The invention relates to a power nut runner of the type comprising
a rotation motor for driving an output shaft via a torque responsive release clutch
including a driving clutch half, a driven clutch half, and a cam mechanism for
transferring torque between the driving clutch half and the driven clutch half,
wherein a first one of the driving and driven clutch halves is axially moveable
by the cam mechanism in a release direction, from an engagement position to a
release position, a spring arranged to bias and displace the first clutch half
in an engagement direction, and a clutch release detecting device cooperative
with the movable first clutch half.
In prior art nut runners of the above type, described for instance
in US Patents 4,838,364 and 5,201,374, there are provided clutch release detecting
devices including an activation rod extending axially through the rotor of the
motor. Since the activation rod is not rotationally locked to the rotor, there
will always be a difference in speed between the rod and the rotor which inevitably
results in a frictional wear of the rod.
Moreover, in most tools of this type there is employed a speed reduction
gearing between the motor and the clutch, which means that there is also a difference
in speed between the rotor and the activation rod support point on the clutch.
This amplifies the problem of frictional wear of the rod, and despite an adequate
surface hardening of the rod, there is still a problem with a limited service
life of the device.
In US Patent 4,231,270, there is described a power screw driver in
which a micro switch is actuated by one part of a torque limiting clutch provided
between the ring gear of a planetary reduction gearing and the tool housing. This
concept, however, is less advantageous in that the release detecting switch is
activated by a clutch which is not an in-line clutch, i.e. the clutch does not
transfer the driving torque. This results in a slower and less accurate release
action and release detection.
In US Patent 3,608,686, there is described a torque responsive in-line
clutch with an overload detecting micro-switch intended for preventing damage on
machine tool parts by initiating disconnection of a drive motor. A disadvantage
inherent in this device refers to an indistinct action of the switch due to a rather
short and slow axial release movement of the clutch part. This results in a less
accurate release detecting signal. Another disadvantage of this known device is
the continuous sliding action between the stationary micro-switch arm and the
rotating clutch, which inevitably results in a frictional wear of these parts.
A disadvantage also relating to the clutch operated switch shown
in US Patent 4,231,270 resides in the fact that the release movement of the clutch
is rather short and that the switch has to be activated somewhere during that movement.
Due to this short activation movement, the switch has to be very carefully adjusted
to ensure a proper activation. This makes the release detecting and power shut-off
mechanism rather sensitive and less reliable.
The primary object of the invention is to accomplish a power nut
runner by which the above mentioned problems are avoided by providing a torque
transferring clutch with a release detecting device which provides a distinct and
prompt power shut-off initiating movement.
Further objects and advantages of the invention will appear from
the following specification and claims.
A preferred embodiment of the invention is below described in detail
with reference to the accompanying drawing figures.
On the drawings:
Fig.1 shows a side view, partly in section, of a power nut runner
according to the invention.
Fig. 2a shows schematically and on a larger scale a longitudinal section through
the torque release clutch and shut-off initiating mechanism of the power tool in
Fig. 1 and illustrates the mechanism in a low torque condition. Fig. 2b illustrates
the mechanism in Fig. 2a in a release position.
Fig. 2c illustrates the mechanism in Fig. 2a in a shut-off initiating position.
Fig. 3a shows schematically and on a larger scale a fractional view of the torque
transferring balls and clutch pockets in their low torque transferring positions.
Fig. 3b shows the view in Fig. 3a, but illustrates the beginning of the release
displacement of the clutch.
Fig. 3c shows the view in Fig. 3a, but illustrates the fully released position
of the clutch.
The power nut runner shown in Fig. 1 comprises a battery powered
electric motor 10 controlled by a manually operated on/off switch (not shown) and
an automatically operated shut off switch (not shown).
The nut runner further comprises a housing 11 divided into a forward
section 12 and a rear section 13. In the rear section 13, there is located a double
planetary type reduction gearing including a first stage 14 with a sun gear 15
rotated by the output spindle 16 of the motor 10, a number of planet wheels 17
journalled on a planet wheel carrier 18, and a ring gear 19 secured in the housing
11. A second stage 20 includes a sun gear 21 formed on the planet wheel carrier
18, a number of planet wheels 22 journalled on a planet wheel carrier 23 and engaging
the ring gear 19 which, accordingly, is common to both gearing stages. As illustrated
in Fig. 1, the planet wheel carrier 23 of the second stage 20 is connected via
a coupling member 24 to a driving clutch half 25 of a torque responsive release
The release clutch 26 also comprises a driven clutch half 27 which
is formed integrally with the output shaft 28 and the screw bit attachment 29 of
the nut runner. The driven clutch half 27 includes an annular thrust element 30,
a number of torque transferring balls 31, a bias spring 32 for pre-loading the
thrust element 30 onto the balls 31, and an adjustable spring support 33. The latter
is axially supported by a ring nut 34 threadingly engaging the output shaft 28.
The thrust element 30 as well as the driving clutch half 25 are provided with pockets
35 and 36, respectively, for receiving the balls 31 and for transferring torque
between the clutch halves 25 and 27, in a conventional way. The pockets 35,36 are
formed with slanted side walls which together with the balls 31 accomplish an
axial displacement of the thrust element 30 as the clutch halves 25,26 are rotationally
displaced relative to each other at a certain predetermined torque load. See Figs.
Moreover, between the driven clutch half 27 and the thrust element
30 there is provided a ball spline connection 38 for enabling a simultaneous torque
transfer and axial displacement between the thrust element 30 and the driven clutch
The thrust element 30 is provided with a sleeve member 39 which extends
rearwardly from the thrust element 30 into an abutting engagement with an activation
element 40. The latter is made of steel and comprises a rearwardly extending sleeve
portion 41 and a flange portion 42. The sleeve portion 41 is movably guided on
an outer cylindrical surface 43 of the ring gear 19, and an inner part of the
flange portion 42 is intended to be abuttingly engaged by the rear end of the sleeve
member 39. A number of magnets 44 are mounted in a common plane and in a circle
at the rear end of the forward housing section 12 and arranged to generate an
attraction force on the flange portion 42 of the activation element 40.
In the rear housing section 13, there is mounted a shut-off switch
46 which is arranged to be activated by the activation element 40. Preferably,
the shut-off switch 46 is of the non-contact Hall-element type which is triggered
by the mere presence of the sleeve portion 41 of the activation element 40.
In operation of the nut runner, during the initial running down phase
of a screw joint tightening process, the torque delivered by the motor 10 via the
output spindle 17 is transferred through the reduction gearing stages 14,20 and
the coupling member 24 to the driving clutch half 25. Then, the torque is transferred
via the balls 31 and the thrust element 30 to the output shaft 28 and further to
the screw joint being tightened via the screw bit attachment 29.
Initially, the reaction torque from the screw joint is low enough
not to make the balls 31 climb the slanted walls of the pockets 35, 36 in the thrust
element 30 and the driving clutch half 25, respectively, against the bias load
of the spring 32. At this stage of the tightening process, which is illustrated
in Fig. 2a, the activation element 40 occupies its inactive position in which it
is drawn against the end of the sleeve member 39 by the magnets 44.
As the reaction torque from the screw joint has increased to a certain
level, the camming action between the slanted walls of the pockets 35,36 and the
balls 27 will make the thrust element 30 move axially (to the right in Figs. 2a-c)
against the bias load of the spring 32. This results in a subsequent movement
of the sleeve member 39 as well as the activation element 40 under the action of
the magnets 44 until the activation element 40 gets into contact with the magnets
44. This position is illustrated in Fig. 2b.
At continued rotation of the driving clutch half 25, each one of
the balls 31 will climb up the slanted walls of the pockets 35,36 in the driving
clutch half 25 and the thrust element 30 and pass an apex before falling into the
next pockets, in a way common to this type of clutch. However, when the balls
31 fall into the next pockets, the thrust element 30 is accelerated very abruptly
by the force of the spring 32. This means that the thrust element 30, the sleeve
member 39 and the activation element 40 are abruptly accelerated as well, and when
the thrust element 30 and the sleeve member 39are stopped as the balls 31 reach
the bottoms of the new pockets, the activation element 40 will continue its movement,
to the left in Figs. 2a-c, as a result of its inertia, i.e. the kinetic energy
gained during the return movement of the thrust element 30.
Now, the activation element 40 will reach its active position, beyond
its inactive low torque position as shown in Fig. 2a, such that the sleeve portion
41 gets into a position opposite the sensor 46, thereby making the latter deliver
a signal for initiating shut-off of the motor 10. See Fig. 2c.
After having been thrown backwards to its active switch triggering
position, the activation element 40 returns immediately to its inactive position,
as shown in Fig. 2a, by the attraction force of the magnets 44. In this position
the activation element 40 re-assumes its abutting engagement with the sleeve member
By arranging the activation element 40 freely movable in the re-engagement
direction of the clutch there is obtained a distinct and extended activation movement
of the activation element 40 such that the triggering of the shut-off initiating
switch 46 distinctly and safely indicates that the clutch has been released and
that the intended torque level has been obtained.
Although in the above described embodiment of the invention there
is used a Hall-type switch for accomplishing a contact-less activation, the invention
is not limited to this type of switch. However, a contact-less activation is preferred
because it does not suffer from mechanical wear.
Neither is the invention limited to the use of magnets for biassing
the activation element 40 towards the inactive position of the latter. Alternatively,
some type of spring may be used. Magnets are preferred though, because they are
not exposed fatigue stresses.