Tools – Wrench – screwdriver – or driver therefor – Rigid jaws
Reexamination Certificate
1998-07-01
2001-04-03
Smith, James G. (Department: 3723)
Tools
Wrench, screwdriver, or driver therefor
Rigid jaws
C081S437000, C081S124100, C081S119000
Reexamination Certificate
active
06209424
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to improvements in clamping screws or shear bolts used to exert a clamping force on an object or objects by the screwing and tightening of the bolt thereon.
As a conventional bolt is tightened to exert a clamping force on an object the torque required to further tighten the bolt increases. When the torque reaches a required level to provide the clamping force the conventional shear bolts are provided with reduced necks and shear at this point such that the head portion is removed. The shearing action of the bolt is commonly succesful but the position of the shear is entirely dependent upon the positioning of the neck.
This causes problems in many uses and when the article upon which the bolts are used is an electrical cable connector and the objects to be clamped are cable cores, the shank of the bolt which remains can protrude above the surface of the connector. When a connector is used, portions of the two cable cores to be connected are placed in the connector and covered and shear bolts are applied to the connector to be screwed into threaded bores therein to provide a clamping force onto the cores and hold the same in place. When a certain clamping torque is reached the bolt shears at the neck and can leave part of the bolt shank protruding above the surface of the connector. The existence of these protrusions cause particular concern when used with high voltage cable connectors as each of the protrusions can cause what is known as a “corona” effect to occur. A corona effect occurs due to the attraction of high voltage stresses at points which are of a different form than the connector such as the protrusions caused by the sheared bolts. The corona effect leads to the insulation material around and adjacent the protrusions being eroded and destroyed by the high voltage stresses.
Further problems are caused when it is desired that the connector be coated by a heat shrinkable insulating material or placed in a close fitting sleeve of insulating material to protect the same once the cable cores are in position. The existence of the protrusions on the connector can produce protrusions on the insulating material which are undesirable or may prevent the fitting of the sleeve.
In an attempt to overcome these problems it is possible to use packing pieces in conjunction with the bolt when the cable core to be clamped is a relatively small diameter cable compared to the bore of the connector and/or to use differing lengths of bolts for each diameter of cable core to be clamped. These are not ideal solutions as the use of additional components adds time to the fitting of the bolts and the range of sizes of bolts which now exist is excessively large and expensive to stock.
The aim of the present invention is to provide a bolt or screw clamping means for use in clamping an object in position, said bolt being provided with means to allow at least a portion to shear off once a predesignated clamping torque is reached and to provide drive means for the same, the said apparatus of a form to allow the position and plane of the shear to be controlled and predicted thereby eliminating the production of protrusions on the surface of the article when the bolt shears.
SUMMARY OF THE INVENTION
The present invention provides a screw clamping apparatus comprising a screw shank and a drive means connectable to the shank for transmission of torque from the drive means to the shank to screw it into a bore in an article to apply a clamping effect, and when the shank applies a predetermined turning torque to produce a clamping force the shank shears in a transverse shear plane with continued turning of the drive means, wherein the axial position of the shank is movable relative to the drive means and the drive means can seat on the article adjacent the bore, whereby said shear plane is created substantially in the plane of the surface of the article around said bore regardless of the extent to which the shank, within a predetermined range, enters the bore.
According to a first embodiment, the drive means comprises a drive rod of non circular section slidable in a recess axially of the shank whereby the shank is driven by rotation of the drive rod, and a support rotatably supports the drive rod and is adapted to seat the article surface adjacent the bore.
In this embodiment the screw clamping means comprises the threaded shank and the drive means, the drive means including a drive rod for engagement in the axial recess in said shank and a support means, wherein the support means and the drive rod are rotated together to tighten the shank to bring the support means into contact with the surface of the article adjacent the bore; continued driving of the drive means causing the shank to shear when a predesignated clamping force is reached. Alternatively the support means is held in a fixed position and the drive rod only is driven.
Typically the shank is caused to shear on a plane substantially defined by the end of the drive rod of the drive means which is positioned in the axial recess of the shank. In this embodiment it is the position of the end of the drive rod which determines the plane of shear of the shank. The position of the end of the rod may be provided to lie directly along the plane of the surface of the article or slightly below the plane but still substantially in the same plane of the surface of the article. This arrangement ensures that the top of the sheared shank does not protrude above the surface of the article and, if the article has a curved surface, that the top of the sheared shank lies on or below the lowest point of the bore edge.
In one embodiment the support means comprises a plurality of downwardly depending legs the ends of which rest on the article to which the shank is applied.
Alternatively the support means is a downwardly depending skirt which passes substantially around the drive rod.
Typically the drive rod and the axial recess of the shank have a polygonal cross section such as, for example, hexagonal or 12 sided.
Typically the length of the drive rod relative to the article determines the plane of shear upon a predesignated clamping torque level being reached. In all cases the shank will shear in a plane substantially parallel with the end of the drive rod.
In one embodiment the drive means is provided in the form of a drive socket and said shank is provided on the outer surface thereof with a drive formation for the location of drive means therewith. In one embodiment the drive formation is in the form of a head at the end of the shank which is of polygonal cross section and may also have an aperture for the reception of an allum key and in an alternative form the engagement portion is formed along part of the length of the outer surface of the shank.
In one embodiment the axial recess passes through the length of the bolt and the end of the shank is plugged. In one embodiment the plug is formed of brass to provide electrical contact with the clamped article. In one embodiment there is provided an insert which contacts with the bore as it passes therethrough and when the shank is fixed into the bore prevents release of the same due to vibratory action. Typically the insert is a washer held between the plug and the shank at the end thereof.
The shear torque or clamping force at which the shank is designed to shear is defined by the relative sizes of the axial aperture and outside diameter of the shank and the material from which the shank is manufactured.
In any embodiment the shank is preferably provided with at least one weakened point along the length thereof to encourage the shank to shear at those points. Typically the weakened points are notches cut into either or both of the outer surface of the shank or along the wall of the axial recess in the shank.
When used on an electrical cable core connector, the cable cores are clamped in position and the notches are preferably positioned to ensure the shank shears at known positions and these positions can be calculated when used in a connector of known size
Croton David
Elam Trevor
Sicame Electrical Developments Ltd.
Smith James G.
Wilson Lee
Zarley McKee Thomte Voorhees & Sease
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