Note: Descriptions are shown in the official language in which they were submitted.
-- 1 --
SHEET METAL SC~EW
Back~round and a Brief Description of the Invention
The present invention is directed to a sheet me~al screw.
More particularly, the presen~ invention i.s directed to
5 a screw for attaching two thin panels together, which
screw will exhibit increased stripping torque over o~her
known sheet metal screws
With currently available sheet metal screws, there is
but a very small torque range between drive torque and
strip torque. Thus, it is virtually impossible, given
dimensional tolerances and variances in materials, to
find a single torque setting on power screw drivers
which will drive the hardest-to-drive screw without
stripping the most strippable. This means a compara-
tively high number of screws will be stripped which re-
sults in reduced clamp load or requires substantial
operator time for removal and replacement.
The screw of the present invention reduces the above-
stated problem by increasing ~he size of th~ range be-
tween drive and strip torque. The sheet metal screw of
the present invention has a recess in the lower side of
the head surroundLng the upper region of the shank.
Within this recess and for some distance beyond, the
shank has formed thereon a helioal ramp which may form
an extension of the thread on the shank. Thîs ramp can
readi`ly be formed during the heading process. The ramp
~k
-- 2
preferably has teeth for~ed thexeon which further incxease
stripping torque or increase backout resistance depending on
the orientation of these teet~.
The invention, in its broades~ aspect, contemplates a rotary
fas-tener for use in attachment of a ~irst panel with a clearance
hole to a second panel. The fastener comprises a shank having
an axis and an enlarged head having drive-inducing surfaces on
one end of the shank. The head has an under surface, and the
shank has at least one helical thread thereon. The at least
one helical thread extends outwardly from the shank for a pre-
determined height that extends from a thread root diameter to
a thread crest diameter. A ramp begins substantially at one
termination point of the at least one helical thread and forms
a continuation thereof. The ramp comprises a runner surface
adjacent a face surface, with the runner surface being generally
perpendicular to the shank and generally advancing helically
along the shank. The runner surface is generally perpendicular
to the face surface.
In a further embodiment, the invention contemplates a rotary
fastener for use in attachment of a first panel with a clearance
hole to a second panel. The fastener comprises a shank having
an axis and an enlarged head having drive~inducing surfaces on
one end of the shank. The head has an under surface, and the
shank has at least one helical thread thereon. The at least one
helical thread extends outwardly from the shank for a predetermined
height that extends from a thread root diameter to a thread crest
diameter. A ramp begins substantially at one termination point
of the at least one helical thread and forms a continuation
thereof. The ramp comprises a runner su face generally advancing
helically along the shank, and the ramp further comprises a face
surface. The face surface is adjacent the runner surface and
gerlerally parallel to the axis. The face surface establishes a
first boundary of the runner surface. A second boundary of the
runner surface comprises the sha~k, with the ramp having a radially
spiral configuration and the face surface having a taper. The ramp
has a mini~um radial extent at a point on the ramp opposite the
under surface and the face surface has a maximum axial length at
the point of the minimum radial extent of the ramp.
;3
-- 3 --
Additionally, there is provided a rota~ fastener for use in
attachment of a first panel with a clearance hole to a second
panel. The fastener comprises a shank having an axis, and an
enlarged head having drive-induci~g surfaces on one end of -the
shank. The head has an under surface, and the shank has at least
one helical thread thereon. The at least one helical thread
extends outwardly from the shank and having a generally constant
height, the thxead having a thread root diameter and a thread
crest diameter. A ramp has a face portion and a runner portion,
with the face portion being bounded by t~e under surface at a firs-t
end and by the runner portion at a second end. The runner portion
is bounded by the face portion at a first edge and by the shank at
a second edge. The face portion is oriented such that, at any
poin-t of the face portion, a plane tangent to the face portion will
be generally parallel to the axis of the shank. The ramp extends
helically along the shank to the under surface, with the ramp
having a spiral cross-section with an initial radial extent less
than half of the crest diameter and a final radial extent greater
than half of the crest diameter.
Other characteristics, features and advantages of the present
invention will become apparent after a reading of the following
specification.
Brief Description of the Drawings
FIG. 1 is an enlarged side view in partial section of the head
region of the sheet metal screw blank used to form the screw of
the present invention;
FIG. 2 is a lateral cross-sectional view of the screw blank shown
in FIG. 1 as seen along line 2-2;
FIG. 3 is a side view in partial section showing the screw of the
present invention in operative engagement with two sheet metal
panels;
FIG. 4 is a lateral cross-sectional view similar to FIG. 2 showing
the helical ramp with strip-torque-increasing teeth;
FIG. 5 is a lateral cross-sectional view similar to FIG. 2 showing
the helical ramp with backout-resistant tee-th;
FIG. 6 is a lateral cross~sectional view similar to FIG. 2 showing
first and second helical ramps;
FIG. 7 is a somewhat schematic view of a modification of the
invention with variation in the ramp;
FIG~ 8 is a view si~ilax -to FIG. 7 showin~ ~othe~ modi~ication
with a further modification of the ramp;
FIG. 9 is a fragmentary view generally similar to FIG~ 1 and
showing another modification of the invention;
FIG~ lO is a cross-sectional view ta~en substantiall~ on the line
10-10 in FIGo 9;
FIG. 11 is a rragmentary view partially in section showing the
screw of FIG. 9 with a metal beari~g panel and a plastic second
panel; and
FIG. 12 is a view similar to FIG~ but with a plastic bearing
panel and a metal second panel.
Detailed Description of the Present Invention
The rotary fastener, or sheet me-tal screw, of the presen-t invention
is shown generally at 10. Head 12 has a flange 14 which extends
laterally from a generally cylindrical shank 15. The lower side
16 of head 12 is undercut forming a recess 18. The upper side of
head 12 has drive inducing surfaces 19 thereon. ~ helical ramp 20
extends from a point outside the recess 18 upwardly into the recess.
Threads 22 shown in phan-tom in FIG. 1, will extend laterally about
shank 15 such that helical ramp 20 will Eorm a con-tinua-tion thereof.
The thread 22 will be formed partially from the lower extremity of
the ramp and, since the ramp exceeds -the blan~ diameter of the
shank, additional material is provided to insure fuller thread
formation in this region. In conventional sheet metal screws,
this upper thread tends to be only partially Eormed due to the fact
-that the thread rolling dies cannot reach into the recess and can,
therefore, only c3ather material from one side of the upper thread.
The ramp 20 is comprised of an axial face por-tion 33 facing
radially outwardly such that planes tangent to -the face por-tion are
generally parallel to the axis of the screw. The face surface 33
is bounded on its upper side by the under surface 16 of the screw
head 12 and by a runner surface 35 on its lower side. The runner
surface 35 extends from its intersection with the face surface 33
radially inwardly to the shank of the screw. The teeth 2~ are
comprised of a generally radially oriented sur~ace 25 and a
generally circumferentially oriented surface 27.
It should be noted that the axial length of the ~ace surface 35
of the ramp decreases as it approaches the under surface 16 of
the screw. At the same time, the radial distance between the
axis of the screw and the face surface 35 increases as the ramp
extends between the thread 22 and the under surface 1~. The
radial extent of the ramp is initia~ly significantly less than
that of the thread crest a-t its beginnin~ point adjacent the
thread. ~owever, adjacent the under surface 16, the ramp has a
radial extent significantly greater than the thread crest. This
is possible because the ramp is ~ormed during a heading operation
as opposed to thread rolling operation. The simultaneously
spiralling and tapering face portion engages workpiece material
to increase the amount of torque required to strip or remove the
screw.
This helical ramp 20 is preferably formed during heading and
can be provided with whatever configuration desired.
.. i ~,G
/, .
~ 3
For example, the ramp 20 may be serrated so as ~o have
teeth 24 which point in the same direction as the screw's
rotational direc~ion (FIG. 4) or opposite to that rota-
tional direction (FIG 5). Teeth 24 will, then, increase
_ 5 stripping torque or backout resis~ance, respectively. A
second helical ramp 30 (FIG. 6) may be provided which
can have teeth 24 which projec~ in a direction opposite
to those of ramp 20 so as to provide both increased
s~rip torque and backout resistance,
In operation, as best shown in FIG. 3, screw lO attaches
a first or bearing panel 26 to a second or tapping panel
28. The first panel 26 may typically be provided with a
clearance hole 31 which has a dimension exceeding that
of threads 22. As the screw 10 reachPs its fully seated
position where the material of the second panel 28 would
ordinarily strip out, the inner edge 32 is instead
brought into engagement with helical ramp 20. As the
edge 32 climbs higher on ramp 20, the diameter of the
ramp, and accordingly, the magnitude of the stripping
torque for the screw, increases, As shown in FIG. 4,
helical ramp 20 may be so dimensioned as to engage (and
in the case of the toothed ramp), bite into the inner
edge of clearance hole 31. This will further increase
the value for the strip torque.
By the way of example, the helical ramp preferably has
a length equal to one and one-quarter times the thread
pitch. .375 of a pitch is preferably within the
confines of recess 18 with the remaining ,875
pitch length extending beyond the recess to form the
linkup with thread 22. The minimum height of the helical
ramp 20 will be the blank diameter which approximates
the diameter of the root of the thread. The maxim~m
height of the ramp 20 (as measured from the root dia-
~ 3
-- 7
meter) will be at least equal to the thread crest height(The thread crest diameter less the root diameter).
Preferably, the ramp has a maximum height which is
generally twice that of the thread crest height.
Tests indicate that the screw of the presPnt invention
has s~rip torque values significantly greatPr than
either conventional sheet metal scr~ws or screws pro-
vided with a recess beneath the head but, having only
a cylindrical unthreaded region instead of the ramp.
In fact, this latter screw produces no increased strip
torque values over other conventional sheet metal screws.
This is not surprising, since this recessed screw is
not intended to increase s~rip torque. In fact, this
screw is intended to strip the threads in the tapping
panel in order to prevent removal. Such a screw can
only be effectively used with specific panel thicknesses.
If the two panel thicknesses vary rom this specific di-
mension, the screw will fail to produce ~he desired
anti-removal results or will have a significantly
reduced clamp load. The screw of the present invention
can, on the other hand, be used with a range of different
panel thicknesses.
In the preceeding forms of the invention the helical
ramp has been o;E constantly increasing diameter, start-
ing from its inception to its outer extremity. Contact
with the clearance hole in the first or bearing panel
thus is only with the outer extremity of the ramp, i.e.
an essentially point contact. A modification of the
invention is shown in FIG. 7 in which the ramp is of
limited arcuate extent with a further circumferential
portion forming a conti.nuation of the ramp which pro-
vides a larger area contact with the clearance hole.
With specific reference to FIG. 7 wherein similar parts
are identified by similar numerals with the addition of
'JF,~
-- 8
the suffix _ there will be seen a screw lOa in which the
ramp 20a (the tee~h being omi~ted for simplicity of
illustration3 extends over 270 of arc, reaching its
maximum radial dimension at point 32 which is Z70 from
_ 5 the 0 starting point 34. The remaining 90~ forms a
continuation 36 of the ramp which is of constant dia-
meter, thereby providing approximately 90~ of contact
with the clearance hole. This has some value in apply-
ing stopping torque to the screw, in resisting with-
drawal, and in preventing relative movement of the twoplates transversely of the screw.
A further embodiment of the invention is shown in FIG. 8
in which similar parts are identified by similar numerals
with the addition of the suffix b. In this ins~ance the
rising portion of the ramp 20b extends from the 0 posi-
tion to 180, i.e from point 34b to point 32b. The
constant diameter extended portion 36b thus extends
through 180, presenting an even greater area of en~age-
ment with the ecLge of the clearance hole than in FIG. 7.
The embodiments of the invention as heretofore shown anddescribed are particularly advantageous for securing
together thin pieces of sheet metal. It is sometimes
desired to secure together adjacent sheets of thin
sheet metal and heavier gauge sheet plastic material.
A screw particularly adapted to this purpose is shown
in FIGS. 9 12, wherein similar numerals again are
utilized to identify similar parts, this time with the
addition of the suffix c. The essential difference in
the present embodiment of the inventi~n is that the
lower side 15c of the head is flat with the exception
of the shank 16c and the ramp 20c. When the screw of
FIG. 9 is passed through the clearance hole 31c in a
thin metal bearing plate 26c and screwed through a
.~ .
g
plastic second or tapping plate 28c a certain amount of
the plastic is forced or extruded 38 into the clearance
hole 31c by the ramp 20c, and impinges against the flat
underside 16c of ~he head. If the he~d were recessed
as in FIGS. 1 and 3 it is probable ~hat the extruded
plastic portion would simply be torn away from the plas-
tic tapping plate, whereby the present screw provides
a better stopping torque ~o gtall the screw when driven
home by a pneumatic screw driver.
When the pla~es are reversed as shown in FIG.. 12 with
a plastic bearing plate 26d on top and a thinner sheet
metal tapping plate 28d on the bottom the clearance
hole 31d is larger ~han the thread crest diame~er and
the plastic material is simply compressed by the ramp
20c, However, some of the metal surrounding tha tapping
hole in the tapping plate 28d is deflected upwardly at
32c by the screw threads 22c, and part thereof is engaged
by the ramp 20c to aid in stalling the screw and pneumatic
driverO
Actual test results with the configuration of FIG. 12
indicate tha~ the stall torque is nearly six times the
driving torque, whereby the sheet metal screw and the
driver are readily stalled, The stalled torque is nearly
twice that of a conventional ''ABI' screw without the
ramp provided in accordance with the present invention.
While the present invention has been described in
accordance with specific embodiments, various changes,
alterations and modifications will become apparent
following a reading of the foregoing specification.
For example, while the present invention states that the
screw has a "generally cylindrical shank", it is intended
that such terminology include such lobular c~nfigurations
'`;3
-- 10 --
as are now conventional screw forms. Further, although
the screw has been depicted as a gimlet pointed screw,
it will be understood that the inventive concept of
the present invention can be used on drill screws, with
special thread configurations or for a~taching a plastic
panel to sheet metal or to a second plastic panel.
Accordingly, it is intended that all such changes,
alterations and modifications as come within the scope
of the appended claims be considered part of the pre~
sent invention.
~`
