Conventional economy brand caulking guns are generally manual trigger-operated devices incorporating a unidirectional gripping assembly which urges a piston rod forward to eject the compound from a cartridge. As shown in FIG. 1 (prior art), a common gripping assembly includes a piston rod 4 passing through a gripping plate 6 that, in turn, is controlled by the operation of a trigger 8 to grip and move the piston rod 4 in a forward direction for dispensing the caulking composition. The trigger 8 cants the gripping plate 6 clockwise when the trigger is actuated until it grips the piston rod 4, thereafter urging the piston rod 4 forward. Upon release of the trigger 8, the gripping plate 6 is returned to a neutral upright position to allow retraction of the piston rod 4. However, the piston rod 4 also passes through a release plate 10 that prevents undesired retraction of the piston rod 4. The release plate 10 is commonly located at the rear of the gun and remains disengaged from the piston rod 4 during dispensation. However, when the trigger 8 is released, the release plate 10 rides the piston rod 4 rearwardly and becomes increasingly canted relative to the piston rod 4 until it engages the piston rod to thereby prevent inadvertent retraction. The user must depress the release plate 10 to free the piston rod 4 for full retraction.
Over time the alternating clockwise and counterclockwise torques imposed on the steel thrust components of the above-described mechanism tend to create wear and tear tolerances. Specifically, the hole through the gripping plate 6 and the release plate 10 become enlarged. These wear tolerances combine with the inherent manufacturing tolerances to the point that they effect the operation of the gripping plate and release plate. The effects includes excessive trigger slack and "piston rod lock". With regard to FIG. 1, piston rod lock occurs when the hole in the gripping plate becomes so large that the gripping plate tends to return past its normally-neutral upright position (.o slashed.1) to an oppositely-canted position (.o slashed.2) Because of the undesired cant of angle (.o slashed.2), the rod 4 again grips the piston rod 4, and the rod 4 cannot be pulled back even if the release plate 10 is released.
There have been previous attempts at implementing wear compensating devices to prevent the above-described problem. For instance, U.S. Pat. No. 5,370,282 to Sedlmeier shows an advancing mechanism (6) with two pivoting clamping levers (12, 13) mounted on the piston rod (7). Clamping lever (12) is for forward advancement while lever (13) prevents retreat. Two springs (14, 15) are required, each in contact with a corresponding clamping lever (12, 13). In addition, an unlocking lever (17) operates via a transmission element 18 to substantially simultaneously unlock both of the clamping levers (12, 13) to free the piston rod (7). The clamping lever 13 rests in a pendular manner in a recess of the frame 5. When the piston rod 7 moves forward, the clamping lever 13 pivots forward along with rod 7 to a perpendicular position (clockwise about the top). When the rod 7 is released, the clamping lever 13 pivots backward (counterclockwise about the top) until it inclines enough to bite the rod 7, thereby locking it in position. When the cartridge is spent, the unlocking lever (17) operates through the transmission element 18 to kick clamping lever 13 forward to release the rod. Thus, the use of the transmission element 18 to unlock both of the clamping levers (12, 13) somewhat resolves the problem of a worn gripping plate that returns past its normally-neutral upright position to improperly regrip the piston rod. However, the '282 transmission element 18 is a small tube that rides the piston rod. It is only capable of pushing against the immediate periphery of the hole in the clamping lever 13. This gives the transmission element 18 very poor leverage against the rod, and as wear becomes worse the transmission element 18 loses its ability to return the clamping lever 13 to a vertical position. The problem of piston rod lock reoccurs. Moreover, the thrust mechanism is complex inasmuch as it requires two separate pivoting clamping levers (one for moving the piston rod forward and one for locking the piston rod in the forward position), two springs each contacting one of the clamping levers, plus a release lever, and the transmission element. The additional parts and labor greatly increases manufacturing costs and assembly time.
Similarly, U.S. Pat. No. 5,823,403 to Schneider shows a drive mechanism that provides a double-lever action to bias the gripping plate into a neutral position in a controlled manner to permit the piston rod to be retracted. A gripping (or recoil) plate 52 is normally biased by spring 50 into a canted position to prevent rearward movement of the piston rod 26. However, when the piston rod 26 moves forwardly the recoil plate 52 pivots forward along with rod 26 to a perpendicular position (counter-clockwise about the bottom) and rod 26 becomes free to slide through the recoil plate 52. The recoil plate 52 is situated rearwardly of the frame and this necessitates a backplate 40. The '403 patent appears to be functionally similar to the above-described '282 patent except that the release lever 17 of the '282 gun kicks both levers 12 and 13 out via sleeve 18, while the '403 lever 62 only kicks out the recoil plate 52. Otherwise, both mechanisms requires three separate pivoting levers (one for moving the piston rod forward, one for locking the piston rod in the forward position, and a release plate). Moreover, both require two springs each contacting one of the clamping levers, plus a transmission element. The sleeve 18 only pushes against the immediate periphery of the hole (giving it very poor leverage against the rod), and this provides a temporary solution at best to the wear problem because sleeve 18 eventually loses its ability to prevent piston rod lock.
It would be advantageous to provide a more reliable solution to piston rod lock and other wear-and-tear problems to ensure proper advancing and releasing operation in an economical two-lever drive assembly.