This invention relates to a portable power tool as per the preamble
of claim 1.
An example of such power tool is disclosed by DE 41 05 340 A.
In particular, the invention concerns a power tool of the above type
provided with a ball-type balancing device interconnected with the output spindle
for automatic balancing of the output spindle and the working implement attached
One problem inherent in this type of tools is the difficulty to obtain
a rigid connection and a perfect centering of the balancing device in relation to
the output spindle. Another problem is to accomplish a compact power tool design
where the overall axial dimension of the output spindle and the balancing device
is small and where the axial distance between the working implement and the forward
bearing of the output spindle is small.
The primary object of the invention is to create a power tool of the
above type in which the identified problems are avoided. This is accomplished by
the invention as it is defined in claim 1.
A preferred embodiment of the invention is described below with reference
to the accompanying drawing figures.
On the drawings:
- Fig. 1 shows a longitudinal section through the output end of an angle grinder.
- Fig. 2 shows a side elevation, partly in section, of a clamping element and
a working implement clamping screw.
- Fig.3 shows a fractional section of the balancing device.
- Fig.4 shows a cross section along line III - III in Fig. 1.
- Fig.5 shows, on a larger scale, a fractional section through the rear bearing
and retaining means of the output spindle.
- Fig.6 shows, on a larger scale, a side elevation of the coupling element.
The power tool shown in the drawing figures is an angle grinder which
comprises a housing 10, a rotation motor 11 and an output spindle 12. The latter
is drivingly coupled to the motor 11 by means of an angle drive 13 which comprises
a pinion 14 connected to the motor 11 and a bevel gear 15 connected to the output
spindle 12. The output spindle 12 is journalled relative to the housing 10 by a
forward ball bearing 17 and a rear ball bearing 18. The outer race of the forward
bearing 17 is supported in a detachable wall section 19 of the housing 10.
At its forward end, the output spindle 12 is provided with an automatic
ball type balancing device 21 and a mounting means 22 for a wheel type of grinding
tool (not shown).
The grinding tool mounting means 22 comprises a threaded coaxial bore
23 in the output spindle 12 for receiving a clamping screw 24, a radial support
shoulder 25, a threaded socket portion 26 coaxial with the bore 23, and a disc shaped
grinding tool support element 28. The latter is formed with a rear threaded neck
portion 29 for engagement with the socket portion 26. The pitch of this thread,
however, is bigger than the pitch of the thread of the clamping screw 24, which
means that the clamping screw 24 and the support element 28 can not be untightened
The support element 28 also comprises a forwardly directed tubular
neck portion 27 for centering cooperation with a corresponding central opening in
the grinding tool.
Moreover, the support element 28 has a coaxial opening 30 which is
provided with axially directed splines 31. A clamping element 32 is arranged to
cooperate with the clamping screw 24 to clamp the grinding tool against the support
element 28. The clamping element 32 has a tubular neck portion 33 provided with
splines 35 for cooperation with the splines 31 in the opening 30 of the support
element 28. See Fig. 4.
Due to the locking action of the spline connection between the support
element 28 and the clamping element 32 on one hand and due to the difference in
pitch between the threads on the clamping screw 24 and the support element 28 on
the other hand the grinding tool is prevented from comming loose as a result of
any accidental relative rotation between the grinding tool and the output spindle
The support element 28 is formed with a flat radial flange 36 which
is intended to be sandwiched between the support shoulder 25 on the output spindle
12 and the grinding tool. The support shoulder 25 forms together with the support
element 28 an axial backing means for the grinding tool as the latter is secured
by tightening of the clamping screw 24.
When after some service time the support element 28 has become worn
down to a certain extent it is easily exchanged by a new one. Without the employment
of a separate support element 28, the shoulder 25 on the output spindle 12 itself
would be subjected to the inevitable mechanical wear caused by the grinding tool.
An exchange of the entire output spindle 12 would be a much more costly operation.
The separate support element 28 also makes it possible to accomplish a simple adaptation
of the mounting means 22 to differently shaped grinding tools.
The balancing device 21, which is intended to compensate for dynamic
unbalance forces arrising in the grinding tool during its service life, comprises
a circular peripheral wall 38, a transverse end wall 39, an annular closure member
40, and a number of steel balls 41 freely movable along the peripheral wall 38.
For accurate guidance of the balls 41, the peripheral wall 38 is provided with an
internal part-spherical contact surface 43 of a very high quality as regards centering
visavi the rotation axis of the output spindle 12 and smoothness. This type of balancing
device is previously known per se and is described in for instance GB 832 048.
In the power tool, however, the transverse end wall 39 and the peripheral
wall 38 are formed integrally with each other as well as with the output spindle
12, and the transverse end wall 39 forms the radial support shoulder 25 of the grinding
tool mounting means 22.
Also formed in one piece with the output spindle 12 is a coaxial cylindrical
surface 42 located radially inside the balls 41 and having a smaller axial extent
than the peripheral wall 38.
The annular closure member 40 has a substantially L-shaped cross sectional
profile and is clamped by elastic expansion between the peripheral wall 38 and the
cylindrical surface 42. For securing the closure member 40 in this position, the
rear portion of the peripheral wall 38 is formed with an internal shoulder 44 for
cooperation with the outer rim portion of the closure member 40, thereby locking
the closure member 40 against rearward axial movement. See Fig. 3. O-rings 45, 46
are fitted in grooves 47, 48 in the peripheral wall 38 and the inner cylindrical
surface 42, repectively, for sealing cooperation with the closure member 40.
Close to the end wall 39, the output spindle 12 is formed with a radial
shoulder 49 and a cylindrical surface 50 for locating the inner ball race of the
forward bearing 17. Since the diameter of the cylindrical surface 42 is bigger than
the outer diameter of the bearing 17, it is possible to have the closure member
40 located partly outside the bearing 17. This means in turn that the forward end
section of the output spindle 12, the balancing device 21 included, is axially very
Further to the rear, the output spindle 12 comprises another radial
shoulder 52, a cylindrical surface 53 for guidingly supporting the bevel gear 15,
a spline portion 54, a further cylindrical surface 55 and a threaded portion 56.
The rearmost cylindrical surface 55 supports guidingly the inner race of the rear
bearing 18 and the threaded portion 56 is engaged by a clamping nut 58.
On the spline portion 54, there is supported an annular coupling element
59 which is formed with internal splines for driving connection with the spline
portion 54 and with forwardly extending coupling teeth 60. See Fig. 6. The latters
engage mating dog means 61 on the bevel gear 15 for transferring a driving torque
between the bevel gear 15 and the coupling element 59.
The inner ball race of the rear bearing 18, the coupling element 59
and the bevel gear 15 are axially clamped to a rigid unit between the clamping nut
58 and the shoulder 52. By this arrangement it is made possible to use a light fit
between the bevel gear 15 and the output spindle 12, which fascilitates dismantling
of the output spindle assembly.
The bevel gear 15 is formed with a forwardly extending neck portion
62 on which is mounted a sleeve element 63 for cooperation with a seal ring 64 mounted
in the housing 10. The purpose of the seal ring 64 is to prevent escape of the lubricating
grease originally applied to the angle drive 13.
The output spindle 12 together with the rear bearing 18, the coupling
element 59, the bevel gear 15 and the forward bearing 17 are axially clamped to
the housing 10 by means of a retainer element 65 located beneath the rear bearing
18 and secured to the housing 10 by means of two screws 66. See Fig. 4. A clamping
force is applied on the outer race of the rear bearing 18 by means of a washer type
spring 67 inserted between the bearing 18 and the retainer element 65. See Fig.
The axial clamping force exerted by the spring 67 is transferred to
the output spindle 12 via the rear bearing 18 and further to the housing 10 via
the output spindle 12, the forward bearing 17 and the wall section 19. By this arrangement
there is obtained an axial pretensioning of the ball bearings 17, 18 such that the
bearing plays are eliminated and the rotation accuracy of the output spindle 12
is very high.