GB2131907A

GB2131907A - Screw

Info

Publication number: GB2131907A
Application number: GB08331727A
Authority: GB
Inventors: James Lawrence Seymour; George Edward Hardwick
Original assignee: GKN Screws and Fasteners Ltd
Current assignee: GKN Screws and Fasteners Ltd
Priority date: 1982-12-17
Filing date: 1983-11-28
Publication date: 1984-06-27
Also published as: GB8331727D0; GB2131907B; ES285333U; EP0161250A1; ZA839333B; NO842746L; SG51387G; HK77787A; ES285333Y; NZ206599A; KR840007454A; IT8368312A0; WO1984002380A1; IT8354049V0

Abstract

A screw usable as either a wood screw or a self-tapping screw comprises a two start helical thread (12) having a helix angle of 20<o> or more with an included angle to the thread profile at its crest of 40<o> or less. The threads are raised rapidly from a conical un-threaded entry point (14) and the thread crest diameter is greater than that of the unthreaded shank portion (10) of the screw.

Description

SPECIFICATION A screw This invention relates to a screw for use in wood, particle board, plastics or in metal.

Many different designs of screw point and screw thread configuration are of course well known in the art. In particular, it is known to provide a wood screw wherein the helix angle of the thread is between 14" and 160 and wherein the included angle ofthethread profile at its crest is 50". It is known to provide such a screw with a double threadto give a twin start facilitating its initial insertion into wood.

It is also known from GB-A-1357720 to provide a self-tapping screw having a single or multi-start helical thread wherein the induced angle ofthe thread profile at its crest is 40 but wherein the axis of symmetry ofthe thread profile is inclined towards the screw head by at least 5" from the perpendicularto the screw axis. Such a type of self-tapping screw is also known to be used in chipboard and is provided with a helix angle of between 100and 12".

Furthermore itis known from GB-A-1564904to provide a self-tapping screw having a thread helix angle of between 17"and 19" and an included angle of the thread profile at its crest of between 45" and 55".

Although such a screw is said to usable in both metal and plastics materials, it is not said to be usable as a wood screw and the specification of G B-A-1564904 teaches that the above limits on helix and thread crest angles are critical.

The traditional methods of manufacturing screws by cutting threads have been largely replaced by current production methods involving rolling a thread onto the shank of a headed cylindrical metal blank.

Such a rolling operation produces a thread crest diameter greater than the diameter of the original blank so that, in orderto achieve a particularthread crest diameter, one starts with a smaller blank diameter. This leaves any un-threaded portion of the shankwith a smaller diameter than the thread crest diameter and such screws are known in the artas scant shank screws.

Ascant shankwood screw is described in GB-A2072781 butthis specification is especially concerned withthe provision of an enlargediocation collar immediately underthe head ofthe screw.

Itis is an object of the present invention to provide an itnproved screw which, by virtue of its particular thread form, will be capable ofadvantageous application as a wood screw for use in woods or plastics or as a self-tapping screwfor use in metals.

In accordance with the invention there is provided a hardened steel screw having a head at one wend, an entry portion at the other end and a threaded shank portion extending from the entry portion towards the head characterised in that the thread profile is of helical form wherein the helix angle is 20 or more, the included angle of the thread profile at is crest is 40 or less and the thread profile is symmetrical about an axis perpendicular to the longitudinal axis of the screw.

The said helix angle ofthe thread conveniently lies between 20 and 25 and the said included angle conveniently lies between 300 and 40". Preferably the said helix angle is substantially equal to 20 and the said included angle is substantially equal to 400.

The threaded shank portion may extend from the entry portion to a position closely adjacent the underside of the head or may terminate short ofthe head to provide an unthreaded shank portion of a smaller diameter than the thread crest diameter of the threaded portion dependent upon the overall length of the screw and the type of screw required.

Thethread preferably comprises a two startthread and is preferably raised rapidlyto the full crest diameter from the entry portion ofthe screw. The entry portion ofthe screw may comprise a tapered point or may be a blunt end and the threaded portion may extend to a position adjacentthe end ofthe entry portion. Atapered entry portion may comprise a conical unthreaded point wherein the included angle ofthe unthreaded point is substantially equal to 300.

The diameter ofthe shank atthe base ofthe thread conveniently lies within the range of 60% to 70% of the crest diameter of thethread.

A raised unthreaded location portion, or collar, may be provided immediately below the screw head having a diameter substantially equal to the crest diameter ofthethread.

Other features ofthe invention will becomeapparentfrom the following description given herein solely by way of example with reference to the accompanying drawings wherein: FIGURE 1 is a sideelevation of a countersunkscrew in accordancewith the invention, the head being provided with a driving recess; FIGURE 2 is a side elevation ofthe head and unthreadedshankportion ofacountersunkscrew similar two that shown in Figure 1 but having a cross slot in the head.

FIGURE 3 is an enlarged cross-sectional detail ofthe thread profile; and FIGURE 4 is a graph showing the relationship between torsional strength and nominal screw diameter (screw gauge) for hardened and non-hardened screws.

The screw shown in the drawing is manufactured from a cylindrical metal blank, usually of a low carbon steel, which is itself cropped to length from wire stock and then cold headed. The screw comprises an un-threaded shank portion 10 corresponding to the diameter ofthe headed blank adjacentto which a rolled threaded portion 12 extends towards the entry portion 14 of the screw. At the other end ofthe screw there is provided a head 16, shown herein as a counter-sunk head and having either a driving recess (Figure 1) or a cross-slot (Figure 2) therein for driving engagement by a driving tool. It will be appreciated that, dependant upon length and application requirement, the threaded portion 12 may extend substantially unto the underside of the head 16from the entry portion 14.

The entry portion 14 ofthe screw comprises an un-threaded conical point having an included angle substantially equal to 30 from which a rapidly raised two start helical thread extends overthethreaded portion 12 to the unthreaded portion 10 of the shank.

In this examplethe thread profile is provided with steeply inclined flanks 20to define therebetween an includedangletothethread profile at its crestwhich is substantially equal to 400 with the edge of the thread at the crest having a maximum thickness of 0.008 inches.

The helix angle of the helical thread, in this example, substantially equal to 20 and the thread profile is symmetrical about an axis perpendiculartothe longitudinal axis ofthe screw.

In accordance with the invention the helix angle of the thread is always 200 or greater but preferably lies within the range 200 to 250 whilst the included angle of the thread profile at its crest is aiways 40 or less and preferably within the range 300 to 40". It is found that, in use, the provision of a low included angle to the crest in combination with the high helix angle of the thread greatly facilitates the insertion of the screw by reducing the insertion torque which has to be applied.

Comparativetestshave been carried outon2 inch screws of Number 10 Screw Gauge driven into beech wood to a depth of 30.5mm using a pilot hole in the wood of 3.2 diameter. All the screws used in the tests were bright zinc plated and case hardened and were not provided with any low friction coating.The results of the tests are tabulated belowwhich show that the screw according to the invention, which was provided with a two startthread having a 20 helix angle and 40 included crest angle, resulted in a substantially lower insertion torque than that required to drive the known forms of screw:: Conventional Conventional Conventional Screw of Wood Screw Self-Tapping Two Start the invention toBS1210 Screwto Wood Screw BS4174 Newton Newton Newton Newton Metres Metres Metres Metres insertion insertion insertion insertion torque torque torque torque 5.15.5 4.6-4.7 5.3-5.5 3.5-4.0 Tests have also been carried out on the insertion speed of the screws described above as measured by the number of turns required to fully apply the screw, i.e. to bury the countersunk head in the wood::- No. 10 Screw Gauge Z' Screw Conventional Conventional Conventional Screwofthe Wood Screw Self-Tapping Two Start Invention toBS1210 Screwto Wood Screw BS4174 26turns 32turns 15turns 11.1 turns No. 12 Screw Gauge 3 Screw Conventional Conventional Conventional Screwofthe Wood Screw Self-Tapping Two Start Invention to BS1210 Screwto WoodScrew BS4174 33 turns 42 turns 19.5 turns 14.3 turns A screw may be produced in accordance with the invention by conventional cold heading and thread rolling techniques and it will be appreciated that the screw may have a cross slot or recess in its head and that such head may be of any suitable form.The screw in accordance with the present invention is preferably provided with a two startthread which thread may extend from the unthreaded shank portion 10 eitherto the end orto the start ofthe entry portion 14. Thus, although the specific example described hereinbefore is provided with an entry portion comprising a conical unthreaded point, such entry portion may be threaded to the end thereof and may eithertaperto a point or may comprise a relatively blunt end.

As mentioned hereinbefore,the screw need not be provided with the unthreaded shank portion 10, wherebythe threaded portion 12may extend from the entry portion 14 substantially up to the underside of the head 16. Also, a raised unthreaded location portion in the form of a cylindrical orfrusto-conical collar may be provided immediately below the screw head 16, such collar having a maximum diameter substantially equal to the crest diameter ofthe threaded portion 12.

Inthe screw ofthe present invention the diameter ofthe shankatthe base ofthethread preferably lies within the range of 60% to 70% ofthe crest diameter of the thread dependant upon the Screw Gauge number as follows Thread Base/Thread Screw Gauge Number Crest Diameter 3 64% 4 62% 5 62% 6 68% 7 68% 8 68% 9 63% 10 63% 12 63% 14 63% Screws produced in accordance with the invention are always hardened, either case hardened or through hardened, in order to give them sufficient torsional strength for use in their various applications i.e. in both hard and soft woods, man-made board materials, plastics or metals. It is of course conventional practice to harden self-tapping screws but wood screws are not usually hardened and reference may be had to Figure 4 which shows the relationship between torsional strength and nominal screw diameter (i.e. screw gauge number) for hardened and non-hardened screws.

Claims

1. A hardened steel screw having a head at one end, an entry portion at the other end and a threaded shank portion extending from the entry portion the head, characterised in that the thread profile is of helical form wherein the helix angle is 20" or more, the included angle ofthethread profile at its crest is 400 or less and thethread profile is symmetrical about an axis perpendiculartothe longitudinal axis of the screw.

2. A screw as claimed in Claim 1 furthercharacterised in thatthe said helix angle ofthethread is between 20" and 25" and the said included angle is between 30" and 40".

3. A screw as claimed in Claim 2 further characte- rised in that the said helix angle is substantially equal to 20" and the said included angle is substantially equal to 40".

4. Ascrewasclaimed in any one of the preceding Claims further characterised in that the thread comprises a two startthread.

5. Ascrewas claimed in any one of the preceding Claims further characterised in thatthe threaded shank portion thereof extends to the end ofthe entry portion.

6. Ascrewasclaimed in anyone of Claims 1 to 4 further characterised in that the threaded shank portion thereof extends to the start of the entry portion.

7. A screw as claimed in anyoneofthe preceding Claims further characterised in that the entry portion is tapered.

8. Ascrew as claimed in Claim 7 further characte- rised in thatthe thread is raised rapidly from the tapered entry portion to the full thread crest diameter.

9 Ascrew as claimed in Claim 8further characterised in that the tapered entry portion comprises a conical unthreaded point.

10. A screw as claimed in Claim 9 further char- acterised in that the included angle ofthe conical un-threaded point is substantially equal to 30".

11. Ascrewasclaimed in anyone ofthe preceding claims fu rther characterised in that an unthreaded shank portion is provided between the threaded portion and the head, the diameter of the portion being less than the crest diameter of the threaded portion.

12. A screw as claimed in any one of Claims 1 to 10 further characterised in that the threaded portion extends from the entry portion substantially up to the underside of the head.

13. A screw as claimed in any one of the preceding claims further characterised in thata raised unthreaded location portion is provided immediately below the head, said portion having a maximum diameter substantially equal to the crest diameterofthethreaded portion.

14. A screw constructed and arranged substantially as hereinbefore described with reference to the accompanying drawings.