Orbital sander

An orbital sander having a sanding plate braced via elastic elements, the plate being capable of being set into reciprocating motion by an eccentric drive element that can be set into rotation by the armature shaft of the drive motor. The eccentric drive element is an outer wheel of an asymmetrical planetary gear, with a sun wheel disposed on the drive shaft.

BACKGROUND OF THE INVENTION 
The invention relates to an orbital sander and more particularly to an 
orbital sander having a sanding plate capable of being set into 
reciprocating motion by an eccentric drive element driven either directly 
or indirectly by the armature shaft of a drive motor. 
Conventional orbital sanders of this type typically have no reduction gear 
between the armature shaft of the drive motor and the drive shaft of the 
sanding plate, so that the armature shaft and drive shaft are identical. 
However, orbital sanders of this generic type are also known in which the 
rpm of the drive motor is stepped down by means of a conventional gear and 
the oscillation frequency of the sanding plate is correspondingly reduced. 
The gears used for this purpose occupy a not inconsiderable amount of 
installation space, however, and they are also expensive. 
It is an object of the invention to provide a gear design that with minimum 
installation space and at minimal cost enables a reduction, selectable 
within a relatively wide range, of the rpm of the armature shaft of the 
drive motor. 
SUMMARY OF THE INVENTION 
This object is attained in accordance with the invention by providing that 
the eccentric drive element consists of an outer wheel of an asymmetrical 
planetary gear having a sun wheel disposed on the drive shaft. With this 
provision, the gear and the eccentric element form a structural unit which 
is compact and economical. 
In accordance with another feature of the invention the outer wheel has 
internal teeth, the sun wheel of the planetary gear is embodied by the 
drive pinion of the armature shaft of the drive motor, and the drive 
pinion is in engagement with at least two planet wheels of different 
diameters that mesh with the internal teeth of the outer wheel, wherein 
all the gear wheels have the same modulus. A further feature of the 
invention is that the planetary gear includes three planet wheels in 
engagement with the sun wheel and the internally toothed outer wheel, of 
which planet wheels one has a larger diameter than the other two. 
In an alternate embodiment, the sun wheel, the planet wheels and the outer 
wheel may be embodied as a friction wheel system.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The asymmetrical planetary gear shown in schematic fashion in FIGS. 1-3 has 
gear wheels of the same modulus and includes a sun wheel 2, mounted on the 
armature shaft or drive shaft 1 of the drive motor (not shown), with which 
three planet wheels 4, 5 and 6 that engage internal teeth of an outer 
wheel 3 mesh. Of these three planet wheels, the wheel 4 has a larger 
diameter than the other two wheels 5 and 6. 
The term "modulus" as used herein means a quantity related to the pitch of 
the gears. More specifically, the modulus M is given by M=t/.pi. where t 
is the pitch (spacing) of the gear teeth. The pitch or modulus of all 
gears (2-6) must be the same. 
In such an orbital sander using a Type VS 14 W 500 motor, an idle speed of 
25,000 rpm, a reduced rpm of 7000 and an overall sanding stroke of 5 mm, 
the gears may be provided with a modulus of 1.25 mm and an optimum gear 
ratio results with the larger planet wheel 4 having a diameter of 15 mm, 
the sun wheel 2 having a diameter of 10 mm and the outer wheel 3 having an 
inner diameter of 35 mm, so that the positive and negative strokes 
(eccentricity) of the outer wheel relative to its center of motion is 2;5 
mm. 
Upon rotation of the sun wheel 2, the three planet wheels roll on it and on 
the internal teeth of the outer wheel 3, which may optionally also be 
elastically deformable within limits. Because of the different sizes of 
the planet wheels and the unchanging location of the drive shaft 1, the 
outer wheel 3 executes a rotary movement about the drive shaft axis 1', 
which is eccentric to the center point M of the outer wheel 3. Because of 
this eccentric motion, there is a positive and negative stroke e of the 
outer wheel 3 in the longitudinal direction (see the double arrow) whith 
respect to the "starting position" shown in FIG. 1 (center line 7), and 
overall, there is an orbital movement of the outer wheel 3, as illustrated 
in principle in FIG. 4. The starting position of the outer wheel 3 in FIG. 
4 corresponds to that of FIG. 1 (0.degree./360.degree.). 
As FIGS. 5 and 6 show, the motion of the eccentric element embodied by the 
outer wheel is transmitted to the sanding plate 8, which has a circular 
recess 9 in which the outer wheel 3 is disposed and mechanically firmly 
joined to the sanding plate. 
The reduction according to the invention of the rpm of the drive shaft 
enables a considerable increase in the rpm of the drive motor and hence, 
if the structural size of the motor remains the same, an increase in the 
drive output as well. 
Instead of the planetary gear with gear wheels of the same modulus, a 
planetary arrangement of friction wheels can also be provided, even for 
the sun wheel 2 (with the outer wheel having a frictional internal 
surface). The schematic drawings of Figs. 1-6 also illustrate such an 
embodiment. Optionally, regardless of whether a planetary gear with gear 
wheels or friction wheels is used, it is also possible for only two planet 
wheels, of different diameters, to be provided. 
Although the preferred embodiment as disclosed herein has three planet gear 
wheels with one wheel having a larger diameter than the other equal 
diameter wheels, it is also permissible that all three planet wheels have 
different diameters. 
The present disclosure relates to the subject matter of out patent 
application No. 3630155.8 filed in the Federal Republic of Germany on 
Sept. 4, 1986, the entire specification of which is incorporated herein by 
reference. 
It is to be understood that various other modifications will be apparent to 
(and can readily be made by) those skilled in the relevant art without 
departing from the scope and spirit of this invention. Therefore, it is 
not intended that the scope of the claims appended hereto be limited to 
the description as set forth herein, but rather that the claims be 
construed as encompassing all the features of patentable novelty.