Vacuum pump having an rpm-measuring device

A vacuum pump has a housing, a pump shaft having a terminal shaft portion, a bearing member mounted in the housing and supporting the pump shaft and a rotor affixed to the pump shaft. The housing has a housing wall forming a side chamber with the bearing member. The terminal shaft portion projects through the bearing member into the side chamber. There is further provided a device for measuring the rpm of the rotor. The device comprises an inductive sensor held in the housing wall and a ferromagnetic member mounted on the terminal shaft portion for rotation in unison therewith. The ferromagnetic member induces electric pulses in the inductive sensor at a rate proprotionate to the rpm of the rotor. A non-magnetic partition separates the inductive sensor from the side chamber.

BACKGROUND OF THE INVENTION 
This invention relates to a vacuum pump which has a device for measuring 
the rpm of at least one pump piston or pump rotor. The shaft of the piston 
or rotor is supported in a bearing member (bearing plate or bearing block) 
and the terminus of the shaft projects into a side chamber of the pump. 
Lobed rotary piston pumps (Roots pumps) constructed according to current 
technology have a split tube motor drive and are completely encapsulated 
so that all rotary components are accommodated within the pump housing. 
Further, the side chambers of the Roots pumps are, as a rule, under vacuum 
as the pump operates. Roots pumps are often used to drive corrosive fluids 
whose penetration into the side chambers cannot be prevented. A direct 
measurement of the rpm by means of a mechanical coupling with the shaft 
stubs is therefore feasible only with difficulty. Even in case of 
turbomolecular vacuum pumps, the rotor shaft usually terminates in a 
chamber, for example, a motor chamber in which usually a pre-vacuum 
(fore-vacuum) prevails during the operation of the pump. 
SUMMARY OF THE INVENTION 
It is an object of the invention to provide a vacuum pump with an improved 
rotor rpm-measuring device which is simple, robust and needs no mechanical 
coupling with the pump rotor. 
This object and others to become apparent as the specification progresses, 
are accomplished by the invention, according to which, briefly stated, the 
pump shaft stub carries a rotary element made of a ferromagnetic material 
(soft iron, steel or the like) having peripheral gaps. A sensor equipped 
with permanent magnets is supported in the housing wall which forms the 
lateral pump chamber (gear chamber), at the height of the peripheral gaps 
of the rotary element. The sensor is separated from the inside of the side 
chamber by a partition made of a non-magnetic material. 
In an rpm-measuring device of the above-outlined type no mechanical 
coupling or sealed passage of the shaft are necessary. The rotary element 
induces, by virtue of the peripheral gaps, current pulses in the coil of 
the sensor, permitting an rpm measurement. By virtue of the separation of 
the side chamber from the chamber in which the sensor is located, the 
danger that the sensor and the associated electronic components contact 
the liquid medium handled by the pump is eliminated.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Turning to FIG. 1, there is illustrated therein a Roots pump 1 having a 
housing ring 2 and side-by-side arranged lobed, interengaging rotary 
pistons 3 and 4. The cross sections 3' and 4' of the respective pistons 3 
and 4 are illustrated in a plane turned 90.degree. into the plane of the 
drawing figure. The shafts 5 and 6 of the respective pistons 3 and 4 are 
supported in lateral bearing plates of which only the gear-side bearing 
plate 7 is shown. The housing ring 2 and the bearing plates form the work 
chamber 8 of the Roots pump 1. 
A cover 9 and the bearing plate 7 together form a gear chamber (side 
chamber) 11 which accommodates meshing gears 12 and 13 mounted on the 
respective piston shafts 5 and 6. The gears 12 and 13 serve for 
synchronizing the motion of the pistons 3 and 4. The side chamber 11 has 
an oil sump 14. An oil scattering disc 15 which is, together with the gear 
13, affixed to the shaft 6 is partially submerged into the oil held in the 
oil sump 14. The disc 15 supplies the bearings of the shafts 5 and 6 as 
well as the meshing zone of the gears 12 and 13 with lubricant. 
To the terminus of the shaft 6 there is secured a disc 17 which rotates in 
unison with the shaft 6 and which has peripheral discontinuities (gaps) 
18. At the height of the discontinuities the cover 9 is provided with an 
aperture 19 through which a bowl-shaped member 21 projects, with its base 
20 first, into the side chamber 11. Within the bowl-shaped member 21 there 
is located a sensor which is generally designated at 22 and which 
comprises an annular coil and a magnetic core arranged therein. At least 
the base 20 of the bowl-shaped member 21 is made of a non-magnetic 
material. Upon rotation of the apertured disc 17, current impulses are 
induced in the coil of the sensor by virtue of the discontinuities 18. The 
pulses are electronically amplified and utilized for measuring the rpm of 
the rotor 4. Instead of an apertured disc 17, a toothed annulus or a 
polygonal member may be used. 
The bowl-shaped member 21 comprises a flange 23 which is secured 
vacuumtight to the outside face of the cover 9. To the bowl-shaped member 
21 there is attached a housing 24 which defines a chamber 25 accommodating 
electronic components such as a pulse amplifier. Expediently, the 
bowlshaped member 21, the flange 23 and the housing 24 constitute a 
one-piece construction. In order to secure the electronic components in 
the chamber 25 and the sensor 22 in the bowlshaped member 21, it is 
expedient to embed these components in resin. The amplified pulses are 
transmitted to a nonillustrated display device by a cable 26. The cable 26 
is thus a pulse output means for the sensor 22. 
Turning now to FIG. 2, there is shown therein a combined 
molecular/turbomolecular vacuum pump 29 whose housing is designated at 31. 
There is provided a central bearing block 32 which extends into the 
housing in a bushinglike manner and in which there is supported a shaft 33 
by means of pin bearings 34. With the shaft 33 there are coupled the 
armature of a drive motor 35, the rotor 36 of the molecular pump stage and 
the rotor 37 of the turbomolecular pump stage. 
The rotor 37 is provided with impellers 38 which, together with stator 
plates 39 supported in the housing 31 constitute the turbomolecular pump 
stage. By means of a flange 41 the pump is coupled to the receptacle to be 
evacuated. 
The molecular pump stage comprises the bell-shaped rotor 36 surrounding the 
bearing chamber 42 and having at its outer face thread-like grooves 43. 
During operation of the pump the grooves 43 deliver gas from the high 
vacuum side to the fore-vacuum side. The rotor 36 cooperates with a stator 
44 which has approximately the same axial length as the rotor 36. 
In order to measure the rpm of the rotor 36 or 37 there is provided a 
device 18-26 of a construction identical to that described in connection 
with FIG. 1. The rotary element 17 is secured at the end of the shaft 33 
passing through the motor 35. The motor chamber is encapsulated by the 
cover 45. The bowl-shaped member 21 accommodating the sensor 22 extends 
into the motor space of the pump through an opening 19 provided in the 
cover 45. During rotation of the apertured member 17 current pulses are 
induced in the coil of the sensor 22 due to the gaps 18 provided therein. 
These pulses are amplified and are utilized for electronically measuring 
the rpm of the rotor 36 or 37. 
While the invention was described in connection with a Roots pump and a 
turbomolecular vacuum pump, it will be understood that the rpm-measuring 
device may be utilized in any other vacuum pump such as a rotary vane pump 
a claw type pump or the like. 
The present disclosure relates to subject matter contained in Federal 
Republic of Germany Patent Application No. G 87 03 108.6 (filed Feb. 28, 
1987) which is incorporated herein by reference. 
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.