Screening machine

In a screening machine with a three-dimensional screening motion, a screen carrier for at least one screen plate is provided. The screen carrier is movable relative to a housing from which it is supported by springs. The screen carrier is driven by an electromagnetic drive device. The amplitude of oscillation of the screen carrier should be accurately readable, on the one hand, and, on the other hand, made adjustable for comparative tests. For this purpose, between the housing and the screen carrier an inductive displacement pickup for determining the amplitude of oscillation of the screen carrier is provided. This pickup is connected to a control unit which represents the determined amplitude of oscillation on an optical display and/or controls the electromagnet drive device to obtain a predetermined amplitude of oscillation.

BACKGROUND OF THE INVENTION 
The invention relates to a screening machine with a three-dimensional 
screening motion, having a carrier for at least one screen plate, wherein 
the screen carrier that is movably arranged relative to a housing and is 
driven by an electromagnet is supported at the housing by spring means. 
A screening machine of this type is described in the company brochure 
"Retsch-Labor Siebmaschine, Vibro, 006/1985" (Retsch-Laboratory Screening 
machine, Vibro, 006/1985). In this screening machine the screen carrier 
includes a three-armed spider with its spider arms being supported on 
springs arranged at the housing so that it is movable relative to the 
housing. The springs, in a tangential path, are arranged in an inclined 
position relative to the vertical axis of the screening machine. An 
electromagnet arranged below the screen carrier, generates, by acting on 
the spider of the screen carrier, a vertical amplitude of oscillation and 
this vertical motion of the screen carrier is converted via the motion 
along the axis of the inclined springs into a three-dimensional motion of 
the screen plates carried by the screen carrier. 
The accuracy of a grain size analysis to be undertaken with a screening 
machine of the aforementioned kind is determined by adhering to or knowing 
the following parameters: correct mesh size of the screen plates according 
to DIN (German Industrial Standard); analysis time; frequency of the 
screen motion; amplitude of oscillation of the screen motion. In order to 
guarantee the comparability of various analyses these parameters must be 
defined for the analyses carried out and must be reproducible for further 
analyses in order to arrive at a reliable result of analysis. 
However, such a screening machine disadvantageously does not meet these 
requirements, in particular with respect to the determination of the 
amplitude of oscillation. The amplitude of oscillation may only be 
determined by means of a scale read according to the principle of optical 
illusion. For this purpose a triangle extending transverse to the 
oscillating axis is attached to the screen carrier, the sides of which, 
when the screen machine is in operation, apparently converge at one point. 
This point, on a correspondingly determined scale, should correspond to 
the dimensions of the amplitude of oscillation to be detected. It will be 
understood that this type of determining the amplitude of oscillation must 
be insufficient with regard to its accuracy, because the optical reading 
of a measuring point is dependent on the observation and the assessment of 
the operator. A further disadvantage is that with various assemblies of 
screen plates in the screening machine, a desired or previously adjustable 
amplitude of oscillation for each assembly is only obtainable by manually 
readjusting the driving energy of the electromagnet, but accordingly the 
desired measuring point cannot be adjusted with the desired accuracy due 
to the manual regulation of the electromagnetic drive device. 
The object of the present invention is therefore to improve the screening 
machine of the aforementioned kind, so that the amplitude of oscillation 
during an analysis procedure may be read with sufficient accuracy. A 
further aspect of the present invention is that a certain or predetermined 
amplitude of oscillation may be obtained during operation of the screening 
machine, irrespective of the selected assembly of screen plates in the 
screening machine. 
SUMMARY OF THE INVENTION 
According to the present invention a screening machine with a 
three-dimensional screening motion is provided, comprising: a screen 
carrier for at least one screen plate, whereby the screen carrier that is 
movable relative to a housing is supported by resilient means at the 
housing; an electromagnetic drive device for moving the screen carrier; an 
inductive displacement pickup between the housing and the screen carrier 
for determining the amplitude of oscillation of the screen carrier; and a 
control unit connected to the pickup for representing the determined 
amplitude of oscillation at an optical display and/or for controlling the 
electromagnetic drive device for obtaining a desired amplitude of 
oscillation. 
Thus the invention proceeds from the fundamental concept that arranged 
between the housing and the screen carrier an inductive displacement 
pickup for detecting the amplitude of oscillation of the screen carrier is 
provided, that is connected to a control unit which has an optical display 
for the representation of the established amplitude of oscillation and/or 
controls the required energy supply for the electromagnet drive device 
with respect to a given amplitude of oscillation. The advantage is that 
the amplitude of oscillation is measured physically accurately and is, by 
way of the correspondingly adjusted control unit, transformed into a 
readable representation, which may be directly read without requiring a 
subjectively influenced judgement of the measuring point by the operator. 
A further advantage results from the fact that the amplitude of 
oscillation can be predetermined, whereby the control unit in cooperation 
with the inductive displacement pickup controls the supply of energy 
required for the excitation of the drive electromagnet such that the 
predetermined amplitude of oscillation is accurately obtained. It is of 
particular advantage that this adjustment or input of an amplitude of 
oscillation can occur without consideration of the assembly of screen 
plates in the screening machine, since due to the greater weight of a 
larger assembly the driving energy for the motion of the screen carrier is 
still controlled until the predetermined amplitude of oscillation 
corresponds to the actual value measured by the inductive displacement 
pickup in each case. 
With a preferred exemplary embodiment a restraint guide, which is free from 
play, is provided for the screen carrier. With the restraint guide the 
motion of the screen carrier is controlled relative to the housing via the 
oscillatory path of the screen carrier. The advantage is that the vertical 
component of the three-dimensional screening motion is accurately defined 
as a gauge for the amplitude of oscillation to be determined and therefore 
may also be measured exactly via the inductive displacement pickup. 
Falsifications of the vertical oscillating component of the screening 
motion through overlapping oscillations within the three-dimensional 
screening motion, due to the effects of the spring guide, are thus 
avoided. 
According to a preferred exemplary embodiment of the invention the screen 
carrier is guided along a bolt arrangement, that is clamped fast with the 
housing and extends in the direction of the spring path, via a 
form-locking sliding guide, which is preferably a spherical roller sleeve 
arrangement, connected to the screen carrier and embracing the bolt 
arrangement.

DESCRIPTION OF PREFERRED EMBODIMENTS 
Referring to the FIGS., arranged in a housing 10 provided with feet 11, 
there is a base 12 that serves as the support of a drive device and a 
screen tower. The base 12 is supported on springs 13 in order to dampen 
the vibrations emanating from the drive device and/or the screen tower 
that would otherwise affect the housing 10. On the base 12 an 
electromagnet 14 is centrally positioned and serves as the drive device, 
which is provided with electrical energy by way of supply lines not 
represented. 
A screen carrier 15 is furthermore disposed on the base 12 such that it is 
positioned above the electromagnet 14 and spaced at a distance therefrom. 
The screen carrier 15 consists of a spider 16 arranged in the interior of 
the housing 10 and of a holder 17 for screen plates 40, disposed 
externally above the housing and penetrating the housing. The holder 17 is 
connected to the spider 16 by way of a screw 18. The holder 17 carries two 
vertically extending guide rods 19, on which are mounted the screen plates 
40 with respective openings and arranged above each other in the form of a 
tower. The assembly of the screen plates 40 for the screening machine is 
sealed by a clamping cover 20, which has openings 21 receiving the guide 
rods 19. On the free ends of the guide rods 19 toggles 22 are screwed in 
order to clamp the screen plates against each other and to the cover 20, 
so that during the analysis procedure a closed screen tower is provided. 
The spider 16 as part of the screen carrier 15 is guided with its three 
arms 23 on guides 24, arranged at the outer periphery of the base 12 and 
enclosing the electromagnet 14. The axes 25 of the guides 24 are 
tangentially inclined to the vertical axis of the screening machine (FIG. 
3). Each guide 24 consists of a bolt 26, anchored in an inclined position 
at the base 12, on which a spherical roller sleeve 27 is slidable with 
freedom of play. The sleeve is firmly connected to the associated arm 23 
of the spider 16. Above and below each spherical roller sleeve 27 there is 
arranged a compression spring 28, so that the motion of the spider arm 23 
in both directions of movement along the bolt 26 occurs against the force 
of both springs 28. 
At the housing 10 a carrier 29 is arranged fast with the housing. The 
carrier 29 extends below the screen plate holder 17, on which an inductive 
displacement pickup 30 is arranged such that it detects the vertical 
oscillatory movements of the screen carrier 15 respectively of its holder 
17. In the housing 10 there is located a control unit 31, which, on the 
one hand, is connected to the inductive displacement pickup 30 via a 
signal line 32 and, on the other hand, via line 34 to an optical display 
33, that is attached to the outside of the housing, as well as via a line 
35 to the supply of energy for the electromagnet 14. 
For an analysis to be carried out with the screening machine the 
electromagnet 14 is excited with a predetermined frequency, so that this 
electromagnet 14 excites the spider 16 of the screen carrier 15, arranged 
above it, to oscillate. This vertical oscillation is transformed via the 
guide of the spider arms 23 along the inclined bolt 26 into a 
three-dimensional screening motion. Because of the guiding restraint of 
the screen carrier 15 at the bolts 26 the vertical amplitude of 
oscillation of the screen carrier can be accurately detected, without 
overlapping oscillations occurring which could also be amplified by an 
irregular loading of the screening machine with the material being 
screened. This vertical amplitude of oscillation of the screen carrier is 
measured by the inductive displacement pickup 30, arranged fast with the 
housing, which transmits its signals via the signal line 32 to the control 
unit 31, in which the actual value for the amplitude of oscillation is 
detected and supplied via the signal line 34 to the optical display 33, so 
that in an analogous form the amplitude of oscillation measured according 
to the physical law can be read accurately by the operator. 
The control unit 31 is also connected via the control line 35 to the supply 
of energy for the electromagnet 14, so that an amplitude of oscillation 
may be input via the display 33 and the control unit 31, by comparing the 
predetermined amplitude of oscillation with the measured values detected 
by the inductive displacement pickup 30, can control the driving energy 
for the electromagnet 14 until, in dependence on the particular assembly 
of screen plates in the screening machine, the predetermined amplitude of 
oscillation is accurately obtained. 
In this way the analysis conditions may be established relative to the 
amplitude of oscillation without question, but also a reproducibility of 
the analysis conditions with the screening machine according to the 
invention is given. 
The present invention is, of course, in no way restricted to the specific 
disclosure of the specification and drawings, but also encompasses any 
modifications within the scope of the appended claims.