The invention relates to a method for detecting the attainment of
a predetermined target torque level when tightening screw joints by means of a
power wrench having a mechanical override type release clutch, and a power wrench
including that type of clutch as well as means for detecting the release of the
clutch and, hence, the attainment of the predetermined target torque level.
In one type of prior art power wrenches, as shown for instance in
U.S. Patent No. 5,201,374, a predetermined release torque level is detected by
a mechanical device connected to one of the clutch halves for transferring a release
movement to a power shut-off device. This mechanical type of target torque detection
suffers from the drawback of an undesirable mechanical influence on the clutch
operation in that the release torque level to some extent is dependent on the frictional
resistance in the release detecting mechanism.
In another type of prior art wrenches, as shown in for instance U.S.
Patent No. 5,868,208, the attainment of a predetermined target torque level is
indicated by a micro switch indicating a release movement of one of the clutch
parts. The switch is usually connected to an electric control unit for initiating
power shut-off as a clutch release is indicated. This type of clutch release sensing
device is disadvantageous in that it is exposed to a mechanical wear thereby having
a limited service life.
In still another type of prior art wrenches the clutch release is
indicated by a Hall-element located close to the clutch for detecting the release
movement of one part of the clutch and delivering an electric signal in response
thereto. This type of release detection, however, requires a specific clutch design
providing an extended or amplified release movement for ensuring a proper activation
of the Hall-element.
In order to avoid the problems and drawbacks of the above described
previous devices for detecting the target torque level and clutch release point
the invention provides a technique which does not have any influence mechanically
or otherwise on the release movement of the clutch and which requires very little
space and which does not require any special clutch design.
A general object of the invention is to provide a technique where
the actual release of the clutch is detected. This is in contrast to most other
methods where the change in position of a moving part of the clutch is detected,
which might not correctly reflect the fact that the clutch has actually released.
Especially at low output shaft rotation speed the inertia of the motor and gear
mechanism might not be enough to ensure clutch release after a specific position
for the clutch has been detected.
So, the main object of the invention is to provide a method and a
power wrench where the attainment of a predetermined target torque is detected
as a release sound of a torque responsive release clutch of the override type comprised
in the power wrench.
Further objects of the invention will appear from the following specification
A preferred embodiment of the power wrench according to the invention
is described below with reference to the accompanying drawing.
The drawing shows schematically a power wrench according to the invention
and comprising a housing 10 with a rear handle 11 and a forward angle head 12.
An output shaft 14 is journalled in the angle head 12 and is arranged to carry
a nut socket for connection to a screw joint for tightening same. Within the housing
10, there is supported an electric rotation motor 15, a reduction gearing 16 and
an override type torque release clutch 17.
Within the handle 11 there is located an electronic control unit
20 including a motor operation governing means. The control unit 20 is connected
on one hand to the motor 15 and on the other hand to a power source via a supply
cable 21. The latter is connected to the rear end of the handle 11 and communicates
electric power from the power source, i.e. a network, to the motor 15 via the control
At its motor end of the control unit 20 there is provided a microphone
22 utilised for detecting the characteristic release sound generated by the clutch
17. The microphone 22 delivers electric signals to the control unit 20, and the
latter includes a signal identifying device which is arranged to accomplish braking
of the motor 15 as soon as it recognises the very signal caused by the clutch 17
during the release movement.
Depending on the clutch design, there is generated a single or a
double "click" as the torque transferring rolling elements and cam profiles of
the clutch override at the pre-set target torque level. This "click" sound has
a characteristic frequency and duration, and in the case of a double "click" there
is a characteristic short interval between the "clicks". These identification characteristics
are programmed into the signal identifying device and when there is a coincidence
between the programmed data and the signals actually received from the microphone
22 the identifying device delivers an output signal to make the control unit 20
stop the motor 15. The task of the identifying device is to detect and separate
the "click" sound of the release clutch from other spurious signals caused by
sounds of other frequencies and duration occurring in the power wrench, for instance
from the motor 15 and the reduction gearing 16.
In order to get a safer identification and separation of the clutch
release "click" related signals the identifying device comprises filters for suppressing
all signals related to other inadequate sounds. Sounds generated by the motor
15 and the gearing 17 are usually of a lower frequency and are easy to separate
from the "click" sound related signals. Some sounds emanating from strikes on the
tool housing 10 and/or on the output shaft 14 are often more alike the "click"
sound. In particular, the strike sound generated when a nut socket fitted to the
output shaft 14 falls down into its proper engagement with the screw head or nut
under axial load at some point during the tightening process. Such nut socket fall
down may occur when reaching the torque snug level of the joint and the torque
resistance in the screw thread increases.
If a single "click" type clutch is used there has to be a very careful
identification of the signals to safely establish whether a clutch release has
actually taken place. Should a signal emanating from an external strike on the
power tool be mistaken for a clutch release signal there would be generated an
erroneous stop signal interrupting the tightening process at a premature stage.
That would be a serious error, because the screw joint might not be tightened
at all. If, however, a double "click" type of clutch is used it is much easier
to separate the clutch release signal from occurring strike related signals, because
in this case you can safely identify the double "click" signal.
The control unit 20 also comprises a brake sequence for accomplishing
braking of the motor 15 to a quick stop as the "click" sound related signal is
detected, which means that possible torque overshoots due to remaining kinetic
energy in the rotating parts will be avoided.
It is to be noted that the invention is not limited to the above
described example but can be freely varied within the scope of the claims. For
instance the location of the microphone may be another than what is described.
From the sound recording point of view it might be advantageous to locate the
microphone closer to the release clutch. The technique according to the invention
may be used in power wrenches of different types, for instance in power wrenches
having different configurations like straight, angular and pistol handle type,
mains or battery powered.