This invention concerns a portable power grinder of the type including
a housing, a rotation motor with a rotor shaft having a rear end portion extending
out of the motor, an output shaft drivingly coupled to the motor rotor and intended
to carry a grinding tool, and a speed governor for providing a constant motor speed.
A serious problem to deal with at rotating power grinders is the
risk for accidentally reaching overspeed and a resulting fast desintegration or
blowing up of the grinding wheel. This may occur as a result of malfunction of
the speed governor of the grinding tool or, perhaps more commonly, as a result
of using a grinding wheel intended for a much lower maximum speed.
One reason why an improper grinding wheel may be mounted on the output
shaft is that the maximum speed marking on the tool housing is incorrect. The marking
may not inform of the maximum speed level provided by the speed limiting means
actually mounted in the tool. Most suppliers of power grinding tools offer a number
of different speed limiting means fitting into the same tool housing but providing
different maximum speed levels, and if a tool is assembled with a speed limiting
means which provides a higher maximum speed level than what is informed of by
the tool housing marking there is a great risk that the grinding wheel attached
to the output shaft will not withstand the inertia forces generated at this higher
It is an object of the invention to create a solution to the above
problem by suggesting an improved power grinder design which ensures that the maximum
speed level provided by the speed limiting means does not exceed the speed level
informed of by the tool housing marking.
A previously known power grinder having a speed marking which safely
corresponds to the speed limiting means actually fitted in the tool housing is
shown in U.S. Patent No. 4,444,272. In this prior art tool, the speed marking
is located on top of a non-rotating cup-shaped member forming an integral part
of the speed governor and overspeed safety device of the tool.This cup-shaped
member is exposed through an aperture in the tool housing such that the speed marking
is readable from outside the housing.
This known speed marking arrangement, however, is restricted in its
applicability to power grinders having fly-weight actuated speed governors and
overspeed safety devices which are mounted at the rear end of the motor shaft.
This restricted applicability is a drawback which is avoided by a
power grinder according to the invention.
Other features and advantages of the invention appear from the following
specification and claims.
A preferred embodiment of the invention is described below with reference
to the accompanying drawing.
In the drawing figure, there is shown a longitudinal section through
a portable power grinder according to the invention. The shown example comprises
an air turbine 10 as rotation motor which is located in a housing 11 and supplied
with motive pressure air through an inlet passage 12.
The housing 11 comprises a detachable lid 15 which covers the rear
end of the turbine 10 and which forms a part of the air inlet passage 12. When
the lid 15 is removed the air inlet passage 12 is open to the atmosphere and no
motive pressure air can reach the turbine 10. The lid 15 forms the upper part
of the tool housing 11 and carries the speed marking 21 of the tool. The speed
marking 21 informs of the maximum speed level of the output shaft 22 and is crucial
for the choice of grinding wheel to be mounted on the output shaft 22.
The turbine comprises a rotor shaft 13 and a turbine wheel 14. The
latter is formed with a peripheral row of blades 16 on which motive pressure air
is directed from a number of nozzles 17. Pressure air is supplied to the nozzles
17 via the inlet passage 12 and a speed governor valve 18. Downstream of the turbine
wheel 14 there is an exhaust passage 19 and a silencer 20.
The speed limiting means of this tool comprises both governor valve
18 which is controlled by a certain speed related control pressure obtained in
the turbine 10 and a reduction gearing 23 with an exchangeable gear ratio. The
governor valve 18 is arranged to maintain the turbine speed constant. Different
output shaft speed levels are accomplished by changing the gear ratio of the reduction
There is, however, only one version of the governor valve 18 available,
which means that overspeed can not be the result of fitting the tool with a governor
valve operating at too a high speed level in relation to the speed marking 21.
Since the speed governor valve 18 per se is not an essential part
of the invention it is not described in detail.
The turbine rotor shaft 13 is drivingly coupled to the output shaft
22 via the reduction gearing 23 which comprises a first gear 24 on the rotor shaft
13 and a second gear 25 on the output shaft 22. Both gears 24 and 25 are integral
with their respective shafts 14 and 22. The output shaft 22 is provided with a
mounting means 26 for attachment of a grinding wheel.
In a tool of this type having an air turbine as rotation motor it
is necessary to use a reduction gearing, because the turbine rotation speed of
about 60 000 rpm has to be reduced 7 to 10 times to be useful for grinding wheel
operation. Once you have a reduction gearing it is more beneficial to change the
gear ratio than fitting different speed governors 18 to obtain different output
speed levels. Accordingly, it is possible to have tools with different output
speed levels, although the turbine 10 and the speed governor 18 are all the same.
Changing the reduction gear ratio means that the rotor shaft 13 as
well as the output shaft 22 are exchanged for another set of shafts having gears
of other sizes.
The rotor shaft 13 is formed with a rear end portion 27 the length
of which varies with the size of the integral gear 24 such that a longer end portion
27 corresponds to a bigger gear 24.
Opposite the rotor shaft end portion 27, the lid 15 is formed with
an internal projection 28 the height of which is related to the speed level informed
of by the speed marking 21 on top of the lid 15. This means that there is availiable
a number of interchangeable lids 15 with different speed markings 21 and differently
shaped internal projections 28.
The speed markings 21 and internal projections 28 of the lids 15
are matched to the rotor shaft rear end portions 27 and, accordingly, to the gear
ratio of the reduction gearing 23. This means that a lid 15 with a certain speed
level marking can only be mounted on a tool having a reduction gearing 23 that
provides an output shaft speed that is equal to or lower than speed level informed
of by the speed marking 21. So, if one tries to mount a lid with a certain speed
marking 21 and a corresponding projection size on a housing containing a gearing
with too a low reduction ratio, i.e. a turbine rotor shaft 13 with too a big gear
24 and too a long rear end portion 27, the projection 28 will abut against the
rotor shaft end portion 27 and make a correct lid mounting impossible. The lid
15 will not land properly on the housing 11 and the pressure air inlet passage
22 will not be sealed off from the atmosphere so as to enable starting of the
The above arrangement means that a lid 15 with a certain speed marking 21 can never
be fitted to a tool containing an output shaft 22 which runs faster than the maximum
speed level informed of by the speed marking 21.