Note: Descriptions are shown in the official language in which they were submitted.
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'I']~e ~ sel-t inv(:~nt-iOIl re1.~1es to a screw ~oior machil1e
and more speci~ically to tl1e shai~e of the intermeshing ro~ors for
snch a machine.
The rol:ors for such a machine cornprise a female rotor
provided with helical lancls and in~ervening grooves having gener-
ally concave flanks at least the major portions of which are
; located inside the pitch circle of the rotor, and a male rotor
provided with helical lands and intervening grooves having ''
generally convex flanks at least the major portions of which
are located outside the pitch circle of the rotor.
In most cases each land of the female rotors extends
a short distance outside the pi,tch circle forming an addendum ',
~, and each groove or the male rotor extends a short distance ~,'
'~ inside the pitch circle forming a dedendum corresponding to
said addendum. The flanks of the addendums and dedendums form
minor portions of the complete flanks of the corresponding lands
~ and grooves. Up to now each of said minor flank portions has
j been shaped so as to follow a circular arc having its centre on '~
the pitch circle. In order to guarantee non-interference bet-
ween cooperating minor flank portions the radius of the circular
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arc of the male rotor has been made considerably larger than , ,~ ~ -
that of the female rotor, said differ'ence between the radii
being of the order of l to 2 Qo . Owing to the short extent of
said minor flank portions and their location close to the pitch
circles where the sliding between the flanks has a minimum the
leakage areas so obtained have been negligible.
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~ctew (ornL)~ ors i~av:irlc3 such f~ank profiles oE the
add~ndunls have run quite satisEact-orily when the ro~ors have ~ ;
been interconnected by means of synchronizing gears. Also in
screw compressors wi~h direct contact between the ~lanks such
addendum and dedend~lm profiles have worked satisfactorily when
the male ro-tor has been conllected with a power source even
though the torque transmitting surfaces of the ma~or flank
portions adjacent the pitch circles have been oflimi,ed size. ~
Those surfaces have normally in order to obtain flank contact ~-
adjacent to the pitch circles only, been provided with a smaller
interflank clearance than that of the remaining flank portions
in the way well known since the early thirties from the screw
rotor pump field. In screw compressors with direct flank con~
tact and the female rotor connected with a power source addendum
and dedendum profiles of the earlier known type some prohlems
have arised partly owing to the larger torque to be transmitted
between the rotors, partly owiny to the fact that the points of -~
contact of the driving female rotor flank lie inside the pitch
circle whereas the points of contact of the driven male rotor
flank lie outside the pitch circle, which means that in the
points of con~act the peripheral speed of driving flank is
lower than that of the driven flank.
Another disadvantage with the described addendum and
dedendu~ flank profiles is that they are complicated ~o manufac~
ture and almost impossible to cut properly by a hob milling
process.
One aim of the present invention is to extend the
area of the power transmitting flank surfaces of a pair o
intermeshing rotors.
Another aim of the invention is to shape the rotors
such that at least a portion of the power transmitting surface~
are so located in relation to the respective pitch circles that
driving flank has a higher speed than that of the driven flank. -~
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A third aim of the invention is to ishape the rotors so
that they can be easily cut in a hob milling machine.
According to the present invention there is provided a
pair of intermeshing rotors having helical lands and intervening
grooves and adapted for rotation about parallel axes within a `~
working space of a screw rotor machine, one of the rotors being
of female rotor type and shaped such that each groove flank
thereof has a major portion located inside the pitch circle of
the rotor and a minor portion located outside thereof, the pro-
file of said minor portion being in any plane perpendicular to
the rotor axis a curve extending from the pitch cixcle outwardly
and towards the adjacent groove, and the other of said rotoris
being of male rotor type and shaped such that each land flank
thereof has a major portion located outside the pitch circle of ~ `~
the rotor and a minor portion located inside thereof, character
ized in that in a plane perpendicular to the rotor axes said `
minor flank portion of at least one flank of each female rotor
groove and the minor portion of the cooperating flank of a male
rotor land, each comprises a segment located adjacent to the
related pitch circle, said segme~ts being travelling generated
one by another, and that said flank segment and the continuing
major portion at least of the female rotor flank at their com~on
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point on the pitch circle have different radii of curvature and
a common tangent, said common tangent forming an angle of less
than 30 with a radial line drawn through said common point of
tangency from the center of the rotor.
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' A preferred embodiment of the invention will now be
described by way of example with reference to the accompanying `~
drawings, in which:
Fig. 1 shows a vertical section through a screw com-
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pressor taken on line 1-1 in Fig. 2,
Fig. 2 shows a transverse isection through the compres-
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sor of Fig. 1 -taken on line 2-2 in Fig. 1~ and Fiy. 3 shows a
detail of Fig. 2 on a laryer scale.
The screw compressor shown in Figs. 1 and 3 comprises
a casing 10 forming a working space 12 substantially in the form
of two intersecting cylindrical bores having parallel axes. The
casing 10 is further provided with a low pressure channel 14 and
a high pressure channel 16 for the working fluid which channels
communicate with the working space 12 through a low pressure port
18 and a high pressure port 20, respectively.
10In the compressor shown the low pressure port 13 is
located in its entirety in the low pressure end wall 22 of the ~.
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working space 12 and extends mainly on one side of the plane con-
taining the axes of the bores. The high pressure port 20 of the
compressor shown is located partly in the high pressure end wall -
24 of the working space 12 and partly in its barrel wall 26 and
it is in its entirety located on the side of the plane through
the bore axes opposite to the low pres,sure port.
In the working space 12 are provided two cooperating ; `
rotors, viz. a male rotor 28 and a female rotor 30, located with
their axes coinciding with the bore axes. These rotors are
~ journaled in the casing 10 in cylindrical roller bearings 32 ~
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in the low pressure er~d wall c~ l in pa.irs of bal:L ~earin~,s 34
with shou.l(lers in the high pressure end wall 24. The fernale rotor
30 is Eurther prov.ided w.ith a stub shaft 36 projecting outside
the casing 10.
The male rotor 28 has four helical lands 38 and inter-
vening grooves 40 having a wrap angle of about 300. The female
rotor 30 has six helical lands 42 and in-tervening grooves 44
having a wrap angle of about 200. The female rotor lands 42
are provided with addendums 48 located radially outside the
pitch circle 46 of the female rotor 30 and the male rotor
grooves 40 are provided with corresponding dedendums 52 located
radially inside the pitch circle 50 of the male ro-tor 28. .
In the barrel wall 26 of the working space 12 are provided
a plurality of oil injection channels 54 opening at the inter-
section line 56 between the two bores forming the working space
12. These channels 54 form communica-tions be-tween an oil supply
chamber 58 and the working space 12. Oil is supplied to this
chamber 58 froM a pressure oil source not shown through a supply
opening 60 under a pressure higher than -the pressure prevail.ing
in the working space 12 at the openings of the channels 54. :.
Each female rotor groove 44 comprises a first flank 62, being
the leading flank when disposed in a compressor, and a second, ~
trailing flank 64. Each of said flanks 62, 64 extends from a
radially innermost point 66 of the groove 44 out to a poin-t 68
and 70, respectively on the crest 72 of the adjacent land 42 :
which is located outside the pitch circle 46 of the rotor 30,
which circle intersects the flanks 62, 64 in the points 74 and 76, .
respectively. Each flank 62, 64 has thus a major portion 66-74,
66-76 inside the pit~h circle 46 and a minor portion 74-68,
76-70 outside the pitch circle 46.
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5'7252
The major portioll 66-74 oi the leading f:lal-lk 8~ is composecl
of three different sections. The first sect:ion 66-78 is shaped as
a circular arc extending over an angle "a" of about 10, and
having i-ts cen~re 80 located on the pitch circle 46, and a
radius "r" with a length of about 40~ of tha-t of ~the pitch radius
of the rotor 30. The second section 78-82 is shaped as a c~rcular
arc having its centre 84 located outside the pitch circle 46 on
an extension vf the straight line 78-80, and a radius "R" having
a length of abou-t four thirds of tha-t of the radius "r". The
third section 82-74 follows a straight line which in the point 82
forms a tangent to the circular arc with the radius "R" and in
the point 74 on the pitch circle 46 forms an angle "~" of about
20 with a radial line drawn from the centre 86 of the rotor 30.
The major portion 66-76 of the trailing flank 6~ is also
composed of three different sections. The first section 66-88
is a mirror image of the first section 66-78 of the leading flank
62. The second section 88-90 has in its inncr end point 88 a
tangent common to tha-t of the first section 66-88. The outer end
point 90 of the section is disposed on a circle having the pi-tch
radius 86-80 as a diameter. Between those two points 88, 90
the section follows a curve o* epicycloidal type which will be
described later. The third section 90-76 follows a straight
radial line drawn from the centre 86 of the rotor 30.
Each male rotor land 30 comprises in the same way a leading
flank 92 and a trailing flank 94. Each of said flanks 92~ 94
extends from a summit 96 to a point 98 and 100, respectively, on
the bottom portion 10? of the adjacent groove 40 which is located
inside the pitch circle 50 of the male rotor 28, which circle
intersects the flanks 92, 94 in the points 104 and 106, respectively. ;
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liclcil fL~Ir~k 92, 9l~ h~ls ;hus a Inajor por-tion 96-.l.Ol~, 96-106 ou~side
the pitch circle S0 arld a m:i.rlor por-tion 104-98, lOG-100 inside
the pitch circle 50.
The major portion 96-104 of -the leading flank 92 is composed
of three different sections. The first section 96-108 is, except
for a relieved portion 110 in order to provide a sealing s-trip at
the su~nit 96, shaped as a circular arc conforming with the first
section 66-78 of the leading flank 62 of the female rotor 30. -
The second sec-tion 108-112 is genera-ted by the second section
78-82 of the leading flank 62 of the female rotor 30. The third
sec-tion 112-104 is generated by the third section 82-74 of the
leading flank 62 of the female rotor.
The major portion 96-106 of the -trailing flank 94 is also Y;
composed of three different sections. The first section 96-114
is a mirror image of -the first section 96-108 of -the leading
flank 92. The end point 114 of this first section further
generates -the...e-picycloidal, second section 88-90 of -the trailing
flank 64 of the female ro-tor 30. The second sectlon 114-116 is :~
generated by the point 90 on the trailing flank 64 of -the female ;~
rotor 30. The third section 116-106 is genera-ted by the third
section 90-76 of the trailing flank 64 of the female rotor 30. ~:
The minor portion 104-98 of the leading flank 92 of the male~:
rotor 28 is composed of two different sections. The first section ~-~
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104-118 follows a s~raight line forming a tangent to the -third
section 112-104 of -the major portion 96-104 of the flank 92
in the point 104 on the pitch circle 50. The second section 118-98
is generated by the outermost point 68 of the leading female
rotor groove flank 62. The point 118 of intersection between the
flank sections 104-~18, 118-98 is so located that the line 104-118
forms a tangent to the curve 118~98 in said point 118. ?
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Tl~` millOr' pOl'tiOIl ~/4-6~ oL t:he lecldirlg fJaoIc ~i2 of the female
rotor 30 fo:llows a curv~ generatecI by -the first sec~ion 104-118 of
the minor portion 104-98 of the leading flank 92 of -the male rotor
28.
The minor portion 106-100 of the trailing flank 94 of the
male ro-tor 28 is composed of two differen-t sec-tions. The first
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section 106-120 follows a straight line the ex-tens:ion of which
passes through the centre 122 of the male rotor 28. This straight
line 106-120 forms a tangent to the third section 116-106 of the
ma~or portion 96-106 of the trailing flank 94 in the poin-t 106
on the pltch circle 50. '~le second section 120-100 is generated
by the outermost point 70 of the trailing female rotor groove
flank 64. The point 120 of intersection be-tween the flank
sections 106-120, 120-100 is so located tha-t the line 106-120
forms a tangent to the curve 120-100 in said point 120.
The minor por-tion 76-70 of the trailing flank 64 of the
female rotor 30 follows a curve generated by the first section
106-120 of the minor portion 106-100 of the trailing flank 94
of the male rotor 28.
The crest 72 of each female rotor land 42 is provided with
a sealing strip 124 for improvement of the sealing against the
barrel wall of the casing. The bottom portion 102 of each male
rotor groove 40 is further provided with a slot 126 into and
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out of which the sealing strip 124 of the cooperating female
rotor land 42 passes as the rotors 30, 28 revolve.
The rotor profiles discussed above all relate to the
theoretical shape thereof. However, in order to guarantee mechani-
cal reliability of the machine in which the rotors are mounted,
certain clearances ~us-t be provided between the rotors within
the portions thereof that shall not directly contact each other.
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In order to o~td:in Suc}l cle~lL~clllc~es o~e or ~oth rotor profi:les
mus~ be some~Ji~a~ modi~ied ~o deviate from the theoretical shape
thereof. 1'he shapes and sizes of those devia-tions differ for
different -types of machines depending upon the working conditions, ~-
i.e. compression or expansion, the type of gas -to be ac-ted upon,
the temperature of the gas depending of the cooling syste/m
of the machine i.ncluding liquid injection and the system for
transmitting torque to the rotors, such as separate synchronizing
gears for interconnection o~ the ro-tors, direct flank contact
with driving male rotor and direc-t flank con-tac-t with driving
female rotor, as well as the dimensions of the rotors.
In the drawing the deviation from the theoretical rotor
profiles have been disposed completely in the female rotor 30.
The shape o:E the deviations are indicated by the dotted line 128 .
which for the sake of clearness have been disposed on a very
exaggerated dis-tance from the theoretical flank profile. In .
the drawing the devia-tions have been shown for a compressor
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with direct flank contact and driving female rotor 30. The
devia-tions from the theore-tical shape over -the first and second
sections 66-78, 66-88 and 78-82, ~18-90 of -the major portions
of the flanks 62 and 64, respectively, have a constant first siZe
measured perpendicularly to the flank 62, 64. The deviation over
the third section 82-74 of the major portion 66-74 o -the
leading flank 62 diminishes continuously from the size in point
82, being equal to -that over the adjacent second section 78-82,
to a second minimum size in point 74 on the pitch circle 46.
The deviation.over the minor flank portion 74-68 is of constant
size equal to said second size in point 74 when measured per-
pendicularly to th~ flank. The deviations over the third sectiron
90-76 of the major portion 66-76 of the trailing flank 64 and .
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over t~,e rninor port:;on /6-70 o~: Lhe trailir~g ~larlk 6'~ are of a
third size. The deviation over the crest 72 excep-t for the
sealing strip 12l~ is of a cons-tant fourth size when measur-ed
perpendicularly to the crest 72, i.e. in radial direction of
the ro-tor. In an ac-tual application rela-ted to a compressor for
air of atmospheric pressure and temperature, having a pressure
ratio of about 8:1, provided with oil injection for cooling7
sealing and lubrication, direct flank contact with driving
female rotor 30, and a rotor diameter of abou-t 200 mm, the
value of said first, second, third and fourth sizes of deviation
being .06-.10 mm, .0~-.05 mm, zero, and .06-.10 mm, respectively.
For direct flank contact drive and driving male rotor the
values of the second and third sizes of deviation should be
interchanged. For drive through synchronizing gears the second
and third sizes of deviations should have posi-tive values smaller
than the first size of deviation.
By disposing all -the clearance producing deviations in one
rotor only, preferably the female one, the o-ther rotor will be
identical in shape for all different embodiments. In this way
the same cutting tool can be used for said other rotor independent
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of application, and when shifting -the production only one new
cutting tool has to be prepared which is essen-tial especially
from economical point of view.
The angle "~" may vary between 0 and 30. The sarnller
this angle is the better the contact conditions will be between
the cooperating flanks 62, 92. On the other hand the larger
this angle is the be-tter are the conditions for a cutting tool,
,
especially when used in a hob milling machine, which means that
more rotors can be ~cut by the same cutting tool without any
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rec,llarF,i rling ~:he~eof~. In oL~ r worcls the prodllctioll of. the rotors
will be faster ancl cheaper. 'l'he s.i~e of -the angle "f3" -to be
about 20 as shown in the drawing seerns to be about an optimum
a-t leas-t in the described embodiment of a compressor with
direct î lank contact and drivi.ng fernale ro-tor.
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