GB2100824A

GB2100824A - Screws for engaging sheet metal

Info

Publication number: GB2100824A
Application number: GB08217862A
Authority: GB
Inventors: Gerald D Barth
Original assignee: Illinois Tool Works Inc
Current assignee: Illinois Tool Works Inc
Priority date: 1981-06-22
Filing date: 1982-06-21
Publication date: 1983-01-06
Also published as: FR2508122B1; ES272908Y; FR2508122A1; CA1184053A; IT8221941A0; IT1152247B; GB2100824B; ES272908U

Abstract

A screw providing increased strip torque in sheet metal panels, comprises a helical ramp 20 formed as an extension to the thread 22 and partially surrounded by a recess 18 in the lower surface 16 of the head 12. The material of the tapping panel 28 is drawn upwards through the clearance hole 31 in the bearing panel 26 into engagement with the helical ramp 20. The farther up the surface of the ramp the material of the tapping panel is drawn, the greater the magnitude of the stripping torque. <IMAGE>

Description

SPECIFICATION Sheet metal screws The present invention is directed to a sheet metal screw. More particularly, the present invention is directed to a screw for attaching two thin panels together, which screw will exhibit increased stripping torque over other 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 comparatively high number of screws will be stripped which results in reduced clamp load or requires substantial operator time for removal and replacement.

The screws of the present invention reduce the above-stated problem by increasing the size of the range between drive and strip torque. The shank has formed thereon a helical ramp which may form an extension of the thread on the shank. This ramp can readily be formed during the heading process. The ramp preferably has teeth formed thereon which further increase stripping torque or increase backout resistance depending on the orientation of these teeth.

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-torqueincreasing teeth; FIG. 5 is a lateral cross-sectional view similar to FIG. 2 showing the helical ramp with backoutresistant teeth; 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 similar to FIG. 7 showing another modification 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. 10 is a cross-sectional view taken substantially on the line 10-10 in FIG. 9; FIG. 11 is a fragmentary view partially in section showing the screw of FIG. 9 with a metal bearing panel and a plastic second panel; and FIG. 12 is a view similar to FIG. 11, but with a plastic bearing panel and a metal second panel.

Detailed Description of the Present Invention The blank for forming a sheet metal screw of the present invention is shown generally at 10.

Head 12 has a flange 14 which extends laterally from a generally cylindrical shank 1 5. The lower side 1 6 of head 1 2 is undercut forming a recess 18. The upper side of head 12 has drive inducing surfaces 19 thereon. A helical ramp 20 extends from a point outside the recess 1 8 upwardly into the recess. Threads 22 shown in phantom in FIG. 1 , will extend laterally about shank 1 5 such that helical ramp 20 will form a continuation thereof. The thread 22 will be formed partially from the lower extremity of the ramp and, since the ramp exceeds the blank 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 formed due to the fact that the thread rolling dies cannot reach into the recess and can, therefore, only gather material from one side of the upper thread.

This helical ramp 20 is preferably formed during heading and can be provided with whatever configuration is desired. For example, the ramp 20 may be serrated so as to have teeth 24 which point in the same direction as the screw's rotational direction (FIG. 4) or opposite to that rotational direction (FIG. 5). Teeth 24 will, then, increase stripping torque or backout resistance, respectively. A second helical ramp 30 (FIG. 6) may be provided which can have teeth 24 which project in a direction opposite to those of ramp 20 so as to provide both increased strip torque and backout resistance.

In operation, as best shown in FIG. 3, screw 10 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 reaches 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 ofthe 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 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 1 8 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 maximum height of the ramp 20 (as measured from the root diameter) 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 present invention has strip torque values significantly greater than either conventional sheet metal screws or screws provided 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 strip 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 from this specific dimension, the screw will fail to produce the 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 preceding forms of the invention the helical ramp has been of constantly increasing diameter, starting 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 continuation of the ramp which provides 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 the suffix a there will be seen a screw 1 Oa in which the ramp 20a (the teeth being omitted for simplicity of illustration) extends over 2700 of arc, reaching its maximum radial dimension at point 32 which is 2700 from the 00 starting point 34.

The remaining 900 forms a continuation 36 of the ramp which is of constant diameter, thereby providing approximately 900 of contact with the clearance hole. This has some value in applying stopping torque to the screw, in resisting withdrawal, and in preventing relative movement of the two plates 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 instance the rising portion of the ramp 20b extends from the 0 position to 1 800, i.e. from point 34b to point 32b. The constant diameter extended portion 36b thus extends through 1 800, presenting an even greater area of engagement with the edge of the clearance hole than in FIG. 7.

The embodiments of the invention as heretofore shown and described 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 invention is that the lower side 1 sic of the head is flat with the exception of the shank 1 6c and the ramp 20c.

When the screw of FIG. 9 is passed through the clearance hole 31 c in a thin metal bearing plate 26c and screwed through a plastic second or tapping plate 28c a certain amount of the plastic is forced or extruded 38 into the clearance hole 31 c by the ramp 20c, and impinges against the flat underside 1 6c of the head. If the head were recessed as in FIGS. 1 and 3 it is probable that the extruded plastic portion would simply be torn away from the plastic tapping plate, whereby the present screw provides a better stopping torque to stall the screw when driven home by a pneumatic screw driver.

When the plates 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 31 d is larger than the thread crest diameter and the plastic material is simply compressed by the ramp 20c. However, some of the metal surrounding the 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 driver.

Actual test results with the configuration of FIG. 12 indicate that 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 "AB" 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 configurations 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 attaching 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 present invention.

Claims

1. A rotary fastener for use in attachment of a first panel with a clearance hole to a second panel, said fastener comprising a generally cylindrical shank, an enlarged head having drive-inducing surfaces on one end of said shank, said head having an under surface, said cylindrical shank having at least one helical thread thereon, said thread extending outwardly for a predetermined height that extends from a thread root diameter to a thread crest diameter, a helical ramp beginning substantially at one termination point of said helical thread and forming a continuation thereof, said ramp extending along the shank to said head under surface, whereby said helical ramp will deflect outwardly some of the material of at least one of said panels thereby greatly increasing the stripping torque of the screw relative to the panel.

2. 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.

3. The rotary fastener of claim 2 wherein the maximum height of the helical ramp is generally equal to twice the thread crest height.

4. The rotary fastener of claim 1 wherein the helical ramp has a number of serrations thereon.

5. The rotary fastener of claim 4 wherein the serrations form a plurality of teeth which point in a direction generally the same as the rotational direction of the screw, said teeth further increasing stripping torque.

6. The rotary fastener of claim 4, wherein the serrations form a plurality of teeth which point in a direction generally opposite to the rotational direction of the screw, said teeth increasing backout resistance.

7. 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.

8. The rotary fastener of claim 1, in combination with a first panel, wherein the predetermined thread crest height is insufficient to contact the edges of the clearance hole in said first panel.

9. The rotary fastener of claim 1, in combination with a first panel, wherein the helical ramp has a sufficient height to engage the edges of the clearance hole in said first panel.

10. A rotary fastener as set forth in claim 1, wherein said under surface includes a recess beneath said head, said helical ramp deflecting material of said second panel through said clearance hole up into said recess.

11. A rotary fastener as set forth in claim 1, wherein said under surface is substantially flat and said helical ramp deflects material of at least one of said panels transversely of said shank.

12. A rotary fastener as set forth in claim 1, wherein said helical ramp has an arcuate extent of less than 3600 followed by a continuation of substantially constant radius.

13. A rotary fastener as set forth in claim 12 wherein said ramp and said continuation together have an arcuate extent of substantially 3600.

14. A rotary fastener as set forth in claim 1 3 wherein said ramp has an arcuate extent of substantially 2700 and said continuation has an arcuate extent of substantially 900.

1 5. A rotary fastener as set forth in claim 13 wherein said ramp has an arcuate extent of substantially 1 800 and said continuation has an arcuate extent of substantially 1800.