CA1184053A

CA1184053A - Sheet metal screw

Info

Publication number: CA1184053A
Application number: CA000402097A
Authority: CA
Inventors: Gerald D. Barth
Original assignee: Illinois Tool Works Inc
Current assignee: Illinois Tool Works Inc
Priority date: 1981-06-22
Filing date: 1982-04-30
Publication date: 1985-03-19
Also published as: ES272908U; FR2508122A1; IT1152247B; GB2100824A; ES272908Y; GB2100824B; IT8221941A0; FR2508122B1

Abstract

Abstract A sheet metal screw providing increased strip torque.
A helical ramp is formed as an extension to the thread and is partially surrounded by a recess in the lower surface of the head. The material of the tapping panel is drawing upwardly through the clearance hole in the bearing panel into engagement with the helical ramp.
The farther up the surface of the ramp the material of the tapping panel is drawn, the greater the magnitude of the stripping torque.

Description

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 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 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.

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~ 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 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,~

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 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.

~`

Claims

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A rotary fastener for use in attachment of a first panel with a clearance hole to a second panel, said fastener comprising a shank having an axis, an enlarged head having drive-inducing surfaces on one end of said shank, said head having an under surface, said shank having at least one helical thread thereon, said at least one helical thread extending outwardly from said shank for a predetermined height that extends from a thread root diameter to a thread crest diameter, a ramp beginning substantially at one termination point of said at least one helical thread and forming a continuation thereof, said ramp comprising a runner surface generally advancing helically along said shank, said ramp further comprising a face surface, said face surface being adjacent said runner surface and generally parallel to said axis; said face surface establishing a first boundary of said runner surface; a second boundary of said runner surface comprising said shank, said ramp having a radially spiral configuration and said face surface having a taper, said ramp having a minimum radial extent at a point on said ramp opposite said under surface, said face surface having a maximum axial length at said point of said minimum radial extent of said ramp.

2. A rotary fastener as recited in Claim 1 wherein said ramp increases spirally in transaxial dimension as said ramp advances helically along said shank.

3. A rotary fastener as recited in Claim 2 wherein said face surface has a plurality of serrations thereon.

4. A rotary fastener for use in attachment of a first panel having a clearance hole to a second panel, said fastener comprising a shank having an axis, an enlarged head having drive inducing surfaces at one end of said shank, said head having an under surface, said shank having at least one helical thread thereon, said at least one helical thread extending outwardly for a predetermined height that extends from a thread root diameter to a thread crest diameter and having a termination point axially displaced from said head, at least one ramp extending helically along the shank from said thread to said under surface, said at least one ramp having a face portion and a runner portion, said face portion being bounded by said under surface at a first end and by said runner portion at a second end, said runner portion being bounded by said face portion at a first boundary and by said shank at a second boundary, said face portion being oriented such that, at any point of said face portion, a plane tangent to said face portion will be generally parallel to the axis of said shank, said face portion having an initial radial extent adjacent said thread, said initial radial extent being less than half of said crest diameter and said face portion having a final radial extent adjacent said under surface, said final radial extent being greater than half of said crest diameter, said face portion tapering and having a maximum axial length where said portion has said initial radial extent and said face portion having a minimum axial length where said face portion has said final radial extent.

5. A rotary fastener as recited in Claim 4 wherein said ramp increases spirally in transaxial dimension as said ramp extends toward said under surface.

6. A rotary fastener as recited in Claim 5 wherein said face portion has a plurality of serrations thereon.

7. A rotary fastener for use in attachment of a first panel with a clearance hole to a second panel, said fastener comprising a shank having an axis, an enlarged head having drive-inducing surfaces on one end of said shank, said head having an under surface, said shank having at least one helical thread thereon said at least one helical thread extending outwardly from said shank and having a generally constant height, said thread having a thread root diameter and a thread crest diameter, a ramp having a face portion and a runner portion, said face portion being bounded by said under surface at a first end and by said runner portion at a second end, said runner portion being bounded by said face portion at a first edge and by said shank at a second edge, said face portion being oriented such that, at any point of said face portion, a plane tangent to said face portion will be generally parallel to the axis of said shank, said ramp extending helically along the shank to said under surface, said ramp having a spiral cross-section with an initial radial extent less than half of said crest diameter and a final radial extent greater than half of said crest diameter.

8. A rotary fastener as recited in Claim 7 wherein said ramp increases spirally in transaxial dimension as said ramp extends helically along the shank.

9. A rotary fastener as recited in Claim 8 wherein said ramp has an initial transaxial dimension generally equal to said thread root diameter, said transaxial dimension increasing to a maximum dimension at least equal to said thread crest diameter.

10. A rotary fastener as recited in Claim 9 wherein said maximum dimension is generally equal to twice the thread crest diameter.

11. A rotary fastener as recited in Claim 7 wherein said face portion has a plurality of serrations thereon.

12. A rotary fastener as recited in Claim 11 wherein said plurality of serrations forms a plurality of teeth which extend in a direction generally the same as a rotational direction of the fastener appropri-ate to advance the fastener into a workpiece, said plurality of teeth further increasing stripping torque of the fastener.

13. A rotary fastener as recited in Claim 11 wherein said plurality of serrations forms a plurality of teeth which extend in a direction generally opposed to a rotational direction of the fastener appropriate to advance the fastener into a workpiece, said plurality of teeth increasing backout resistance of the fastener.

14. A rotary fastener as recited in Claim 7 wherein the fastener further comprises a second ramp beginning at a point generally diametrically opposed to the beginning of said first ramp; said second ramp having a second face portion and a second runner portion, said second face portion being bounded by said under surface at an upper end and by said second runner portion at a lower end, said second runner portion being bounded by said second face portion at a first boundary and by said shank at a second boundary, said second face portion being oriented such that, at any point of said face portion, a plane tangent to said face portion will be generally parallel to the axis of said shank, said second ramp extending helically along the shank to said under surface, said second ramp having a spiral cross-section with an initial radial extent less than half of said crest diameter and a final radial extent of said second ramp being greater than half of said crest diameter.

15. A rotary fastener as recited in Claim 7 wherein said under surface includes a recess beneath said head, said ramp deflecting material of said second panel through said clearance hole into said recess during installation of the fastener.

16. A rotary fastener as recited in Claim 7 wherein said under surface is substantially flat and said ramp deflects materials of at least one of said first panel and said second panel transversely of said shank during installation of the fastener.

17. The rotary fastener of Claim 1 wherein said helical ramp has an initial height approximately that of the thread root diameter, said height increasing to a maximum height which is at least equal to the thread crest diameter.

18. The rotary fastener of Claim 1 wherein the maximum height of the helical ramp is generally equal to twice the thread crest height.

19. The rotary fastener of Claim 1 wherein each serration is comprised of a generally radial surface and a generally circumferential surface.

20. The rotary fastener of Claim 1 or Claim 19 wherein the radial surface faces a rotational direction of motion required to install the screw, said teeth further increasing stripping torque.

21. The rotary fastener of Claim 1 or Claim 19 wherein radial surface faces a rotational direction of motion required to remove the screw, said teeth increasing backout resistance.

22. The rotary fastener of Claim 1 further comprising a second helical ramp beginning at a point which is generally diametrically opposed to the initial point of said first helical ramp.

23. The rotary fastener of Claim 1 wherein the pre-determined thread crest height is insufficient to contact the edges of the clearance hole in said first panel.

24. The rotary fastener of Claim 1 wherein the helical ramp has a sufficient height to engage the edges of the clearance hole in said first panel.

25. A rotary fastener as set forth in Claim 1 wherein said helical ramp has a pitch substantially equal to the pitch of said thread.