Conveyors: power-driven – Conveyor section – Screw
Reexamination Certificate
1999-06-11
2001-09-04
Ellis, Christopher P. (Department: 3651)
Conveyors: power-driven
Conveyor section
Screw
Reexamination Certificate
active
06283275
ABSTRACT:
BACKGROUND
1. Field of the Invention
This invention relates to a novel screw construction having both a shafted portion and a shaftless portion. The shafted portion includes a torque limiter tube and tension rod in the center of the spiral of a vertical or steeply inclined shaftless screw conveyor for conveying wastewater treatment solids and other types of wet and/or sticky solids. This invention also relates to an improved vertical or steep inclined shaftless screw conveyor by increasing the permissible capacity and permitting longer conveyor lengths using the novel screw construction.
2. State of the Art
Solids can be conveyed vertically, up steep inclines, with both conventional shafted screw conveyors and standard shaftless screw conveyors. However, vertical or steeply inclined conveying (greater than 60-degrees from the horizontal) presents problems for both technologies (types of screws). Conventional shafted screw conveyors require high rotational speeds to convey the material; the result of high speed is increased shear, and wet solids material sometimes becomes thixotropic (i.e., thickens) and/or sticks to the screw, which decreases system capacity. The spiral within standard shaftless screw conveyors used in vertical or steep incline situations can easily become unevenly loaded, causing stresses on portions of the screw greater than the safe design load of the screw metal material and thus deforming (and ruining) the screw.
More particularly, it has been found that when the frictional forces between the auger and product (P
pa
) are significantly less than the frictional forces between the tube and product (F
pt
), the product will convey. Thus, when the relative friction (F
r
=F
pa
/F
pt
) between the auger and the tube is significantly less than one (1), the product will convey vertically. Conversely when the relative friction (F
r
) is close to or equal to one (1) the product will not convey vertically.
The relative frictional force, F
r
, has been found to be approximately proportional to the ratio of the surface area of the auger (S
a
) (flights and center pipe to which it is welded) and the surface area of the tubular section (S
t
), F
r
≈S
a
/S
t
. If S
a
≧S
t
then F
pa
≧F
pt
and the product will rotate with the auger and will not convey upwards. The relative frictional forces, F
r
, between the auger and tubular section is therefore approximately proportional to the relative surface area between the auger Sa (flights and center pipe) and tube S
t
(i.e., F
r
≈S
a
/S
t
).
To convey vertically or on a high incline with a conventional screw or auger, higher rotational speeds of 40 to 100 RPM (revolutions per minute) and greater are used to create a greater centrifugal force, which “throws” material outward against the conveying tube and reduces the dynamic forces of friction between the auger and the product (F
pa
), but not the frictional forces between the product and the tube (F
t
). Under higher rotational speed F
pa
becomes significantly less than F
pt
, thereby reducing the relative frictional forces (F
r
) between the product/auger and product/tube.
Convention Shafted Screws
To convey wastewater solids (and/or other wet and/or sticky solids) with conventional shafted screw conveyors, the frictional force between the shafted screw and the wastewater solids must be significantly less than the frictional force between the conveyance tube (in which the screw conveyor is housed) and the conveyed solids. This condition permits the shafted screw to rotate through the conveyed solids moving the material up on the flights of the shafted screw. In situations when this frictional condition does not exist, the conveyed solids rotate with the screw (i.e., friction between the solids and the screw is high) and so the solids are not moved upward. Therefore, when the ratio between these two frictional forces is significantly less than one, the material can be conveyed vertically. This ratio is termed herein Relative Friction. Meeks (U.S. Pat. No. 1,906,395) shows a conventional shafted screw conveyor.
To achieve a Relative Friction which is significantly less than one, either the ratio between (i) the shafted screw and conveyed solids and (ii) the conveyance tube and the conveyed solids must be less than one and minimized to the greatest extent possible, or the rotational speed of the shafted screw must be increased. The physical construction of shafted screws does not permit the minimization of the surface area ratios and therefore the speed of the shafted screw must be increased. However, the increase in speed of the shafted screw results with the material conveyed changing its rheologic characteristics as it is overworked by the rotating screw (e.g., the material to be conveyed is dilatant or rheopectic). Such types of conveyed solids result in increased friction between the shafted screw and the material as the screw is rotated, significantly reduce the capacity of the system, can prevent conveyance totally, and/or are undesireable for their intended end use.
The center pipe around which the flights are attached in a conventional auger (screw) performs a stabilizing and strengthening function for the auger. The center pipe transmits the total product weight being conveyed (W) plus the axial friction load (L
a
) and axial shear stresses (A
s
) from the flights and evenly distributes these loads throughout the length of the auger to the drive shaft. The total force being transmitted is defined by (F
was
)=(W)+A
s
+L
a
. The axial load (A
s
) and axial shear stress (L
a
) are a functions of and proportional to the frictional forces (F
pa
) and (F
t
) and the gravitational load or weight (W) of the spiral auger and product. The center shaft in a conventional screw prevents over-stressing through stretching (elongation) and distortion of the spiral flights. Small thickness (thin) flights are possible because of the center shaft of a conventional shafted screw conveyor.
Standard Shaftless Screws
Shaftless screw conveyors, which use centerless (shaftless) spirals to convey wet and/or sticky (e.g., wastewater) solids, are subject to three main stresses: stress due to torsional resistance; stress due to frictional resistance; and stress due to axial load of the material being conveyed. These stresses are all directly proportional to the angle of the conveyor which, as it increases to a steep incline or vertical conveyor, results in disproportionally high stresses at specific locations along the length of the spiral. These high stresses can result in spiral failure through elongation or breakage. Somers (U.S. Pat. No. 3,802,551) and Bruke (EP 333 682 A1) provide examples of shaftless screws.
Shaftless screws, augers without center pipes, have been used to reduce the surface area of the auger, S
a
, relative to the surface area of the tube, S
t
, thereby reducing F
r
as described above. The reduced F
r
has permitted lower rotational speeds, e.g., 20 RPM, thereby reducing significantly the degradation of the product.
A loading screw or push screw to feed product into the vertical screw is used as a necessary part of vertical or steep incline conveying. The product is then forced into the vertical screw forming a “plug” that compacts the product throughout the centerless void of the shaftless screw. The product remains compacted throughout the full length. This compaction force (Cf) results in an increase in both frictional forces F
pa
and F
pt
(and A
s
and L
a
). The continued feeding of product, and plug forming due to centerless compaction and the upward pull of the shaftless spiral auger, propels the product upward.
Shaftless screws, by their very nature, have no center pipe to evenly distribute and transmit axial shear stresses (A
s
), axial loads (L
a
), product, and auger weight (W
pa
) along the full length of the auger. Both A
s
and L
a
loads are increased by the degree of compaction. The full weight, axial load, and shear stresses (W
pa
L
a
+A
s
=F
was
) with a shaftless auger are transmitted successively along
Hoogendonk Pieter
Morris C. Edward
American Bulk Conveying Syst.
Ellis Christopher P.
Hopgood, Calimafde Judlowe & Mondolino
Tran Khoi H.
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