[0001] The invention relates to a sieving device comprising a pervious sieve surface adapted
to rotate about an upwardly extending rotary axis.
[0002] Such a sieving device is known from Dutch Patent Application 7605572. In this known
sieving device the sieve surface forms part of the outer surface of a cone, in which
the part nearest the rotary axis at the lower level and the part of the sieve surface
furthest remote from the rotary axis being at the higher level. During operation the
material to be sieved has to move outwardly along the sieve surface, the fine particles
to be separated have to fall down through the sieve surface and the coarse particles
have to be ejected from the outer periphery of the sieve surface.
[0003] In this known construction the velocity of the material to be sieved increases according
as the material gets further away from the rotary axis, whilst also the size of the
sieve surface increases quadratical- ly with the distance from the rotary axis.
[0004] When the speed of rotation is chosen low, the speed of movement of the material to
be sieved towards the outer periphery of the sieve is proportionally low near the
rotary axis so that comparatively only a small amount of material can be processed
per unit time. However, when the speed of rotation of the sieve is raised, the product
will pass through the major part of the sieve surface at a distance from the rotary
axis with such a velocity that an effective sieving is out of the question.
[0005] A further disadvantage of this known sieving device is that particles can easily
stick in the perforations of the sieve surface, from which they can no longer part,
since on the contrary due to the effect of the centrifugal force or force of gravity
they clamp tight in the perforations.
[0006] The invention has for its object to provide a sieving device of the kind set forth
by which the disadvantages inherent to the known sieving device can be avoided.
[0007] According to a first aspect of the invention this can be achieved by shaping the
sieve surface with a curved sectional area so that parts of the sieve surface located
near the rotary axis and near the outer circumference of the sieve surface are located
at a higher level than the intermediate parts of the sieve surface. By this design
it can be ensured that in the proximity of the rotary axis the centrifugal force acting
on the particles of the material is strengthened by the component of the force of
gravity affecting the particles extending parallel to the sieve surface, whereas near
the outer circumference of the sieve surface the centrifugal force is counteracted
by the component of gravity extending parallel to the sieve surface. In this way by
the design of the curved sieve surface the velocity of the movement of the particles
of material along the sieve surface can be influenced to an extent such that the optimum
sieving effect is obtained.
[0008] According to a second aspect of the invention the sieve surface is rotatable about
two at least substantially parallel rotary axes. As a result an additional rotational
movement is superimposed on the conventional rotation of the sieve surface so that
the sieve surface performs, so to say, small circular movements below the material
to be sieved, which counteracts obturation of the sieve due to particles sticking
in the perforations of the sieve surface.
[0009] According to a third aspect of the invention means are provided for causing the sieve
surface to perform a tilting movement about a pivotal axis extending transversely
of the rotary axis of the sieve surface. As a result the sieve surface moves up and
down, the deflection of the sieve surface increasing with the distance from the rotary
axis. Also in this way obturation of the sieve surface is effectively counteracted.
[0010] The invention will be described more fully hereinafter with reference to the accompanying
Figures.
Fig. 1 is a schematic cross-sectional view of an embodiment of a sieving device in
accordance with the invention.
Figs. 2 to 4 show different embodiments of shafts with the aid of which the sieve
surface can be supported.
[0011] The sieving device shown in Fig. 1 comprises a circular-section housing 1, the lower
part of which is tapering in downward direction. In the housing 1 is arranged a supporting
assembly 2 as well as a stationary, continuous gutter 3. With the gutter communicates
the top end of an outlet pipe 4, which extends beyond the housing 1.
[0012] To the frame 2 is secured a housing 5, in which a vertical, shaft 6 is journalled
with the aid of bearings 7. The lower end of the shaft 6 is provided with a gear wheel
8, which is coupled with the aid of a toothed rope 9 with a toothed wheel 10 fastened
to the shaft of a motor 11 supported by the supporting frame 2. The shaft 6 is integral
with a shaft 12 extending above the shaft 6, on which shaft 12 a hood 13 is rotatably
journalled with the aid of bearings 14. The hood 13 is integral with a toothed wheel
15, which is coupled with the aid of a toothed belt 16 with a toothed wheel 17 fastened
to the output shaft of an electric motor 18, which is also secured to the supporting
frame 2.
[0013] To the hood 13 is fastened a plate 19 forming part of a conical surface and being
downwardly and outwardly inclined away from the hood 13. The lower rim of the conical
plate 19 is secured by means of a few strips 20 to a continuous sheath 21. Between
the top rim of the sheath 21 and the hood 13 is provided a sieve surface 22. From
Fig. 1 it will be apparent that this sieve surface is curved so that the rim of this
annular sieve surface fastened to the hood 13 and the circumferential rim of this
sieve surface 22 fastened to the top rim of the sheath 21 are located at a higher
level than the part of the sieve surface in between the former.
[0014] On the top side the housing has a central inlet port 23 for the introduction of the
material to be sieved.
[0015] Potential dispositions of the shafts 6 and 12 forming one unit are shown in detail
in Figs. 2, 3 and 4.
[0016] Fig. 2 shows that the centre lines of the shafts 6 and 12 may be parallel to one
another and be spaced from one another by a distance e. Preferably said distance e
is, for example, 2 mms.
[0017] As an alternative the centre lines of the two shafts may be at an angle m to one
another, said angle α preferably being 35°. The centre line of the shaft 12 may intersect
the centre line of the shaft 6 in the plane in which the two shafts join one another
as is shown in Fig. 3 or in this plane the shafts may again be spaced apart by a distance
e as shown in Fig. 4.
[0018] In operation material will be fed through the inlet port 23 and arrive at the sieve
surface. With the aid of the motor 18 the sieve surface is rotated about the rotary
axis of the shaft 12 so that the material striking the sieve surface is subjected
to a centrifugal force and will move outwardly along said sieve surface. The fine
particles of the material can pass through the perforations of the sieve surface and
be guided or not guided by the conical plate 19 to the gutter 4. With the aid of the
strips 23 this material is displaced towards the opening where the pipe 4 adjoins
the gutter 4 so that the fine material can be conducted away through the pipe. The
coarse particles of the material is displaced outwardly along the sieve surface 22,
be ejected across the outer rim of the sieve surface and be conducted away through
a delivery port 24 in the underside of the housing 1.
[0019] Since initially the supplied material moves along a downwardly inclined part of the
sieve surface 22, the movement of this material is further accelerated by the action
of the resolved component of gravity extending parallel to the sieve surface. This
component of gravity thus supports the centrifugal force which brings about an outward
displacement of the material.
[0020] After having passed beyond the lowermost point of the sieve surface, designated by
A, the material moving along the sieve surface has to move upwards towards the outer
rim of the sieve surface. During this upward movement the component of the force of
gravity extending parallel to the sieve surface will counteract the centrifugal force.
Therefore, by a suitable design of the curved sieve surface 22amost effective displacement
of the material along the sieve surface can be ensured, preferably in a manner such
that the velocity with which the particles of the material move outwards away from
the centre line of the sieve surface remains at least substantially constant, viewed
in a horizontal direction.
[0021] In the preferred embodiment of the invention described and illustrated herein the
sieve surface rotates not only about the rotary axis formed by the centre line of
the shaft 12, but also the shaft 6 carrying the shaft 12 is rotated with the aid of
the electric motor 11. When the shafts are disposed as is shown in Fig. 2, the sieve
surface will perform a swinging movement in a horizontal' sense, the amplitude of
the sieve surface being equal to twice the distance e between the centre lines of
the two shafts. Preferably the speed of rotation of the shaft 6 appreciably exceeds
that of the sieve surface about the shaft 12 so that the sieve surface will perform
a strong rocking movement, which will counteract obturation of the pores of the sieve
surface.
[0022] When the shafts are disposed as is shown in Figs. 3 and 4, the sieve surface will,
in addition, perform a tilting movement about an imaginary tilting axis at right angles
to the centre line of the shaft 6. It will be obvious that as a result the sieve surface
will perform upward and downward movements during operation, the deflection of the
sieve surface increasing with a growing distance from the rotary axis. This will intensify
the sieving effect and contribute to avoidance of clogging of the sieve surface.
[0023] The figures used in the claims are only meant to explain more clearly the intention
of the invention and are not supposed to be any restriction concerning the interpretation
of the invention.
1. A sieving device comprising a pervious sieve surface adapted to rotate about an
upwardly extending rotary axis characterized in that the sieve surface has a curved
sectional area in a manner such that parts of the sieve surface located near the rotary
axis and near the outer periphery are at a higher level than the parts of the sieve
surface located there between.
2. A sieving device as claimed in Claim 1 characterized in that the sieve surface
is curved in a manner that, when the sieve surface is rotating with the normal operational
speed the displacement of the material to be sieved by the action of the centrifugal
force and the component of the force of gravity extending parallel to the sieve surface
takes place with at least substantially constant speed in a radial direction outwards
from the middle of the sieve surface.
3. A sieving device comprising a pervious sieve surface adapted to rotate about an
upwardly extending rotary axis characterized in that the sieve surface is adapted
to rotate about two at least substantially parallel rotary axes.
4. A sieving device comprising a pervious sieve surface adapted to rotate about an
upwardly extending rotary axis characterized in that means are provided with the aid
of which a tilting movement about a pivotal axis extending transversely of the rotary
axis of the sieve surface can be imparted to the sieve surface.
5. A sieving device as claimed in anyone of the preceding Claims characterized in
that the sieve surface is rotatably mounted on a shaft supporting the sieve surface
and is coupled with a driving source, whilst the shaft supporting the sieve surface
is fastened to a second shaft which is also coupled with a driving source.
6. A sieving device as claimed in Claim 5 characterized in that the interconnected
shafts extend parallel to one another.
7. A sieving device as claimed in Claim 5 characterized in that the interconnected
shafts are at an angle to one another.