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Nov. 13, 1962
I 3,063,379
Filed Feb. 23, 1960
F G. Z
United States Patent
Patented Nov. 13., 1962
In these equations:
i1 is the number of threads of the middle screw
i2 is the number of threads of the side screws
61 is the thread top angle of the middle screw (in degrees)
S1 is the thread lead of the middle screw
Carl Oscar Torsten Montelius, Stockholm, Sweden, as
signor to De Laval Steam Turbine Company, Trenton,
N.J., a corporation of New Jersey
Filed Feb. 23, 1960, Ser. No. 10,398
Claims priority, application Sweden Feb. 23, 1959
\//1 is the half angle of intersection (in degrees) of the
bores of the casing as seen from the center of the
middle screw.
It is of interest to note that when these equations are
1 Claim. (Cl. 103-128)
The present invention relates to an improvement in 10
used, the length of the screws will not be equal to the
screw pumps of the type which comprises an assemblage
length of the casing. The difference between the two
of screws including a middle screw with convex thread
lengths will be equally apportioned to the ends of the
?anks and one or more side-screws meshing with the
screw assemblage.
middle screw and having concave thread ?anks, the
threads being of such con?guration that they seal against
The “length” of the screws in this
15 connection refers to the e?ective length of the threads,
the circumference of cooperating screws, all said screws
being sealingly enclosed in a casing formed with inter
secting bores for the screws.
In the operation of such a pump, the liquid being
pumped is progressed axially from the inlet end or suc 20
i.e. the length in which they mesh with each other. The
“length” of the casing refers to the portion thereof which
sealingly surrounds the screws.
If the pump is to operate at a higher pressure, it is
necessary to increase the lengths of the screws and
the casing, so that there are more seals between the inlet
and outlet ends. In such cases, the lengths can be se
lected so that the number of seals is constant during
formed by the thread ?anks and bottoms of the screws and
the operation of the screws. This can be achieved in
the enclosing casing. Such a chamber is formed at‘ the
suction side of the screw assemblage when thescrews 25 diiferent ways.
tion side of the screw assemblage to the outlet end or
pressure side thereof in essentially closed chambers
It the number of seals in each thread groove is to
be constant, the lengths of the casing and the screws will
be increased by an amount equal to
are rotated and is ?lled with liquid while it is formed.
When the chamber is fully formed, it is closed toward
the suction side and on continued rotation of the screws
it travels axially along the screw assemblage toward the
outlet end of the screw assemblage where it is opened
and the liquid is discharged. The screw assemblage may
be such that a chamber starts to open toward the outlet ; . where i, is the number of seals desired.
(Here, as well
as in connection with ‘Equations V and VI below, the
expression “thread groove” relates to the thread grooves
momentarily a fully closed chamber ‘between the inlet 35 of the screw having the greatest number of threads.)
end as soon as it is closed toward the inlet end. This
can be considered as a limit case whereinthe're is only
' ‘Under certain circumstances, however, it may be ad
end and the outlet end. In such a ca'se,‘ho'wever, the
inlet end and the outlet end are‘ always’separated' by a
closure or seal formed by the screw threads. .x The volume
vantageous to permit di?erent numbers of seals between
the inlet and the outlet end in the various thread grooves,
the sum of the number of seals in the various thread
of the chamber is unchanged while it progresses from
the inlet to the outlet end, and provided that. the pump
grooves being constant for all angular positions of'the
is ideally tight, the liquid in the chamber is throughout
this travel subjected 'to' the pressure prevailing at the
inlet end to be subjected to the higher pressure prevailing
at the outlet end only when the chambervis opened at
a certain amount of leakage will occur on account of
in, is the greatest number of seals in any thread groove
The amount by which the casing and screw
lengths will be increased will then be equal to
the outlet end. In an actual pump of this type,-of course, 45
unavoidable tolerances in the manufacture which re
sult in a certain amount of play as between the screws
and as between each screwpand the casing, wherefore a
in any position of the screws
m is the number of thread grooves having im seals in
during travel along the screw assemblage. - With a su?i
1'12 is the greatest of i1 and i2
small increase in the pressure- on the liquid will occur 50
such position
cient accuracy in manufacture, however, this increase in
pressure will be so small that substantially the entire "I‘ In this case the total number of seals will be
increase in pressure occurs'at the outlet end.
, ,
If the screws and the casing have a certain minimum 55
length, the above limit case will be obtained, wherein
there is constantly a single seal between thelinlet end
and the outlet end.
This minimumf-length is determined in the following
in other ways, so that the above equations are not satis
?ed, then a varying number of seals will be obtained in
different angular positions of the screws. ‘Such lengths
manner: The minimum length (Ls) of the screws to 60 mayv be advantageous from other points of view, e.g. to
achieve sufficiently large bearing surfaces of the screws.
obtain only one seal between the suction side and the
The seals between the pressure side and the suction
pressure side is the highest of the values determined by
are of di?erent types. There are seals between the
the two equations
peripheral surfaces of the screws and the casing, and
65 there are seals between the peripheral surfaces and cores
of the screws and between the peripheral surfaces and
thread ?anks. However, as mentioned above when there
is more than one seal, in cases where for some reason or
and the minimum length of the casing is determined by
the equation
another the screw length is selected so that the above
70 Equations IV, V and VI are not satis?ed, the number of
seals varies with the angular positions of the screws
during each revolution of the middle screw. As above
mentioned, on account of unavoidable plays the seals
cannot be made perfect in practise, but an internal leak
age must always occur. When the number of seals be
tween the pressure side and the suction side varies
during each revolution, this means that the internal leak
age will also vary during each revolution. Since a posi
tive pump of this type gives a capacity which is equal to
a. theoretical quantity per revolution reduced by the in
ternal leakage, it will be understood that the rate of flow
through the pump will ?uctuate or pulsate slightly due 10
bearing surfaces on the middle screw at opposite sides
of the groove. The distance (x in FIGURE 1) from one
end of the middle screw shall be equal to the minimum
screw pump of the type referred to which has a non
by Equation I or II above. The distance x may be taken
from either end of the screws.
If the groove is disposed at either end of the screw, the
‘to this variation of the internal leakage. It is desirable,
however, to obtain a ?ow which is free'from pulsation.
The object of the present invention is to provide a
The groove 14 may be disposed at either end of (the
threaded portion of) the middle screw, but this gives
substantially the same result»v as if the length of the screw
had been selected according to the above formula for
obtaining a constant number of seals. It is more advan
tageous to provide the groove intermediate the ends of the
threaded portion of the screw, so as to obtain satisfactory
length LS for obtaining constantly one seal, as determined
pulsating ?ow by keeping the number of seals constant 15 width (W) thereof will-be equal to
for each revolution. It has been found that this can be
achieved by providing a shallow circumferential groove
in any thread of the middle screw extending from one
flank to the other of such thread in such a manner that
one seal between the suction side and the pressure side 20 L—_—the actual effective length of the screws
is destroyed in certain angular positions of the revolu
Ls==the minimum length of the screws to obtain one seal
tion of the screw.
(according to Equation I or II‘)
The invention will be described more in detail with
n=a positive integer selected‘ so that O§W1<S1/i1
reference to the accompanying drawings, wherein:
vFIGURE 1 shows an embodiment of the invention in 25 The case, where n can be selected so that W1 is equal to
0 occurs when the length of the screws is such as to corre
elevation, partly in section;
spond to a constant number of seals in which case no
FIGURE 2 shows a cross-section taken along line
groove is necessary. This case is included above for the
sake of completeness to facilitate the understanding of the
The pump illustrated on the drawings is of the type
comprising a driven middle screw 1 and two side-screws 30 following.
However, when the groove is disposed intermediate the
2, 3, the middle screw 1 having two threads with convex
ends of the screw, as is preferred, the width W must be
?anks and the side screws also having two threads each
increased by an amount equal to
but with concave ?anks and of oppposite hand to the
threads of the middle screw, the threads being formed
in: a manner known per se so as to be in sealing rela 35
tionship with each other. The screw assemblage com
prising the middle screw 1 and the side-screws 2, 3 is
enclosed in a casing 4 which sealingly surrounds the
screw assemblage, said casing having at its lower end
two symmetrical ports 5 through which the entrance of 4-0
?uid to the screw assemblage takes place, the fluid being
discharged through the open top end of the casing. The
casing 4 hasv an attachment ?ange 6 for attaching it by
as compared to W1. The reason therefor is that in the
contacting surfaces between‘ the middle and side screws,
the threads of the screws engage each other in an axial
direction. Thus, in this case the width will be equal to
means of screws 7 in a pump housing 8 so that the
lower end of the casing 4 with the ports 5 is disposed
in the inlet chamber 9 of the pump housing, and the 45
upper end is disposed in the discharge chamber 10 of
On account of the axial engagement of the threads with
the pump housing.
each‘ other, as- mentioned above, it will be necessary, even
The middle screw 1 is driven by a motor (not shown)
when the groove is‘ to be disposed intermediate the ends
through a driving shaft 11 which is journaled in a bearing
of the screws, to determine whether WI will be 0, in which
12 attached to the top end of the casing 4 and extends 50 case the length‘ of the screws is such as to correspond to a
through a' cover -13 attached to the pump housing.
As best seen in FIGURE 2, the casing is formed with
a larger central bore for the middle screw 1 and two
smaller bores for the side-screws 2, 3, each of the two
latter bores intersecting with the ?rst bore.
constant number of seals and the groove will be unneces
sary. If W1>0, the width of the groove will be deter
mined according to Equation VIII.
For screws having a‘ screw length L such that
The screw pump shown in FIGURES 1 and 2 rep
resents a type which is common in practice and is -
(in which case‘ Equation VII above gives a negative value
dimensioned so that the effective‘ length of the casing as
for W1), the width can be chosen freely, but should be
well as the effective length of the screws (the distance
chosen as small as possible (although of course not so
L in FIGURE 1) is equal to 3D, where D is the ex‘ 60 small that the desired function of the groove is not
ternal diameter of the middle screw. This represents
achieved) in order that the bearing surfaces shall be as.
only an example and is not intended to limit the inven
large as possible‘.
tion. In a screw assemblage formed in this manner it
The angles l//1, and 01 referred to in connection with the
will be found that during a part of each revolution of
the middle screw there is one seal between the suction 65 Equations I, II and III above are indicated in FIGURE 2.
It will be understood that the invention is not restricted
side and the pressure side, while during another part
the embodiment shown comprising a driven two
there are two seals. The leakage will therefore vary
threaded middle screw and two two-threaded side-screws,
during each revolution so that a pulsating ?ow is ob
but is generally applicable to other screw-pumps having
tained. However, by destroying one seal, in accordance 70 a different number of side-screws and a different number
with the present invention, so that there is only one seal
of threads, the shape of the threads and the number of
during each revolution, the leakage is equalized resulting
in a nonpulsating ?ow from the pump. This is. achieved
in accordance with this invention by providing a groove
14 in the thread circumference of the middle screw.
side-screws as well as the number of threads of the screws
being so selected in relation to each other, in a manner
known per se, that in each position of the screws there is
at least one seal between the ends of the screw assem~
that any thread of said middle screw is provided with a
blage. It is known to those skilled in the art that this can
be realized if the threads obtain a certain mathematically
shallow circumferential groove at its periphery extending
de?ned geometrical shape, and if the condition
from one ?ank to the other of said thread, located be
tween the ends of said middle screw and between the ends
is satis?ed, where G is the number of threads of the mid
dle screw, 12 is the number of side-screws and g is the
groove has such axial extent and is so located as to inter
of said continuously, closed length of the bores, which
rupt the complete closure of chambers during portions of
revolutions of the screws to maintain constant the num
number of threads of each side-screw. The invention is
ber of closures existing throughout each revolution of
applicable to all screw pumps of this type, including such
screw pumps of this type where not only the middle screw 10 the screws.
but also the side-screws are driven.
References Cited in the ?le of this patent
I claim:
A positive screw pump of the type comprising a screw
assemblage including a middle screw with convex thread
?anks and at least one side screw with concave thread 15
Whit?eld ____________ __ June 23, 1942
Lysholm et a1. ________ _._ July 14, 1942
having intersecting bores for accommodating said screws,
said casing providing inlet and outlet passages, the screws
sealing against each other and against the casing to form
Sennet _______________ __ Aug. 7, 1945
Lysholm ____________ __ Dec. 28, 1948
Whit?eld ____________ _._ June 14, 1949
Berck _______________ __ Nov. 28,
Nilsson ______________ __ Dec. 9,
Nilsson _____________ _._ Dec. 23,
Whit?eld ____________ __ Jan. 26,
Sennet _______________ __ Feb. 9,
Wildhaber ___________ _._ Sept. 13,
?anks and a casing enclosing said screw assemblage and
constant volume closed chambers within which the 20
pumped ?uid is conveyed axially along the screw assem
blage from its inlet passage to its outlet passage, the
length of the screws and a continuously closed length of
the bores in the casing being such that there is always at
least one closure in the screw assemblage, the last ar~ 25
rangement being such that, without the characteristics
speci?ed hereafter the number of closures would vary
during rotation of the screws, characterized by the fact
Great Britain _________ __ July 18, 1956
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