Note: Descriptions are shown in the official language in which they were submitted.
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METHOD FOR REGULATING THE THROW ANGLE OF A VIBRATING SIEVE OR FEEDER
The invention relates to a method which is usable in those driving
means for achieving shaking or oscillating movements, in which two out-of-
balance weights are rotated in opposite directions around individual shafts
and are provided with separate drive motors.
There are two types of this sort of driving unit. In one of them,
which has been known for a long time, the two out-of-balance weights are of
the same size and weight. A theoretical treatment of this case is to be found
in Aufbereitungs-Technik for 1976, pages 108-114, for example. In this first
type of drive unit, a linear, reciprocal movement can be obtained along the mid-
point normal to a straight line connecting the two shafts.
The other type of drive unit was invented by us and is described in
United States Patent No. 4,212,731 of Wallin, issued July 15, 1980. According
to this invention, an elliptical stroke is obtained by making the out-of-balance
weights of different size, under the precondition that the center of gravity of
the oscillating system lies essentially on an Appollonios' circle to the axes
of rotation, so determined that the ratio of the distances from the center of
gravity to the axes of rotation is inversely proportional to the products of
the weights of the respective oscillation
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masses and their mean distance to the respective axis of
rotation.
The latter invention was developed from an in-depth
theoretical study of a previously known system with two
different sized oscillation masses, compulsorily coupled
together with a toothed gearing. The research began with a
calculation of the reaction moments on the shafts to the
out-of-balance weights, where, inter alia, the following
result was obtained:
(2ml rl~)2 sina cosa (a sin ~t + b cos ~t)2
Ml-M2 = '--
Y (1)
where Ml and M2 are the reaction moments around the
respective rotational axes; ml and rl are the weight of one
out-of-balance weight and its distance from its rotational
axis, respectively; a and b are the Cartesian coordinates
in a Cartesian system of coordinates with the center of
gravity of the oscillating system as the origin;
a is the angle between the direction of stroke and a line
from the origin to the point (a,b); ~ is the angular
frequency for the rotation of the out - of-balance masses;
and y is the mass moment of inertia of the system.
Ml-M2 is then a restoring moment which acts to return
the system to the direc-tion a = 0.
The basic idea which is the first step in the present
invention is the insight that the equation (1) is valid even
when reversed so that if an external moment is applied, the
throwing angle is affected -to a corresponding degree, that
is to say, artificially it is possible to achieve a value
for a which is non-zero. A precondition for balance in such
a system is that the work of the external moment be equal to
the work of the sychronizing moment. The cycle for the
synchronizing moment is 2rr. If we integrate Ml-M2 over one
revolution, we obtain from equation (l):
2~ (2ml rl ~) (a +b ) ~r
I (Ml-M2)d~t = sin 2a =
o 2 y
= CONST. sin 2a. (2
-2-
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One can see from this that a numerical increase or decrease in an
external moment results in an increase or decrease in ~ up to the limit ~45 ,
whereafter stability can no longer be expected.
In the absence of the external moment, as the description of the
invention in United States Patent No. 4,212,731 reveals, a purely translational
shaking movement is obtained, without rotation (tilting). The introduction
of an external moment disturbs this "equilibrium", and a superimposed tilting
movement is obtained. However, it has been shown both theoretically and in
practice that this is of completely negligible size.
It is now possible to superimpose an "external" moment in many dif-
ferent ways. For example, it is theoretically possible by using springs or
weights fastened to the screening box, to apply a couple of forces which tends
to turn the suspended mass. However, we feel that it is most advantageous to
act on the two motors which drive the out-of-balance weights.
The present invention provides a method for regulating the throw angle
of a vibrating device such as a sieve or feeder of the type which has a drive
means with out-of-balance means around two shafts which are parallel to each
other, characterized in that the two out-of-balance means are of different dim-
ensions, are driven by individual motors and have their shafts arranged in
relation to the center of gravity of the device in such a way that the center
of gravity lies on an Apollonios' circle to the two shafts, so determined that
the ratio of the di6tances from the center of gravity to the axes of rotation
is inversely proportional to the products of the out-of-balance weights of the
respective out-of-balance means and their mean distance to the respective axis
of rotation, the throw angle being affected by changing the power of one of
the motors in relation to that of the other.
The regulation can include the possibility of compensating for uneven
load due to uneven distribution of the material to be screened.
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The invention will now be described with reference to an embodiment
shown in the drawings.
Figure 1 shows a perspective view of a horizontal sieve.
Figure 2 shows a schematic sectional view of one oE the out-of-balance
devices in the sieve according to Figure 1.
The sieve in Figure 1 consists of a screen box 1, with a first
screening level 7 and a second screening level 8, which is suspended on a bed
9 by means of springs 2.
Two motors 3 each drive one of a pair of out-of-balance weight means
5, each pair mounted in a dust-proof casing with an intermediate shaft. As
can be seen from the detailed drawing in Figure 2, each motor drives, via a
belt transmission 11, mounted in a protective casing 6 (Figure 1), and via
flexible couplings 12 (in casing 4), a shaft with out-of balance weights 10
pairwise rotatably journalled in the walls of the screening box 1. The motors
and the belt transmissions are rigidly joined to the bed 9 and thus do not
vibrate. Movement in the drive train is accommodated by the flexible couplings.
The inventive effect is achieved by virtue of the fact that the
motors 3, which are short-circuit asynchronous motors with the same rated speed,
can be imparted different propelling forces. If the motors receive current
directly from a common alternating current mains source, the oscillation masses
10 (Figure 2) in the two units, which rotate in opposite directions, will be
synchronized and an elliptical thrust will be obtained. Through a transfornler
unit of a type well-known to the person skilled in the art (either stepwise or
continuously variable), the supply voltage to one of the motors can be reduced.
Since the two motors are loaded, they will have a certain lag in relation to the
synchronous r.p.m. Due to the difference in voltage, the driving force of one
of the motors will drop, and the lag in that motor will increase somewhat.
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(Other types of motors and other control mechanisms which are well-known to
the personskilledin the art are of course conceivable.)
It has been shown, quite surprisingly at first, that it is possible
to obtain with this simple method a displacement of the direction of the stroke,
up to about 45 in either direction, depending on which of the two motors has
its supply voltage reduced.
The theoretical explanation above is rather sketchy, but we did not
feel it necessary to have a more careful calculation of the dynamic relations.
We were satisfied with finding, by tests with models (regardless of the explan-
ation one wishes to give to the phenomena) that the inventive effect is
achieved if one proceeds as specified in the accompanying claims.
The advantages of the invention are apparent. In both a feeder anda sieve, the transport speed can be varied as needed. It is also possible to
compensate for uneven load.
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sy optimizing the throw angle it is also possible to obtain
an improved screening effect and control the screening so
that the screen meets the material, when falling down, in
such a way that the material and the screen have opposite
movements, thus achieving the best sorting effect and
avoiding plugging.
