselves spaced radially from or outwardlv of the adjacent surface of the cylinder plate body of the cylinder screen, that is. radially of the accepts surface of the screen plate. A further important characteristic that may be applied to each of the embodiments is the positioning of the torque elements at any given position so as to be rotationally offset from axially spaced torque elements.

While the forms of apparatus herein described constitute preferred embodiments of this invention, it is to be understood that the invention is not limited to these precise forms of apparatus, and that changes may be made therein without departing from the scope of the invention which is defined in the appended claims.

What is claimed is: