Automatic particle size analyzer using stacked sieves

The size ranges of particles separated on a stack of sieves of different mesh sizes is measured automatically. The stack is clamped together and shaken as a unit to separate the respective fractions, following which the sieves are sequentially separated from the stack and inverted one at a time to dump the fraction retained on each, onto a scale. Each fraction is weighed separately and their relative proportions can be calculated automatically. In a preferred embodiment the sieves are individually cantilevered from a vertical conveyor. The sieves are inverted one by one to dump their contents by advancing them around a horizontal roll at a lower end of the conveyor.

FIELD OF THE INVENTION 
This invention relates to an apparatus and method for automatically 
measuring the weights and/or relative proportions of the size ranges of 
particles in particulate mixtures. 
BACKGROUND OF THE INVENTION 
Particle size analyses, that is, measurements of the relative proportions 
by weight of particles of a given sample in different size (diameter) 
ranges, are widely used in process control and optimization. The size 
range of a given fraction may be characterized, for example, as being 
between 0.01 and 0.05 inch, which means that the particles in that range 
are retained on a screen having openings smaller than 0.01 inch but pass 
through a sieve having openings larger than 0.05 inch. Such analyses are 
frequently performed with sieves (screens) of progressively finer mesh 
sizes, such as the well-known U.S. Standard testing sieves. The sample to 
be analyzed is placed on the coarsest sieve at the top of a stack of 
sieves and the entire stack is shaken, particles of different size ranges 
being retained on different sieves. The sieves are then removed one by one 
from the stack and the fractions on them are emptied onto a scale and 
weighed to determine the proportion of the fraction relative to the total 
sample weight. If the analysis is carried out manually, as is often done, 
the procedure is slow and labor intensive. A mechanical shaking device 
such as the "Ro-tap" shaker made by W.S. Tyler, Inc., of Mentor, Ohio, can 
be used to apply standardized shaking schedules to the stack of sieves, 
but nevertheless each sieve must be taken manually from the stack, the 
retained fraction emptied from it and weighed, and the emptied sieves 
restacked in proper sequence for the next analysis. 
In many laboratories and manufacturing processes it is necessary to make 
particle size range analyses frequently and routinely. It may, for 
example, be desirable to monitor the proportion of "fines" (particles 
below some predetermined minimum size), or the proportion of coarse 
"overs"; or the relative distribution among size ranges may be important 
for process control. Because manual particle size range analyses are time 
consuming, in those applications where they must be performed frequently 
and routinely there exists a need for an automatic particle size analyzer 
which will separate the various size range fractions, and empty and weigh 
them individually with no or minimal manual control and manipulation. 
Any automatic particle size analysis using a graduated set of sieves 
necessarily requires separately weighing the particles retained on each 
sieve. Automatic apparatus for carrying out a separation, and weighing and 
calculating the fractions is known and is commercially available. The 
"Gradex" particle size analyzer, which is described in Marrs U.S. Pat. No. 
4,487,323 and which is made and sold by the assignee of this application, 
is one such analyzer. In that apparatus the sample to be analyzed is fed 
into a horizontal polygonal drum having sieves of progressively coarser 
mesh sizes on its side faces. The drum is first indexed or positioned 
rotationally so that the finest sieve is at the bottom, and the sample to 
be analyzed is deposited on that sieve. The entire drum is shaken, so that 
particles finer than the mesh of the sieve pass through and fall onto a 
weigh pan and are weighed automatically by an electronic scale. The drum 
is then indexed rotationally so that the next finest sieve is at the 
bottom; the particles retained on the first sieve fall onto the second 
sieve. The drum is again shaken and the particle fraction which can pass 
through the second sieve is thereby separated. The process of drum 
indexing, shaking, and weighing is continued automatically until the 
sample has been screened on each sieve. The fraction weights may be 
totaled by a computer and their relative percentages determined and 
displayed in a readout. 
The Gradex machine is expensive and comparatively slow by reason of the 
polygonal drum which must be indexed to and shaken at each rotational 
position. Moreover the analyses it provides do not always correlate 
directly with analyses made with a more conventional stack of standard 
screens. It has therefore been desirable to provide a machine which is 
comparatively simpler and faster, and which will provide accurate analyses 
using a standard stack of sieves rather than a polygonal drum, and which 
can be operated entirely automatically with virtually no operator 
attention. 
SUMMARY OF THE INVENTION 
Rather than a horizontal polygonal drum, the apparatus and method of this 
invention utilize a stack of sieves for making particle size analyses. The 
sample to be analyzed is introduced onto the topmost (coarsest) sieve of 
the stack and the entire stack is vibrated or shaken as a unit to separate 
the fractions retained on the respective sieves. At the bottom of the 
stack is a pan (which is referred to and treated herein as a "sieve" even 
though it is actually imperforate) which catches and holds the fines that 
have fallen through all the other sieves. The individual sieves are then 
automatically and sequentially removed from the stack and emptied onto a 
weighing apparatus which sequentially records the weight of each fraction, 
from which the weight percentage proportions of the respective fractions 
can be calculated. In a preferred embodiment, each sieve of the stack is 
separately mounted or cantilevered from a conveyor having a vertically 
oriented section or run and is movable around a horizontal bottom roll 
below the vertical run. The conveyor is advanced or indexed downwardly to 
move the sieves downwardly to swing the sieves sequentially around the 
bottom roll. Each sieve is inverted by its movement around the bottom 
roll, and dumps the retained particles onto an electronic weigh scale. The 
diameter of the bottom roll is such that a sieve moving around the bottom 
roll is tipped sufficiently to dump the particles retained on it before 
particles are dumped from the next sieve. The sieves are thereby emptied 
individually, and the various fractions are weighed separately. After all 
the sieves have been emptied, the conveyor reverses to return the empty 
sieves back around the roll, where they are again aligned as a stack to 
receive and analyze a subsequent sample. The entire operation is 
automatic. 
To insure complete emptying of each fraction from its sieve before 
weighing, the apparatus optionally but preferably includes an automatic 
sieve cleaner. The cleaner engages each sieve while it is inverted over 
the weighing pan, after most of the particles have fallen from it. The 
cleaner operates a brush or other cleaning device whereby remaining 
adherent particles are dislodged and emptied from the sieve for weighing 
with the rest of the fraction separated on it. 
Because the sieves are cantilever-mounted, they tend to tip or sag under 
gravity on the vertical run and thereby become cocked or disaligned with 
one another. This would prevent accurate fraction separation. The 
invention includes optional but preferred means for automatically 
temporarily clamping the sieves together, in alignment on a vertical axis, 
for shaking. The clamping means is released after shaking so that the 
sieves can be individually moved around the bottom roll. 
It is an advantage of the apparatus of the invention that it can utilize 
U.S. Standard, Tyler, or other sieves, in their pre-existing sizes and 
configurations. Many users are accustomed by long practice to performing 
particle size analyses on U.S. Standard or Tyler screens; frequently, 
production parameters are specified in size ranges as determined by such 
analyses. An analysis performed on a different type of analyzer, no matter 
however accurate, does not usually correlate identically with an analysis 
made with standard screens. In this invention, standard stacking sieves 
can be used in the apparatus to make the separations. Close correlation of 
an analysis provided by this invention with pre-existing stack screen 
analyses is thereby achieved.

DETAILED DESCRIPTION 
The preferred embodiment of the automatic particle size analyzer 10 which 
is shown in the drawings is housed in a cabinet 12 having a hinged door 
14. The apparatus utilizes a series or set of graduated sieves, seven in 
the embodiment shown, comprising sieves designated as 16a, b, c, d, e, and 
f, and an imperforate bottom pan 18 (see FIG. 7). Each sieve comprises a 
stackable cylindrical skirt 20 with axially spaced upper and lower 
peripheral flanges, and a screen or mesh 22 mounted inside skirt 20 (see 
FIGS. 2, 3 and 8). Typically, although not necessarily, the skirts are 
circular, about 8 inches in diameter and approximately 3 inches high, and 
may be conventional commercial screens. The skirts are of uniform diameter 
and the sieves may differ only in the size of the screens 22 which they 
mount. The respective sieves are progressively finer in the downward 
direction as viewed in FIG. 4, the topmost sieve 16a having the coarsest 
mesh. An application might for example utilize U.S. Standard testing 
sieves Nos. 30, 35, 40, 50, 70, and 120 as the sieves 16a-16f. The 
apparatus shown thus separates seven fractions (including the fines which 
are collected in bottom pan 18) which is sufficient for most analyses; if 
fewer fractions are needed, one or more sieves can be replaced with a 
dummy skirt having no sieve or a coarse sieve. 
Each sieve 16a-f and 18 is seated on and secured to a modular sieve holder 
or bracket 24 (see FIGS. 8 and 9). The bracket comprises a flat plate 
having a center opening which is sized to receive the lower portion of 
sieve skirt 22. Each sieve is removably secured in its holder 24, by a 
pair of swingable, spring loaded retainers 26, only one of which is shown. 
The holders 24 are cantilevered (mounted at one side only) to a conveyor 28 
preferably in the form of a wide, endless belt as shown in FIG. 3 of the 
type ordinarily used for horizontal conveyors. Each sieve holder 24 is 
secured by one or more bolts 34 through the conveyor (see FIG. 8). 
Conveyor 28 passes around an upper roll 30 and a lower or bottom roll 32 
(FIG. 3). The region traversed between upper roll 30 and lower roll 32 is 
referred to herein as vertical run 33. A back run 35 extends parallel to 
run 33 on the other side of the rolls. As will be explained, when all the 
holders are on the vertical run portion 33 of the conveyor, the sieves and 
holders are nestable to form a vertically aligned stack 31 (see FIG. 4) in 
which each sieve 16b-f and 18 seats against the bottom of the holder of a 
sieve above it. This stack can be shaken as a unit to make the separation; 
particles fall from one sieve directly into the next sieve below it and 
cannot escape laterally from the stack. 
Lower roll 32 is mounted for rotation in journals secured to cabinet 12. 
Upper roll 30 is journaled on an upper roll support arm 36 which is 
pivoted to the cabinet 12 by pivot 38. Spring means 40, which may be a 
coil spring as shown or a selectively operable air spring, biases support 
arm 36 upwardly about pivot 36 to maintain tension on conveyor 28. 
Conveyor drive means in the form of an electric motor 41 with a speed 
reducer and brake is mounted on support arm 36 and is connected to turn 
upper roll 30 by a timing belt 42. Tension on belt 42 is maintained by an 
adjusting screw 44 (see FIG. 2). 
Cabinet 12 includes a top portion 52 which can be removed from a lower 
portion 53 for access to the top of the operating mechanism. The sample to 
be analyzed is introduced through an opening 46 in top portion 52 and 
falls through a funnel 48 which in turn leads to a tubular chute 50 (FIG. 
4). Funnel 48 separates from chute 50 if the cabinet top portion is 
removed. Chute 50 is mounted on a shaker top plate or mounting plate 54 
which in turn is supported at one side by and on two parallel vertical 
leaf springs 56, 56 which project upwardly from bracket 58 in the cabinet. 
Leaf springs 56, 56, which may be resiliently flexible fiberglass strips, 
support top plate 54 and the sieves and other structure supported from it 
for vibration (FIG. 2). 
The Shaking Means 
Screening motion is imparted to the stack of sieves by drive means 
indicated generally by 60, see FIGS. 2, 4 and 7. In the embodiment shown 
drive means 60 includes a motor 62 on an adjustable bracket which in turn 
is mounted to the cabinet wall. A screw adjuster 69 bears between the 
bracket and the cabinet wall to tension the belt (FIG. 4). Motor 62 is 
connected by a timing belt 64 to turn an eccentric pin 66, which is 
journaled in and supports top plate 54 at the side thereof opposite leaf 
springs 56. Operation of motor 62 turns eccentric pin 66 in a circular 
orbit and thereby imparts a screening motion to plate 54 and the sieves 
suspended from it. The pin-engaging side of plate 54 (the right side as 
seen in FIG. 4) is moved in a circular orbit; the other side is 
constrained by the leaf springs 56, 56 to move in a more linear path. 
Eccentric pin 66 may, for example, move in an orbit of 1-3/16 inch 
diameter at a rate of 280 rpm, and thereby generate a lateral sieve 
acceleration of 1.3 g. Shaking cycle times in the range of 3 to 10 minutes 
are sufficient for many purposes. 
The motion just described is preferred because it approximates that used in 
"Ro-tap" machines. However, it is pointed out that other screening motions 
may be used; the invention does not require the use of a particular 
gyratory, vibratory or other movement, provided the movement is sufficient 
to separate the particles on the various sieves. (The term "shaking" as 
used herein is meant to include all such types of screening movement, 
whether or not in the plane of the screen.) Optionally, a "tapper" or 
vertically reciprocating piston 67 (FIG. 4) may be mounted to top plate 
54, to apply a repetitive "tapping" pulse or impact to the stack of sieves 
during shaking. The tapper is an air cylinder which is rapidly 
reciprocated to strike the plate, for instance at 200 cycles per minute. 
This assists in separating the particles and further simulates the tapping 
movement that is applied in mechanical shakers. 
The Stack Leveling and Clamping Means 
Because sieves 16a-f and 18 are individually cantilevered from conveyor 28 
and are supported by it only at one side, they tend to sag downward under 
gravity if not further supported (note the tilt of the sieves on the right 
side of the conveyor in FIG. 6). Such sagging would be disadvantageous 
during shaking, because the central vertical axes of the individual sieves 
would be disaligned from one another and gaps could open between adjacent 
sieves and holders through which particles being screened could escape 
over the rims of the skirts 20. It has been found highly effective to 
provide means which position the sieves horizontally when they are to be 
shaken, and additionally to clamp them together in vertical alignment for 
shaking so that there is minimal or essentially no relative motion between 
the sieves during shaking. The sieves need not, however, be clamped 
together during the time that conveyor 28 is moving them, and they must of 
course be free to separate as they move about bottom roll 32. For this 
purpose there is provided a sieve holding back plate 68, shown in FIGS. 2, 
3 and 4, which is mounted vertically from top plate 54 and which presents 
stop ledges that coact with a series of stop pins 70 on the respective 
sieve holders 24. Pins 70 extend from the sieve holders, through conveyor 
28 (see FIG. 4) toward plate 68. The pins are simultaneously engageable 
against a series of sequentially offset stop ledges 72 in holding plate 68 
as the sieves approach their topmost positions on vertical run 33. The 
stop ledges 72 are offset laterally like "stairs," along a side edge of 
plate 68 and are spaced apart vertically according to the distance between 
the respective pins 70 (see FIG. 3) so that as the conveyor moves 
upwardly, the pins engage the respective ledges. As conveyor 28 moves 
upwardly around rolls 30, 32 (counterclockwise as seen in FIG. 3) the stop 
pins, which project angularly upwardly by reason of the downward tilt of 
the respective holders, are moved upwardly with the conveyor. The pins 
essentially simultaneously engage the respective stop ledges, which 
arrests their further upward movement; short final upward travel of the 
conveyor thereby tilts the holders upwardly (counterclockwise in FIG. 5) 
from their sagged positions to the substantially horizontal positions 
shown in FIG. 4. Conveyor drive motor control means 98 or a limit safety 
switch 73 (FIG. 4), stops operation of motor 41 at the conveyor position 
at which the sieves are substantially horizontal. It is important that the 
sieves be level for shaking (for the same reason, cabinet 12 may have 
leveling screws 71 at its base to level the cabinet itself). 
The sieves are then aligned laterally with one another and are clamped 
axially (vertically) for shaking. This alignment and clamping is 
preferably provided by double-functioning sieve aligning and clamping 
means, generally designated by 74, best shown in FIG. 7. The means 74 
operates to press opposed V-sectioned lateral clamps 76, 76 diametrically 
toward the sieves, into engagement with corresponding V-shaped notches 78, 
78 on opposite sides of the sieve holders 24. The clamps 76, 76 move 
toward one another in a vertical plane, generally parallel to the plane of 
vertical run 33, to cam the sieve holders laterally (horizontally) into 
alignment with one another. 
As indicated above, the double-acting means 74 also clamps the sieves 
together vertically as well as aligning them horizontally. For this 
purpose the means 74 also operates diametrically opposed vertically 
swingable clamp arms 80, 80 to apply a lifting force to the bottom holder 
of the stack (see FIG. 7). When actuated, the vertical clamping arms 80 
lift the entire stack of sieves upwardly until the top sieve 16a abuts and 
is clamped against the underside of top plate 54 which acts as a stop (see 
FIG. 7). Thus clamping means 74 constrains the stack of sieves both 
laterally and vertically. 
The two sets of lateral and vertical clamp arms 76, 80 are preferably 
operated by double acting clamp cylinders 82, 82. At an upper end, each 
clamp cylinder 82 is pivotally connected to an upper clamp swing arm 84, 
which at an outer end is connected by a pivot 85 to a clamp means mounting 
bracket 86 carried from top plate 54 (see FIG. 2). The other end of upper 
clamp swing arm 84 is pivotally secured to lateral clamping arm 76. 
Cylinder 82 operates a piston rod 88, the lower end of which is connected 
to swing the vertical clamp arm 80 about its pivot 90 (see FIG. 7). Clamp 
arm 80 is pivoted to a bracket on side plate 92. Side plate 92 is 
connected to top plate 54, as is bracket 86. Extension of piston 88 from 
its cylinder 82 rotates clamp arm 80 about its pivot 90 and brings roller 
94 into lifting engagement beneath the bottom holder 24 (see FIG. 7). It 
can be seen that in operation cylinder 82 both causes the lateral clamping 
arm 76 to be moved in a horizontal direction and the roller 94 to be moved 
upwardly. The mechanism operates both the lateral clamp arms and the 
vertical arms, moving each until stopping resistance is encountered. 
Control means 98 causes pressure fluid to be supplied to the clamp piston 
82 when the sieves are at their upper positions and after they have been 
brought into horizontal position by engagement of their respective stop 
pins 70 with the stop ledges 72. Specifically, when fluid pressure 
(pneumatic pressure is preferred) is supplied into cylinder 82, piston rod 
88 is extended, which swings both the upper clamp swing arm 84 and the 
vertical clamp arm 80 about their respective pivots. Referring to the 
right cylinder 82 in FIG. 7, its upper clamp swing arm 84 swings in a 
clockwise direction about its pivot 85, as indicated by the arrow 87, 
thereby moving lateral clamp arm 76 to the left, into the notches 78 of 
the sieve holders 24. At the same time, extension of piston 88 swings 
vertical clamping arm 80 clockwise about its pivot 90, bringing the roller 
94 thereof upwardly against the bottom of the stack. To maintain 
parallelism and avoid cocking of the clamp arm 76, a lower clamping arm 
link 96 is pivotally connected between the lower end of lateral clamping 
arm 76 and a mounting bracket on side plate 92. The two arms 84, 96 
establish a parallelogram-type movement which insures that clamp arm 76 
remains vertical as it is moved laterally (FIG. 7). The sieves remain 
clamped only during the shaking cycle. (Because the clamping means is 
suspended from top plate 54, it moves with the sieves during the shaking 
cycle.) 
The clamping means on the opposite side of the stack may be a mirror image 
of that just described and operates in a similar manner. 
Sieve Emptying 
At the completion of the shaking cycle, the control means 98 directs fluid 
pressure in the opposite direction to retract piston arms 88, and thereby 
essentially simultaneously disengages the lateral clamping arms and 
vertical clamping arms from the stack of sieves. Drive motor 40 is then 
energized to move the stack of sieves slowly downwardly toward lower roll 
32. Movement is gradual, at a rate that does not throw particles off the 
respective sieves, for example 6.5 feet per per minute. As downward 
movement continues, bottom pan 18 is the first to move around roll 32. 
Because of the small diameter of roll 32 in relation to the larger size of 
the sieves, relatively short conveyor travel achieves a large angular 
swing of the respective sieve through an essentially vertical position, to 
an inverted or dump position such that the particles in that sieve fall 
out of it. By this means the sieve is inverted before the next sieve 
starts to empty (see FIG. 5). It is preferred that each sieve be stopped 
in a position in which it is angulated at about 120.degree. to 140.degree. 
with respect to its horizontal stacked position. Stopping is controlled by 
a switch 101 (FIG. 7), which de-energizes motor 40 when the sieve holder, 
at the dump position, engages the switch. The motor brake promptly stops 
and holds the belt with the sieve in the dump position. As it is inverted 
by movement around the bottom roll 32, each sieve dumps its contents into 
a weigh pan 100 (see FIG. 5). Weigh pan 100 rests on an electronic weigh 
scale 102 which provides a readout of the weight of particles discharged 
into it from each pan. Scales suitable for this purpose are commercially 
available, for example Toledo Scale Corporation Model SM 6000. The scale 
may reset or "zeroize" after recording the weight of the fraction; or 
preferably the computer 99 records the successively increasing weights in 
the pan and obtains the individual fraction weights by sequential 
subtraction, in known manner. 
Pan 100 can be removed from scale 102 through door 14, for emptying. It is 
not necessary to empty the pan after each sieve has been dumped into it, 
or even after an entire sample analysis has been completed. An analysis 
usually requires samples of only a few hundred grams; pan 100 may be sized 
to hold many such samples. 
The Sieve Cleaning Means 
Although most of the particles retained on a sieve will fall from it as it 
is inverted, nevertheless some particles may adhere to or be lodged in its 
screen 22, especially particles whose size closely approximates the size 
of the mesh openings. For this purpose it is desirable to provide sieve 
cleaning means to brush or knock particles from each sieve (preferably 
including bottom pan 18) while it is inverted over the weigh pan 100 so 
that such particles can be included in the weight of the respective 
fraction. 
The sieve cleaning means designated generally by 104 basically comprises a 
rotary brush 106 which is automatically moved from an inactive position 
shown in FIG. 5, into a cleaning position shown in FIGS. 6 and 8 in which 
the brush brushes the bottom (lower) side of the mesh of a sieve in the 
dump position. Brush 106 is dimensioned to engage substantially the entire 
area of mesh 22, to brush particles from the mesh so that they will fall 
into weigh pan 100 (see FIG. 8). Brush 106 is rotated by a brush drive 
motor 108, which is automatically energized at the appropriate time by 
control 98. The brush 106 is operated to brush the screen preferably by 
rotation in both directions; the cleaning means is removed to its inactive 
position before the conveyor is operated to move the just-cleaned sieve 
from the dump position and the next sieve into that position. 
For movement of the brush and motor between the inactive position and the 
cleaning position, they are mounted on a cleaner swing arm 110 which is 
journaled to the cabinet base at pivot 112. The swing arm 110 is turned 
about pivot 112 by dual pneumatic cleaner positioning cylinders 114, one 
of which is shown in FIG. 5. When extended the piston of cylinder 114 
positions the cleaning means in the inactive position; when retracted 
(FIG. 6) the piston swings arm 110 to bring brush 106 into approximate 
planarity with the mesh 22 of the sieve in the dump position. As shown in 
FIG. 8, brush 106 is yieldably mounted on motor shaft 115 and is biased 
outward (toward the sieve) by springs 116. Some movability of brush 106 on 
shaft 115 is provided by a pivot-in-slot connection 117. This provides a 
certain amount of yieldability and flexibility so that the brush will be 
brought more gently into contact and alignment with the screen 22. 
I have found it desirable that each sieve be held rigidly while it is being 
cleaned so that it does not move away from the brush. For that purpose 
cleaner clamping means 118 is provided to engage the respective sieve 
holder 24 in the dump position and pull the holder and sieve in a 
direction toward the brush, as indicated by arrow 119 in FIG. 8. The 
cleaner clamping means has a clamp pad 122 which is operated by a 
pneumatic cylinder 120 mounted on and moved with arm 110. Clamp pad 122 is 
extended while the respective sieve is moving toward dump position so that 
holder 24 and its sieve will clear the extended clamp pad as the holder is 
swinging around roll 32. When retracted, pad 122 engages the edge of the 
holder (FIG. 8) and pulls the holder and sieve toward the brush for 
cleaning, against a stop 121 which is mounted by and moves with cleaner 
swinging arm 110. (It is not necessary to brush bottom pan 18 since it has 
no screen, but it is preferred that the brush at least strike the pan to 
knock loose any residual particles.) 
Weighing of the fraction is delayed until the respective sieve has been 
cleaned so that the cleared particles will be included in the weight. 
After cleaning, cleaning means 104 is retracted and conveyor 28 is operated 
to move the emptied sieve past the dump position and to advance the next 
sieve to that position. Just as the sieves tend to tilt downwardly on run 
33, they again tend to tilt when they are on back run 35 after they have 
been emptied. The lowermost tilted sieve could interfere with movement of 
cleaning means 104. To provide clearance the inverted emptied sieves are 
lifted during cleaning by a back run sieve lifter 126 (FIGS. 2, 4, and 6). 
This comprises a pneumatic cylinder 128 which is swingably suspended in 
cabinet 12, and which operates a pivoted lifting arm 130 that is lifted to 
engage beneath the lowermost sieve on back run 35, to pull the sieves 
upward sufficiently for brush 106 to pass beneath them. The lifting 
movement is illustrated in FIGS. 2, 5 and 6. 
The sequence of individual sieve emptying, cleaning, fraction weighing, and 
movement up back run 35 continues until the entire stack of sieves has 
been moved from the starting or shaking position shown in FIG. 4, in which 
all are on vertical run 33, to a finish position at which all have been 
emptied and are inverted on the back run 35. A safety switch 124 (FIG. 4) 
is desirable as a failsafe, to limit conveyor movement up the back run 
after all the sieves have been cleaned. Computer 99 (FIG. 1) is programmed 
to count the sieves as they are cleaned, and signals control 98 to reverse 
the motor after all have been cleaned, thereby to return the empty sieves 
to starting position. (The sieves need not be cleaned on the return.) 
Control 98 acts as a power relay, to control the application and cut-off 
of operating power to the various motors and solenoid operated valves. It 
is responsive to low power signals from computer 99 and the limit switches 
.