US3214104A

US3214104A - Paper making apparatus

Info

Publication number: US3214104A
Application number: US206642A
Authority: US
Inventors: Breuninger Wolfgang; Bahr Theodor
Original assignee: JM Voith GmbH
Current assignee: JM Voith GmbH
Priority date: 1961-07-07
Filing date: 1962-07-02
Publication date: 1965-10-26
Anticipated expiration: 1982-10-26
Also published as: FI43948B; DE1411893A1; SE310837B; AT235126B; GB1007176A; ES278367A1; CH403465A

Description

25, 1965 w. BREUNINGER ETAL 3,214,104

PAPER MAKING APPARATUS Filed July 2, 1962 2 Sheets-Sheet 1 Fig. 7

WOLF GANG BREUN/NGEI? 77/6000? 5/4/10? Afomeys Oct. 26, 1965 Filed July 2, 1962 W. BREUNINGER ETAL PAPER MAKING APPARATUS Fig.8

2 Sheets-Sheet 2 Fig.9

//7 venfors WOLFGANG BfiEU/V/NGER THEODOR BA/1R m ml/ Alfomcys United States Patent 3,214,104 PAPER MAKING APPARATUS Wolfgang Breuninger and Theodor Biihr Heidenhelm (Br-em), Germany, assignors to J. M. Vorth G.m.b.H., Heidenheim (Brenz), Germany Filed July 2, 1962, Ser. No. 206,642 Claims priority, application Austria, July 7, 1961, A 5,235/61 12 Claims. (Cl. 241-146) The partly prepared pulp of cellulose, wood, waste paper, rags or the like, or felted layers thereof, before being used for making paper, must first be subjectedto mechanical disintegration in order to separate the 1ndividual fibers from one another. The usual contrivances for this purpose operate with a beating action, the effect of which is additionally to cut the individual fibers into shorter pieces. Such an effect often is undesirable because the fibers of such a partly prepared pulp are already of the proper freeness and do not need any further shortening.

The apparatus for converting the partly prepared pulp into the finished pulp, consists generally of two relatively rotatable members, one of which is driven while the other remains stationary. These two members are provided with opposite working surfaces which are formed with oppositely acting working edges. With deflakers, e.g. with disk-type deflakers, these working edges are closely spaced and capable of accurate adjustment toward and from each other so that shortening of the fibers can be largely avoided. In contradiction thereto with beaters, e.g. with disk-type beaters, the working edges of the two members came into actual contact with each other, so that the fibers are cut up and shortened during the operation of the beater.

In some converting apparatus the partly prepared pulp is delivered to the working edges through openings in the stationary member, while in others it is introduced centrally in the axial direction in such a manner that it will be encountered and moved in the outward radial direction by the working edges on rotating disks. In these and many other constructions the working edges were easily damaged by foreign objects, and the subsequent resharpening of the edges, if at all possible, was difficult.

The purpose of this invention is therefore to give the working parts of a defiaking equipment such a structure that the fibers will not be cut and, hence, the lengths of the fibers will not be reduced during the converting operation, which structure is easy to manufacture, not likely to become damaged by foreign objects, and easy to resharpen.

According to this invention, the above purposes are accomplished by providing the interacting working surfaces with recesses of such design that these recesses leave between them ribs which form a grating, the walls of the recesses being perpendicular to the working surface or not inclined thereto more than 30. By the interaction of two such working surfaces in a deflaking equipment, experiments have shown that a further shortening of the fibers is almost entirely avoided, but nevertheless such working surfaces are very effective. With a pair of disks 30 cm. in diameter, one of which is rotated 2900 r.p.m. while the other remains stationary, the material between them will be subjected to 2 million abrupt changes of direction of motion per second, and during each of which the material will be subjected to impact forces 'ice which will cause the fibers to become separated from one another, without causing any undesirable shortening or cutting up of the individual fibers.

Such a defiaking apparatus can be manufactured very cheaply, especially if, according to a further aspect of this invention, the recesses are in the form of cylindrical or conical boreholes.

Since the working surface of this sort does not present any projecting edges, ribs or teeth, it will not be damaged by foreign bodies such as nails, staples, etc., but even if a few of the edges do become dulled, the efficiency of the equipment will not be seriously reduced, because it operates mainly by impact action. Furthermore, because of the simplicity of the construction, the working edges can be formed of hardened metal, without danger of breakage during operation. The working edges can also be repeatedly re-sharpened by grinding down of the working surfaces.

In another form of this invention the recesses are arranged in concentric circular rows about the axis of rotation. The pulp will then have to pass successively through wide and narrow passageways. The circular rows of recesses in adjacent disks may also be staggered relative to one another so as to produce violent turbulence and vorticity while passing from the recesses in one disk to those of an adjacent disk.

For the introduction of crude material, one of the disks, usually the stationary one, is provided with a central opening about equal in diameter to that of the supply pipe.

In another form of construction the recesses which are adjacent the inlet opening are made larger in crosssection than the recesses which are near the regions of discharge. In this manner it will be possible not only to prevent the larger particles from getting permanently wedged in the recesses and thus close up the same, but it will then also be possible to provide working surfaces of limited dimensions with the maximum total number of recesses, and therefore with the greatest total length of working edge. Since the crude fiber bundles containing material undergoes continually increased disintegration from the inlet opening to the region of discharge, the cross-sectional dimensions of the recesses in the concentric circular rows are made gradually smaller throughout the entire distance from the inlet opening to the region of discharge. In this manner it will be possible to make the greatest and most eflicient use of working surfaces of limited dimensions.

, In some cases it is advantageous to provide the inlet passageway itself with comminuting means for initial subdivision of the larger fiber bundles into sizes which can easily be handled by the deflaking apparatus to which they are about to be delivered.

It may also be desirable to form the recesses of the innermost row in such a manner that their openings will be directed not only toward the recesses of the other working surface, but also toward the inlet opening.

The discharge of the finished pulp from the working surfaces canbe facilitated by forming the recesses in the outermost row in such a manner that their openings will be directed, not only toward the recesses of the other working surface, but also toward the discharge passageway.

The same advantages which can be obtained with con centric circular series of recesses can be obtained as well or better with spiral series. The recesses can all be arranged on one spiral of several convolutions, or they can be distributed over several spirals, each of which makes only a portion of a convolution.

This invention is not limited to deflaking apparatus with flat Working surfaces, but is also applicable to working surfaces which are lens-shaped or conical. The recesses in each case may have their walls perpendicular or inclined to the working surfaces, but should not be inclined more than 30 to the perpendicular.

The working parts of such deflaking apparatus can also be formed of two combined units which may be supplied with crude material from either a single source or from separate sources. Such a duplex arrangement may be more efiicient and more versatile than a simplex construction.

With a duplex arrangement comprising three closely spaced disks, the central disk which may be the rotary disk is provided on both cylindrical faces with recesses interacting with correspondingly arranged recesses in the working surfaces of the outer disks which may be stationary.

The depths of the recesses will depend on the state of disintegration of the material. In order to keep the thickness of the central disk at a minimum, the circular rows of recesses on both surfaces of this disk may be staggered so that the recesses of one face will overlap those of the opposite face. Thus the effective depth of each recess may be increased as compared with a construction in which the central disk is combined of two individual disks each of which is provided with recesses only in the outward directed face.

In another form of this invention, the rotary disk is provided with openings which pass completely through the same which, in this manner, also has working edges on each face. A stationary disk with registering working recesses is arranged closely adjacent to each surface of the rotary disk. Such a construction has the advantage of greatly increased capacity with only slightly increased cost of manufacture.

For efficient operation such deflakers have to be operat ed at high speed, which results in much wear of the working parts. To make the entire disks of hard wearresistant material would add greatly to their cost, but the increased wear-resistance can be achieved by providing the disks with hardened bushes embedded therein to form the walls of the recesses. The disks may then be made of tough refined steel or bronze, while the bushes may be made of wear-resisting stainless steel. The bushes may be inserted into boreholes in the disks with drive fits, but this requires careful machining of the boreholes and subsequent smoothing of the working surfaces of the disks.

These difficulties can, however, be avoided if the bushes are slitted longitudinally so that they can be sprung into place. After one end of a bush becomes worn, it can be reversed end-for-end, and after the other end becomes worn, it can be replaced by a new bush.

The features which make up this invention can also be applied to defiakers in which the relatively rotatable working surfaces are cylindrical in form. They may also be in the form of truncated cones of small apex angle, the pulp during its passage from one end to the other being subjected to the disintegrating action of opposed working edges on the adjacent surfaces. If the working surfaces are brought into actual contact under pressure, then such equipment may be used to increase the freeness of the material passing therethrough.

If the recesses in the working surfaces are made very shallow, then such an apparatus can be used for fiberpreserving surface treatment of cellulose fibers Examples of such deflaking devices are shown on the drawings, wherein FIGURE 1 is a cross-section of a deflaker of duplex construction with central supply from one side and three cooperating disk-like Working members;

FIGURE 2 is a partial crosssection of two cooperating deflaking members shown on a slightly larger scale than the corresponding parts in FIGURE 1;

FIGURE 3 is a partial side view of one of the disks in FIGURE 2, viewed in the direction of the arrow IIIIII;

FIGURE 4 is a view similar to FIGURE 3, but showing a modified form of disk;

FIGURE 5 is a partial side view of one of the disks in FIGURE 2, and viewed in the direction of the arrow VV, but in which the recesses are arranged spirally;

FIGURES 69 :show further modifications in which the recessed disks are fiat, concavo-convex, or conical;

FIGURE 10 is a partial cross-section showing certain details of a duplex deflaker of the type shown in FIG- URE 1;

FIGURE 11 is a partial cross-section of a modified form of duplex defiaker, including one rotary and two stationary disks, similar to the arrangement shown in FIGURE 1; and 7 FIGURE 12 shows on a larger scale a fragmentary cross-section of a duplex deflaker, the recesses of which are provided with removable bushes.

In the deflaker shown in FIGURE 1, the housing 1 is secured to the side of the base 2, in which the drive shaft 3 is journalled. This shaft carries at one end thereof the rotary disk 4, while its opposite end 5 is adapted to be power driven. The two stationary disks 6 and 7 which cooperate with the rotary disk 4 are secured to the inside of the inside walls of the cover plates 8 and 9 of the housing 1. The plate 8 has a central opening 10 for receiving the crude material that is to be fed to the machine. The crude material which has entered the deflaker through the central opening 10 then passes in radial outward directions between the interacting surfaces of the disks, and after disintegration into individual fibers finally leaves through the discharge conduit 12. For the sake of clarity, the clearances between the working surfaces of the rotary and stationary disks have been exaggerated on the drawing.

The

disks

13 and 14 of FIGURE 2 are provided in their adjacent surfaces with

pocket boreholes

17, 17a, 17b and 18, 18a. The crude material is passed through the central opening 19 and from there into the spaces between the disks. The opening 19 is provided around its periphery with inwardly projecting radial fins 20 whose front ends are slanted off sufficiently to deflect any solids that come into contact therewith, while at the same time subjecting large fiber bundles to a partial subdivision. The crude material is then delivered to the recesses 18a of the innermost row of the rotary disk 14. These recesses overlap both the central opening 19 and the first row of recesses 17a of the adjacent stationary disk 13. The

recesses

17a, 18a of the

disks

13 and 14 are larger other in the two disks overlap one another throughout the entire radial dimensions of the disks. The material during its passage through the deflaking equipment is therefore transferred along from one recess to another while at the same time it is subjected to sudden impacts and accelerations which cause a further disintegration of the fiber agglomerations. In this manner even very hard fiber bundles are converted into individual fibers during just a single passage through the machine. In order to facilitate the discharge of the finished pulp from the space between the disks to the discharge passageway, the recesses 17b of the outermost series of the stationary disks are open not only toward the working surface of the ad jacent rotary disk, but also in the radially outward direction toward the discharge passageway.

In FIGURE 3 the

recesses

18, 18a in the face of the disk are arranged in concentric rows around the axis of rotation. A slightly different arrangement of the same recesses is shown in FIGURE 4. Here the recesses in successive rows are arranged in such a manner that they are staggered with respect to the recesses of adjacent rows.

In FIGURE 5 the

recesses

17, 17a and 17b of the stationary disk 13 are arranged similarly in concentric rows, but also in congruent spirals s. It will be observed that the remaining structural material between the recesses which presents the working edges is also in the form of a grating, somewhat similar to that of FIGURES 3 or 4. The

recesses

17a, 18a of the

disks

13 and 14 are larger in cross-section than the remaining

ones

17, 17b and 18. Furthermore, the length of working edge per square centimeter of cross-sectional area of the

recesses

17, 17a, 17b and 18, 18a increases in the radially outward direction.

In FIGURE 6 the recesses 22 of the disk 21 are at an acute angle to the working surface 23. The angular inclination of these recesses to the working surface of the disk is so chosen that the walls of the recesses do not make an angle of more than 30 with the normal to the working surface.

In FIGURE 7 the recesses 30 in the working surface 28 of the disk 26 are inwardly enlarged. These conical recesses may be formed from the rear side of the disk 26, and then covered by a disk 34 secured to the disk 26 by means of screws 36.

In FIGURE 8 are shown two cooperating lens-shaped

disks

37, 38, the stationary disk 37 presenting a concave working surface 39 while the rotating disk 38 presents a similarly curved convex working surface. The axes of the recesses in these disks are normal to the working surfaces. During operation the material is forced in radial outward directions by going back and forth from one recess to another in the adjacent disks.

' The two disks in FIGURE 9 have conical working surfaces 48 and 49, the one being concave while the other is convex. In this construction the axes of the recesses are parallel to the axis of rotation of the rotary disk. The walls of the recesses do not bear an inclination of more than 30 to the normals to the conical surfaces, the half angle at the apex of the cone being more than 60. In this construction the recesses of the innermost series of the stationary disk are open not only toward the working surface of the rotary disk, but are open in the radially inward direction toward the inlet passageway.

The defiaking apparatus in FIGURE is a duplex type, similar to that of FIGURE 1. It comprises a cylindrical disk 59 with two working

surfaces

60, 61 provided with

recesses

62, 63. These recesses are given a depth of more than half the thickness of the disk, which is acc'omplished by giving the recesses on opposite sides of the disk a staggered arrangement.

j FIGURE 11 shows a duplex arrangement in which the central rotary disk 64 has transverse recesses 67 extending entirely through the same, while the two

stationary disks

81, 82 have the recesses in their working

surfaces

65, 66 overlapping with the transverse recesses 67 in the central rotary disk 64. Furthermore, the disk 64, near its axis of rotation, has transverse openings 69 extending through this disk and in direct communication with the central inlet passageway for delivering crude material to the recesses on the opposite side, interacting with the recesses in the stationary disk 82.

In FIGURE 12 the working edges and the recesses are provided by longitudinally split

bushes

73, 74 sprung into

boreholes

75 and 76 of the

disks

71, 72.

With respect to the process which utilizes the novel apparatus of this invention, it is to be recognized that a partly prepared pulp of cellulose, wood, waste paper, rags or the like is conventionally produced by slushing the material with water in a pulper to produce a pumpable suspension consisting of generally small fiber bundles capable of being distintegrated into individual fibers in the novel apparatus.

The resultant pulp generally comprises a solids concentration of 30 to 60 parts by weight of fibers to one liter of solution, the fibers generally having a length of 0.05 to about 0.3 centimeter. The pulp is passed through the apparatus of this invention at about a rate of 200 to 1200 liters per minute, whereas the rotating member of the apparatus revolves at a speed corresponding to a circumferential velocity of about 30 to 55 meters per minute, preferably 47 meters per minute.

A specific example of the process of this invention is as follows30 kilograms of waste paper per minute with an average fiber length of approximately 1.5 millimeters is mixed in a pulper with approximately 600 liters of water per minute, and mechanically preslushed by the slushing disk of the pulper so as to shred the Waste paper into finger-nail-sized pieces. The resultant pumpable suspension which has a solids concentration of approximately 5 percent by weight is then forced by a conventional high-consistency stock pump through the deflaker of this invention. This apparatus is a duplex-type deflaker having three 30 centimeters diameter deflaker disks in parallel arrangement, the central disk being driven at 2900 r.p.m. while the two outer deflaker disks are stationary. With this arrangement, the stock through-put through the deflaker is 600 liters per minute, the power input being approximately 55 kw. On being passed through the deflaker, the waste paper shreds are disintegrated into individual fibers without being cut or shortened. Thus, the pulp finished in the deflaker has the original average fiber length of approximately 1.5 millimeters of the furnished waste paper and, after appropriate dilution and after separation of undesirable foreign bodies (if required), is suitable for further processing on the paper making machine. Paper machine broke can be similarly prepared.

Paper stock that in this manner has been reclaimed from printed waste paper must in general be freed from any printing-ink particles before it can be processed on the paper making machine. For this purpose, a flotation deinking equipment is arranged after the deflaker. Since the deflaker not only disintgrates the waster paper pieces into individual fibers, but it also separates printing-ink particles from the fibers, the resultant stock contains the printing-ink as free particles and is thus excellently suited for the removal of these particles by flotation.

With a through put of less than 500 liters per minute, the high-consistency stock pump ahead of the deflaker may be omitted, since the deflaker is capable of automatically handling such small through-puts.

Felted layers of long-fibered virgin cellulose will deliver the maximum yield of usable fibers, if they are disintegrated in a deflaker of this invention prior to the shortening of the fibers in a fiber-cutting equipment to an optimum fiber length for further processing on a paper making machine, in general approximately to 1.5 millimeters.

It will be understood that various changes and modifications of this invention may be made by those skilled in the art to suit various conditions and uses to which the invention may be put. It is accordingly contemplated that such changes may be made within the spirit and scope of this invention and which is more particularly set forth in the appended claims.

What is claimed is:

1. A defiaking apparatus comprising two relatively rotatable members with working surfaces closely adjacent to each other, said surfaces being provided with separate recesses spaced from one another, said recesses being circular in cross section and being lined with wear-resistant bushes, the margins of said bushes forming circular working edges, and the walls of said bushes making with the normal to the working surface an angle of not more than 30.

2. The apparatus of claim 1 in which the bushes are longitudinally split.

3. A defiaking apparatus comprising a rotatable disk and a stationary disk on each side thereof, said rotatable disk having separate spaced recesses in both sides thereof, the recesses in one side of said rotatable disk being staggered in relation to the recesses in the other side of said rotatable disk so as to permit increase in depth of said recesses above half the thickness of said rotatable disk, the recesses of both sides of said rotatable disk thus overlapping in depth without interfering with one another, and the working marginal edge of each recess being endless and closed.

4. A defiaking apparatus comprising a rotatable disk and a stationary disk on each side thereof, said rotatable disk having recesses extending completely through the disk, each stationary disk having recesses in the side adjacent said rotatable disk, the recesses in said stationary disks being radially staggered with the recesses in said rotatable disk, one of the stationary disks having a central inlet opening, and the recesses of said rotatable disk next to the axis of rotation of said rotatable disk partially overlapping the central inlet opening as well as the innermost recesses of the other stationary disk, and the marginal edge of each recess being endless and closed.

5. A defiaking apparatus comprising an inlet chamber, an outlet chamber, two relatively rotatable disk-shaped working members arranged between said inlet chamber and said outlet chamber, so as to separate the said chambers from one another, each working member having a working surface having therein structurally separated substantially cylindrical recesses having closed bottoms and spaced from one another and distributed over the entire working surface, the said two working surfaces having substantially the same geometrical shape and being closely adjacent each other in confronting relation, the said recesses of one working member being in staggered relation to the recesses of the opposite working member in the direction of movement of the suspension when passing from said inlet chamber to said outlet chamber, the margin of each recess forming an endless edge that is closed upon itself, the portions of the working surface which remain between the recesses forming a material grating, and the walls of the recesses forming with the normal to the working surface an angle of approximately 30.

6. A defiaking apparatus comprising an inlet chamber, an outlet chamber, two relatively rotatable disk-shaped members arranged between said inlet chamber and said outlet chamber so as to separate the said chambers from one another with confronting working surfaces having substantially the same geometrical shape and being closely adjacent to each other, said surfaces being provided with separate substantially cylindrical recesses having closed bottoms and spaced from one another, the recesses of one working member being in staggered relation to the recesses of the opposite Working member in the direction of movement of the suspension when passing from said inlet chamber to said outlet chamber, the margins of said recesses forming circular working edges, the recesses next to the outlet rim of one member being open toward the discharge chamber and the walls of said recesses making with the normal to the working surface an angle of approximately 30.

7. A defiaking apparatus comprising an inlet chamber, an outlet chamber, two relatively rotatable disk-shaped members arranged between said inlet chamber and said outlet chamber so as to separate the said chambers from one another with confronting working surfaces having substantially the same geometrical shape and being closely adjacent to each other, said surfaces being provided with separate substantially cylindrical recesses having closed bottoms and spaced from one another and arranged in a plurality of circular series around the axis of rotation of the rotatable member, the recesses of one working member being in staggered relation to the recesses of the opposite working member in the direction of movement of the suspension when passing from said inlet chamber to said outlet chamber, the margins of said re cesses forming closed integral working edges, the recesses next to the outlet rim of one member being open toward the discharge chamber and the walls of the said recesses making with the normal to the working surface an angle of approximately 30.

8. A defiaking apparatus comprising two relatively rotatable disk-shaped members with confronting working surfaces closely adjacent to each other, each said member consisting of a perforate cylindrical disk covered by a cover plate, said disk being provided with separate perforations structurally spaced from one another, the portions of said disk which remain between the perforations form-' ing a material grating, said perforations being covered on the side opposite the Working surface by said cover plate so as to form recesses on the working surface, said recesses only open on one side and toward the opposite working member, the margins of said recesses forming closed integral working edges, and the Walls of said recesses making with the normal to the working surface an angle of not more than 30.

9. A defiaking apparatus comprising a rotatable disk and a stationary disk on each side thereof, said rotatable disk having recesses in both sides thereof, the depth of each recess in the rotatable disk being greater than onehalf the thickness of the disk, the recesses in one side of said rotatable disk being staggered in relation to the recesses in the other side of said rotatable disk, each stationary disk having recesses in the side adjacent the rotatable disk, and one of the stationary disks having a central inlet opening.

10. A defiaking apparatus comprising a rotatable disk and a stationary disk on each side thereof, said rotatable disk having a plurality of substantially circular series of recesses extending completely through the disk, each stationary disk having also a plurality of substantially circular series of recesses in the side adjacent said rotatable disk, the series of recesses in the stationary disks being in staggered relation to the recesses in said rotatable disk, the edge of each of said recesses defining a closed working edge, and one of the stationary disks having a central inlet opening.

11. A defiaking apparatus comprising a rotatable disk and a stationary disk on each side thereof, the rotatable disk having recesses in both sides thereof, the depth of each recess in the rotatable disk being greater than onehalf the thickness of the disk, each stationary disk having recesses in the side adjacent said rotatable disk, the recesses in each stationary disk being radially staggered in relation to the recesses in the adjacent side of said rotatable disk.

12. A defiaking apparatus comprising an inlet chamber, an outlet chamber, two relatively rotatable working disk-shaped members arranged between said inlet chamber and said outlet chamber so as to separate the said chambers from one another, each working member having a working surface having therein structurally separate substantially cylindrical recesses having closed bottoms and closely spaced from one another and distributed over the entire working surface, the said two working surfaces having substantially the same geometrical shape and being closely adjacent each other in confronting relation, the said recesses of one working member being in staggered overlapped relation to the recesses of the opposite working member in the direction of movement of the material when passing from said inlet chamber to said outlet chamber, the cross-sectional dimensions of the recesses at the inlet being greater than the cross-sectional dimensions of the recesses at the outlet, the margin of each of said recesses forming an endless edge that is closed upon itself, the portions of said working surface which remain between the recesses forming a material grating, and the Walls of said recesses forming with the normal to said working surface an angle of approximately 30.

References Cited by the Examiner UNITED STATES PATENTS (Other references on following page) 9 UNITED STATES PATENTS Bidwell 241-260 Graffljn 241-296 Swartz 241-146 Sutherland 241260 5 Jones 24126O Gustke 241--163 Honeyman 241-260 Johnson et a1 241--260 Heintz et a1. 241-260 10 1 0 FOREIGN PATENTS 5/50 Canada. 11/60 Germany.

Examiners.

Claims

1. A DEFLAKING APPARATUS COMPRISING TWO RELATIVELY ROTATABLE MEMBERS WITH WORKING SURFACES CLOSELY ADJACENT TO EACH OTHER, SAID SURFACES BEING PROVIDED WITH SEPARATE RECESSES SPACED FROM ONE ANOTHER, SAID RECESSES BEING CIRCULAR IN CROSS SECTION AND BEING LINED WITH WEAR-RESISTANT BUSHES, THE MARGINS OF SAID BUSHES FORMING CIRCULAR WORKING EDGES, AND THE WALLS OF SAID BUSHES MAKING WITH THE NORMAL TO THE WORKING SURFACE AN ANGLE OF NOT MORE THAN 30*.