A vacuum pump includes a first pump housing defining a pumping chamber; a pump rotor received in the pumping chamber; a drive motor operatively connected to the pump rotor; an oil casing receiving the first pump housing; and a second pump housing surrounding the first pump housing and the drive motor.

CROSS REFERENCE TO RELATED APPLICATION 
This application claims the priority of German Application No. P 40 17 
193.0 filed May 29, 1990, which is incorporated herein by reference. 
BACKGROUND OF THE INVENTION 
This invention relates to a vacuum pump having a housing, a drive motor and 
an oil casing. 
Vacuum pumps, particularly oil-lubricated, rotary vane vacuum pumps have 
low-noise characteristics provided that they are driven with a low rpm, 
that they have a relatively large mass and that the coolant air stream is 
weak. Further, to enhance low-noise characteristics, the "oil-knock" 
(caused by compression in the pumping chamber, particularly in the final 
pressure phase) is avoided by the admixture of leakage air. 
The above-outlined measures, however, are inconsistent with the low-cost 
manufacture and good power data of the vacuum pump. Vacuum pumps operating 
at low rpm's and weak coolant air flow have a relatively low power 
density, that is, they have a large structural volume. An elimination of 
the oil-knock by the admixture of leakage air adversely affects the final 
pressure. 
SUMMARY OF THE INVENTION 
It is an object of the invention to provide an improved vacuum pump of the 
above-outlined type which is economical to manufacture, which has good 
power data and yet has superior low-noise characteristics. 
This object and others to become apparent as the specification progresses, 
are accomplished by the invention, according to which, briefly stated, the 
vacuum pump has an additional, outer housing, preferably constituted by a 
hood or cover. 
An outer housing provided according to the invention has a reducing effect 
not only on noises generated by the pumping body itself but also as 
concerns the noises generated by the coolant air flow, in case the housing 
simultaneously serves for guiding the air. It is of particular advantage 
that, with the aid of the hood, the coolant air can be purposely guided to 
the surfaces to be cooled. It is generally known to generate an air flow 
by the motor fan in small rotary vane pumps and to use such air for 
cooling the pump. Without particular guide arrangements, however, the 
risks are high that the coolant air flow is dispersed (destroyed) by the 
structure. Such risks are eliminated by the invention. Further, the 
coolant air distribution is independent from the location of installation 
of the pump.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
Turning to FIGS. 1 and 2, the two-stage rotary vane vacuum pump shown 
therein and generally designated at 1 includes a pump housing 2, an oil 
casing 3 surrounding the pump housing 2, a drive motor 4 and an outer 
housing or pump cover (hood) 5. The pump housing 2 and the drive motor 4 
are mounted on a separating shield 6 which, in turn, is secured to a base 
plate 7 supported on a floor. 
The pump housing 2 includes a one-piece pump ring 8 which has a 
throughgoing opening having three consecutive zones 11, 12 and 13 of 
different configurations. In the ring opening 11, 12, 13 there is disposed 
a one-piece rotor assembly 14 which has axially consecutive parts 14a, 14b 
and 14c. The two outer parts 14a and 14c are provided with vane slots 15, 
16 at their end faces and form the armature of the high vacuum stage 
(hereafter "HV-stage") and the pre-vacuum stage (hereafter "PV-stage"), 
respectively. 
The mid portion 14b of the rotor assembly 14 corresponds in length and 
diameter to the mid zone 12 of the opening of the pump ring 8 such that 
this zone functions as a slide bearing for the rotor assembly 14. The 
opening zone 13 of the pump ring 8 is enlarged relative to the zone 12 and 
constitutes, together with the shield 6, the pump chamber 17 of the 
HV-stage in which the rotor part 14c, carrying a vane 18, is disposed. The 
zone 11 of the pump ring 8 constitutes, together with a frontal plate 19, 
the pump chamber 21 of the PV-stage in which the rotor part 14a, carrying 
a vane 22, is disposed. The HV-stage has an inlet channel 23, and a 
channel 24 extends from the outlet of the HV-stage to the inlet of the 
PV-stage. The PV-stage has an outlet channel 25, associated with an outlet 
check valve 26 which is situated at the upper part of the pump housing 2 
and, in case of a pump breakdown, preserves the vacuum in the vessel under 
evacuation. A depression 27 is provided in the top face of the housing 2 
for serving as an intermediate oil sump during the operation of the vacuum 
pump, as it will be explained later. 
An inlet nipple 31 of the vacuum pump is secured to the shield 6 in which a 
port 32 is provided, connecting the inlet nipple 31 to the inlet channel 
23 of the HV-stage. The shield 6 also carries an outlet nipple 33 which is 
connected by a non-illustrated port that is similar to the port 32, with 
the inner space of the oil casing 3. 
Also referring to FIG. 3, the oil casing 3 has an end face structure 35, 
whose central portion 36 is transparent and serves for monitoring the oil 
level in the oil casing 3. The narrow, vertical end face structure 35 has 
an approximately semicircular horizontal cross section, whose wide side is 
oriented towards the oil casing 3. The end face structure 35 extends over 
the entire height of the oil casing 3 and is provided with an oil filling 
opening 37 and an oil drain 38. 
The motor 4 is provided with a blower 41 at its end face oriented away from 
the pump structure proper. The cold air flow generated by the blower 41 
serves for cooling both the motor 4 and the pump. The housing of the motor 
4 and the oil casing 3 are both provided with axially or horizontally 
oriented cooling ribs 42 and 43, respectively. 
Referring once again to FIG. 3, the hood 5 covers the oil casing 3 and the 
drive motor 4 and is provided with sound dampening layers 45, 46 made of 
soft foam to thus ensure an effective suppression of sounds derived from 
body vibrations. The free zones of the base plate 7 are also covered by a 
dampening layer 47. 
A handle 48 is pivotally mounted on the hood 5 and is secured to tie bars 
51, 52 which in turn are releasably attached by screws 53, 54 to the base 
plate 7. If the pump is carried by the handle 48, the tie bars 51, 52 take 
up the required load (carrying forces) so that the other zones of the hood 
5 need not have load-carrying strength and stability. 
The front faces 55, 56 of the hood 5 are ribbed, and coolant air passages 
are provided by leaving the space between the ribs partially open. A 
coolant air inlet opening 57 provided in the hood 5 faces a fan 41 mounted 
on the shaft of the drive motor 4. The fan 41 is situated at that end of 
the motor 4 which is oriented away from the pump proper. The cooling air 
passes through the hood 5 and exits at the end face 56 thereof. The end 
face 56 has, laterally of end structure 35, vertical, inwardly angled 
portions 58, 59 which, together with a lower slot 67, are provided with 
cooling air passages. This arrangement ensures that the coolant air flows 
essentially entirely around the oil casing 3. Further, the outflowing 
cooling air first impinges on the end construction 35 which projects 
outwardly from the housing 5 through a vertical, elongated opening 
provided therein, and on the floor and is deflected thereby which further 
contributes to noise suppression. 
In the zone of the generally rectangular oil casing 3 the cooling air 
channels are formed by cooling ribs 43 provided on the outer faces of the 
oil casing 3 and the inner wall of the lining 46 of the hood 5. In order 
to effectively cool the cross-sectionally circular drive motor 4, a shaped 
component 61 is provided which is preferably made of hard foam. In the 
zone of the inlet opening 57 the component 61 has an inner wall face 62 
which defines the air inlet opening 57 of the hood 5 and through which the 
cooling air gains access to the fan 41. The shaped component 61 also 
assumes the function of a fan hood and thus such a component is not needed 
as a separate, independent part. The shaped component 61 straddles the 
motor 4 and forms with its ribs 42 the required cooling air channels. 
After cooling the motor 4, the cooling air is introduced into the zone of 
the oil casing 3 and exits through the slot 67 in the zone of the curved 
end construction 35. 
The mounting and insulation of electrical components such as condensers 63, 
cables, a switch 64 and the like are generally involved with significant 
expense in vacuum pumps, particularly if for one pump different motor 
variants are used. Since the shaped part 61 is made of an electrically 
insulating material, it may be used for supporting the electrical 
components. For this purpose, in the shaped body 61 cutouts or recesses 
65, 66 are provided which, together with the hood 5 form hollow spaces. 
After positioning the hood 5, a secure and electrically insulating support 
for the electrical components is ensured. 
It will be understood that the above description of the present invention 
is susceptible to various modifications, changes and adaptations, and the 
same are intended to be comprehended within the meaning and range of 
equivalents of the appended claims.