Roots-type fluid machine

A Roots-type rotary fluid machine having resin-coated rotors fluid-tightly accomodated in a chamber formed in a housing structure which consists of a generally cup-shaped main housing body having a bore closed at its one end and open the other end, and a covering member which closes the open end of the bore in the housing body. The housing body includes a fillet portion which bridges an inner flat surface defining the bottom of the bore and facing one end face of each rotor, and an inner peripheral surface defining the periphery of the bore and facing the outer peripheral surface of each rotor. The fillet portion is formed along a line of intersection of extensions of the inner flat surface and the inner peripheral surface of the housing body, and inwardly into the bore from this line of intersection. The fillet portion has a generally triangular shape in cross section taken in a direction perpendicular to the above line of intersection.

BACKGROUND OF THE INVENTION 
1. Field of the Art 
The present invention relates generally to a rotary fluid machine of a 
Roots type having resin-coated rotors, and more particularly to a 
technique for improving displacement efficiency of such type of rotary 
fluid machines. 
2. Related Art Statement 
In a rotary fluid machine of a Roots type such as rotary lobe-type pump or 
air compressor or blower, two lobes or rotors supported for rotation in 
opposite directions are accommodated in a chamber formed in a hollow 
housing structure, such that there exist small clearances between the 
outer surfaces of the two rotors, and between the outer surfaces of the 
rotors and the inner surfaces of the housing structure. It is desired to 
keep these clearance to a minimum, for ensuring maximum displacement 
efficiency of the fluid machine. 
Referring to FIG. 4, there are partly shown one of two rotors 10 and a 
housing structure of a known fluid machine of the type indicated above. 
The housing structure consists of three pieces, that is, a main housing 
body 14 which has a bore which is open at its opposite ends, and a pair of 
covering members 16, 16 which close the opposite open ends of the bore and 
which cooperate with the housing body 14 to define a chamber 12 in which. 
the rotors 10 are accommodated. In this arrangement, the blanks for the 
housing body 14 and the covering members 16, 16 may be readily machined or 
processed so that the inner surfaces of the covering members 16, 16 
defining the opposite ends of the chamber 12 are at right angles to the 
inner peripheral surface of the housing body 14. Accordingly, a clearance 
"a" between the outer peripheral surface of each rotor 10 and the inner 
surface of the housing body 14, and a clearance "b" between the end faces 
of the rotor 10 and the inner surface of the covering members 16, may be 
made comparatively small. 
PROBLEM SOLVED BY THE INVENTION 
Another known fluid machine uses a two-piece housing structure partly shown 
in FIG. 5. This modified housing structure consists of a generally 
cup-shaped main housing body 18 having a bore which is closed at its one 
end and closed at the other end, and a covering member (not shown in the 
figure) which closes the open end of the housing body 18. The housing body 
18 and the covering member cooperate to define a chamber 20 for 
accommodating the rotors 10. In this case, it has been a common practice 
to form a recess 22 in the inner surface of the blank for the housing 
structure, for facilitating the machining of the blank so as to form the 
bottom and inner peripheral surfaces of the chamber 20. While the 
clearances "a" and "b" between the rotors 10 and the inner surfaces of the 
housing body 17 may be made relatively small as in the arrangement of FIG. 
4, the recess 22 results in reducing the capability of sealing between the 
rotors 10 and the housing body 17, as compared with the sealing capability 
provided in the fluid machine using the three-piece housing structure. 
Thus, the arrangement of FIG. 5 suffers from comparatively low 
displacement efficiency. 
SUMMARY OF THE INVENTION 
It is therefore an object of the present invention to provide a rotary 
fluid machine of a Roots type having a housing structure including a 
generally cup-shaped main housing body, which is improved in displacement 
efficiency. 
According to the invention, there is provided a rotary fluid machine of a 
Roots type including a housing structure which consists of a generally 
cup-shaped main housing body having a bore closed at one end thereof and 
open at the other end, and a covering member which closes the open end of 
the bore and which cooperates with the main housing body to define a 
chamber in which rotors are accommodated in a substantially fluid tight 
condition with respect to each other and to the housing structure, wherein 
each of the rotors has an outer surface which consists of opposite end 
faces, one of which faces an inner flat surface of the main housing body 
defining a bottom of the bore, and an outer peripheral surface facing an 
inner peripheral surface of the main housing body defining a periphery of 
the bore. The outer surface of the rotor is coated with a resin layer. 
According to the invention, the main housing body includes as an integral 
part thereof a fillet portion which bridges the inner flat surface and the 
inner peripheral surface thereof, which is formed along a line of 
intersection of extensions of the inner flat surface and the inner 
peripheral surface, and which projects inwardly into the bore with respect 
to the line of intersection, the fillet portion having a generally 
triangular shape in cross section taken in a direction perpendicular to 
the line of intersection. 
In the rotary fluid machine of the present invention constructed as 
described above, the fillet portion of a substantially trangular shape is 
formed so as to connect the bottom wall of the bore in the main housing 
body, and the adjacent end of the peripheral wall which defines the bore. 
That is, the main housing body of the instant rotary fluid machine is not 
formed with a recess as provided at the bottom of the bore in the main 
housing body of the two-piece housing structure of the known rotary fluid 
machine previously described. Consequently, the displacement efficiency of 
the machine according to the invention is significantly improved, as 
compared with that of the known machine. 
According to an advantageous feature of the invention, the generally 
triangular shape of the fillet portion is generally an equilateral 
triangle having an apex at the aforementioned line of intersection, and a 
base which consists of a pair of circular arcs connected to each other. 
Each circular arc has a center located within the bore. 
According to another advantageous feature of the invention, the fillet 
portion is formed such that it slightly interferes with the resin layer of 
each rotor when the rotors are installed in position in the chamber during 
assembly of the rotary fluid machine. Namely, the resin-coated rotors are 
positioned such that the resin coating at the edge of the rotor 
elastically yields by a small amount, while being depressed against the 
fillet portion. In this condition, the clearances between the inner 
surfaces of the housing body and the rotors are small enough to assure a 
high level of displacement efficiency of the fluid machine. With the 
rotors operated in this condition during a run-in period of the fluid 
machine, the resin coating at the edge of the rotor wears until the 
elastic force between the fillet portion and the resin coating of the 
rotor is reduced below a certain level. 
The resin layer preferably has an approximate thickness within a range from 
0.3 mm to 1.5 mm. In one preferred form of the fluid machine, the resin 
layer is formed of a fluorocarbon resin. 
The fillet portion is preferably continuous and coextensive with the line 
of intersection.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
To further clarify the principle of the present invention, the preferred 
embodiment of the invention will be described in detail, referring to the 
accompanying drawings. 
Referring first to FIGS. 1 and 2, there is shown a rotary fluid machine of 
a Roots type in the form of a supercharger 26 used for an engine of an 
automotive vehicle to increase volumetric efficiency by forcing a greater 
quantity of air into the cylinders. The housing structure of the 
Roots-type supercharger 26 consists of a generally cup-shaped, hollow main 
housing body 28, and a covering member in the form of an end plate 32. The 
main housing body 28 has a bore 30 with an elliptical transverse cross 
sectional shape. The bore 30 is closed at one end and open at the other 
end. The open end of the bore 30 is closed by the end plate 32 which is 
secured to the housing body 28 and to a rear cover 34, by means of bolts 
36. Thus, the end plate 32 cooperates with the main housing body 28 to 
define a fluid-tight pump chamber 38. 
The housing structure rotatably supports a pair of parallel support shafts 
40, 42 which support a corresponding pair of lobe-type rotors 60, 62 
accomodated in the pump chamber 38. Described in more detail, one of 
opposite ends of each shaft 40, 42 is rotatably supported by the main 
housing body 28, via respective bearings 44, 48 fixed in the bottom wall 
of the housing body 28 which defines the bottom of the bore 30, while the 
other end of each shaft 40, 42 is supported by the end plate 32, via 
respective bearings 46, 50 fixed in the end plate 32. Reference numeral 52 
designates sealing members which are fluid-tightly mounted on the opposite 
end portions of the support shafts 40, 42, axially inwardly adjacent to 
the bearings 44, 46, 48, 50. The end portion of the support shaft 40 
supported by the bearing 44 extends outwardly through the bottom wall of 
the housing body 28, and carries a pulley 54 fixed thereto outside the 
housing structure. The pulley 54 is connected by V-belts (not shown) to a 
suitable drive source such as the vehicle engine, or a dedicated drive 
motor. The other end portion extending through the end plate 32 carries a 
timing gear 56 fixed thereto. This timing gear 56 meshes with a timing 
gear 58 fixed to the end portion of the other support shaft 42. The two 
timing gears 56 and 58 have the same number of teeth. With the support 
shaft 40 driven via the a pulley 54, the two support shafts 40, 42 are 
rotated at the same angular velocity in opposite directions. 
Each of the rotors 60, 62 has a pair of lobes which give the rotor a 
transverse cross sectional shape similar to the shape of a cocoon or 
peanut shell, as shown in FIG. 2. Each lobe-type rotor 60, 62 consists of 
a metallic core member 64 made of an aluminum alloy, iron or other 
suitable metals, and a resin layer 66 of a suitable thickness which covers 
the external peripheral surface and the opposite end faces of the core 
member 64. The resin layer 66 preferably has an approximate thickness in a 
range between 0.3 mm and 1.5 mm, and is formed of a fluorocarbon resin, 
for example, a copolymer of tetrafluoroethylene and ethylene. The two 
rotors 60, 62, which have the same dimensions and shape, are so disposed 
that there exists only a small clearance therebetween. With the two rotors 
60, 62, rotated in the opposite directions upon rotation of the 
corresponding support shafts 40, 42, air is sucked into the pump chamber 
38 through an inlet 68 formed in the housing body 28, and the air 
compressed by the rotors 60, 62 is discharged from the pump chamber 38 
through an outlet 70 also formed in the housing body 28, whereby the 
compressed air is supplied to the vehicle engine. 
Referring further to FIG. 3, the bottom wall of the main housing body 28 
has an inner flat surfaces 74 which defines the bottom of the bore 30, and 
which faces the end face of each rotor 60, 62. Further, the main housing 
body 28 has an inner peripheral surface 76 which defines the periphery of 
the bore 30 and which faces the outer peripheral surface of each rotor 60, 
62. The peripheral edge of the inner flat surface 74 (bottom of the bore 
30) is connected or bridged to the end of the inner peripheral surface 76 
adjacent to the bottom of the bore 30, by means of a fillet portion 72. 
Described in greater detail, an extension 72' of the inner flat surface 74 
and an extension 76' of the inner peripheral surface 76 intersect each 
other at right angles, at an imaginary line of intersection 78 which 
extends parallel to the peripheral surface 76 which defines the elliptical 
periphery of the bore 30 or pump chamber 38 as seen in FIG. 2. The fillet 
portion 72 is formed along the line of intersection 78, and projects 
inwardly of this intersection line 78 with respect to the bore 30 or pump 
chamber 38. As shown in FIG. 3, the fillet portion 72 has a generally 
triangular shape in cross section. More specifically, the triangulas shape 
of the fillet portion 72 is a generally equilateral-triangular cross 
section, taken in a direction perpendicular to the intersection line 78, 
such that an apex of the equilateral triangle is positioned at the 
intersection line 78, while a base 80a, 80b of the triangle connects the 
peripheral edge of the inner flat surface 74 and the end of the inner 
peripheral surface 76. 
As indicated in FIG. 3, the base consists of a pair of circular arcs 80a, 
80b connected to each other. Each circular arc 80a, 80b has a center 
located within the bore 30. That is, the circular arcs 80a, 80b are convex 
toward the apex of the triangle or the intersection line 78. More 
specifically, the fillet 72 is left uncut when the surfaces 74, 76 are 
finished with suitable cutting tools (not shown). The radius of the 
circular arc 80a, 80b is equal to a radius of a cutting edge of the finish 
cutters used to finish the bore 30. 
In this connection, it is noted that the rotors 60, 62 are installed in the 
pump chamber 38, at the time of assembly of the supercharger 26, such that 
the base 80a, 80b of the fillet portion 72 interferes with the edge of the 
resin layer 66 of each rotor 40, 42 by a small amount. In other words, the 
resin layer 66 of each rotor 60, 62 installed in position in the chamber 
38 elastically yields a small amount while being depressed against the 
base 80a, 80b of the fillet portion 72. The cross sectional area of the 
fillet portion 72 as seen in FIG. 3, and the amount of interference of the 
resin layer 66 and the fillet portion 72, are determined so that the 
clearance "a" between the peripheral surface of the rotor 60, 62 and the 
opposite peripheral surface 76 of the housing body 28, and the clearance 
"b" between the flat surface 74 and the opposite end face of the rotor 60, 
62 are small enough to assure a satisfactory level of displacement 
efficiency of the supercharger 26. The edge of the resin layer 66 
depressed against the base 80a, 80b of the fillet portion 72 wears during 
a run-in period of the supercharger 26, until the elastic force between 
the resin layer 66 and the base 80a, 80b of the fillet portion 72 has been 
reduced below a certain level. 
As described above, the fillet portion 72 formed at the intersection of the 
inner flat surface 74 and the inner peripheral surface 76 of the main 
housing body 28 replaces the recess 22 (FIG. 5) as formed in the 
cup-shaped main housing body of the two-piece housing structure of the 
conventional fluid machine of the same type. Therefore, the displacement 
efficiency of the fluid machine in the form of the supercharger 26 is 
significantly improved owing to increased sealing between the rotors 60, 
62 and the main housing body 28. 
While the resin layer 66 is elastically depressed at its edge against the 
fillet portion 72 at the time of initial installation of the rotors 60, 
62, as indicated in FIG. 3, so as to maintain the desired small clearance 
"a" and "b", the elastically depressed portion of the resin layer 66 may 
be easily removed due to wear in a run-in period of the supercharger 26. 
Thus, it is not necessary to form a recess (22) which had previously been 
considered necessary. Accordingly, the cost of forming the recess (22) in 
the housg body 28 is saved. In the present arrangement, the clearances "a" 
and "b" can be made as small as the clearances in the conventional fluid 
machine. 
The above arrangement is compared with an arrangement of FIG. 6 which is 
not constructed according to the principle of the invention but provided 
for the purpose of explaining the principle of the invention. In the 
arrangement of FIG. 6, the rotor 10 is not coated with a resin layer, 
while the main housing body 18 is formed with a fillet 24 so as to connect 
the bottom and perpheral surfaces of the bore 20. In this case, the edge 
of the rotor 10 must not interfere with the fillet portion 24, since the 
rotor 10 is not covered with an elastic resin layer. Hence, the clearances 
"a" and "b" can not be made sufficiently small. Although it is possible to 
form the fillet portion 24 with dimensions small enough to obtain 
sufficiently small clearances "a" and "b" as indicated in FIG. 6, this 
requires highly accurate machining of the housing body 18 and the rotors 
10 to within very tight tolerances, and high accuracy of positioning of 
the rotors 10 with respect to the fillet 24. This creates a high failure 
or defect rate, resulting in increased material and manufacturing costs of 
the supercharger. Hence, the arrangement of FIG. 6 is not a practical 
solution. It will be understood that the substitution of the fillet 72 of 
the illustrated embodiment for the recess 22 of the conventional 
arrangement requires the rotors 60, 62 to be coated with the resin layer 
66. 
Further, as compared with the bore 20 having the recess 22 of the 
conventional arrangement, the bore 30 can be cut or finished with a 
shorter feed length of the cutting tools and in a shorter length of time, 
since the fillet portion 72 is left uncut when the surfaces 74 and 76 are 
machined. This suggests a reduced machining cost and a reduced cost of the 
machining equipment. 
While the fillet portion 72 provided in the illustrated embodiment takes 
the form of a generally equilateral triangle in cross section and its base 
consists of the two circular arcs 80a, 80b, the fillet portion 72 may take 
other substantially triangular forms, or the base of the triangle may 
consist of a single or three or more circular arcs. The principle of the 
invention may be practiced by other variations, provided the fillet 
portion 72 is formed inwardly of the line of intersection 78 with respect 
to the bore 30. 
While the present embodiment has been described in its preferred embodiment 
with a certain degree of particularity for illustrative purposes only, it 
is to be understood that the invention is by no means confined to the 
precise details of the disclosure contained herein, but may be embodied 
with various changes, modifications and improvements which may occur to 
those skilled in the art, in the light of the foregoing teaching, without 
departing from the spirit and scope of the invention defined in the 
appended claims.