In this way the discharge variations, which with three lobed rotors occur in cycles of 60° of rotor revolution, will be smoothed out and the helical angle given to the rotors will eliminate both 5 sharp cut off and sharp opening of the entire rotpr length and the sudden back flow into the low pressure chambers which goes therewith and at the same time reduce the extent or degree of fluctuations in the cyclic pulsations.
10 It has been found that these results are best attained when the helical angle is such that the projection of one end of the rotor in a plane at right angles to the axis of the rotor is displaced angularly relatively to the other end of
15 the rotor a number of degrees equal to half the angle between adjacent teeth on the rotor, i.'e. the "helical angle" should be a function of the number of teeth on the rotors and equal to 360° divided by twice the number of teeth on each
20 rotor. With three lobes at 120° on each rotor
this "helical angle" will be 60° and a seal of 180°
around the housing will be required for each
One dimension of the port areas must be di
25 vided equally above and below the transverse geometrical center line of the blower for equal displacement from each rotor and for maximum port areas with a seal of 180° this dimension will be equal to the distance between rotor cen
30 ters for each port if the housing seals for each
. rotor are divided symmetrically 90° on either side
of the line of centers of the rotors in a plane at
right angles to the rotor axes. The number of
degrees required for housing seal in a plane at
35 right angles to the rotor axes is the number of degrees between lobes plus the number of degrees of angular displacement of one end of the rotor relatively to the other end and If this total is greater than 180° the housing can not
40 have two sides defined by the common, tangents to the two rotor tip circles.
It will thus be seen that a three lobed rotor with one end angularly displaced 60" relatively to the -other best fulfills the conflicting require
45 ments for a blower of given dimensions, of maximum displacement, maximum number of pulsations of minimum magnitude per revolution, and continuity of discharge with gradual intake, exhaust, back flow and cut off, contributing to
50 a smoother and more constant output.
A blower built in accordance with the invention and capable of operating at speeds up to. 9.000 R. P. M. with a capacity of 900 cubic feet per minute and pressures up to 14 or 17 Ibs. per
55 square inch above atmospheric pressure may have a temperature difference between inlet and outlet which are only 4" or 5" apart, in the neighborhood of 300° F. The intake air, particularly if mixed with gasoline, may be 15° P. below at
00 mospheric temperature while the heat of compression may raise this temperature to 350° P. at the outlet. The housing is water jacketed for the purpose of reducing this temperature difference, but it is not practicable so to cool the
65 rotors. For this reason it has been 'found expedient to make the rotors and the housing of materials of different coefficient of expansion such that although the housing is cooled it will expand approx'mately the same amount as the
70 uncooled' rotors. For instance, cast iron may be used for the rotors and aluminum for the housing.
The water jacketing of the housing is insufficient to equalize the temperature difference
75 between inlet and outlet to keep the aluminum
housing from unequal expansion out of shape. The hotter outlet side expands, giving more clearance between itself and the rotors and the inlet side buckles in towards the rotors. In order always to maintain the necessary clearance 5 between rotors and housing within the desirable maximum and minimum limitations, and in order that these clearances may be approximately equal and as small as possible, particularly on the outlet or pressure side to reduce back flow 10 between the rotors and their housing for the average operating conditions of speed and pressure and hence of temperature difference, the housing is preferably bored eccentric to the line of centers of the rotor shafts, so as to allow more 15 clearance on the intake side when the housing has an even temperature throughout and there may be a slight excess of clearance on the outlet side when the temperature difference is a maximum. 20
In order to reduce mechanical noise, the rotors are synchronously driven through helical gears at each end of the rotors. The helical angle of the gear at one end of each rotor is opposed to that at the other end in order to balance end 25 thrust. The use of such gears at each end of the rotors prevents torslonal displacement or twist of one end of each rotor relative to the other end and it maintains the rotor ends in registry independently of the rotors themselves. 30
The accompanying drawings show a Roots type blower constructed in accordance with the invention.
In the drawings:
Figure 1 is a part cut away, part sectional ele- 35 vation of the blower.
Figure 2 is an enlarged end elevation showing the mounting of the driving gears.
Figure 3 is a section on line 3—3 of Figure 2.
Figure 4 is a section on line 4—4 of Figure 1. 40
Figure 5 is an enlarged part detail sectional view of the sealing rings between the rotating spindles of the rotors and the housing to prevent the escape of fluid out from the blower and the leakage of lubricating oil for the gears and 45 the bearings into the blower.
The rotors I and 2 each with three lobes, or teeth, J, 4, 5 and 6, 7, 8 of right and left hand spiral form respectively, the projection of one end of each rotor in a plane at right angles to 50 the axis of the rotor being displaced 60° angularly relatively to the other end, are provided with splined stub shafts such as 9, 10, M, splined in the rotor hubs and pinned against endwise movement relative to the rotors as by a pin 12. 55
.The rotor housing 13 closely encircles 180° of each roiot and follows the two tangents common to both rotor tip circles. It is provided with intake and output ducts 14 and IS leading to inlet and outlet ports IS and 17, extending a distance 60 equal in length to the distance between centers of the rotors, divided equally above and below the transverse geometrical center line of the blower along each of the tangents common to the two rotor tip circles. A water cooling space 65 19 around the blower is provided. End plates 20 and 21 form the end closures for the rotor housing II.
Splined on the rotor shafts are gear hubs 22, ^ 23, 24, and 25 for the right and left hand spiral 7° gears 26 and 27 of the rotor I and the left and right hand spiral gears 20 and 29 of the rotor 2.
On the rotor shafts between the rotor hubs and gear hubs are end thrust collars 30 forming ^ part of an oil seal later to be described. The end <5