This invention relates to fasteners.
The invention has particularly been devised for use where it is intended to secure a roofing or cladding panel to a purlin or stanchion, with a layer of insulation between the two. It is desirable that the insulation material should not be crushed or compressed excessively and hence the fastener required to secure the panel in position must be capable of supporting the panel at a position spaced from the purlin or other structural member to which it is to be secured, as well as performing the actual securing operation.
A previously proposed type of fastener has comprised a self-tapping screw having an enlarged head, a resilient annular washer disposed beneath the head and being too large to pass through the hole provided in the panel, a rigid expander disposed below the washer and being of annular shape with an inwardly tapering lower end, and a rigid support sleeve having a slightly flared upper end, co-operable with the taper on the expander. The arrangement was such that the screw head and resilient washer remained on the outer side of the panel whilst the sleeve and the expander passed through the hole in the panel. As the self-tapping screw was screwed into the structural member, the spacer sleeve was trapped above the structural member and forced against the expander so that the flared upper end of the sleeve was forced up the taper and expanded further. Eventually, when the fastener was fully tightened, the flared end of the spacer sleeve was sufficiently enlarged to be pressed against the underside of the panel while the resilient sealing washer was firmly pressed against the upper side, sealing the aperture in the panel.
This previously known fastener had a disadvantage that it was possible accidentally to start expanding the upper end of the sleeve whilst the flared portion was above, rather than below, the level of the panel. In this case, the panel would not be properly supported and might not even be properly secured to the structural member underneath. Furthermore, the seal between the resilient washer and the panel was wholly dependent on there being sufficient pressure exerted by the head to hold the underface of the washer against the upper face of the panel in sealing engagement.