A number of pneumatically or hydraulically operated impact tools have been designed for driving nails into metal/metal or metal/concrete workpieces, with the fasteners being fed from a spring-loaded magazine. As used herein, the term "nail" is intended to include various type of fastening pins that are designed to be driven into a workpiece. The term "workpiece" denotes a multi-layered arrangement of separate members that are to be fastened together by fastening pins, e.g., a corrugated steel decking plate overlying a framework (substrate) formed of interconnected beams, e.g., I-shaped or H-shaped beams or bar joists (2 L-shaped angle irons secured back to back). Such impact tools, often called "powered drivers" or "powered nail drivers" and exemplified by U.S. Pat. Nos. 4,253,598 4,227,637, 4,196,833, 4,040,554, 3,952,398, 3,711,008, 3,638,532 and 3278103 and the prior art cited therein, generally comprise a housing having a hammer guide track in the form of a bore along which a fastening pin driver element (also known as a "hammer") is reciprocated rapidly on command, and a side opening intersecting the guide track which functions as a fastening pin feed port, whereby a fastening pin may be introduced from a magazine into the guide track in the path of the driver element immediately after the driver element has completed a cycle of operation consisting of a drive stroke and a return stroke. While it is possible to load the magazine with loose fastening pins on a one by one basis, loading is facilitated if the fastening pins are pre-assembled in a plastic strip so as to form a clip which can be easily and rapidly inserted into the magazine, and the magazine is provided with a spring biased pusher element for advancing the clip along a guideway in the magazine through the fastening pin feed port into the hammer guide track.
Examples of clips of fasteners that have been used in powered nail-driving tools are disclosed in U.S. Pat. No. 3954176, issued on May 4, 1976 to Harry M. Haytayan, U.S. Pat. Nos. 4106618 and 4106619, both issued on Aug. 15, 1978 to Harry M. Haytayan; and U.S. Pat. No. 4106819, issued on Aug. 16, 1978 to Harry M. Haytayan; and the prior art cited in the aforementioned patents.
Prior to this invention, the preferred form of nail clip was one that comprised a plastic strip in the form of a row of interconnected sleeves, with a nail extending through and gripped by portions of each sleeve. Such plastic strips are arranged so that the leading sleeve may be sheared off from the remainder of the strip when the nail carried by that sleeve is driven by the hammer of the driver.
However, providing strips of nail-supporting sleeves that can be sheared off as intended without jamming the tool has presented problems. It is essential that the plastic strip be designed so as to avoid or minimize any tendency of the leading nail to tilt as it is introduced into the hammer guide track or while it is in the hammer guide track, since whenever a nail is not aligned with the axis of the guide track, there is an increased likelihood that such nail will jam the tool when the hammer commences its drive stroke.
Accordingly it is known that each sleeve should be sized so as to make a close sliding fit in the hammer guide track. Typically the guide track is a bore of circular cross-section and the sleeves have a generally cylindrical shape with a maximum outer diameter ("o.d.") about 0,010" less than the diameter of the hammer guide track. This requirement is particularly acute for nail clips where the plastic sleeves do not surround the nail shanks for their full lengths and are spaced from one another by interconnecting web sections.
Another known design technique for reducing the likelihood of jamming is to connect them with web sections that minimize the spacing or gap between adjacent sleeves but extend for substantially the full height of the sleeves, whereby the web sections are better able to keep the sleeves perpendicular to the longitudinal axis of the nail strip. Also the web sections are made thin to minimize the shear force required to separate the leading nail sleeve from the next nail sleeve. Typically the web sections have a length measured along the length of the strip of about 0.015 to 0.020" and a width (thickness) of about 0.035".
Another design guide rule to prevent tilting of the nail disposed in the hammer guide track is to make the strip of a relatively high density plastic. As used herein the term "high density plastic", e.g., high density polyethylene, means a material with a durometer value of at least 70 on the A scale. A strip of the type described made of a 70-80 durometer plastic provides a stiffness adequate to keep the leading nail straight in the hammer guide bore, thus minimizing jamming of the tool. However, using a high density plastic nail strip suffers from the handicap that the plastic sleeves do not squash as readily as they would if made of a low density material, so that often a substantial portion of the squashed sleeve will be trapped between the workpiece and the driven nail.
The stiffness of the nail strip is more critical in steel decking applications where metal deck fastening pins (nails) are required to penetrate a relatively thin steel decking, such as a 0.029" thick (22 gage) corrugated steel sheet, and anchor it to a steel substrate, e.g., to the flange of a bar joist as previously described, such flange typically having a thickness of 0,125" . If a plastic sleeve holding the decking pins is made of a relatively stiff material such as an 80 durometer polyethylene, when a decking pin is driven into the steel decking, the plastic sleeve surrounding that pin tends to act as a piston, deforming the decking and thereby transmitting a force that causes the substrate (bar joist) to (a) separate from the decking so as to leave a gap therebetween and/or (b) deform the substrate by bending and twisting the leg portion of the angle rods that make up the bar joists, and/or (c) preventing the pin from being driven fully into the substrate. In that particular circumstance, the plastic sleeve surrounding the driven nail may or may not completely disintegrate or squeeze out from between the nail head and the steel decking; but regardless of what happens to the squashed sleeve, the holding power of the pin is diminished, frequently enough to require application of another pin to assure that the decking is adequately secured to the substrate. In order to achieve maximum holding force, and thus transmit maximum shear strength to the deck complex, it is essential that the underside of the head of the pin (nail) be in full contact with the steel deck when the steel deck is fastened to the bar joist.
Making the strips of a relatively low density polyethylene, e.g., a 40-60 durometer polyethylene, allows the decking pins to be driven so as to achieve maximum holding power. However, the softer low density strips are plagued by the aforementioned tendency of the leading nail to tilt in the hammer guide bore, thereby increasing the likelihood of jamming. This tendency is increased when the tool is used outdoors in hot weather, since a thermoplastic material such as polyethylene will soften with increasing temperature, thereby decreasing the stiffness of the nail strip. In connection with the concern about softening of the nail strip due to increasing temperature, it is important to appreciate that the pushing force exerted on the nail clip by the clip pusher member of the magazine of a tool such as is shown in U.S. Pat. No. 4253598 tends to compress and hence deform the leading sleeve that is disposed in the hammer guide track, and, depending on the ambient temperature and how long the tool lays idle in the sun, the deformation can be sufficient to cause the nail carried by the leading sleeve to be misaligned in the hammer guide track bore, with the result that the tool will jam the next time it is operated.