Driving unit with an electric driving motor and a worm gear connected downstream to said motor

The invention relates to a drive unit (10) having an electric drive motor (12) and a subsequent connected worm gear (14), which serves for instance to drive the wiper system of a motor vehicle. The drive unit (10) has a drive shaft (16), which includes both the motor armature shaft (18) and the worm shaft (20) of the gear. A unit housing (22), which receives the electric motor (12) and the worm gear (14), is provided with spring means (26) that to overcome the axial play of the drive shaft (16) press in prestressed fashion against the free end face of the worm shaft (20). An especially simple and economical way of overcoming longitudinal shaft in the drive shaft, in a way suitable for robot assembly, is obtained if the spring means (26) are embodied by a leaf-springlike spring element (26) oriented crosswise to the axis of rotation of the worm shaft (20), the spring element being fixed by opposed end edges to shoulders (48, 50) of the housing and its middle portion, located between the end edges, engaging the end face of the worm shaft.

BACKGROUND OF THE INVENTION 
Drive units of the generic type in question are used to operate accessory 
devices in motor vehicles. When used for instance in motor vehicle 
windshield wiper systems, the drive shaft, formed by the armature shaft 
and the worm shaft, are subjected to an alternating axial load during 
operation, which varies from 0--when the wiper blades are at the turning 
points--to a maximum--when the wiper blades are being accelerated on 
leaving the turning points or being slowed down on approaching them. This 
axial load induced into the drive shaft by the worm gear leads to 
undesired knocking noises in the drive unit, if the drive shaft has 
longitudinal play between an axial stop toward the motor and an axial stop 
toward the gear. 
For long-term elimination of this drive shaft longitudinal play, in a 
commercially available drive unit a threaded bore has been disposed on the 
side toward the worm shaft, penetrating the unit housing in the extension 
of the drive shaft axis of rotation, and a threaded pin is rotated in this 
bore. The threaded pin has a central blind bore, open toward the drive 
shaft, in such a helical compression spring is disposed that presses with 
prestressing against a mushroom-shaped stop placed on the face end of the 
shaft toward it. The prestressing of the helical compression spring, which 
belongs to the spring means, must be greater than the maximum axial load 
acting on the drive shaft, which load, because of the existing worm pitch 
direction, is always oriented toward the free end of the worm shaft 
portion of the drive shaft. This known type of eliminating drive shaft 
longitudinal play is cost-intensive, because of the disposition of the 
threaded bore in the unit housing. The known version also requires at 
least one manual and hence expensive assembly step. 
SUMMARY OF THE INVENTION 
In the drive unit according to the invention, which according has the 
characteristics of the body of the main claim, the threaded bore can be 
omitted. Neither a mushroom-shaped stop nor the threaded pin are required 
either, because their functions are taken over by the leaf-spring-like 
spring element. Furthermore, the fixation of the spring element in the 
housing can be designed to be performed by robot, thus further reducing 
production costs for the drive unit. 
If in a further feature of the invention the end edges of the spring 
element are fixed on opposed housing shoulders, and furthermore the 
spacing between the housing shoulders is less than the spacing between the 
two edges of the spring element, the spring element necessarily tenses 
upon assembly, if it is inserted between the shoulders of the housing, 
toward the face end of the drive shaft on the gear side. 
An especially simple assembly suitable for robots is obtained if the 
housing shoulders are disposed on inner walls of the housing, and on the 
side remote from the worm shaft, in terms of the spring element, at least 
one of the housing walls is provided with a setback, so that in this 
region the spacing between the inner housing walls is greater than the 
spacing between the two end edges of the spring element. 
Suitably, the spring element in plan view has the shape essentially of a 
rectangle, whose end edges are formed on the shorter sides of the 
rectangle, and the two end portions, with the end edges of the rectangle, 
are bent outward from the plane of the middle portion to the same side by 
an angle of at least 30.degree.. Such an embodiment of the spring element 
makes it easier to install in the unit housing. 
To stabilize the spring element, its peripheral regions forming the two 
long sides of the rectangle are bent by approximately 90.degree. to the 
same side as are the end portions along with the end edges. 
Advantageously, the two peripheral regions are narrower than the two end 
portions. If furthermore, the cutting edge, located on the side of the 
worm shaft, of the end edge is provided with a ridge, then this cutting 
edge digs into the housing shoulders in such a way that separate fastening 
means for the spring element can be omitted. 
Especially simple production of a stable, operationally reliable spring 
element is assured if it is made from a spring band steel. 
In a further feature of the invention, the end face of the worm shaft 
pressing against the spring element is embodied as spherical, resulting in 
a low-friction contact, because it is at least approximately punctuate, 
between the spring element and the drive shaft. 
If the end portions each have one longitudinal slit, then inaccuracies in 
the surface of the housing shoulders and bevels can be compensated for 
thereby. 
Further advantageous features of and improvements to the invention may be 
learned from the drawing and the associated description of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
A drive unit 10 shown in FIG. 1 includes an electric drive motor 12 and a 
step-down gear 14 connected subsequent to it and embodied as a worm gear. 
A drive shaft 16 of the drive unit 10 includes an armature shaft 18, which 
is part of the electric drive motor 12, and a worm shaft 20, belonging to 
the worm gear 14, which forms an extension of the motor armature shaft 18. 
The electric drive motor 12 and the worm gear 14 are accommodated in a 
unit housing 22. A wiper device, not shown in detail, belonging to a motor 
vehicle and embodied for instance as a pendulum-type wiper device (FIG. 
1), is connected to a driven shaft 24 protruding from the unit housing 22. 
It can also be seen from FIG. 1 that the free face end of the drive shaft 
16 toward the motor is supported in the axial direction on a bearing plate 
25 belonging to the housing 22. On the other end of the drive shaft 16, 
toward the worm shaft, spring means 26 retained on the unit housing 22 are 
provided, embodied by a leaf-springlike spring element. This spring 
element 26 is seated in the unit housing 22 and is prestressed in such a 
way that it urges the drive shaft 16 toward the bearing plate 25. 
The embodiment of the spring plate will be described in further detail 
below in conjunction with FIGS. 4-6. As FIG. 5 shows, it has an 
essentially rectangular shape seen in plan view. Its two shorter end edges 
28 and 30 extend parallel to one another. It can also be seen from FIG. 4 
that the two end portions 32 and 34 together with the end edges 28 and 30 
are bent outward, relative to a middle portion 36 of the spring element 
26, to the same side out of the plane of the middle portion by an angle 
.alpha. and have a slit 36. FIGS. 4 and 6, in particular, also show that 
the two peripheral regions 30 and 40 of the spring element 26 that form 
the long sides of the rectangle are bent to the same side as the end 
portions 32 and 34 by approximately 90.degree., resulting in a dishlike 
indentation 42 that is rectangular in plan view. It can also be seen that 
the two peripheral regions 38 and 40 are narrower than the end portions 32 
and 34. In the production of the spring element 26, care is taken so that 
what are now the outer corners 44 and 46 of the end edges 28 and 30 are 
embodied as a cutting edge. In other words, these corners should be 
especially sharp-edged. It is advantageous if they are even provided with 
a ridge that is created when the end edges 28 and 30 are cut. 
As FIGS. 1 and 2 show, the unit housing 22, viewed in the extension of the 
worm shaft 20, has two spaced-apart housing shoulders 48 and 50 extending 
parallel to one another; the spacing 52 between the housing shoulders 
formed on inner walls of the gear housing 22 is less than the spacing 54 
between the two end edges 28 and 30 or the two corners 44 and 46 of the 
spring element (FIG. 5). As FIG. 2 in particular shows, the shoulders 48 
and 50 each have a setback 58 in their course leading away from the worm 
shaft 20, producing a spacing 60 between the two inner housing walls in 
the region of the setbacks 58 that is greater than the spacing 54 between 
the two end edges 28 and 30 of the spring element 26. The result is 
accordingly a chamber 62, into which the spring element 26 can be inserted 
into the unit housing 22 without tension, in the direction of the arrow 64 
in FIG. 3, crosswise to the axis of rotation of the drive shaft 16. 
Subsequently, the spring element 26--which in FIG. 2 is shown in 
dot-dashed lines in a preassembly position--is moved i the direction of 
the arrow 66 (FIG. 2) into its operating position represented by solid 
lines in FIG. 2. In the process, the end portions 32 and 34 of the spring 
element 26 deflect elastically outward in the direction of the arrows 68 
(FIG. 4), so that in the operating position of the spring element 26 they 
press in prestressed fashion against the inner walls of the unit housing 
22 and thus also against the shoulders 48 and 50 of the housing. The 
corners 44 and 46 formed as a cutting edge or provided with a ridge dig 
into the housing shoulders 48 and 50 in the process, so that the spring 
element 26 can no longer be displaced counter to the direction of the 
arrow 66. By means of the slits 35, the end portions 32 and 35 adapt to 
the surface of the housing shoulders 48 and 50. As FIG. 2 also shows, the 
middle portion 36 of the spring element 26 is indented somewhat in its 
operating position, which is effected by a spherically embodied face end 
70 of the drive shaft 16. Thus the middle portion 36 of the spring element 
is likewise prestressed in the direction of the arrow 66, and it presses 
the drive shaft 16 by its other free end against the bearing plate 24. The 
prestressing is dimensioned such that on the one hand no successive 
increase in current consumption of the drive motor 12 comes about, but on 
the other a certain amount of wear on the face ends of the drive shaft 16 
is still compensated for. 
The drive unit 10 accordingly has prestressed spring means 26, which are 
realized in the form of a leaf-springlike spring element preferably made 
from spring band steel, which is inserted into the unit crosswise to the 
axis of rotation of the worm shaft 20. This spring element 26 is fixed by 
opposed end edges 28, 30 and 44, 46, respectively, to shoulders 48, 50 of 
the unit housing 22. A middle portion 36 of the spring element 26 located 
between these end edges 28, 30 engages the end face 70 of the worm shaft 
20 facing it in prestressed fashion and loads the drive shaft 16 in such a 
way that the drive shaft, with its other face end toward the motor, comes 
to rest on the bearing plate 25 of the drive motor 12. The bearing plate 
25 belonging to the unit housing 22 thus forms an axial stop for the other 
end, toward the motor, of the armature shaft 18 or driven shaft 24. The 
result is simple, effective elimination of longitudinal play for the drive 
shaft 16, which can also compensate for a certain amount of wear to the 
two axial supports.