Note: Descriptions are shown in the official language in which they were submitted.
BACKGROUND OF THE INVENTION
This invention pertains to improvements in torc~le limiting
or overload release clutches of the type which are used in power
tools for tightening threaded fasteners and the like. The develop-
ment of torque limiting clutches for power tools has resulted in
numerous inventions which are generally characterized by a driving
clutch member having an external polygonal cam surface and a
plurality of drive balls or rollers retained in a driven member
in yieldable engagement with the cam surface. At a predetermined
torque the balls or rollers ride up and over the peaks of the
cam surfaces to allow relative rotation between the driving and
driven members to limit the torque output of the tool. The im-
pulses caused by the balls riding over the cam peaks are normallysensed by the tool operator and serve as an indication that the
limit .orque output of the tool has been achieved. Examples of
the types of torque limiting clutches referred to above are dis-
closed in U.S. Patents 2,957,323 to C.A. Elliott et al; 2,978,082
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to B.R. Better, and 3,005,325 to R.E. Eckman. A variation of
the aforementioned clutch design is disclosed in U.S. Paten~
No. 3,937,036 to W.D. Sauerwein.
In the first three above mentioned patents the driving clutch
member is characterized by an external hexagonal cam configuration.
One disadvantage of this type of cam is that the peaks formed by
the intersection of the sides of the hexagon are subject to con-
centrated stresses which can result in rapid wear and early failure
of the mechanism. Other torque limiting clutch designs character-
ized b~ members which have sharp edged recesses from which thedrive balls are displaced also have the disadvantage of being
subject to rapid wear and causing poor torque control because of
the impacts or torque pulses which are delivered to the driven
member as the balls or rollers move in and out of the recesses.
SUMMARY OF THE INVENTION
The present invention provides for a torque limiting clutch
for a power tool or the like which is operable to allow the tool
motor and drive train ahead of the clutch to continue operating
when a limit torque has been exceeded without causing excessive
wear on the clutch members and while maintaining superior control
of the predetermined limit torque over an extensive torque range.
In the torque limiting clutch of the present invention im-
proved cperating life of the clutch is realized ~y a design wherein
one of the clutch members includes cam surfaces for engagement with
a plurality of drive balls which cam surfaces are not subject to
concentrated stresses and the rapid wear associated therewith.
In the present invention the driving or driven clutch member may
be characterized by a bore having an inner surface geometry or
profile which is operable to permit movement of the balls over
the cam surfaces at an accurately controlled predetermined torque
and with an unexpected improvement in the operating life of the
cam.
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In accordance with one broad aspect, the invention
relates to a torque limiting clutch for use in a power operated
tool for tightening threaded fasteners and the like, said
clutch including: a rotatable driven member; a driving
member rotatable with respect to said driven member at a
predetermined torque transmitted by said clutch to a driven
spindle; one of said members being characterized by a
continuously curved internal cam surface including a plurality
of spaced apart recesses extending radially outwardly with
respect to the axis of rotation of said spindle and in a plane
perpendicular to the axis of rotation of said one member said
recesses are formed by substantially concave curved surface
portions which are interconnected by substantially convex
curved surface portions disposed radially inwardly of said
concave curved surface portions with respect to the axis of
rotation of said one member; the other of said members including
a radially, outwardly extending rim having at least one
transverse face and a plurality of radially extending grooves
in said rim opening to said face and being spaced apart so as
to be respectively alignable with said recesses; movable
elements disposed in said grooves and operable to be biased
radially outwardly into said recesses for coupling said driving
and driven members for rotation together to transmit torque
to said spindle; a third member having a surface disposed
adjacent said transverse face on said other member and engaged
with said movable elements; and, resilient means engaged with
said third member for causing said third member to yieldably
urge said movable elements into said recesses in said one member
and whereby at a predetermined limit torque said elements are
forced radially inwardly in said grooves by said cam surface
to allow said driving member to rotate with respect to said
driven member.
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One embodiment of the present invention also provides a
torque limiting clutch which may be adapted to operate over a
large range of limiting torques by interchanging an inexpensive
member of the clutch assembly.
The present invention further provides for a torque limiting
clutch which has a relatively few working parts, is inexpensive,
and is easy to manufacture.
BRIEF DESCRIPTION OF THE DRAWINGS
Fig. 1 is a lcngi~udinal sectional view of one embodiment
of a torque limiting clutch of the present invention;
Fig. 2 is a section view taken along the line 2-2 of Fig. l;
Fig. 3 is a longitudinal sectional view of an alternate em-
bodiment of the torque limiting clutch;
Fig. 4 is a section view taken along the line 4-4 of Fig. 3;
and,
Fig. 5 is a perspective view of an alternate torque control
ring for use with the clutch embodiment shown in Figs. 1 and 2.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
Referring to Fig. 1 a torque limiting clutch in accordance
with the present invention is generally designated by the numeral
10 and is shown disposed in a housing 12 comprising a portion of
a power tool for tightening threaded fasteners and the like. The
housing 12 is adapted to be removably connected to a tool motor
unit, not shown, in a conventional manner. The housing 12 includes
a bore 14 comprising bearing means for supporting a rotatable
spindle 16. The spindle 16 includes a removable sleeve 18 having
a polygonal bore 20 engaged with one end of the spindle and adapted
to receive a tool bit or the like, not shown. The opposite end of
the spindle 16 includes a cylindrical bearing portion 22 on which
is mounted a driving member 24 of the torque limiting clutch 10.
The member 24 is retained on the bearing portion 22 by a retaining
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ring 26 and is operable to rotate relative to the spindle 16,
a portion of which comprises the driven clutch member to be
described herein, when a limit torque is exceeded.
The driving member 24 includes axially projecting teeth 28
which are cooperable with similar interfitting teeth 30 formed
on a drive shaft 32 which is adapted to be driven by the tool
motor in a known way. The positive engaging clutch formed by
the teeth 28 and 30 becomes engaged in response to axial pressure
exerted on the spindle 16 by the tool operator which will move
the spindle and the clutch 10 into position to engage the teeth
28 with the teeth 30 on the drive member 32. When this axial
pressure is relieved a spring 34 disposed in a bore 36 in the
spindle 16 will return the spindle to the position shown in Fig. 1,
disengaging the member 24 from the member 32.
Referring to Fig. 2 also, the driving member 24 of the torque
limiting clutch is further characterized by an integral axially
projecting cylindrical part 38 having an internal cam surface
formed by a plurality of equally spaced concavely curved recesses
40 extending radially outwardly with respect to the axis of rota-
tion 42 of the spindle 16. The cam surface is further characterized
by convex curved portions 44 interposed between adjacent recesses
40 and preferably tangent to the concave curved surfaces at their
respective meeting points. Alternatively, the portions 44 may be
flat or slightly concave surfaces.
The spindle 16 has an integral rim portion 46 disposed acjacent
the internal cam surface on the driving member 24 and having a
plurality of radially extending equally spaced grooves 48 which
open to the circumference of the rim and also to one transverse
face 50 of the rim. As shown in Fig. 2 also, the torque limiting
clutch 10 includes spherical balls 52 disposed in each of the
grooves 48 which are equal in number to the recesses 40. The
balls 52 are retained in the grooves 48 by a ring 54 disposed on
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the spindle 16 and biased into engagement with the balls by a
compression coil spring 56. The spring 56 is retained on the
spindle 16 by a nut 58 engaged with a threaded portion 60 of
the spindle. The compression force exerted by the spring 56 on
the ring 54 may be varied by changing the position of the nut.
An axially slidable washer 62 is interposed between the spring
56 and the nut 58 and includes a detent ball 64 which engages a
recess 66 in the nut to prevent unwanted rotation thereof with
respect to the spindle 16.
The ring 5~ includes an axially sloping or frustoconical
surface 66 which is in engagement with the balls 52 and biases
the balls radially outwardly into the recesses 40 as shown in
Fig. 2. The angle that the surface 66 forms with respect to the
axis of rotation 42 of the spindle 16 affects the torque value
at which the balls 52 may be forced radially inwardly in the
grooves 48 to allow the member 24 to rotate with respect to the
spindle 16 to limit the torque transmitted by the clutch. The
torque limit setting of the clutch 10 is also controlled, of
course, by the compression of the spring 56 and the axial force
exerted thereby on the ring 54. The compression of the spring
56 may be adjusted by turning the nut 58 in one direction or the
other.
The torque range of the clutch 10 may be extended by inter-
changing the ring 54 with, for example, a ring 70 as shown in
Fig. 5 which has an axially sloping surface 72 which forms an
angle with the central axis of the ring different from the surface
66. Since the force transmitted from the balls 52 to the ring 54
is normal to the surface 66 the axial force component of the net
force transmitted to the ring increases with the slope of the
surface 66. Accordingly, a ring similar to the ring 54 with a
more steeply sloping ball engaging surface will cause the clutch
10 to slip at a lower torque for a given setting of the compression
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of the spring 56. Accordingly, by merely interchanging the ring
54 with the ring 70 the overall range of torques for which the
clutch may be operated may be extended.
In the operation of a hand-held power operated tool including
the clutch 10 the clutch formed by the teeth 28 and 30 is engaged
by applying axial pressure to the spindle 16 through a bit or the
like disposed in the socket 20. When the driving torque of the
motor transmitted by the clutch 10 exceeds the limit value the
balls 52 are forced radially inwardly in the grooves 48 by the
i0 cam surfaces 40 and 44 on the driving member 24 as the ring 54 is
forced axially away from the grooves, and the driving member
rotates with respect to the spindle 16 which normally stalls at
the limit torque. The continued rotation of the driving member
24 imparts gentle reaction impulses to the tool housing which are
sensed by the tool operator to indicate that the tool operating
cycle is complete. The smooth curved contour of the internal cam
surface formed by the recesses 40 and the convex portions 44
eliminates any highly stressed areas on the cam and promotes
longer service life of the clutch parts. The cam surfaces on
the driving member 24 also reduce the magnitude of the impacts
or torque pulses which are transmitted by prior art clutches and
which result in inaccurate final torques applied to the fastener.
Referring to Figs. 3 and 4 of the drawings an alternate em-
bodiment of a torque limiting clutch in accordance with the present
2~ invention is shown and generally designated by the numeral 80. The
torque limiting clutch 80 is disposed in a housing 82 including
bearing means 84 for supporting a rotatable and axially slidable
spindle 86. The spindle 86 includes a socket 88 for receiving the
shank of a screwdriver bit or the like, not shown. The spindle
86 also includes an axially projecting trunnion 88 which partially
supports a rotatable shaft 90 adapted to be suitably connected
to a motor, not shown. A coil spring 92 disposed in a bore 94
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in the shaft 90 biases the axially slidable spindle 86 in the
position shown in Fig. 3.
In the clutch embodiment of Figs. 3 and 4 a driven clutch
member 96 is mounted on the shaft 90 and is rotatable with respect
to the shaft on suitable bearing balls 98 disposed in cooperating
grooves on the shaft and the driven member, respectively. The
member 96 is characterized by an axially extending cylindrical
portion 100 having an internal cam surface formed by concavely
curved recesses 102 which are spaced apart equidistant from each
other and are interconnected by flat surface portions 104. The
cam surface on the driven member 100 is disposed around a radially
extending rim 106 on the shaft 90 which includes a plurality of
equally spaced grooves 108 equal in number to the recesses 102.
The grooves 108 open to the circumference of the rim 106 and also
to a transverse face 110 of the rim. As shown in Fig. 3 the
grooves 108 have an inclined bottom surface 112 which intersects
the face 110 and the circumference of the rim 106.
The clutch 80 is further characterized by a flat sided ring
114 which is disposed adjacent to the face 110 and is biased
toward the face by a coil spring 116 disposed around the shaft 90.
The spring 116 is retained on the shaft by a washer 62 and a nut
58 whereby the compression of the spring may be varied to control
the limit tor~ue of the clutch 80 in the same manner as the clutch
10. The ring 114 is engaged with spherical balls 118 disposed one
in each of the grooves 108. The balls 118 are biased radially
outwardly to be seated in the recesses 102 by the force of the
spring 116 acting through the ring 114.
The clutch embodiment shown in Figs. 3 and 4 includes a posi-
tive engaging clutch comprising axially projecting interfitting
teeth 120 and 122 disposed on the spindle 86 and the clutch
member 96.
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The operation of the clutch 80 is similar to the clutch 10
shown in Figs. 1 and 2. With the teeth 120 and 122 interengaged,
as a result of axial force applied to the spindle 6, the member
96, shaft 90 and the spindle may be rotated in unison. At a pre-
determined limit torque the balls 118 will move radially inwardlyin the grooves 108 as they are forced out of the recesses 102 and
along the flat surface portions 104. The shaft 90 may then con-
tinue to rotate while a gentle impulse action is sensed by the
operator of the tool to indicate that a limiting torque has been
applied to the spindle 86. The clutch 30 includes the advantages
of the embodiment shown in Figs. 1 and 2. The internal cam sur-
face formed by the concavely curved recesses 102 and the inter-
connecting f-lat surface portions 104 does not have any sharp
projections or portions which are highly stressed. Moreover,
the contour of the cam surface reduces the sharp impacting and
torque variations normally encountered with prior art torque
limiting clutches.
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