GB2111157A

GB2111157A - Improvements to screws

Info

Publication number: GB2111157A
Application number: GB8234370A
Authority: GB
Inventors: Denis Norman Pearce; David Austin Burt
Original assignee: ITW Ltd
Current assignee: ITW Ltd
Priority date: 1981-12-04
Filing date: 1982-12-02
Publication date: 1983-06-29

Abstract

A screw S for assembling a compressible workpiece L, G to a thin rigid support F, such as a door frame for a refrigerator, includes a threaded shank 10 provided with a radially enlarged head 12 and a self-tapping tip 14 and a stop 18 having a shoulder 20 spaced axially from the head 12, the shoulder 20 having a radial dimension greater than that of the thread 16 yet less than that of the head 12. In use, when the stop shoulder 20 abuts the support F, there is a steep increase in the frictional resistance to driving, giving a tangible indication of the optimum driven position of the screw S. The stop 18 may be formed integrally with the shank 10 as a circumferentially continuous rib, or as a series of circumferentially spaced teeth, or alternatively not be formed integrally with the shank but as an annular ring carried in an annular groove, or as a tubular sleeve carried around the shank. <IMAGE>

Description

SPECIFICATION Improvements to screws The present invention is concerned primarily, but not exclusively, with overcoming the problems met in securing a readily compressible part to a thin supporting part, by means of a screw.

An example of a situation where such parts are utilised is a door for a refrigerator.

A refrigerator door typically has a frame made from sheet steel having a thickness of less than 1 mm. The frame may be attached to a liner and a gasket which are, respectively, typically made from polystyrene and polyvinyl chloride. The liner and the frame may together define an enclosure into which a thermally insulating foam can be introduced. The gasket may itself define an enclosure into which a strip of magnetic material can be introduced for enabling the door to be held shut by magnetic attraction. A plurality of spacedapart screws secure the liner, the gasket and the frame together.

Normally, each of the screws has a self-tapping tip at one end separated from a radially enlarged head at its other end by an elongate threaded shank. The self-tapping tip of each screw is passed sequentially through clearance holes in the liner and the gasket and driven through the frame until such time as the head is brought up against the liner. The gasket is thus firmly sandwiched between the liner and the frame.

It will be appreciated that, as the screw is being driven through the frame, a short length of thread complementary to that of the screw is formed in the thin material of the frame.

A significant problem is that, because said complementary thread in the frame is so short, it is very easily stripped by overtightening the screw, with the result that the liner and the gasket are not then secured to the frame. Another significant problem is that, because the material from which the gasket is formed is so compressible, it is very easy to tighten a series of the screws by different amounts, especially if one is anxious to avoid overtightening the screws. However, uneven tightening of the screws not only results in an unsightly irregular appearance for the gasket but, more importantly, prevents the gasket acting as an effective thermal barrier when the door is shut.

These problems are exacerbated when the screws are driven home semi-automatically by electrically or pneumatically operated power tools.

Most power tools are provided with a drive clutch which disengages when the torque that resists drive of the screw exceeds a predetermined level.

Unfortunately, in the situation described hereinbefore it is found that the torque necessary to start the driving of the screw through the frame is greater than the torque capable of stripping the thread formed in the frame. As the drive clutch must be set at a level which allows tightening of the screw to begin, it cannot subsequently be utilised to disengage the drive when tightening of the screw is to stop.

The fact that the liner is formed of a shiny material, offering little increase in frictional resistance to driving when engaged by the head of the screw, does not help.

According to a first aspect of the present invention, however, a screw comprises an axially extending shank, a radially enlarged head at one end of the shank, a self-tapping tip at the other end of the shank, a thread extending along at least an intermediate part of the shank, and a stop located at that end of the thread closer to the head and presenting a shoulder which is spaced axially by a predetermined distance from the head, the shoulder having a radial dimension greater than that of the thread yet less than that of the head.

In use, the screw is passed through a compressible, fragile, easily damaged or other difficult workpiece and is driven into a support.

Driving is continued until the shoulder has passed through the workpiece and has been brought into abutting engagement with the support. This causes a steep increase in the frictional resistance to driving the screw giving a tangible indication that the screw has been driven to its optimum position. Both overtightening and uneven tightening of the screw are thus efficiently and reliably avoidable.

Preferably, the stop is formed integrally with the shank as a circumferentially continuous rib, or as a series of circumferentially spaced teeth.

However, the stop need not be formed integrally with the shank, but could alternatively be an annular ring carried in an annular groove in the shank, or be a tubular sleeve carried around the shank in abutment with the head.

In accordance with a second aspect of the present invention, an assembly comprises a workpiece secured to a support by a screw, the screw including an axially extending shank, a radially enlarged head at one end of the shank, a self-tapping tip at the other end of the shank, a thread extending along at least an intermediate part of the shank and a stop located at that end of the thread closer to the head and presenting a shoulder which is spaced axially by a predetermined distance from the head, the shoulder having a radial dimension greater than that of the thread yet less than that of the head, with the head of the screw lying against the workpiece, the thread of the screw being in threaded engagement with a complementary thread formed by the self-tapping tip of the screw in the support, and the shoulder presented by the stop of the screw lying within the workpiece and against the support.

Several screws and an assembly incorporating one of the screws, according to the present invention, will now be described, by way of example only, with reference to the accompanying drawings, in which: Figure 1 is a fragmentary axial section through a refrigerator door utilising an assembly according to the present invention; Figure 2 is a side view of a screw according to the present invention forming part of said assembly; Figures 3 and 4 are fragmentary side views of two other screws according to the present invention; and Figures 5. 6 and 7 are respectively front, plan and underneath views of yet another screw according to the present invention.

Part of an edge of a refrigerator door is shown in section in Figure 1. A gasket G extends along the entire periphery of a frame F, which is of rectangular shape, and a liner L covers a central opening presented by inwardly folded edge flanges of the frame F. A plurality of screws S are spaced apart from one another to secure the liner L and the gasket G to the frame F. The liner L and the frame F thus together define an enclosure and the enclosure accommodates foamed thermal insulation I. The gasket G itself defines an enclosure accommodating a strip magnet M.

When the door is held shut, by magnetic attraction, the insulation I and the gasket G together act as thermal barriers, resisting flow of heat through and past the refrigerator door. It is important that the liner L and the gasket G be firmly secured to the frame F. It is also important that upper portion P of the gasket G be as flat as possible to seal uniformly against the main body of the refrigerator when the door of the refrigerator is shut.

Problems have arisen, in the past, resulting from the fact that the frame F is usually formed of thin sheet steel, the screw S is usually also formed of a steel, the gasket G is usually formed of readily compressible polyvinyl chloride, and the liner L is usually formed of polystyrene having low surface friction.

These problems may be overcome, however, by utilising the screw S of the present invention. As shown more clearly in Figure 2, the screw S includes an axially extending shank 10, a radially enlarged head 12 at one end of the shank 10, a self-tapping tip 14 at the other end of the shank 10, a thread 16 extending along at least an intermediate part of the shank 10, and a stop 1 8 located at that end of the thread 1 6 closer to the head 12 and presenting a shoulder 20 which is spaced axially by a predetermined distance from the head 12, the shoulder 20 having a radial dimension greater than that of the thread 1 6 yet less than that of the head 1 2.The stop 1 8 is here shown as a circumferentially continuous rib, which has been formed during rolling of the thread 1 6 by provision of suitably shaped dies, but other arrangements are possible as described hereinafter.

In use, the self-tapping tip 14 is passed sequentially through clearance holes in the liner L and the gasket G (together constituting a workpiece) and is inserted into a location hole in the frame F (constituting a support).

The screw S is driven home by applying driving torque to the head 12, which is shown as having a cruciform shaped recess 22 but could have any other external or internal torque transmitting surfaces. As the screw S is driven, the self-tapping tip 14 acts on the frame F to form a thread therein which is complementary to the thread 1 6. The self-tapping tip 1 4 can be of standard construction, but is preferably formed in accordance with the teachings of Illinois Tool Works Inc's patent GB 201 5402 B. The reader is instructed to refer to the corresponding published patent application GB 201 5402A for full information.

Briefly, GB 201 5402A discloses a particularly beneficial form of screw in which the tip is shaped to be self-extruding as well as self-tapping. In particular, the tip presents a pair of dissimilar opposing peripheral segments, one of the segments having at any axial location a greater radial dimension than the other, with a portion of the two segments nearer to the head carrying a plurality of partially formed thread crests extending circumferentially thereabout, a portion of said one segment further from the head carrying a plurality of incomplete threads and a portion of said other segment further from the head being substantially void of threads, and with said one segment being tapered in the axial direction at an angle not greater than 6". This arrangement produces a substantial backward, as well as forward, extrusion while the screw is being driven into a pre-formed hole. The resultant usable wall area of the hole is at least twice that of the original material thickness. For the avoidance of doubt, this patent application hereby incorporates by specific reference the entire disclosure of said GB 201 5402A.

Eventually, the stop 1 8 is advanced through the clearance holes in the liner Land the gasket G and into abutting engagement with the frame F. This causes a steep increase in the frictional resistance to driving the screw S, and gives a tangible indication that the screw S has been driven to its optimum position. Because of the metal-to-metal contact of the shoulder 20 of the screw S with the adjacent face of the frame F, the torque necessary to continue driving of the screw increases to such a level that the risk of overtightening is virtually eliminated. Indeed, this sudden and significant increase in frictional resistance causes the required drive torque to exceed the torque necessary to start driving of the screws. A conventional drive clutch on a power tool can thus be preset to disengage the drive when the screw S has been driven fully home.

Ciearly, the predetermined distance between the shoulder 20 and the head 12 is calculated to be that which allows the upper portion P of the gasket G to remain substantially flat. More generally, said distance can be predetermined in accordance with the specific requirements of the particular workpiece to be secured to the support.

An assembly worker can thus be assured that exactly the requisite amount of tightening is being applied to the screw S.

In the alternative arrangements shown in Figures 3 and 4, the stop is constituted by, respectively, an annular ring 26 carried in an annular groove in the shank, or a tubular sleeve 28 carried around the shank, with one end (lower as shown) of the sleeve 28 constituting the shoulder, and the other end (upper as shown) of the sleeve 28 being flanged at 30 and abutting the head to determine the distance by which the shoulder is spaced from the head.

Another arrangement is shown in Figures 5, 6 and 7 in which the stop is constituted by a series of eight circumferentially spaced teeth 32, the teeth 32 in fact having been rolled and being equally spaced from one another, with the selftapping tip 14 also being shown, together with the basically cruciform-shaped recess 22.

Although shown as having a double-lead for the thread on the shank other arrangements of thread are also possible. Indeed, other arrangements of self-tapping tip are also possible. For example, the tip can incorporate a self-drilling capability or an S point.

For the convenience of the reader we now recite one of the independent claims of the abovementioned Illinois Tool Works Inc's patent GB 201 5402B: "An extruding and thread forming screw fastener including a shank with a head at one extremity, the shank comprising three regions, namely a first, threaded, holding region adjacent the head with a substantially uniform crest diameter throughout its axial extent, a second point extremity region comprising a substantially conical, threaded gimlet point, the maximum crest diameter of the gimlet point being less than the crest diameter of the first threaded region, and a third, thread forming and extruding region intermediate the first and second regions; the third region comprising a pair of dissimilar, opposing peripheral segments, one of said peripheral segments configured so that at any given axial location along said third region said one peripheral segment extends a greater radial distance from the axis of the fastener than the opposing dissimilar segment, the upper portion of said two dissimilar segments forming said third region having a plurality of partially formed thread crests extending circumferentially thereabout, the lower portion of said one peripheral segment including a plurality of incomplete thread crests which are partially formed in regard to both circumferential and axial extent, said incomplete thread crests foliowing the same helix as the thread formations on the first, second and upper portion of the third region, the lowermost incomplete thread crest being at a distance from the uppermost thread on the second region not exceeding two pitches, the uppermost incomplete thread being spaced from the lowermost partial thread on the upper portion of said third region by a distance substantially equal to the pitch of the thread on said first region, the portion of the other of said opposing dissimilar segments opposite the incomplete threads being substantially void of thread formations, the radial outermost portions of said one peripheral segment being tapered downwardly toward the gimlet point at an angle not greater than 60 to the axis of the fastener, thereby forming a gimlet point fastener with a thread forming and extruding section which minimizes the drive torque required to apply such a fastener to a workpiece."

Claims

1. A screw comprising an axially extending shank, a radially enlarged head at one end of the shank, a self-tapping tip at the other end of the shank, a thread extending along at least an intermediate part of the shank, and a stop located at that end of the thread closer to the head and presenting a shoulder which is spaced axially by a predetermined distance from the head, the shoulder having a radial dimension greater than that of the thread yet less than that of the head.

2. A screw according to claim 1, in which the stop is formed integrally with the shank.

3. A screw according to claim 2, in which the stop is constituted by a circumferentially continuous rib.

4. A screw according to claim 2, in which the stop is constituted by a series of circumferentially spaced teeth.

5. A screw according to claim 1, in which the stop is not formed integrally with the shank.

6. A screw according to claim 5, in which the stop is constituted by an annular ring carried in an annular groove in the shank.

7. A screw according to claim 5, in which the stop is constituted by a tubular sleeve carried around the shank in abutment with the head.

8. A screw according to any preceding claim, in which the tip is shaped to be self-extruding as well as self-tapping.

9. A screw according to claim 8, in which the self-extruding self-tapping tip presents a pair of dissimilar opposing peripheral segments, one of the segments having at any axial location a greater radial dimension than the other, with a portion of the two segments nearer to the head carrying a plurality of partially formed thread crests extending circumferentially thereabout, a portion of said one segment further from the head carrying a plurality of incomplete threads and a portion of said other segment further from the head being substantially void of threads, and with said one segment being tapered in the axial direction at an angle not greater than 60.

1 0. A screw substantially as hereinbefore described with reference to Figure 2, or to Figure 2 as modified by Figures 3 or 4, or to Figures 5, 6 and 7, of the accompanying drawings.

11. An assembly comprising a workpiece secured to a support by a screw, the screw including an axially extending shank, a radially enlarged head at one end of the shank, a selftapping tip at the other end of the shank, a thread extending along at least an intermediate part of the shank, and a stop located at that end of the thread closer to the head and presenting a shoulder which is spaced axially by a predetermined distance from the head, the shoulder having a radial dimension greater than that of the thread yet less than that of the head, with the head of the screw lying against the workpiece, the thread of the screw being in threaded engagement with a complementary thread formed by the self-tapping tip of the screw in the support, and the shoulder presented by the stop of the screw lying within the workpiece and against the support.

12. An assembly according to claim 11, in which the screw is itself in accordance with any one of claims 2 to 10.

13. An assembly according to claim 11 or claim 12, in which the workpiece is readily compressible and the support has a thickness of less than 1 mm.

14. An assembly according to claim 13, in which the workpieces is constituted by a liner and a gasket, and the support is constituted by a door frame for a refrigerator.