GB2266940A

GB2266940A - Making screws captive

Info

Publication number: GB2266940A
Application number: GB9210511A
Authority: GB
Inventors: Kenneth Payne; Trevor Walter Palmer
Original assignee: Ampy Automotion Digilog Ltd
Current assignee: Landis and Gyr Ltd
Priority date: 1992-05-15
Filing date: 1992-05-15
Publication date: 1993-11-17
Anticipated expiration: 2012-05-15
Also published as: GB9210511D0; GB2266940B

Abstract

A screw (22, 24, 26) is made captive in a hole in a plastic housing (14) (such as the cover of an electricity meter) by inserting the screw into the hole in the housing and subjecting the screw head to a downward blow (38) to deform inwardly at (44) the material around the aperture. An anvil (40) with an opening (42) for accommodating the end of the screw may be positioned beneath the cover to support it while the impact force is delivered. <IMAGE>

Description

Title: Retention of screw fixings Field of invention This invention concerns the retention of screw fixings such as are employed to hold captive a threaded screw member allowing the member to be unscrewed from one member and remain fully rotatable but captive in another member such as a lid or removable housing section of a two part housing. The invention is of general application but is particularly applicable to the retention of securing screws in the lids of electricity meters particularly those parts of such housings which are removable to enable conductors to be fitted to or removed from the meter within the main housing.

Background to the invention Forming a retention device for a screw as aforesaid has normally required the use of a screw having a thread profile at its end remote from the screw head and separated from the head by a reduced diameter section which is typically unthreaded. The aperture in the housing member in which the screw is to be held captive is formed with a thread profile which is similar to the thread profile of the threaded aperture in the second housing member which is to be secured to the first, and the screw is held captive in the first housing member by engaging the screw in the first mentioned aperture and turning it until the threaded section of the screw has passed completely through that first aperture.The screw will now be freely rotatable therein but axially retained in the first housing member by virtue of the reduced diameter unthreaded section between the screw threaded region and the head of the screw. Twisting the screw after the threaded section has entered the threaded aperture in the second housing member causes the screw to tighten into the second housing member, to hold the first housing member against the second. Separation of the two housing members is achieved by simply unscrewing the screw until the threaded section of the screw has completely left the threaded aperture in the second housing member.

By virtue of the threaded section being larger in diameter than aperture of the hole in the first mentioned member, the screw is held captive in the second member.

The present invention seeks to provide an improved method of forming a retention facility for axially retaining a metal screw captive in a plastics housing.

Summary of the invention According to the present invention, a method of forming a retention device for holding captive a metal screw in a plastics housing having an aperture formed therein through which the screw can initially pass, the screw being of the type having an enlarged head, a threaded end and a region of reduced diameter between the threaded end and the head, comprises the steps of: 1. inserting the screw into the aperture so that the enlarged head of the screw abuts the plastics material at one end of the aperture, 2. subjecting the screw to an impact force in a direction tending to drive the screw further through the aperture thereby deforming the material forming the aperture wall immediately below the head of the screw to form a region of reduced diameter back through which the threaded section of the screw cannot pass.

In order to provide a better retention facility the screw preferably includes an annular flange spaced from the underside of the screw head and typically located at the junction between the threaded section of the screw and the reduced diameter section thereof.

Typically the diameter of the aperture in the plastics material into which the screw is introduced and prior to impact and deformation, is just slightly larger than the diameter of the larger part of the screw which is to pass therethrough. Where an annular flange is provided as aforesaid, this annular flange determines the maximum diameter of the screw (disregarding the screw head) and the hole must be sized accordingly.

Typically the diameter of the hole is some five thousandths of an inch larger in diameter than the largest diameter section of the screw which has to pass therethrough.

Preferably the housing is located on a metal anvil before the impact force is applied to the screw and in order to accommodate the protruding length of the screw, the anvil is preferably apertured or otherwise cut away as appropriate.

Typically the anvil has a hole which is commensurate in diameter with the diameter of the hole through which the screw extends so that the housing material is fully supported by the anvil around the aperture through which the screw protrudes.

Preferably the anvil includes means for locating the housing relative thereto so that the method includes the step of positioning the housing on the anvil adjacent the locating means so that the aperture in the housing aligns with an aperture or cut-away region of the anvil.

Apparatus for performing the method of the invention comprises an anvil having an aperture to accommodate the protruding end of the screw, positioning means for aligning the housing containing the aperture so that the aperture in the housing aligns with the aperture in the anvil, and hammer means for delivering an impact force in a downward sense to the upper end of the screw located within the aperture in the housing when the latter is correctly positioned.

The apparatus may additionally include means for holding the housing in position on the anvil whilst the impact force is applied.

Where holding means is provided, an interlock may be provided to prevent the impact force being applied until such time as the housing has been correctly positioned and the holding means is in place to secure the housing on the anvil.

The hammer means may be hand operated or powered.

The invention also lies in a housing having a screw rotatably fitted within an aperture in the housing, the screw being held captive therewith in by means of a radial deformation of the wall of the aperture immediately below the head of the screw and formed in accordance with the invention by means of an impact force on the head of the screw whilst the housing is restrained from movement.

The invention also lies in a housing having fitted thereto a screw which is rotatable in an aperture in the housing but is held captive therein so that it cannot axially leave the aperture by means of a radial deformation in the wall of the aperture immediately adjacent the head of the screw formed in accordance with the method of the invention.

The invention is of particular application to the fitting of securing screws to the covers for electricity measuring meters.

The invention will now be described by way of example, with reference to the accompanying drawings in which: Figure 1 is a perspective view of an electricity meter housing in which the cover is retained on the body of the meter by means of two rotatable screws and a supplementary coverplate is fitted over the terminal block assembly and likewise held in place by a similar screw held captive in the coverplate, Figure 2 is a view of the lower part of the meter showing the coverplate in more detail, Figure 3 is a cross-section through a part of the cover in the region where one of the retaining screws extends therethrough, before the screw retention facility has been created by impact in accordance with the invention, and Figure 4 is a similar view to that of Figure 3 showing the radial deformation immediately below the head of the screw after impact has been applied thereto in accordance with the invention.

Figure 1 of the drawings illustrates an electricity measuring meter which happens to be of the type which accepts prepayment cards.

The meter is formed from a base unit 10, a lid 12 and a coverplate 14. The two items 12 and 14 serve to enclose the open face of the base 10 and collectively they house the circuitry and components employed to measure the amount of electricity consumed by a particular load connected to the meter such as a domestic load.

A window 16 enables an LCD display to be seen in which units of electricity purchased and consumed are indicated and in which various other items of information can be displayed from time to time as appropriate.

As mentioned the meter operates to supply electricity only upon presentation thereto of a suitable prepaid card such as 18 which is entered into the slot 20.

In accordance with the invention the lid 12 and the cover 14 are secured in place by means of screws such as 22 and 24 and 26 which are held captive in the respective housing parts. In this way on being unscrewed (to enable for example, the lid 12 to be removed from the base 10), the two screws 22 and 24 are not "lost" but remain captive within the slots in the housing 12 and are available to be screwed into the base once again when the lid is to be refitted.

Figure 2 of the drawings shows the cover 14 in more detail together with the screw 26 holding the cover in place.

Figure 3 is a cross-section view through the central region of the cover 14 before radial deformation required to hold the screw 26 captive has been formed. The cover 14 includes a generally cylindrical upstanding sleeve 28 and a co-axial aperture 30 through which the shank of the screw can pass with a small clearance, typically 5 thousandths of an inch.

The screw shank is divided into two sections, a first threaded section 32, a second unthreaded section 34 and between these two sections is an annular flange 36. The annular flange determines the maximum diameter of the shank and the small clearance previously referred to exists between it and the wall of the aperture 30.

The screw is held captive within the aperture 30 by radially deforming the material at the upper end of the aperture 30 so as to form a reduced diameter neck region through which the flange 36 cannot pass. In accordance with the invention the deformation is achieved by an impact force applied in the direction of arrow 38 to the head of the screw whilst the housing cover 14 is supported on an apertured anvil 40 part of which is shown in Figure 4. The aperture 42 in the anvil allows the protruding screw sections 32 and 36 to move in a downward direction relative to the cover 14 in response to the impact force 38.Surprisingly it is found that a downward force such as that shown at 38 causes the plastics material making up the cover 14 to flow not only in a downward but also a radially inward direction in response to the impact so as to produce a deformation such as that shown at 44 in Figure 4.

Since the screw thread is not engaged and it is free to move, no damage occurs to the screw as a result of the impact force which is preferably applied to the screw through a screwdriver blade shaped tool, in which the blade enters the screwdriver slot across the head of the screw.

In precisely the same way the screws 22 and 24 are held captive in similarly deformed apertures formed in the front wall of the lid 12.

The deformation is achieved provided the material from which the cover or lid is formed will flow under appropriate impact force. Plastics materials have been found to possess this characteristic.

It is also important that the screw does not become deformed or damaged as a result of the impact force and to this end hardened steel screws are preferred.

Since the anvil must withstand the impact force transmitted through the screw head and wall section 14, the anvil is likewise preferably formed from steel or some other non-deformable hard material.

Although described with reference to a card operated meter, it is to be understood that the invention may be applied to any electricity measuring meter whether card operated or otherwise and the invention has general application where metal screws are to be retained in plastics components whether they form parts of electricity meters or not.

Claims

1. A method of forming a retention device for holding captive a metal screw in a plastics housing having an aperture formed therein through which the screw can initially pass, the screw being of the type having an enlarged head, a threaded end and a region of reduced diameter between the threaded end and the head, comprising the steps of: inserting the screw into the aperture so that the enlarged head of the screw abuts the plastics material at one end of the aperture; and subjecting the screw to an impact force in a direction tending to drive the screw further through the aperture thereby deforming the material forming the aperture wall immediately below the head of the screw to form a region of reduced diameter back through which the threaded section of the screw cannot pass.

2. A method according to claim 1 in which, in order to provide a better retention facility, the screw includes an annular flange spaced from the underside of the screw head and typically located at the junction between the threaded section of the screw and the reduced diameter section thereof.

3. A method according to claim 1 or claim 2 in which the diameter of the aperture in the plastics material into which the screw is introduced and prior to impact and deformation, is just slightly larger than the diameter of the larger part of the screw which is to pass therethrough.

4. A method according to claim 3 in which the diameter of the hole is some five thousandths of an inch larger in diameter than the largest diameter section of the screw which has to pass therethrough.

5. A method according to any one of claims 2 to 4 in which the housing is located on a metal anvil before the impact force is applied to the screw and, in order to accommodate the protruding length of the screw, the anvil is apertured or otherwise cut away as appropriate.

6. A method according to claim 5 in which the anvil has a hole which is commensurate in diameter with the diameter of the hole through which the screw extends so that the housing material is fully supported by the anvil around the aperture through which the screw protrudes.

7. A method according to claim 5 or claim 6 in which the anvil includes means for locating the housing relative thereto, and further comprising the step of positioning the housing on the anvil adjacent the locating means so that the aperture in the housing aligns with an aperture or cut-away region of the anvil.

8. Apparatus for performing the method according to any one of claims 1 to 7, comprising an anvil having an aperture to accommodate the protruding end of the screw, positioning means for aligning the housing containing the aperture so that the aperture in the housing aligns with the aperture in the anvil, and hammer means for delivering an impact force in a downward sense to the upper end of the screw located within the aperture in the housing when the latter is correctly positioned.

9. Apparatus according to claim 8 and further comprising holding means for holding the housing in position on the anvil whilst the impact force is applied.

10. Apparatus according to claim 9 in which an interlock is provided to prevent the impact force being applied until such time as the housing has been correctly positioned and the holding means is in place to secure the housing on the anvil.

11. Apparatus according to any one of claims 8 to 10 in which the hammer means is either hand operated or powered.

12. A housing having a screw rotatably fitted within an aperture in the housing, the screw being held captive therewithin by means of a radial deformation of the wall of the aperture immediately below the head of the screw and formed by means of an impact force on the head of the screw whilst the housing is restrained from movement.

13. A housing formed in accordance with the method of claims 1 to 7 and having fitted thereto a screw which is rotatable in an aperture in the housing but is held captive therein, so that it cannot axially leave the aperture, by means of a radial deformation in the wall of the aperture immediately adjacent the head of the screw.

14. A method of forming a retention device substantially as herein described with reference to the accompanying drawings.

15. Apparatus for forming a retention device substantially as herein described with reference to the accompanying drawings.

16. A housing having a screw rotatably fitted within an aperture therein substantially as herein described with reference to the accompanying drawings.