1. Field of the Invention
The present invention pertains generally to the field of rivet fasteners and is more particularly directed to blind rivets for making fluid-tight rivet joints.
2. State of the Prior Art
Ordinary rivets generally consist of a solid cylindrical shank terminating at one end in a rivet head. Two or more structural members, such as sheets of metal, may be fastened together by inserting the rivet shank through aligned holes in the overlapping sheets, and driving the end of the rivet shank with a suitable riveting tool so as to enlarge its diameter. The sheets are thus held tightly together between the rivet head on one side and the enlarged driven end of the rivet on the opposite side. Such rivet fasteners have been long known and due to the high strength and long term reliability of rivet joints they have been used successfully in a wide variety of applications, particularly in the assembly of aircraft frames.
Ordinary rivets, however, cannot be relied upon to make a fluid-tight joint. In certain applications, for example, in wet aircraft wings where the hollow wing also serves as a fuel tank, it was soon discovered that fuel would leak through the rivet joints due to an imperfect seal between the rivet shank and the sheet metal. The rim of the hole into which the rivet shank is inserted often is not perfectly circular and may also have serrations caused by the drilling process. The rivet shank is normally made of relatively hard material and although it may expand somewhat within the hole during the rivet setting process, it does not flow sufficiently to fill small irregularities. As a result, small openings remain in the rivet joints, which when multiplied by the many hundreds of rivets typically used in assembling an air frame, would allow for considerable total leakage. Even very small leaks of fuel or fuel fumes in an aircraft create an unacceptable risk of fire and explosion and must be avoided.
This problem was overcome in U.S. Pat. No. 2,410,398 to Williams Jr. et al by providing a thin, outer jacket of soft aluminum over the shank of an otherwise conventional rivet. Various such outer jacket configurations are known, some extending the full length of the rivet shank, others consisting of an under-head washer covering only the conical inner surface of a flathead or countersink rivet head, and still others having both a shank jacket and an under-head washer. These fluid-tight rivets were developed in response to the demand of the aircraft industry for a dependable leak-proof rivet for use in aircraft wet wing structural assemblies for transport and cargo aircraft during the second World War, and are still in widespread use in the manufacture, maintenance, and repair of commercial and military aircraft. These rivets are self-sealing when installed and driven by normal riveting methods and tools. The self-seal occurs due to the flow characteristics of the softer metal outer jacket and/or under-head washer, without the addition of liquid sealants, caulks or compounds. When the rivet shank expands within the rivet hole, the softer jacket flows under the pressure of the expanding rivet shank so as to fill any minute voids between the hole rim and the shank, thus forming the fluid-tight seal.
Another sometimes problematic characteristic of conventional rivets is that their installation requires access to both sides of the workpiece. This is because the rivet is inserted into the hole on one side of the workpiece, i.e. the side on which the rivet head remains, and the rivet is then driven from the opposite side so as to flatten or expand the other end and thus make the rivet joint. In certain situations arising in the assembly of aircraft structures, easy access to both sides of the workpiece is not possible. In order to meet the need thus created, so-called blind rivets have been devised which allow the rivet joint to be made by working from only one side of the workpiece. Such rivets, of which there are various designs, have been in commercial use for a considerable length of time.
Blind rivets typically comprise a rivet body or shank having a head at one end and an opposite inner end. A pull-stem extends through an axial inner bore in the rivet body and terminates in a stem head on the blind side of the rivet unit. A collapsible barrel is mounted coaxially on the pull-stem between the stem head and the inner end of the rivet body. The outer end of the pull-stem extends from the head end of the rivet shank and can be grasped by a riveting tool which bears against the rivet head while simultaneously pulling on the pull-stem. As the stem is pulled, the stem head drives the collapsible barrel against the inner end of the rivet body. The barrel is forced into the inner end causing the tubular rivet body to expand at its inner end. The barrel's penetration is halted at a point where the structure surrounding the rivet body, i.e. the rim of the rivet hole prevents the rivet shank from sufficiently expanding to allow further penetration by the collapsible barrel. At this stage, as further pulling force is applied to the pull-stem, the barrel collapses under compressive force between the stem head and the rivet body. The stem is pulled until the barrel is fully collapsed and a large bulb is formed as a result of the combined expansion of the rivet shank and collapse of the barrel on the blind side of the structure being fastened. The pull-stem can then be brokenoff flush with the rivet head to thus complete installation of the rivet. This type of rivet possesses expansion characteristics superior to those of other blind rivets.
A more detailed description of such a blind rivet fastener, including specification of dimensions and material characteristics suitable for its manufacture are found in U.S. Pat. No. 4,451,189 issued to Pratt.
A blind rivet which still further improves over the Pratt rivet has recently become available and is characterized by a radial locking groove on the pull-stem within the inner bore of the rivet body. As the rivet body expands during installation it also extrudes inwardly into the locking groove to form a radial flange which mechanically interlocks to prevent subsequent withdrawal of the pull-stem from the rivet body. Provision of this locking groove eliminates the need for a separate locking ring and a flared seat therefor in the rivet head which are found in the Pratt rivet.
A real need has existed for a blind rivet which is also capable of making a fluid-tight joint in applications similar to those in which fluid-tight conventional rivets have been used. In spite of continuing progress in the development of blind rivets the long felt need for a fluidtight blind rivet, particularly in modern jet aircraft construction, remains unsatisfied and all efforts directed towards such development have until now ended in failure. Even though fluid-tight ordinary rivets have been known, accepted and used in the aircraft industry for several decades, no effective fluid-tight blind rivet is known in the industry.