This invention relates to a pneumatic rotation grinder which comprises
a housing with a pressure air inlet passage and a manually operable throttle valve,
a rotation motor drivingly connected to an output shaft on which a grinding tool
is attachable, a safety guard adjustably supported on the housing in a coaxial
disposition relative to the output shaft, and a lock means located between the
safety guard and the housing and arranged to arrest the safety guard in desired
angular positions relative to the housing.
A prior art rotation grinder of the above type is described in E.P.
0 322 626. This previously known tool comprises a safety guard that is rotationally
locked to the tool housing by means of a latch element in selective cooperation
with a number of teeth or holes on the safety guard.
A drawback inherent in this known concept is that the inevitable
play, although small, existing between the safety guard and the housing as well
as between the latch element and the teeth or holes in the safety guard will cause
a rattling of the latter during operation of the tool. This rattling will in turn
result not only in an annoying noise but after some time also in a deformation
of the latch element and the cooperating teeth or holes, which will increase the
rattling, and so on. This problem will be significant if the safety guard is of
the vibration damping type having additional weights supported thereon.
The above problem is solved by the invention as it is defined in
A preferred embodiment of the invention is described below in detail
with reference to the accompanying drawings.
On the drawings:
Fig 1 shows a side view, partly in section, of a rotation grinder
according to the invention.
Fig 2 shows, on a larger scale, a fraction of the section in Fig
Fig 3 shows a top view of the safety guard.
The pneumatic rotation grinder shown in the drawing figures comprises
a housing 10 formed with a handle 11 and containing a pneumatic rotation motor
(not shown). The latter is drivingly coupled to an output shaft 12 which carries
grinding wheel mounting means in the form of a flanged hub 13, a centre bolt 14
and a washer 15.
Pressure air is supplied to the motor in a common way through an
inlet passage and a throttle valve in the handle 11. As this is a common feature
of most pneumatic tools, a detailed illustration and description thereof has not
been included in this specification.
A safety guard 17 of pressed steel has a welded-on mounting ring
18 with flat end surfaces 19, 20 and is rotationally supported on a neck portion
21 on the housing 10. This neck portion 21 is concentric with the output shaft
12 and is surrounded by a flat ring surface 22 on the housing 10. The neck portion
21 has an axial extent substantially equal to the thickness of the mounting ring
18. The latter is axially locked by a retaining ring 23 which is secured to the
housing 10 by a number of screws 24 and which has a flat annular surface 25 for
guiding cooperation with the annular surface 20 of the mounting ring 18.
At its periphery, the mounting ring 18 is provided with a row of
teeth 26, and a latch lever 27 pivotally mounted on the housing 10 is formed with
teeth 28 for locking cooperation with the safety guard teeth 26. See Fig 3. As
being illustrated in Fig 1, the latch lever 27 is biassed by a spring 29 towards
its teeth engaging position.
As illustrated in Figs 1 and 2, the retaining ring 23 is provided
with two concentric circular grooves 31, 32 supporting 0-rings 33 and 34, respectively.
Both of these 0-rings 33, 34, are in sealing contact with the end surface 20 of
the mounting ring 18 and form between them an annular sealed-off area 35, which
is illustrated by a hatched surface in Fig 3.
A U-shaped passage 36 in the retaining ring 23 communicates pressure
air from a passage 38 in the housing 10 to the ring area 35 between the 0-rings
33, 34. The passage 38 communicates with the air inlet passage of the tool, downstream
of the throttle valve.
The operation order of the adjustable safety guard mounting is the
In the rest position of the tool, the safety guard 17 is arrested
against rotation by the positive interengagement of the teeth 28 on the latch lever
27 and the teeth 26 on the mounting ring 18. By manoeuvering the latch lever 27
against the bias force of the spring 29 the locking interengagement of the teeth
28 and 26 is broken and the safety guard 17 is released for angular adjustment.
During operation of the tool, the safety guard 17 is arrested against
rotation not only by the teeth 26, 28 but is frictionally arrested in that the
annular surface 19 of the mounting ring 18 is axially clamped against the annular
surface 22 on the housing 10. This clamping action is obtained by pressure air
supplied through the passage 38 and the U-shaped passage 36 in the retaining ring
23 to the annular pressure area 35 between the O-rings 33, 34.
The frictional arresting of the safety guard 17 is of such a strength
that the positive tooth locking is superfluous as long as the throttle valve is
open and the motor is powered. However, before opening the throttle valve to start
the motor, i.e. when applying the tool to the workpiece, as well as immediately
after the throttle valve is closed, i.e. while the grinding wheel is still rotating,
it is important that the safety guard 17 is safely maintained in its desired position.
This purpose is served by the positive locking obtained by the latch lever teeth
engagement with the teeth on the safety guard.
By the above described arrangement it is possible to have a fairly
light fitting of the safety guard mounting ring 18 between the retaining ring 23
and the housing 10 so as to make an adjustment of the safety guard 17 easy once
you have released the latch lever 27. Any occuring rattling of the safety guard
17 relative to the housing 10 during operation of the tool is prevented by the
pressure clamping means formed by the O-rings 33, 34 and the pressure air supply
passages 36, 38.