Blind rivet and method of making same

Details Blind rivet and method of making same Blind rivet and method of making same

1999-07-15

2001-05-01

Wilson, Neill (Department: 3627)

Expanded, threaded, driven, headed, tool-deformed, or locked-thr

Having separate expander means

Bulged by axially contracting ends

C411S043000, C411S070000, C470S030000

Reexamination Certificate

active

06224310

ABSTRACT:

BACKGROUND OF THE INVENTION
The invention relates to a blind rivet. A blind rivet is one that can be inserted and set by access to one side only of apertured members to be riveted together,
SUMMARY OF THE INVENTION
More particularly the invention relates to a self plugging blind rivet comprising a tubular shell having a preformed radial head at one end, and a stem extending through the tubular shell and having a stem head adjacent the tail end of the shell. In use, the rivet is inserted, shell tail end first, through the apertures in the members to be riveted until the shell head contacts the nearer member and the shell tail end protrudes beyond the further member. In order to set the rivet, the shell head is supported by an abutment and the stem is then pulled so that the stem head compresses the shell against the abutment and causes axial contraction of the shell and radial expansion of at least part thereof co form a blind head which, together with the preformed head, clamps the members together. At least part of the stem adjacent to its head is retained within the shell to plug the latter after the rivet has been set. The remainder of the stem i.e. the tail end remote from the head thereof) is preferably broken off to avoid protrusion from the shell head.
Such self-plugging blind rivets are well known and widely used. In practice it is highly desirable for the set rivet to exhibit a number of different advantageous characteristics, e.g. wide grip range (the range of total thicknesses of members over which identical rivets will perform satisfactorily); good clench characteristics (the ability to pull together members which are initially separated), good hole fill (the ability to fill the apertures in the riveted members); high shear strength; good stew retention; high tension strength of the set rivet throughout the grip range; and no protrusion of the retained portion of the stem from the preformed head of the set rivet.
These characteristics involve different design features, some of which are mutually conflicting. The present invention seeks to provide a rivet which meets these characteristics and which is simple and easy to manufacture. In order to obtain wide grip range the rivet must form a satisfactory blind head in a position which is in contact with the back sheet in minimum grip and also provide a satisfactory blind head in thicker sheets right up to a maximum grip. A good blind head is essential if high tension strength is to be attained. Good stem retention needs to be achieved by positive interlocking of stem and shell, not only to prevent the stem from falling out and spoiling the appearance but also to assist in making the joint watertight. Hole fill can be attained by ensuring that the shank of the rivet is comparatively soft so that during installation the shank of the rivet shortens and expands to fill the hole. This effect also contributes to good clench characteristics. If high shear strength is to be maintained, the retained part of the stem in the set rivet must lie within the riveted members and across their cleavate or contact plane. In turn if at the same time the retained part of the stem is not to protrude beyond the preformed shell head, the total length of the rivet must vary little throughout the grip range.
EP-A-0398512 discloses a self-plugging blind rivet comprising a tubular shell shank having a preformed radially enlarged head at one end, and a region of increased diameter and hardness at the tail end, adjacent the head of the stem. The rivet is formed by compressing the stem and a plain cylindrical shell in a die having a portion of enlarged bore diameter at the tail end region, whereby the shell is expanded into the enlarged bore portion with resulting work hardening. In use, the relatively soft region of the shank adjacent the head preferentially buckles under axial compression forces and forms a blind head to clamp together the workpiece members.
The rivet of EP-A-0398512 requires the use of a split die to enable the rivet to be removed. Split dies are complicated in use and are subject to wear. Also the reduced diameter of the major portion of the shank makes the provision of hole fill more difficult.
One possible solution to both these problems is to use a stem which has a circumferential groove in the stem shank immediately under the head and to form the rivet by axially compressing the stem and a plain cylindrical shell in a solid die so that the material of the shell is forced into the circumferential groove to provide a thickened shell wall of increased hardness. Rivets of this construction are shown in GB 2233059B. As in the case of EP-A-0398512, in use, the relatively soft region of the shank adjacent the head preferentially buckles under axial compression forces and forns a blind head to clamp together the workpiece members.
The provision of a circumferential groove under the stem head reduces the strength of the stem at this point and it is therefore necessary correspondingly to reduce the strength of the breakneck which determines the position at which the stem breaks when the rivet is placed. This effect restricts the workable range of stem shank diameter and shell wall thickness, compromising rivet performance.
Furthermore the axial loads required to fill the groove under the stem head also fill the breakneck and unless the breakneck has minimum length this affects the function of the rivet since during placing the material forced into the breakneck needs to be sheared.
In a first aspect, the present invention provides a self plugging blind rivet for securing together apertured members, the rivet comprising a tubular shell having a tail end face at one end and a preformed radially enlarged head at the other end, and a stem which extends through the shell and has a stem head adjacent the tail end face of the shell, in which the region adjacent the stem head is of increased diameter relative to the rest of the stem shank and has a plurality of axial depressions formed therein, the corresponding region of the shell being arranged to fill the axial depressions in the stem and being harder than the material of the shell in a region adjacent the radially enlarged head, whereby, in use and under axial compression loading of the shell, the shell buckles in the region adjacent the member remote from the preformed head to form a blind head in contact with said member so as to clamp together the apertured members.
Preferably, the cross sectional area and strength of the stem in the region adjacent the stem head are substantially equal to those of the cylindrical part of the stem. The distance across the mouth of the depressions may vary between 25 and 75% of the circumferential width of the land areas between and defining the depressions, preferably between 45 and 55%, for example 50%. According to a second aspect of the invention, a method is provided of assembling a rivet comprising a tubular rivet shell having a preformed radially enlarged head at one end, and a stem comprising a head and a shank having a region of increased diameter formed with a plurality of axial depressions adjacent the head, the method comprising inserting the shell in a die having a bore a first part of which closely fits the outside diameter of the shell, the tail end of the shell protruding into a second part of the bore having a larger diameter, supporting the head of the shell with an abutment, inserting a punch which has a first diameter closely fitting the bore of the rivet shell, a second diameter equal to the increased diameter under the head of the stem and a third diameter equal to the larger bore of the die, applying a compressive force between the punch and the abutment so as to open the bore of the tail end of the shell to be equal to the increased diameter under the head of the stem, and compressing the tail end of the shell so as to increase its wall thickness, removing the abutment and punch, inserting a stem and ejecting the assembled stem and shell from the die. During assembly, the material o the shell is forced into the longitudinal rece

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