Metal working – Method of mechanical manufacture – Assembling or joining
Reexamination Certificate
2001-08-08
2004-06-01
Bryant, David P. (Department: 3726)
Metal working
Method of mechanical manufacture
Assembling or joining
C029S798000, C029S521000, C029S243530
Reexamination Certificate
active
06742235
ABSTRACT:
The present invention relates to a method and apparatus for fastening sheet material by self-piercing riveting or clinching. The term “clinching” is also known as “press joining” or “integral fastening”.
Methods and apparatus for riveting of the kind in which a self-piercing rivet is inserted into sheet material without full penetration, such that the deformed end of the rivet remains encapsulated by an upset annulus of the sheet material are known.
FIG. 1
is a diagrammatic section of an example of a riveted joint made by such a riveting method in accordance with the invention. A rivet
1
has a head
2
and a shank
3
terminating in an annular edge
4
. The shank
3
is initially cylindrical but is flared outwardly into the illustrated shape as the rivet is driven into two overlapping sheets
5
,
6
located on a suitably shaped die. As shown, the shank and the edge of the rivet
1
remain embedded in the sheet material
5
,
6
after the rivet has been set.
An improved self-piercing riveting method is described in our European Patent No. 0675774. In this method the sheet material is clamped in the region around the rivet insertion with substantial force prior to the commencement of insertion of the rivet and then during rivet insertion. Clamping is applied between a nose of the riveting machine and the die in the region around the rivet insertion location so that there is minimal distortion of the sheet material during the riveting operation. The process is achieved by using two concentric and independently operable hydraulic cylinders. An outer cylinder applies the clamping force and the inner cylinder applies force to insert the rivet. This method has been proved to increase the strength of the riveted joint and reduce the depth of the annular valley
7
. However, the relatively high level of clamping force required to achieve the improved joint characteristics means that a significant pressure of hydraulic fluid or a heavy-duty spring is required to apply the force. Furthermore, if reaction forces within the joint resulting from rivet insertion exceed the clamping force, the nose will be pushed back up away from the die. This results in a reduction of the potential residual compressive stress that could be imparted to the region around the rivet.
Although the hydraulic riveting process described in our aforementioned patent is effective in producing distortion free joints that have improved fatigue life and reduced standard deviation in static strength, it is does require a relatively bulky rivet setter. Moreover, the two-stage process of applying the clamping force and then applying the rivet insertion force adds to the cycle time.
In other riveting apparatus the two-stage process is replaced by a single stage operation in which the clamping force is provided by the compression of a single internal spring between the nose and the actuator of the rivet setter (hereinafter referred to as “spring clamping”). In a single smooth stroke the clamping force is applied to the sheet material before insertion of the rivet and is increased as the actuator descends and compresses the spring. After predetermined travel distance the punch fitted to the end of the actuator comes into contact with the rivet and insert it into the she material. Continued compression of the spring occurs during insertion of the rivet so that the clamping force continues to increase.
Spring clamping of this kind has disadvantages in several respects. First, tests have established that the fatigue life of a riveted joint produced according to the method is significantly reduced in comparison that of a joint produced using the two-stage process. Secondly, the life of the spring is relatively short unless it is of a considerable size (and therefore very bulky in comparison to a hydraulic clamp). The life of a spring is dependent on its initial load, its final (fully compressed) load and the length of travel between these two positions. Since effective clamping of sheet material for self-piercing riveting requires forces of around 4 to 8 kN and rivets can be in excess of 15 mm in length the spring must be designed to withstand the repeated application of such loads over such stroke lengths. The life of such a spring is typically 100,000 cycles or less. Such rapid degradation of the spring results in the production of joints of variable and unpredictable quality. The riveting apparatus and process thus require stringent monitoring systems and frequent preventative maintenance. An alternative option is to use a larger spring with a better specification but this is usually too bulky to accommodate in a rivet setting apparatus of reasonable size. Thus the life of the spring is usually compromised.
In tests conducted by the applicant, a pair of aluminum sheets (5000 series) of 2 mm thickness were riveted together using a two-stage hydraulic process hydraulic clamping force applied prior to rivet insertion and another pair of identical sheets were riveted using spring clamping. The respective clamping forces were of identical magnitude. The joint produced with hydraulic clamping was found to have a fatigue life of around 1.2 million cycles when tested at 40% of the tensile load to failure (970 lb in this case). The fatigue test applied a tensile load cycling between 388 lb (40% of the tensile failure load) and 38.8 lb (i.e. 10% of the maximum) at a frequency of 20 Hz. In contrast, the joint produced with spring clamping had a fatigue life of only 0.6 million cycles. A further test established that a riveted joint produced without any significant clamping force (i.e. the force applied is sufficient only to hold the nose of the rivet setter steady against the sheet material during the riveting operation but has no effect on the flow or displacement of material of the sheet during rivet insertion) had a fatigue life of 1.1 million cycles.
Self-piercing riveting is closely related to clinching in which two sheets of metal are deformed into locking engagement using a punch-and-die combination. An improved clinching method is described in our European Patent No. 0614405. In this method a hollow rivet or tubular slug is inserted into a clinched joint between sheets and the inner end of a shank of the rivet is outwardly deformed within the clinched joint in such a way that it does not penetrate the panels.
In both clinching and self-piercing riveting methods a C-frame is used to support the riveting apparatus and die. A lower limb of the C-frame supports the die and, in use, deflects a certain distance during the riveting operation as a result of the rivet insertion and clamping forces. This means that in hydraulic clamping systems top-up hydraulic fluid is generally required to maintain the required level of clamping. The slow response of hydraulic fluid systems to the demand for extra loading leads to relatively long cycle times.
It is an object of the present invention to obviate or mitigate the aforesaid disadvantages and to provide for an improved method and apparatus for fastening sheet material by self-piercing riveting or clinching.
According to a first aspect of the present invention there is provided a method for inserting a fastener into sheet material comprising inserting the fastener into at least one sheet without full penetration such that a deformed end of the fastener remains encapsulated by an upset annulus of the sheet material, the sheet material being disposed between a nose and a die of fastening apparatus and the fastener being inserted into the sheet material by means of a plunger that is reciprocal relative to the nose, characterised in that, during a first stage of fastener insertion the sheet material around the fastener insertion location is displaced towards the fastener by virtue of its insertion, and thereafter during a second stage (being after said first stage) of fastener insertion a clamping force of sufficient magnitude is applied between the nose and the die in the region around fastener insertion location so as substantially to prevent flow of displaced sheet material away from the fast
Blacket Stuart Edmund
Clew Nicholas Richard
Bryant David P.
Henrob Limited
Piper Rudnick LLP
LandOfFree
Fastening of sheet material does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Fastening of sheet material, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Fastening of sheet material will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3302334